Spacecraft missions to nearby Planets
The Library of ancient Alexandria
The human brain
Egyptian Hieroglyphics
The origin of life
The death of the Sun
The evolution of galaxies
The origins of matter, suns and worlds
COSMOS
The story of fifteen billion years of cosmic evolution transforming matter and life into
consciousness, of how science and civilization grew up together, and of the forces andindividuals who helped shape modern science. A story told with Carl Sagan’s remarkableability to make scientific ideas both comprehensible and exciting, based on his acclaimed
television series.

For Ann Druyan
In the vastness of space and the immensity of time,
it is my joy to sharea planet and an epoch with Annie

Carl Sagan was the Director of the Laboratory for Planetary studies and David Duncan
Professor of Astronomy and Space Sciences at Cornell University. He played a leading rolein the Mariner, Viking and Voyager expeditions to the planets, for which he received theNASA medals for Exceptional Scientific Achievement and for Distinguished PublicService, and the international astronautics prize, the Prix Galabert.
He served as Chairman of the Division for Planetary Sciences of the American
Astronomical Society, as Chairman of the Astronomy Section of the American Association
for the Advancement of Science, and as President of the Planetology Section of theAmerican Geophysical Union. For twelve years, he was Editor-in-Chief of Icarus, theleading professional journal devoted to planetary research. In addition to four hundred
published scientific and popular articles, Dr Sagan was the author, co-author or editor of
more than a dozen books, including Intelligent Life in the Universe, The Cosmic
Connection, The Dragons of Eden, Murmurs of Earth, Broca’s Brain and the bestsellingscience fiction novel, Contact.
He was a recipient of the Joseph Priestly Award ‘for distinguished contributions to thewelfare of mankind’, and the Pulitzer Prize for literature.
Carl Sagan died in December 1996.

CARL SAGAN
CosmosCONTENTS
Introduction
1

The Shores of the Cosmic Ocean

2

One Voice in the Cosmic Fugue

3

The Harmony of Worlds

4

Heaven and Hell5

Blues for a Red Planet6

Travelers’ Tales

7

The Backbone of Night

8

Travels in Space and Time

9

The Lives of the Stars

10

The Edge of Forever

11

The Persistence of Memory

12

Encyclopaedia Galactica

13

Who Speaks for Earth?
Appendix 1: Reductio ad Absurdum and the Square Root of Two
Appendix 2: The Five Pythagorean Solids

INTRODUCTION
The time will come when diligent research over long periods will bring to light things
which now lie hidden. A single lifetime, even though entirely devoted to the sky, would not
be enough for the investigation of so vast a subject . . . And so this knowledge will be
unfolded only through long successive ages. There will come a time when our descendants
will be amazed that we did not know things that are so plain to them . . . Many discoveries
are reserved for ages still to come, when memory of us will have been effaced. Our
universe is a sorry little affair unless it has in it something for every age to investigate . . .
Nature does not reveal her mysteries once and for all.
- Seneca, Natural Questions,Book 7, first century
In ancient times, in everyday speech and custom, the most mundane happenings wereconnected with the grandest cosmic events. A charming example is an incantation against
the worm which the Assyrians of 1000 B.C. imagined to cause toothaches. It begins withthe origin of the universe and ends with a cure for toothache:
After Anu had created the heaven,
And the heaven had created the earth,
And the earth had created the rivers,
And the rivers had created the canals,
And the canals had created the morass,
And the morass had created the worm,
The worm went before Shamash, weeping,His tears flowing before Ea:‘What wilt thou give me for my food,
What wilt thou give me for my drink?’‘I will give thee the dried fig
And the apricot.’‘What are these to me? The dried fig
And the apricot!
Lift me up, and among the teeth
And the gums let me dwell! . .
Because thou hast said this, O worm,
May Ea smite thee with the might of
His hand!
(Incantation against toothache.)Its treatment: Second-grade beer . . . and oil thou shalt mix together;
The incantation thou shalt recite three times thereon and shalt put the medicine upon
the tooth.
Our ancestors were eager to understand the world but had not quite stumbled upon
the method. They imagined a small, quaint, tidy universe in which the dominant forceswere gods like Anu, Ea, and Shamash. In that universe humans played an important if not acentral role. We were intimately bound up with the rest of nature. The treatment of
toothache with second-rate beer was tied to the deepest cosmological mysteries.
Today we have discovered a powerful and elegant way to understand the universe, amethod called science; it has revealed to us a universe so ancient and so vast that humanaffairs seem at first sight to be of little consequence. We have grown distant from the
Cosmos. It has seemed remote and irrelevant to everyday concerns. But science has found
not only that the universe has a reeling and ecstatic grandeur, not only that it is accessible
to human understanding, but also that we are, in a very real and profound sense, a part ofthat Cosmos, born from it, our fate deeply connected with it. The most basic human eventsand the most trivial trace back to the universe and its origins. This book is devoted to the
exploration of that cosmic perspective.
In the summer and fall of 1976, as a member of the Viking Lander Imaging Flight
Team, I was engaged, with a hundred of my scientific colleagues, in the exploration of theplanet Mars. For the first time in human history we had landed two space vehicles on thesurface of another world. The results, described more fully in Chapter 5, were spectacular,
the historical significance of the mission utterly apparent. And yet the general public waslearning almost nothing of these great happenings. The press was largely inattentive;
television ignored the mission almost altogether. When it became clear that a definitive
answer on whether there is life on Mars would not be forthcoming, interest dwindled still
further. There was little tolerance for ambiguity. When we found the sky of Mars to be akind of pinkish-yellow rather than the blue which had erroneously first been reported, theannouncement was greeted by a chorus of good-natured boos from the assembled reporters- they wanted Mars to be, even in this respect, like the Earth. They believed that theiraudiences would be progressively disinterested as Mars was revealed to be less and lesslike the Earth. And yet the Martian landscapes are staggering, the vistas breathtaking. I was
positive from my own experience that an enormous global interest exists in the explorationof the planets and in many kindred scientific topics - the origin of life, the Earth, and the
Cosmos, the search for extraterrestrial intelligence, our connection with the universe. And I
was certain that this interest could be excited through that most powerful communications
medium, television.
My feelings were shared by B. Gentry Lee, the Viking Data Analysis and Mission
Planning Director, a man of extraordinary organizational abilities. We decided, gamely, todo something about the problem ourselves. Lee proposed that we form a production
company devoted to the communication of science in an engaging and accessible way. In
the following months we were approached on a number of projects. But by far the mostinteresting was an inquiry tendered by KCET, the Public Broadcasting Service’s outlet in
Los Angeles. Eventually, we jointly agreed to produce a thirteen-part television seriesoriented toward astronomy but with a very broad human perspective. It was to be aimed at
popular audiences, to be visually and musically stunning, and to engage the heart as well as
the mind. We talked with underwriters, hired an executive producer, and found ourselves
embarked on a three-year project called Cosmos. At this writing it has an estimatedworldwide audience of 140 million people, or 3 percent of the human population of theplanet Earth. It is dedicated to the proposition that the public is far more intelligent than it
has generally been given credit for; that the deepest scientific questions on the nature andorigin of the world excite the interests and passions of enormous numbers of people. The
present epoch is a major crossroads for our civilization and perhaps for our species.
Whatever road we take, our fate is indissolubly bound up with science. It is essential as amatter of simple survival for us to understand science. In addition, science is a delight;
evolution has arranged that we take pleasure in understanding - those who understand are
more likely to survive. The Cosmos television series and this book represent a hopefulexperiment in communicating some of the ideas, methods and joys of science.
The book and the television series evolved together. In some sense each is based on
the other. But books and television series have somewhat different audiences and admit
differing approaches. One of the great virtues of a book is that it is possible for the reader toreturn repeatedly to obscure or difficult passages; this is only beginning to become
possible, with the development of videotape and video-disc technology, for television.
There is much more freedom for the author in choosing the range and depth of topics for achapter in a book than for the procrustean fifty-eight minutes, thirty seconds of anoncommercial television program. This book goes more deeply into many topics than does
the television series. There are topics discussed in the book which are not treated in the
television series and vice versa. Explicit representations of the Cosmic Calendar, featured
in the television series, do not appear here - in part because the Cosmic Calendar is
discussed in my book The Dragons of Eden; likewise, I do not here discuss the life ofRobert Goddard in much detail, because there is a chapter in Broca’s Brain devoted to him.
But each episode of the television series follows fairly closely the corresponding chapter ofthis book; and I like to think that the pleasure of each will be enhanced by reference to theother.
For clarity, I have in a number of cases introduced an idea more than once - the firsttime lightly, and with deeper passes on subsequent appearances. This occurs, for example,
in the introduction to cosmic objects in Chapter 1, which are examined in greater detaillater on; or in the discussion of mutations, enzymes and nucleic acids in Chapter 2. In a few
cases, concepts are presented out of historical order. For example, the ideas of the ancient
Greek scientists are presented in Chapter 7, well after the discussion of Johannes Kepler in
Chapter 3. But I believe an appreciation of the Greeks can best be provided after we see
what they barely missed achieving.
Because science is inseparable from the rest of the human endeavor, it cannot be
discussed without making contact, sometimes glancing, sometimes head-on, with a numberof social, political, religious and philosophical issues. Even in the filming of a televisionseries on science, the worldwide devotion to military activities becomes intrusive.
Simulating the exploration of Mars in the Mohave Desert with a full-scale version of theViking Lander, we were repeatedly interrupted by the United States Air Force, performing
bombing runs in a nearby test range. In Alexandria, Egypt, from nine to eleven A.M. every
morning, our hotel was the subject of practice strafing runs by the Egyptian Air Force. In
Samos, Greece, permission to film anywhere was withheld until the very last moment
because of NATO maneuvers and what was clearly the construction of a warren ofunderground and hillside emplacements for artillery and tanks. In Czechoslovakia the useof walkie-talkies for organizing the filming logistics on a rural road attracted the attentionof a Czech Air Force fighter, which circled overhead until reassured in Czech that no threat
to national security was being perpetrated. In Greece, Egypt and Czechoslovakia our filmcrews were accompanied everywhere by agents of the state security apparatus. Preliminary
inquiries about filming in Kaluga, U.S.S.R., for a proposed discussion of the life of theRussian pioneer of astronautics Konstantin Tsiolkovsky were discouraged - because, as welater discovered, trials of dissidents were to be conducted there. Our camera crews met
innumerable kindnesses in every country we visited; but the global military presence, thefear in the hearts of the nations, was everywhere. The experience confirmed my resolve totreat, when relevant, social questions both in the series and in the book.
The essence of science is that it is self-correcting. New experimental results andnovel ideas are continually resolving old mysteries. For example, in Chapter 9 we discuss
the fact that the Sun seems to be generating too few of the elusive particles called neutrinos.
Some proposed explanations are listed. In Chapter 10 we wonder whether there is enoughmatter in the universe eventually to stop the recession of distant galaxies, and whether theuniverse is infinitely old and therefore uncreated. Some light on both these questions may
since have been cast in experiments by Frederick Reines, of the University of California,
who believes he has discovered (a) that neutrinos exist in three different states, only one ofwhich could be detected by neutrino telescopes studying the Sun; and (b) that neutrinos unlike light - have mass, so that the gravity of all the neutrinos in space may help to close
the Cosmos and prevent it from expanding forever. Future experiments will show whether
these ideas are correct. But they illustrate the continuing and vigorous reassessment ofreceived wisdom which is fundamental to the scientific enterprise.
On a project of this magnitude it is impossible to thank everyone who has made acontribution. However, I would like to acknowledge, especially, B. Gentry Lee; the Cosmos
production staff, including the senior producers Geoffrey Haines-Stiles and David Kennard
and the executive producer Adrian Malone; the artists Jon Lomberg (who played a criticalrole in the original design and organization of the Cosmos visuals), John Allison, Adolf
Schaller, Rick Sternbach, Don Davies, Brown, and Anne Norcia; consultants Donald
Goldsmith, Owen Gingerich, Paul Fox, and Diane Ackerman; Cameron Beck; the KCET
management, particularly Greg Andorfer, who first carried KCET’s proposal to us, Chuck
Allen, William Lamb, and James Loper; and the underwriters and co-producers of the
Cosmos television series, including the Atlantic Richfield Company, the Corporation for
Public Broadcasting, the Arthur Vining Davis Foundations, the Alfred P. Sloan Foundation,
the British Broadcasting Corporation, and Polytel International. Others who helped inclarifying matters of fact or approach are listed at the back of the book. The final
responsibility for the content of the book is, however, of course mine. I thank the staff at
Random House, particularly my editor, Anne Freedgood, and the book designer, Robert
Aulicino, for their capable work and their patience when the deadlines for the televisionseries and the book seemed to be in conflict. I owe a special debt of gratitude to Shirley
Arden, my Executive Assistant, for typing the early drafts of this book and ushering thelater drafts through all stages of production with her usual cheerful competence. This is
only one of many ways in which the Cosmos project is deeply indebted to her. I am more
grateful than I can say to the administration of Cornell University for granting me a twoyear leave of absence to pursue this project, to my colleagues and students there, and to mycolleagues at NASA, JPL and on the Voyager Imaging Team.
My greatest debt for the writing of Cosmos is owed to Ann Druyan and Steven
Soter, my co-writers in the television series. They made fundamental and frequent
contributions to the basic ideas and their connections, to the overall intellectual structure of
the episodes, and to the felicity of style. I am deeply grateful for their vigorous criticalreadings of early versions of this book, their constructive and creative suggestions forrevision through many drafts, and their major contributions to the television script which inmany ways influenced the content of this book. The delight I found in our many
discussions is one of my chief rewards from the Cosmos project.
Ithaca and Los Angeles

May 1980

CHAPTER I
The Shores of the Cosmic Ocean
The first men to be created and formed were called the Sorcerer of Fatal Laughter,
the Sorcerer of Night, Unkempt, and the Black Sorcerer . . . They were endowedwith intelligence, they succeeded in knowing all that there is in the world. Whenthey looked, instantly they saw all that is around them, and they contemplated inturn the arc of heaven and the round face of the earth . . . [Then the Creator said]:‘They know all . . . what shall we do with them now? Let their sight reach only tothat which is near; let them see only a little of the face of the earth! . . . Are they notby nature simple creatures of our making? Must they also be gods?’- The Popol Vuh of the Quiché Maya
Have you comprehended the expanse of the earth? Where is the way to the dwellingof light, And where is the place of darkness . . . ?
- The Book of Job
It is not from space that I must seek my dignity, but from the government of mythought. I shall have no more if I possess worlds. By space the universe
encompasses and swallows me up like an atom; by thought I comprehend the world.
- Blaise Pascal, Pensées
The known is finite, the unknown infinite; intellectually we stand on an islet in the
midst of an illimitable ocean of inexplicability. Our business in every generation isto reclaim a little more land.
- T. H. Huxley, 1887
The Cosmos is all that is or ever was or ever will be. Our feeblest contemplations of the
Cosmos stir us - there is a tingling in the spine, a catch in the voice, a faint sensation, as if a
distant memory, of falling from a height. We know we are approaching the greatest of
mysteries.
The size and age of the Cosmos are beyond ordinary human understanding. Lost
somewhere between immensity and eternity is our tiny planetary home. In a cosmic
perspective, most human concerns seem insignificant, even petty. And yet our species isyoung and curious and brave and shows much promise. In the last few millennia we havemade the most astonishing and unexpected discoveries about the Cosmos and our place
within it, explorations that are exhilarating to consider. They remind us that humans have
evolved to wonder, that understanding is a joy, that knowledge is prerequisite to survival. I
believe our future depends on how well we know this Cosmos in which we float like a moteof dust in the morning sky.
Those explorations required skepticism and imagination both. Imagination willoften carry us to worlds that never were. But without it, we go nowhere. Skepticism enables
us to distinguish fancy from fact, to test our speculations. The Cosmos is rich beyond
measure - in elegant facts, in exquisite interrelationships, in the subtle machinery of awe.
The surface of the Earth is the shore of the cosmic ocean. From it we have learnedmost of what we know. Recently, we have waded a little out to sea, enough to dampen our
toes or, at most, wet our ankles. The water seems inviting. The ocean calls. Some part ofour being knows this is from where we came. We long to return. These aspirations are not,I think, irreverent, although they may trouble whatever gods may be.
The dimensions of the Cosmos are so large that using familiar units of distance,such as meters or miles, chosen for their utility on Earth, would make little sense. Instead,
we measure distance with the speed of light. In one second a beam of light travels 186,000
miles, nearly 300,000 kilometers or seven times around the Earth. In eight minutes it willtravel from the Sun to the Earth. We can say the Sun is eight light-minutes away. In a year,
it crosses nearly ten trillion kilometers, about six trillion miles, of intervening space. That
unit of length, the distance light goes in a year, is called a light-year. It measures not time
but distances - enormous distances.
The Earth is a place. It is by no means the only place. It is not even a typical place.
No planet or star or galaxy can be typical, because the Cosmos is mostly empty. The only
typical place is within the vast, cold, universal vacuum, the everlasting night ofintergalactic space, a place so strange and desolate that, by comparison, planets and stars
and galaxies seem achingly rare and lovely. If we were randomly inserted into the Cosmos,
the chance that we would find ourselves on or near a planet would be less than one in abillion trillion trillion* (1033, a one followed by 33 zeroes). In everyday life such odds arecalled compelling. Worlds are precious.
* We use the American scientific convention for large numbers: one billion = 1,000,000,000 = 109;
one trillion = 1,000,000,000,000= 1012, etc. The exponent counts the number of zeroes after the one.

From an intergalactic vantage point we would see, strewn like sea froth on the
waves of space, innumerable faint, wispy tendrils of light. These are the galaxies. Some aresolitary wanderers; most inhabit communal clusters, huddling together, drifting endlessly in
the great cosmic dark. Before us is the Cosmos on the grandest scale we know. We are in
the realm of the nebulae, eight billion light-years from Earth, halfway to the edge of the
known universe.
A galaxy is composed of gas and dust and stars billions upon billions of stars. Everystar may be a sun to someone. Within a galaxy are stars and worlds and, it may be, aproliferation of living things and intelligent beings and spacefaring civilizations. But fromafar, a galaxy reminds me more of a collection of lovely found objects - seashells, perhaps,
or corals, the productions of Nature laboring for aeons in the cosmic ocean.
There are some hundred billion (1011) galaxies, each with, on the average, a hundredbillion stars. In all the galaxies, there are perhaps as many planets as stars, 1011 x 1011 =
1022, ten billion trillion. In the face of such overpowering numbers, what is the likelihoodthat only one ordinary star, the Sun, is accompanied by an inhabited planet? Why should
we, tucked away in some forgotten corner of the Cosmos, be so fortunate? To me, it seems
far more likely that the universe is brimming over with life. But we humans do not yet
know. We are just beginning our explorations. From eight billion light-years away we are
hard pressed to find even the cluster in which our Milky Way Galaxy is embedded, muchless the Sun or the Earth. The only planet we are sure is inhabited is a tiny speck of rockand metal, shining feebly by reflected sunlight, and at this distance utterly lost.
But presently our journey takes us to what astronomers on Earth like to call the
Local Group of galaxies. Several million light-years across, it is composed of some twenty

constituent galaxies. It is a sparse and obscure and unpretentious cluster. One of thesegalaxies is M31, seen from the Earth in the constellation Andromeda. Like other spiralgalaxies, it is a huge pinwheel of stars, gas and dust. M31 has two small satellites, dwarf
elliptical galaxies bound to it by gravity, by the identical law of physics that tends to keep
me in my chair. The laws of nature are the same throughout the Cosmos. We are now two
million light-years from home.
Beyond M31 is another, very similar galaxy, our own, its spiral arms turning
slowly, once every quarter billion years. Now, forty thousand light-years from home, wefind ourselves falling toward the massive center of the Milky Way. But if we wish to findthe Earth, we must redirect our course to the remote outskirts of the Galaxy, to an obscurelocale near the edge of a distant spiral arm.
Our overwhelming impression, even between the spiral arms, is of stars streamingby us - a vast array of exquisitely self-luminous stars, some as flimsy as a soap bubble andso large that they could contain ten thousand Suns or a trillion Earths; others the size of asmall town and a hundred trillion times denser than lead. Some stars are solitary, like theSun. Most have companions. Systems are commonly double, two stars orbiting one
another. But there is a continuous gradation from triple systems through loose clusters of afew dozen stars to the great globular clusters, resplendent with a million suns. Some doublestars are so close that they touch, and starstuff flows beneath them. Most are as separated as
Jupiter is from the Sun. Some stars, the supernovae, are as bright as the entire galaxy that
contains them; others, the black holes, are invisible from a few kilometers away. Someshine with a constant brightness; others flicker uncertainly or blink with an unfalteringrhythm. Some rotate in stately elegance; others spin so feverishly that they distort
themselves to oblateness. Most shine mainly in visible and infrared light; others are also
brilliant sources of X-rays or radio waves. Blue stars are hot and young; yellow stars,
conventional and middle-aged; red stars, often elderly and dying; and small white or blackstars are in the final throes of death. The Milky Way contains some 400 billion stars of allsorts moving with a complex and orderly grace. Of all the stars, the inhabitants of Earthknow close-up, so far, but one.
Each star system is an island in space, quarantined from its neighbors by the lightyears. I can imagine creatures evolving into glimmerings of knowledge on innumerable
worlds, every one of them assuming at first their puny planet and paltry few suns to be allthat is. We grow up in isolation. Only slowly do we teach ourselves the Cosmos.
Some stars may be surrounded by millions of lifeless and rocky worldlets, planetarysystems frozen at some early stage in their evolution. Perhaps many stars have planetarysystems rather like our own: at the periphery, great gaseous ringed planets and icy moons,and nearer to the center, small, warm, blue-white, cloud-covered worlds. On some,intelligent life may have evolved, reworking the planetary surface in some massiveengineering enterprise. These are our brothers and sisters in the Cosmos. Are they very
different from us? What is their form, biochemistry, neurobiology, history, politics, science,technology, art, music, religion, philosophy? Perhaps some day we will know them.
We have now reached our own backyard, a light-year from Earth. Surrounding our
Sun is a spherical swarm of giant snow-balls composed of ice and rock and organic
molecules: the cometary nuclei. Every now and then a passing star gives a tiny gravitationaltug, and one of them obligingly careens into the inner solar system. There the Sun heats it,
the ice is vaporized, and a lovely cometary tail develops.
We approach the planets of our system, largish worlds, captives of the Sun,gravitationally constrained to follow nearly circular orbits, heated mainly by sunlight.
Pluto, covered with methane ice and accompanied by its solitary giant moon Charon, is
illuminated by a distant Sun, which appears as no more than a bright point of light in apitchblack sky. The giant gas worlds, Neptune, Uranus, Saturn - the jewel of the solar
system - and Jupiter all have an entourage of icy moons. Interior to the region of gassy
planets and orbiting icebergs are the warm, rocky provinces of the inner solar system. Thereis, for example, the red planet Mars, with soaring volcanoes, great rift valleys, enormous
planet-wide sandstorms, and, just possibly, some simple forms of life. All the planets orbit
the Sun, the nearest star, an inferno of hydrogen and helium gas engaged in thermonuclear
reactions, flooding the solar system with light.
Finally, at the end of all our wanderings, we return to our tiny, fragile, blue-whiteworld, lost in a cosmic ocean vast beyond our most courageous imaginings. It is a worldamong an immensity of others. It may be significant only for us. The Earth is our home, ourparent. Our kind of life arose and evolved here. The human species is coming of age here. It
is on this world that we developed our passion for exploring the Cosmos, and it is here that
we are, in some pain and with no guarantees, working out our destiny.
Welcome to the planet Earth - a place of blue nitrogen skies, oceans of liquid water,
cool forests and soft meadows, a world positively rippling with life. In the cosmic
perspective it is, as I have said, poignantly beautiful and rare; but it is also, for the moment,
unique. In all our journeying through space and time, it is, so far, the only world on which
we know with certainty that the matter of the Cosmos has become alive and aware. There
must be many such worlds scattered through space, but our search for them begins here,with the accumulated wisdom of the men and women of our species, garnered at great costover a million years. We are privileged to live among brilliant and passionately inquisitive
people, and in a time when the search for knowledge is generally prized. Human beings,
born ultimately of the stars and now for a while inhabiting a world called Earth, have begun
their long voyage home.
The discovery that the Earth is a little world was made, as so many important
human discoveries were, in the ancient Near East, in a time some humans call the third
century B.C., in the greatest metropolis of the age, the Egyptian city of Alexandria. Here
there lived a man named Eratosthenes. One of his envious contemporaries called him‘Beta,’ the second letter of the Greek alphabet, because, he said, Eratosthenes was second
best in the world in everything. But it seems clear that in almost everything Eratosthenes
was ‘Alpha.’ He was an astronomer, historian, geographer, philosopher, poet, theater critic
and mathematician. The titles of the books he wrote range from Astronomy to On Freedom
from Pain. He was also the director of the great library of Alexandria, where one day heread in a papyrus book that in the southern frontier outpost of Syene, near the first cataractof the Nile, at noon on June 21 vertical sticks cast no shadows. On the summer solstice, thelongest day of the year, as the hours crept toward midday, the shadows of temple columns
grew shorter. At noon, they were gone. A reflection of the Sun could then be seen in thewater at the bottom of a deep well. The Sun was directly overhead.
It was an observation that someone else might easily have ignored. Sticks, shadows,
reflections in wells, the position of the Sun - of what possible importance could such simple
everyday matters be? But Eratosthenes was a scientist, and his musings on thesecommonplaces changed the world; in a way, they made the world. Eratosthenes had thepresence of mind to do an experiment, actually to observe whether in Alexandria verticalsticks cast shadows near noon on June 21. And, he discovered, sticks do.

Eratosthenes asked himself how, at the same moment, a stick in Syene could cast no
shadow and a stick in Alexandria, far to the north, could cast a pronounced shadow.
Consider a map of ancient Egypt with two vertical sticks of equal length, one stuck in
Alexandria, the other in Syene. Suppose that, at a certain moment, each stick casts no
shadow at all. This is perfectly easy to understand - provided the Earth is flat. The Sun
would then be directly overhead. If the two sticks cast shadows of equal length, that also
would make sense on a flat Earth: the Sun’s rays would then be inclined at the same angle
to the two sticks. But how could it be that at the same instant there was no shadow at Syene
and a substantial shadow at Alexandria?
The only possible answer, he saw, was that the surface of the Earth is curved. Not
only that: the greater the curvature, the greater the difference in the shadow lengths. The
Sun is so far away that its rays are parallel when they reach the Earth. Sticks placed at
different angles to the Sun’s rays cast shadows of different lengths. For the observed
difference in the shadow lengths, the distance between Alexandria and Syene had to beabout seven degrees along the surface of the Earth; that is, if you imagine the sticks
extending down to the center of the Earth, they would there intersect at an angle of sevendegrees. Seven degrees is something like one-fiftieth of three hundred and sixty degrees,
the full circumference of the Earth. Eratosthenes knew that the distance between
Alexandria and Syene was approximately 800 kilometers, because he hired a man to pace itout. Eight hundred kilometers times 50 is 40,000 kilometers: so that must be thecircumference of the Earth.** Or if you like to measure things in miles, the distance between Alexandria and Syene is about 500
miles, and 500 miles x 50 = 25,000 miles.

This is the right answer. Eratosthenes’ only tools were sticks, eyes, feet and brains,plus a taste for experiment. With them he deduced the circumference of the Earth with anerror of only a few percent, a remarkable achievement for 2,200 years ago. He was the first
person accurately to measure the size of a planet.
The Mediterranean world at that time was famous for seafaring. Alexandria was thegreatest seaport on the planet. Once you knew the Earth to be a sphere of modest diameter,
would you not be tempted to make voyages of exploration, to seek out undiscovered lands,
perhaps even to attempt to sail around the planet? Four hundred years before Eratosthenes,
Africa had been circumnavigated by a Phoenician fleet in the employ of the Egyptian
Pharaoh Necho. They set sail, probably in frail open boats, from the Red Sea, turned down
the east coast of Africa up into the Atlantic, returning through the Mediterranean. This epic
journey took three years, about as long as a modern Voyager spacecraft takes to fly from
Earth to Saturn.
After Eratosthenes’ discovery, many great voyages were attempted by brave andventuresome sailors. Their ships were tiny. They had only rudimentary navigational
instruments. They used dead reckoning and followed coastlines as far as they could. In an
unknown ocean they could determine their latitude, but not their longitude, by observing,night after night, the position of the constellations with respect to the horizon. The familiar
constellations must have been reassuring in the midst of an unexplored ocean. The stars are
the friends of explorers, then with seagoing ships on Earth and now with spacefaring ships
in the sky. After Eratosthenes, some may have tried, but not until the time of Magellan did
anyone succeed in circumnavigating the Earth. What tales of daring and adventure must

earlier have been recounted as sailors and navigators, practical men of the world, gambled
their lives on the mathematics of a scientist from Alexandria?
In Eratosthenes’ time, globes were constructed portraying the Earth as viewed fromspace; they were essentially correct in the well-explored Mediterranean but became more
and more inaccurate the farther they strayed from home. Our present knowledge of the
Cosmos shares this disagreeable but inevitable feature. In the first century, the Alexandrian
geographer Strabo wrote:
Those who have returned from an attempt to circumnavigate the Earth do not say
they have been prevented by an opposing continent, for the sea remained perfectly
open, but, rather, through want of resolution and scarcity of provision ....
Eratosthenes says that if the extent of the Atlantic Ocean were not an obstacle, wemight easily pass by sea from Iberia to India .... It is quite possible that in thetemperate zone there may be one or two habitable Earths .... Indeed, if [this otherpart of the world] is inhabited, it is not inhabited by men such as exist in our parts,
and we should have to regard it as another inhabited world.
Humans were beginning to venture, in almost every sense that matters, to other worlds.
The subsequent exploration of the Earth was a worldwide endeavor, including
voyages from as well as to China and Polynesia. The culmination was, of course, thediscovery of America by Christopher Columbus and the journeys of the following few
centuries, which completed the geographical exploration of the Earth. Columbus’ firstvoyage is connected in the most straight-forward way with the calculations of Eratosthenes.
Columbus was fascinated by what he called ‘the Enterprise of the Indies,’ a project to reachJapan, China and India not by following the coastline of Africa and sailing East but rather
by plunging boldly into the unknown Western ocean - or, as Eratosthenes had said withstartling prescience, ‘to pass by sea from Iberia to India.’
Columbus had been an itinerant peddler of old maps and an assiduous reader of thebooks by and about the ancient geographers, including Eratosthenes, Strabo and Ptolemy.
But for the Enterprise of the Indies to work, for ships and crews to survive the long voyage,
the Earth had to be smaller than Eratosthenes had said. Columbus therefore cheated on his
calculations, as the examining faculty of the University of Salamanca quite correctly
pointed out. He used the smallest possible circumference of the Earth and the greatest
eastward extension of Asia he could find in all the books available to him, and thenexaggerated even those. Had the Americas not been in the way, Columbus’ expeditions
would have failed utterly.
The Earth is now thoroughly explored. It no longer promises new continents or lostlands. But the technology that allowed us to explore and inhabit the most remote regions of
the Earth now permits us to leave our planet, to venture into space, to explore other worlds.
Leaving the Earth, we are now able to view it from above, to see its solid spherical shape of
Eratosthenian dimensions and the outlines of its continents, confirming that many of theancient mapmakers were remarkably competent. What a pleasure such a view would havegiven to Eratosthenes and the other Alexandrian geographers.
It was in Alexandria, during the six hundred years beginning around 300 B.C., thathuman beings, in an important sense, began the intellectual adventure that has led us to theshores of space. But of the look and feel of that glorious marble city, nothing remains.
Oppression and the fear of learning have obliterated almost all memory of ancientAlexandria. Its population was marvelously diverse. Macedonian and later Roman soldiers,
Egyptian priests, Greek aristocrats, Phoenician sailors, Jewish merchants, visitors from
India and sub-Saharan Africa - everyone, except the vast slave population - lived together
in harmony and mutual respect for most of the period of Alexandria’s greatness.
The city was founded by Alexander the Great and constructed by his former
bodyguard. Alexander encouraged respect for alien cultures and the open-minded pursuit ofknowledge. According to tradition - and it does not much matter whether it really happened- he descended beneath the Red Sea in the world’s first diving bell. He encouraged his
generals and soldiers to marry Persian and Indian women. He respected the gods of othernations. He collected exotic lifeforms, including an elephant for Aristotle, his teacher. His
city was constructed on a lavish scale, to be the world center of commerce, culture andlearning. It was graced with broad avenues thirty meters wide, elegant architecture andstatuary, Alexander’s monumental tomb, and an enormous lighthouse, the Pharos, one of
the seven wonders of the ancient world.
But the greatest marvel of Alexandria was the library and its associated museum(literally, an institution devoted to the specialties of the Nine Muses). Of that legendary
library, the most that survives today is a dank and forgotten cellar of the Serapeum, thelibrary annex, once a temple and later reconsecrated to knowledge. A few moldering
shelves may be its only physical remains. Yet this place was once the brain and glory of thegreatest city on the planet, the first true research institute in the history of the world. The
scholars of the library studied the entire Cosmos. Cosmos is a Greek word for the order ofthe universe. It is, in a way, the opposite of Chaos. It implies the deep interconnectednessof all things. It conveys awe for the intricate and subtle way in which the universe is put
together. Here was a community of scholars, exploring physics, literature, medicine,
astronomy, geography, philosophy, mathematics, biology, and engineering. Science andscholarship had come of age. Genius flourished there. The Alexandrian Library is where wehumans first collected, seriously and systematically, the knowledge of the world.
In addition to Eratosthenes, there was the astronomer Hiparchus, who mapped theconstellations and estimated the brightness of the stars; Euclid, who brilliantly systematizedgeometry and told his king, struggling over a difficult mathematical problem, ‘There is noroyal road to geometry’; Dionysius of Thrace, the man who defined the parts of speech anddid for the study of language what Euclid did for geometry; Herophilus, the physiologist
who firmly established that the brain rather than the heart is the seat of intelligence; Heronof Alexandria, inventor of gear trains and steam engines and the author of Automata, thefirst book on robots; Apollonius of Perga, the mathematician who demonstrated the formsof the conic sections* - ellipse, parabola and hyperbola - the curves, as we now know,
followed in their orbits by the planets, the comets and the stars; Archimedes, the greatest
mechanical genius until Leonardo da Vinci; and the astronomer and geographer Ptolemy,
who compiled much of what is today the pseudoscience of astrology: his Earth-centered
universe held sway for 1,500 years, a reminder that intellectual capacity is no guarantee
against being dead wrong. And among those great men was a great woman, Hypatia,
mathematician and astronomer, the last light of the library, whose martyrdom was bound up
with the destruction of the library seven centuries after its founding, a story to which wewill return.
*So called because they can be produced by slicing through a cone at various angles. Eighteen
centuries later, the writings of Apollonius on conic sections would be employed by Johannes Kepler inunderstanding for the first time the movement of the planets.

The Greek Kings of Egypt who succeeded Alexander were serious about learning.
For centuries, they supported research and maintained in the library a working environment
for the best minds of the age. It contained ten large research halls, each devoted to a
separate subject; fountains and colonnades; botanical gardens; a zoo; dissecting rooms; anobservatory; and a great dining hall where, at leisure, was conducted the critical discussionof ideas.
The heart of the library was its collection of books. The organizers combed all thecultures and languages of the world. They sent agents abroad to buy up libraries.
Commercial ships docking in Alexandria were searched by the police - not for contraband,
but for books. The scrolls were borrowed, copied and then returned to their owners.
Accurate numbers are difficult to estimate, but it seems probable that the Library contained
half a million volumes, each a handwritten papyrus scroll. What happened to all thosebooks? The classical civilization that created them disintegrated, and the library itself wasdeliberately destroyed. Only a small fraction of its works survived, along with a few
pathetic scattered fragments. And how tantalizing those bits and pieces are! We know, forexample, that there was on the library shelves a book by the astronomer Aristarchus ofSamos, who argued that the Earth is one of the planets, which like them orbits the Sun, andthat the stars are enormously far away. Each of these conclusions is entirely correct, but we
had to wait nearly two thousand years for their rediscovery. If we multiply by a hundred
thousand our sense of loss for this work of Aristarchus, we begin to appreciate the grandeurof the achievement of classical civilization and the tragedy of its destruction.
We have far surpassed the science known to the ancient world. But there are
irreparable gaps in our historical knowledge. Imagine what mysteries about our past could
be solved with a borrower’s card to the Alexandrian Library. We know of a three-volume
history of the world, now lost, by a Babylonian priest named Berossus. The first volume
dealt with the interval from the Creation to the Flood, a period he took to be 432,000 years
or about a hundred times longer than the Old Testament chronology. I wonder what was init.
The ancients knew that the world is very old. They sought to look into the distant
past. We now know that the Cosmos is far older than they ever imagined. We haveexamined the universe in space and seen that we live on a mote of dust circling a humdrum
star in the remotest corner of an obscure galaxy. And if we are a speck in the immensity ofspace, we also occupy an instant in the expanse of ages. We now know that our universe or at least its most recent incarnation - is some fifteen or twenty billion years old. This is
the time since a remarkable explosive event called the Big Bang. At the beginning of thisuniverse, there were no galaxies, stars or planets, no life or civilizations, merely a uniform,
radiant fireball filling all of space. The passage from the Chaos of the Big Bang to the
Cosmos that we are beginning to know is the most awesome transformation of matter andenergy that we have been privileged to glimpse. And until we find more intelligent beings
elsewhere, we are ourselves the most spectacular of all the transformations - the remote
descendants of the Big Bang, dedicated to understanding and further transforming the
Cosmos from which we spring.

CHAPTER II
One Voice in the Cosmic Fugue
I am bidden to surrender myself to the Lord of the Worlds.
He it is who created you of the dust . . .
- The Koran, Sura 40
The oldest of all philosophies, that of Evolution, was bound hand and foot and castinto utter darkness during the millennium of theological scholasticism. But Darwin
poured new lifeblood into the ancient frame; the bonds burst, and the revivified
thought of ancient Greece has proved itself to be a more adequate expression of theuniversal order of things than any of the schemes which have been accepted by thecredulity and welcomed by the superstition of 70 later generations of men.
- T. H. Huxley, 1887
Probably all the organic beings which have ever lived on this earth have descended
from some one primordial form, into which life was first breathed .... There is
grandeur in this view of life . . . that, whilst this planet has gone cycling on
according to the fixed law of gravity, from so simple a beginning endless forms
most beautiful and most wonderful have been, and are being, evolved.
- Charles Darwin, The Origin of Species, 1859
A community of matter appears to exist throughout the visible universe, for the stars
contain many of the elements which exist in the Sun and Earth. It is remarkable thatthe elements most widely diffused through the host of stars are some of those mostclosely connected with the living organisms of our globe, including hydrogen,sodium, magnesium, and iron. May it not be that, at least, the brighter stars are likeour Sun, the upholding and energizing centres of systems of worlds, adapted to be
the abode of living beings?
- William Huggins, 1865
All my life I have wondered about the possibility of life elsewhere. What would it be like?
Of what would it be made? All living things on our planet are constructed of organic
molecules - complex microscopic architectures in which the carbon atom plays a central
role. There was once a time before life, when the Earth was barren and utterly desolate. Ourworld is now overflowing with life. How did it come about? How, in the absence of life,
were carbon-based organic molecules made? How did the first living things arise? How did
life evolve to produce beings as elaborate and complex as we, able to explore the mysteryof our own origins?
And on the countless other planets that may circle other suns, is there life also? Is
extraterrestrial life, if it exists, based on the same organic molecules as life on Earth? Do
the beings of other worlds look much like life on Earth? Or are they stunningly different other adaptations to other environments? What else is possible? The nature of life on Earthand the search for life elsewhere are two sides of the same question - the search for who weare.

In the great dark between the stars there are clouds of gas and dust and organic
matter. Dozens of different kinds of organic molecules have been found there by radio
telescopes. The abundance of these molecules suggests that the stuff of life is everywhere.
Perhaps the origin and evolution of life is, given enough time, a cosmic inevitability. On
some of the billions of planets in the Milky Way Galaxy, life may never arise. On others, it
may arise and die out, or never evolve beyond its simplest forms. And on some small
fraction of worlds there may develop intelligences and civilizations more advanced thanour own.
Occasionally someone remarks on what a lucky coincidence it is that the Earth isperfectly suitable for life - moderate temperatures, liquid water, oxygen atmosphere, and soon. But this is, at least in part, a confusion of cause and effect. We earthlings are supremely
well adapted to the environment of the Earth because we grew up here. Those earlier formsof life that were not well adapted died. We are descended from the organisms that did well.
Organisms that evolve on a quite different world will doubtless sing its praises too.
All life on Earth is closely related. We have a common organic chemistry and acommon evolutionary heritage. As a result, our biologists are profoundly limited. They
study only a single kind of biology, one lonely theme in the music of life. Is this faint andreedy tune the only voice for thousands of light-years? Or is there a kind of cosmic fugue,
with themes and counterpoints, dissonances and harmonies, a billion different voices
playing the life music of the Galaxy?
Let me tell you a story about one little phrase in the music of life on Earth. In theyear 1185, the Emperor of Japan was a seven-year-old boy named Antoku. He was thenominal leader of a clan of samurai called the Heike, who were engaged in a long andbloody war with another samurai clan, the Genji. Each asserted a superior ancestral claim to
the imperial throne. Their decisive naval encounter, with the Emperor on board ship,occurred at Danno-ura in the Japanese Inland Sea on April 24, 1185. The Heike were
outnumbered, and outmaneuvered. Many were killed. The survivors, in massive numbers,
threw themselves into the sea and drowned. The Lady Nii, grandmother of the Emperor,
resolved that she and Antoku would not be captured by the enemy. What happened next istold in The Tale of the Heike:
The Emperor was seven years old that year but looked much older. He was solovely that he seemed to shed a brilliant radiance and his long, black hair hung loose
far down his back. With a look of surprise and anxiety on his face he asked theLady Nii, ‘Where are you to take me?’
She turned to the youthful sovereign, with tears streaming down her cheeks,and . . . comforted him, binding up his long hair in his dove-colored robe. Blindedwith tears, the child sovereign put his beautiful, small hands together. He turned
first to the East to say farewell to the god of Ise and then to the West to repeat the
Nembutsu [a prayer to the Amida Buddha]. The Lady Nii took him tightly in herarms and with the words ‘In the depths of the ocean is our capitol,’ sank with him atlast beneath the waves.
The entire Heike battle fleet was destroyed. Only forty-three women survived. These
ladies-in-waiting of the imperial court were forced to sell flowers and other favors to thefishermen near the scene of the battle. The Heike almost vanished from history. But aragtag group of the former ladies-in-waiting and their offspring by the fisher-folk
established a festival to commemorate the battle. It takes place on the twenty-fourth ofApril every year to this day. Fishermen who are the descendants of the Heike dress in hemp
and black headgear and proceed to the Akama shrine which contains the mausoleum of thedrowned Emperor. There they watch a play portraying the events that followed the Battle ofDanno-ura. For centuries after, people imagined that they could discern ghostly samurai
armies vainly striving to bail the sea, to cleanse it of blood and defeat and humiliation.
The fishermen say the Heike samurai wander the bottoms of the Inland Sea still - inthe form of crabs. There are crabs to be found here with curious markings on their backs,
patterns and indentations that disturbingly resemble the face of a samurai. When caught,these crabs are not eaten, but are returned to the sea in commemoration of the doleful
events at Danno-ura.
This legend raises a lovely problem. How does it come about that the face of awarrior is incised on the carapace of a crab? The answer seems to be that humans made theface. The patterns on the crab’s shell are inherited. But among crabs, as among people,
there are many different hereditary lines. Suppose that, by chance, among the distantancestors of this crab, one arose with a pattern that resembled, even slightly, a human face.
Even before the battle of Danno-ura, fishermen may have been reluctant to eat such a crab.
In throwing it back, they set in motion an evolutionary process: If you are a crab and yourcarapace is ordinary, the humans will eat you. Your line will leave fewer descendants. If
your carapace looks a little like a face, they will throw you back. You will leave moredescendants. Crabs had a substantial investment in the patterns on their carapaces. As thegenerations passed, of crabs and fishermen alike, the crabs with patterns that mostresembled a samurai face survived preferentially until eventually there was produced notjust a human face, not just a Japanese face, but the visage of a fierce and scowling samurai.
All this has nothing to do with what the crabs want. Selection is imposed from the outside.
The more you look like a samurai, the better are your chances of survival. Eventually, there
come to be a great many samurai crabs.
This process is called artificial selection. In the case of the Heike crab it was
effected more or less unconsciously by the fishermen, and certainly without any serious
contemplation by the crabs. But humans have deliberately selected which plants andanimals shall live and which shall die for thousands of years. We are surrounded frombabyhood by familiar farm and domestic animals, fruits and trees and vegetables. Where dothey come from? Were they once free-living in the wild and then induced to adopt a lessstrenuous life on the farm? No, the truth is quite different. They are, most of them, made byus.
Ten thousand years ago, there were no dairy cows or ferret hounds or large ears ofcorn. When we domesticated the ancestors of these plants and animals - sometimes
creatures who looked quite different - we controlled their breeding. We made sure thatcertain varieties, having properties we consider desirable, preferentially reproduced. When
we wanted a dog to help us care for sheep, we selected breeds that were intelligent,obedient and had some pre-existing talent to herd, which is useful for animals who hunt inpacks. The enormous distended udders of dairy cattle are the result of a human interest in
milk and cheese. Our corn, or maize, has been bred for ten thousand generations to be more
tasty and nutritious than its scrawny ancestors; indeed, it is so changed that it cannot evenreproduce without human intervention.
The essence of artificial selection - for a Heike crab, a dog, a cow or an ear of corn -is this: Many physical and behavioral traits of plants and animals are inherited. They breed
true. Humans, for whatever reason, encourage the reproduction of some varieties anddiscourage the reproduction of others. The variety selected for preferentially reproduces; it
eventually becomes abundant; the variety selected against becomes rare and perhaps
extinct.
But if humans can make new varieties of plants and animals, must not nature do so
also? This related process is called natural selection. That life has changed fundamentallyover the aeons is entirely clear from the alterations we have made in the beasts andvegetables during the short tenure of humans on Earth, and from the fossil evidence. The
fossil record speaks to us unambiguously of creatures that once were present in enormous
numbers and that have now vanished utterly.* Far more species have become extinct in thehistory of the Earth than exist today; they are the terminated experiments of evolution.
* Although traditional Western religious opinion stoutly maintained the contrary, as for example, the1770 opinion of John Wesley: ‘Death is never permitted to destroy [even] the most inconsiderable species.’

The genetic changes induced by domestication have occurred very rapidly. The
rabbit was not domesticated until early medieval times (it was bred by French monks in the
belief that new-born bunnies were fish and therefore exempt from the prohibitions against
eating meat on certain days in the Church calendar); coffee in the fifteenth century; thesugar beet in the nineteenth century; and the mink is still in the earliest stages ofdomestication. In less than ten thousand years, domestication has increased the weight ofwool grown by sheep from less than one kilogram of rough hairs to ten or twenty kilogramsof uniform, fine down; or the volume of milk given by cattle during a lactation period froma few hundred to a million cubic centimeters. If artificial selection can make such majorchanges in so short a period of time, what must natural selection, working over billions ofyears, be capable of? The answer is all the beauty and diversity of the biological world.
Evolution is a fact, not a theory.
That the mechanism of evolution is natural selection is the great discovery
associated with the names of Charles Darwin and Alfred Russel Wallace. More than acentury ago, they stressed that nature is prolific, that many more animals and plants areborn than can possibly survive and that therefore the environment selects those varieties
which are, by accident, better suited for survival. Mutations - sudden changes in heredity breed true. They provide the raw material of evolution. The environment selects those few
mutations that enhance survival, resulting in a series of slow transformations of one
lifeform into another, the origin of new species.** In the Mayan holy book the Popol Vuh, the various forms of life are described as unsuccessfulattempts by gods with a predilection for experiment to make people. Early tries were far off the mark, creating
the lower animals; the penultimate attempt, a near miss, made the monkeys. In Chinese myth, human beings
arose from the body lice of a god named P’an Ku. In the eighteenth century, de Buffon proposed that the
Earth was much older than Scripture suggested, that the forms of life somehow changed slowly over themillennia, but that the apes were the forlorn descendants of people. While these notions do not preciselyreflect the evolutionary process described by Darwin and Wallace, they are anticipations of it - as are theviews of Democritus, Empedocles and other early Ionian scientists who are discussed in Chapter 7.

Darwin’s words in The Origin of Species were:Man does not actually produce variability; he only unintentionally exposes organic
beings to new conditions of life, and then Nature acts on the organisation, andcauses variability. But man can and does select the variations given to him by
Nature, and thus accumulate them in any desired manner. He thus adapts animals
and plants for his own benefit or pleasure. He may do this methodically, or he may
do it unconsciously by preserving the individuals most useful to him at the time,
without any thought of altering the breed .... There is no obvious reason why theprinciples which have acted so efficiently under domestication should not haveacted under Nature . . . . More individuals are born than can possibly survive . . . .
The slightest advantage in one being, of any age or during any season, over thosewith which it comes into competition, or better adaptation in however slight a
degree to the surrounding physical conditions, will turn the balance.
T. H. Huxley, the most effective nineteenth-century defender and popularizer of evolution,
wrote that the publications of Darwin and Wallace were a ‘flash of light, which to a man
who has lost himself in a dark night, suddenly reveals a road which, whether it takes him
straight home or not, certainly goes his way .... My reflection, when I first made myself
master of the central idea of the ‘Origin of Species,’ was, ‘How extremely stupid not tohave thought of that!’ I suppose that Columbus’ companions said much the same .... The
facts of variability, of the struggle for existence, of adaptation to conditions, were notorious
enough; but none of us had suspected that the road to the heart of the species problem lay
through them, until Darwin and Wallace dispelled the darkness.’Many people were scandalized - some still are - at both ideas, evolution and natural
selection. Our ancestors looked at the elegance of life on Earth, at how appropriate thestructures of organisms are to their functions, and saw evidence for a Great Designer. The
simplest one-celled organism is a far more complex machine than the finest pocket watch.
And yet pocket watches do not spontaneously self-assemble, or evolve, in slow stages, on
their own, from, say, grandfather clocks. A watch implies a watchmaker. There seemed to
be no way in which atoms and molecules could somehow spontaneously fall together tocreate organisms of such awesome complexity and subtle functioning as grace every regionof the Earth. That each living thing was specially designed, that one species did not become
another, were notions perfectly consistent with what our ancestors with their limited
historical records knew about life. The idea that every organism was meticulously
constructed by a Great Designer provided a significance and order to nature and an
importance to human beings that we crave still. A Designer is a natural, appealing andaltogether human explanation of the biological world. But, as Darwin and Wallace showed,
there is another way, equally appealing, equally human, and far more compelling: natural
selection, which makes the music of life more beautiful as the aeons pass.
The fossil evidence could be consistent with the idea of a Great Designer; perhaps
some species are destroyed when the Designer becomes dissatisfied with them, and newexperiments are attempted on an improved design. But this notion is a little disconcerting.
Each plant and animal is exquisitely made; should not a supremely competent Designer
have been able to make the intended variety from the start? The fossil record implies trial
and error, an inability to anticipate the future, features inconsistent with an efficient Great
Designer (although not with a Designer of a more remote and indirect temperament).
When I was a college undergraduate in the early 1950’s, I was fortunate enough towork in the laboratory of H. J. Muller, a great geneticist and the man who discovered that

radiation produces mutations. Muller was the person who first called my attention to the
Heike crab as an example of artificial selection. To learn the practical side of genetics, I
spent many months working with fruit flies, Drosophila melanogaster (which means theblack-bodied dew-lover) - tiny benign beings with two wings and big eyes. We kept themin pint milk bottles. We would cross two varieties to see what new forms emerged from therearrangement of the parental genes, and from natural and induced mutations. The females
would deposit their eggs on a kind of molasses the technicians placed inside the bottles; thebottles were stoppered; and we would wait two weeks for the fertilized eggs to become
larvae, the larvae pupae, and the pupae to emerge as new adult fruit flies.
One day I was looking through a low-power binocular microscope at a newly
arrived batch of adult Drosophila immobilized with a little ether, and was busily separating
the different varieties with a camel’s-hair brush. To my astonishment, I came uponsomething very different: not a small variation such as red eyes instead of white, or neckbristles instead of no neck bristles. This was another, and very well-functioning, kind ofcreature with much more prominent wings and long feathery antennae. Fate had arranged, I
concluded, that an example of a major evolutionary change in a single generation, the very
thing Muller had said could never happen, should take place in his own laboratory. It was
my unhappy task to explain it to him.
With heavy heart I knocked on his office door. ‘Come in,’ came the muffled cry. I
entered to discover the room darkened except for a single small lamp illuminating the stageof the microscope at which he was working. In these gloomy surroundings I stumbled
through my explanation. I had found a very different kind of fly. I was sure it had emerged
from one of the pupae in the molasses. I didn’t mean to disturb Muller but... ‘Does it look
more like Lepidoptera than Diptera?’ he asked, his face illuminated from below. I didn’tknow what this meant, so he had to explain: ‘Does it have big wings? Does it have feathery
antennae?’ I glumly nodded assent.
Muller switched on the overhead light and smiled benignly. It was an old story.
There was a kind of moth that had adapted to Drosphila genetics laboratories. It wasnothing like a fruit fly and wanted nothing to do with fruit flies. What it wanted was thefruit flies’ molasses. In the brief time that the laboratory technician took to unstopper andstopper the milk bottle - for example, to add fruit flies - the mother moth made a divebombing pass, dropping her eggs on the run into the tasty molasses. I had not discovered amacro-mutation. I had merely stumbled upon another lovely adaptation in nature, itself the
product of micromutation and natural selection.
The secrets of evolution are death and time - the deaths of enormous numbers oflifeforms that were imperfectly adapted to the environment; and time for a long successionof small mutations that were by accident adaptive, time for the slow accumulation ofpatterns of favorable mutations. Part of the resistance to Darwin and Wallace derives fromour difficulty in imagining the passage of the millennia, much less the aeons. What does
seventy million years mean to beings who live only one-millionth as long? We are likebutterflies who flutter for a day and think it is forever.
What happened here on Earth may be more or less typical of the evolution of life on many
worlds; but in such details as the chemistry of proteins or the neurology of brains, the storyof life on Earth may be unique in all the Milky Way Galaxy. The Earth condensed out ofinterstellar gas and dust some 4.6 billion years ago. We know from the fossil record that theorigin of life happened soon after, perhaps around 4.0 billion years ago, in the ponds andoceans of the primitive Earth. The first living things were not anything so complex as a
one-celled organism, already a highly sophisticated form of life. The first stirrings weremuch more humble. In those early days, lightning and ultraviolet light from the Sun werebreaking apart the simple hydrogen-rich molecules of the primitive atmosphere, thefragments spontaneously recombining into more and more complex molecules. The
products of this early chemistry were dissolved in the oceans, forming a kind of organic
soup of gradually increasing complexity, until one day, quite by accident, a molecule arosethat was able to make crude copies of itself, using as building blocks other molecules in thesoup. (We will return to this subject later.)This was the earliest ancestor of deoxyribonucleic acid, DNA, the master moleculeof life on Earth. It is shaped like a ladder twisted into a helix, the rungs available in four
different molecular parts, which constitute the four letters of the genetic code. These rungs,
called nucleotides, spell out the hereditary instructions for making a given organism. Every
lifeform on Earth has a different set of instructions, written out in essentially the samelanguage. The reason organisms are different is the differences in their nucleic acid
instructions. A mutation is a change in a nucleotide, copied in the next generation, whichbreeds true. Since mutations are random nucleotide changes, most of them are harmful orlethal, coding into existence nonfunctional enzymes. It is a long wait before a mutation
makes an organism work better. And yet it is that improbable event, a small beneficial
mutation in a nucleotide a ten-millionth of a centimeter across, that makes evolution go.
Four billion years ago, the Earth was a molecular Garden of Eden. There were as
yet no predators. Some molecules reproduced themselves inefficiently, competed forbuilding blocks and left crude copies of themselves. With reproduction, mutation and theselective elimination of the least efficient varieties, evolution was well under way, even at
the molecular level. As time went on, they got better at reproducing. Molecules withspecialized functions eventually joined together, making a kind of molecular collective the first cell. Plant cells today have tiny molecular factories, called chloroplasts, which arein charge of photosynthesis - the conversion of sunlight, water and carbon dioxide into
carbohydrates and oxygen. The cells in a drop of blood contain a different sort of molecularfactory, the mitochondrion, which combines food with oxygen to extract useful energy.
These factories exist in plant and animal cells today but may once themselves have been
free-living cells.
By three billion years ago, a number of one-celled plants had joined together,perhaps because a mutation prevented a single cell from separating after splitting in two.
The first multicellular organisms had evolved. Every cell of your body is a kind ofcommune, with once free-living parts all banded together for the common good. And you
are made of a hundred trillion cells. We are, each of us, a multitude.
Sex seems to have been invented around two billion years ago. Before then, newvarieties of organisms could arise only from the accumulation of random mutations - theselection of changes, letter by letter, in the genetic instructions. Evolution must have beenagonizingly slow. With the invention of sex, two organisms could exchange wholeparagraphs, pages and books of their DNA code, producing new varieties ready for thesieve of selection. Organisms are selected to engage in sex - the ones that find it
uninteresting quickly become extinct. And this is true not only of the microbes of two
billion years ago. We humans also have a palpable devotion to exchanging segments of
DNA today.
By one billion years ago, plants, working cooperatively, had made a stunningchange in the environment of the Earth. Green plants generate molecular oxygen. Since theoceans were by now filled with simple green plants, oxygen was becoming a major
constituent of the Earth’s atmosphere, altering it irreversibly from its original hydrogenrich character and ending the epoch of Earth history when the stuff of life was made bynonbiological processes. But oxygen tends to make organic molecules fall to pieces.
Despite our fondness for it, it is fundamentally a poison for unprotected organic matter. The
transition to an oxidizing atmosphere posed a supreme crisis in the history of life, and agreat many organisms, unable to cope with oxygen, perished. A few primitive forms, suchas the botulism and tetanus bacilli, manage to survive even today only in oxygen-freeenvironments. The nitrogen in the Earth’s atmosphere is much more chemically inert andtherefore much more benign than oxygen. But it, too, is biologically sustained. Thus, 99
percent of the Earth’s atmosphere is of biological origin. The sky is made by life.
For most of the four billion years since the origin of life, the dominant organisms
were microscopic blue-green algae, which covered and filled the oceans. Then some 600
million years ago, the monopolizing grip of the algae was broken and an enormous
proliferation of new lifeforms emerged, an event called the Cambrian explosion. Life hadarisen almost immediately after the origin of the Earth, which suggests that life may be an
inevitable chemical process on an Earth-like planet. But life did not evolve much beyond
blue-green algae for three billion years, which suggests that large lifeforms with specializedorgans are hard to evolve, harder even than the origin of life. Perhaps there are many otherplanets that today have abundant microbes but no big beasts and vegetables.
Soon after the Cambrian explosion, the oceans teemed with many different forms oflife. By 500 million years ago there were vast herds of trilobites, beautifully constructedanimals, a little like large insects; some hunted in packs on the ocean floor. They stored
crystals in their eyes to detect polarized light. But there are no trilobites alive today; there
have been none for 200 million years. The Earth used to be inhabited by plants and animalsof which there is today no living trace. And of course every species now on the planet once
did not exist. There is no hint in the old rocks of animals like us. Species appear, abide
more or less briefly and then flicker out.
Before the Cambrian explosion species seem to have succeeded one another rather
slowly. In part this may be because the richness of our information declines rapidly thefarther into the past we peer; in the early history of our planet, few organisms had hardparts and soft beings leave few fossil remains. But in part the sluggish rate of appearance ofdramatically new forms before the Cambrian explosion is real; the painstaking evolution ofcell structure and biochemistry is not immediately reflected in the external forms revealedby the fossil record. After the Cambrian explosion, exquisite new adaptations followed one
another with comparatively breathtaking speed. In rapid succession, the first fish and thefirst vertebrates appeared; plants, previously restricted to the oceans, began the colonizationof the land; the first insect evolved, and its descendants became the pioneers in thecolonization of the land by animals; winged insects arose together with the amphibians,
creatures something like the lungfish, able to survive both on land and in the water; the firsttrees and the first reptiles appeared; the dinosaurs evolved; the mammals emerged, and thenthe first birds; the first flowers appeared; the dinosaurs became extinct; the earliest
cetaceans, ancestors to the dolphins and whales, arose and in the same period the primates the ancestors of the monkeys, the apes and the humans. Less than ten million years ago, thefirst creatures who closely resembled human beings evolved, accompanied by a spectacularincrease in brain size. And then, only a few million years ago, the first true humans

emerged.
Human beings grew up in forests; we have a natural affinity for them. How lovely atree is, straining toward the sky. Its leaves harvest sunlight to photosynthesize, so trees
compete by shadowing their neighbors. If you look closely you can often see two trees
pushing and shoving with languid grace. Trees are great and beautiful machines, poweredby sunlight, taking in water from the ground and carbon dioxide from the air, convertingthese materials into food for their use and ours. The plant uses the carbohydrates it makes
as an energy source to go about its planty business. And we animals, who are ultimately
parasites on the plants, steal the carbohydrates so we can go about our business. In eating
the plants we combine the carbohydrates with oxygen dissolved in our blood because of ourpenchant for breathing air, and so extract the energy that makes us go. In the process we
exhale carbon dioxide, which the plants then recycle to make more carbohydrates. What amarvelous cooperative arrangement - plants and animals each inhaling the other’s
exhalations, a kind of planet-wide mutual mouth-to-stoma resuscitation, the entire elegant
cycle powered by a star 150 million kilometers away.
There are tens of billions of known kinds of organic molecules. Yet only about fiftyof them are used for the essential activities of life. The same patterns are employed over
and over again, conservatively, ingeniously for different functions. And at the very heart oflife on Earth the proteins that control cell chemistry, and the nucleic acids that carry thehereditary instructions - we find these molecules to be essentially identical in all the plants
and animals. An oak tree and I are made of the same stuff. If you go far enough back, wehave a common ancestor.
The living cell is a regime as complex and beautiful as the realm of the galaxies andthe stars. The elaborate machinery of the cell has been painstakingly evolved over fourbillion years. Fragments of food are transmogrified into cellular machinery. Today’s whiteblood cell is yesterday’s creamed spinach. How does the cell do it? Inside is a labyrinthineand subtle architecture that maintains its own structure, transforms molecules, stores energy
and prepares for self-replication. If we could enter a cell, many of the molecular specks wewould see would be protein molecules, some in frenzied activity, others merely waiting.
The most important proteins are enzymes, molecules that control the cell’s chemical
reactions. Enzymes are like assembly-line workers, each specializing in a particularmolecular job: Step 4 in the construction of the nucleotide guanosine phosphate, say, or
Step 11 in the dismantling of a molecule of sugar to extract energy, the currency that pays
for getting the other cellular jobs done. But the enzymes do not run the show. They receive
their instructions - and are in fact themselves constructed - on orders sent from those incharge. The boss molecules are the nucleic acids. They live sequestered in a forbidden cityin the deep interior, in the nucleus of the cell.
If we plunged through a pore into the nucleus of the cell, we would find somethingthat resembles an explosion in a spaghetti factory - a disorderly multitude of coils andstrands, which are the two kinds of nucleic acids: DNA, which knows what to do, andRNA, which conveys the instructions issued by DNA to the rest of the cell. These are thebest that four billion years of evolution could produce, containing the full complement of
information on how to make a cell, a tree or a human work. The amount of information inhuman DNA, if written out in ordinary language, would occupy a hundred thick volumes.
What is more, the DNA molecules know how to make, with only very rare exceptions,
identical copies of themselves. They know extraordinarily much.
DNA is a double helix, the two intertwined strands resembling a ‘spiral’ staircase. It

is the sequence or ordering of the nucleotides along either of the constituent strands that is
the language of life. During reproduction, the helices separate, assisted by a special
unwinding protein, each synthesizing an identical copy of the other from nucleotidebuilding blocks floating about nearby in the viscous liquid of the cell nucleus. Once theunwinding is underway, a remarkable enzyme called DNA polymerase helps ensure that the
copying works almost perfectly. If a mistake is made, there are enzymes which snip the
mistake out and replace the wrong nucleotide by the right one. These enzymes are amolecular machine with awesome powers.
In addition to making accurate copies of itself - which is what heredity is about nuclear DNA directs the activities of the cell - which is what metabolism is about - bysynthesizing another nucleic acid called messenger RNA, each of which passes to theextranuclear provinces and there controls the construction, at the right time, in the rightplace, of one enzyme. When all is done, a single enzyme molecule has been produced,
which then goes about ordering one particular aspect of the chemistry of the cell.
Human DNA is a ladder a billion nucleotides long. Most possible combinations of
nucleotides are nonsense: they would cause the synthesis of proteins that perform no usefulfunction. Only an extremely limited number of nucleic acid molecules are any good forlifeforms as complicated as we. Even so, the number of useful ways of putting nucleic acids
together is stupefyingly large - probably far greater than the total number of electrons andprotons in the universe. Accordingly, the number of possible individual human beings isvastly greater than the number that have ever lived: the untapped potential of the humanspecies is immense. There must be ways of putting nucleic acids together that will function
far better - by any criterion we choose - than any human being who has ever lived.
Fortunately, we do not yet know how to assemble alternative sequences of nucleotides tomake alternative kinds of human beings. In the future we may well be able to assemble
nucleotides in any desired sequence, to produce whatever characteristics we think desirable- a sobering and disquieting prospect.
Evolution works through mutation and selection. Mutations might occur during
replication if the enzyme DNA polymerase makes a mistake. But it rarely makes a mistake.
Mutations also occur because of radioactivity or ultraviolet light from the Sun or cosmic
rays or chemicals in the environment, all of which can change the nucleotides or tie thenucleic acids up in knots. If the mutation rate is too high, we lose the inheritance of fourbillion years of painstaking evolution. If it is too low, new varieties will not be available toadapt to some future change in the environment. The evolution of life requires a more orless precise balance between mutation and selection. When that balance is achieved,
remarkable adaptations occur.
A change in a single DNA nucleotide causes a change in a single amino acid in theprotein for which that DNA codes. The red blood cells of people of European descent lookroughly globular. The red blood cells of some people of African descent look like sickles or
crescent moons. Sickle cells carry less oxygen and consequently transmit a kind of anemia.
They also provide major resistance against malaria. There is no question that it is better tobe anemic than to be dead. This major influence on the function of the blood - so striking as
to be readily apparent in photographs of red blood cells - is the result of a change in a single
nucleotide out of the ten billion in the DNA of a typical human cell. We are still ignorant of
the consequences of changes in most of the other nucleotides.
We humans look rather different than a tree. Without a doubt we perceive the world
differently than a tree does. But down deep, at the molecular heart of life, the trees and weare essentially identical. We both use nucleic acids for heredity; we both use proteins as
enzymes to control the chemistry of our cells. Most significantly, we both use precisely thesame code book for translating nucleic acid information into protein information, as do
virtually all the other creatures on the planet.* The usual explanation of this molecular
unity is that we are, all of us - trees and people, angler fish and slime molds and paramecia- descended from a single and common instance of the origin of life in the early history of
our planet. How did the critical molecules then arise?
* The genetic code turns out to be not quite identical in all parts of all organisms on the Earth. At
least a few cases are known where the transcription from DNA information into protein information in a
mitochondrion employs a different code book from that used by the genes in the nucleus of the very samecell. This points to a long evolutionary separation of the genetic codes of mitochondria and nuclei, and isconsistent with the idea that mitochondria were once free-living organisms incorporated into the cell in a
symbiotic relationship billions of years ago. The development and emerging sophistication of that symbiosis
is, incidentally, one answer to the question of what evolution was doing between the origin of the cell and theproliferation of many-celled organisms in the Cambrian explosion.

In my laboratory at Cornell University we work on, among other things,
prebiological organic chemistry, making some notes of the music of life. We mix together
and spark the gases of the primitive Earth: hydrogen, water, ammonia, methane, hydrogensulfide - all present, incidentally, on the planet Jupiter today and throughout the Cosmos.
The sparks correspond to lightning - also present on the ancient Earth and on modern
Jupiter. The reaction vessel is initially transparent: the precursor gases are entirely
invisible. But after ten minutes of sparking, we see a strange brown pigment slowly
streaking the sides of the vessel. The interior gradually becomes opaque, covered with athick brown tar. If we had used ultraviolet light - simulating the early Sun - the results
would have been more or less the same. The tar is an extremely rich collection of complex
organic molecules, including the constituent parts of proteins and nucleic acids. The stuff oflife, it turns out, can be very easily made.
Such experiments were first performed in the early 1950’s by Stanley Miller, then agraduate student of the chemist Harold Urey. Urey had argued compellingly that the earlyatmosphere of the Earth was hydrogen-rich, as is most of the Cosmos; that the hydrogenhas since trickled away to space from Earth, but not from massive Jupiter; and that theorigin of life occurred before the hydrogen was lost. After Urey suggested that such gases
be sparked, someone asked him what he expected to make in such an experiment. Urey
replied, ‘Beilstein.’ Beilstein is the massive German compendium in 28 volumes, listing all
the organic molecules known to chemists.
Using only the most abundant gases that were present on the early Earth and almost
any energy source that breaks chemical bonds, we can produce the essential building blocksof life. But in our vessel are only the notes of the music of life - not the music itself. The
molecular building blocks must be put together in the correct sequence. Life is certainly
more than the amino acids that make up its proteins and the nucleotides that make up its
nucleic acids. But even in ordering these building blocks into long-chain molecules, there
has been substantial laboratory progress. Amino acids have been assembled under primitive
Earth conditions into molecules resembling proteins. Some of them feebly control usefulchemical reactions, as enzymes do. Nucleotides have been put together into strands ofnucleic acid a few dozen units long. Under the right circumstances in the test tube, short
nucleic acids can synthesize identical copies of themselves.

No one has so far mixed together the gases and waters of the primitive Earth and at
the end of the experiment had something crawl out of the test tube. The smallest living
things known, the viroids, are composed of less than 10,000 atoms. They cause several
different diseases in cultivated plants and have probably most recently evolved from more
complex organisms rather than from simpler ones. Indeed, it is hard to imagine a still
simpler organism that is in any sense alive. Viroids are composed exclusively of nucleic
acid, unlike the viruses, which also have a protein coat. They are no more than a single
strand of RNA with either a linear or a closed circular geometry. Viroids can be so smalland still thrive because they are thoroughgoing, unremitting parasites. Like viruses, they
simply take over the molecular machinery of a much larger, well-functioning cell andchange it from a factory for making more cells into a factory for making more viroids.
The smallest known free-living organisms are the PPLO (pleuropneumonia-likeorganisms) and similar small beasts. They are composed of about fifty million atoms. Suchorganisms, having to be more self-reliant, are also more complicated than viroids andviruses. But the environment of the Earth today is not extremely favorable for simple formsof life. You have to work hard to make a living. You have to be careful about predators. In
the early history of our planet, however, when enormous amounts of organic molecules
were being produced by sunlight in a hydrogen-rich atmosphere, very simple, nonparasitic
organisms had a fighting chance. The first living things may have been something like freeliving viroids only a few hundred nucleotides long. Experimental work on making suchcreatures from scratch may begin by the end of the century. There is still much to beunderstood about the origin of life, including the origin of the genetic code. But we havebeen performing such experiments for only some thirty years. Nature has had a fourbillion-year head start. All in all, we have not done badly.
Nothing in such experiments is unique to the Earth. The initial gases, and the energy
sources, are common throughout the Cosmos. Chemical reactions like those in ourlaboratory vessels may be responsible for the organic matter in interstellar space and theamino acids found in meteorites. Some similar chemistry must have occurred on a billion
other worlds in the Milky Way Galaxy. The molecules of life fill the Cosmos.
But even if life on another planet has the same molecular chemistry as life here,
there is no reason to expect it to resemble familiar organisms. Consider the enormous
diversity of living things on Earth, all of which share the same planet and an identicalmolecular biology. Those other beasts and vegetables are probably radically different fromany organism we know here. There may be some convergent evolution because there may
be only one best solution to a certain environmental problem - something like two eyes, forexample, for binocular vision at optical frequencies. But in general the random character ofthe evolutionary process should create extraterrestrial creatures very different from any that
we know.
I cannot tell you what an extraterrestrial being would look like. I am terribly limitedby the fact that I know only one kind of life, life on Earth. Some people - science fiction
writers and artists, for instance - have speculated on what other beings might be like. I am
skeptical about most of those extraterrestrial visions. They seem to me to rely too much on
forms of life we already know. Any given organism is the way it is because of a long series
of individually unlikely steps. I do not think life anywhere else would look very much likea reptile, or an insect or a human - even with such minor cosmetic adjustments as green
skin, pointy ears and antennae. But if you pressed me, I could try to imagine somethingrather different.

On a giant gas planet like Jupiter, with an atmosphere rich in hydrogen, helium,
methane, water and ammonia, there is no accessible solid surface, but rather a dense cloudy
atmosphere in which organic molecules may be falling from the skies like manna fromheaven, like the products of our laboratory experiments. However, there is a characteristic
impediment to life on such a planet: the atmosphere is turbulent, and down deep it is very
hot. An organism must be careful that it is not carried down and fried.
To show that life is not out of the question in such a very different planet, myCornell colleague E. E. Salpeter and I have made some calculations. Of course, we cannot
know precisely what life would be like in such a place, but we wanted to see if, within thelaws of physics and chemistry, a world of this sort could possibly be inhabited.
One way to make a living under these conditions is to reproduce before you are
fried and hope that convection will carry some of your offspring to the higher and cooler
layers of the atmosphere. Such organisms could be very little. We call them sinkers. But
you could also be a floater, some vast hydrogen balloon pumping helium and heavier gases
out of its interior and leaving only the lightest gas, hydrogen; or a hot-air balloon, staying
buoyant by keeping your interior warm, using energy acquired from the food you eat. Like
familiar terrestrial balloons, the deeper a floater is carried, the stronger is the buoyant force
returning it to the higher, cooler, safer regions of the atmosphere. A floater might eat
preformed organic molecules, or make its own from sunlight and air, somewhat as plants
do on Earth. Up to a point, the bigger a floater is, the more efficient it will be. Salpeter andI imagined floaters kilometers across, enormously larger than the greatest whale that ever
was, beings the size of cities.
The floaters may propel themselves through the planetary atmosphere with gusts ofgas, like a ramjet or a rocket. We imagine them arranged in great lazy herds for as far as theeye can see, with patterns on their skin, an adaptive camouflage implying that they haveproblems, too. Because there is at least one other ecological niche in such an environment:
hunting. Hunters are fast and maneuverable. They eat the floaters both for their organic
molecules and for their store of pure hydrogen. Hollow sinkers could have evolved into thefirst floaters, and self-propelled floaters into the first hunters. There cannot be very many
hunters, because if they consume all the floaters, the hunters themselves will perish.
Physics and chemistry permit such lifeforms. Art endows them with a certain
charm. Nature, however, is not obliged to follow our speculations. But if there are billions
of inhabited worlds in the Milky Way Galaxy, perhaps there will be a few populated by thesinkers, floaters and hunters which our imaginations, tempered by the laws of physics andchemistry, have generated.
Biology is more like history than it is like physics. You have to know the past tounderstand the present. And you have to know it in exquisite detail. There is as yet no
predictive theory of biology, just as there is not yet a predictive theory of history. The
reasons are the same: both subjects are still too complicated for us. But we can know
ourselves better by understanding other cases. The study of a single instance of
extraterrestrial life, no matter how humble, will deprovincialize biology. For the first time,
the biologists will know what other kinds of life are possible. When we say the search forlife elsewhere is important, we are not guaranteeing that it will be easy to find - only that it
is very much worth seeking.
We have heard so far the voice of life on one small world only. But we have at last
begun to listen for other voices in the cosmic fugue.

CHAPTER III
The Harmony of Worlds
Do you know the ordinances of the heavens?
Can you establish their rule on Earth?
- The Book of Job
All welfare and adversity that come to man and other creatures come through theSeven and the Twelve. Twelve Signs of the Zodiac, as the Religion says, are thetwelve commanders on the side of light; and the seven planets are said to be theseven commanders on the side of darkness. And the seven planets oppress all
creation and deliver it over to death and all manner of evil: for the twelve signs of
the Zodiac and the seven planets rule the fate of the world.
- The late Zoroastrian book, the Menok i Xrat
To tell us that every species of thing is endowed with an occult specific quality bywhich it acts and produces manifest effects, is to tell us nothing; but to derive two or
three general principles of motion from phenomena, and afterwards to tell us how
the properties and actions of all corporeal things follow from those manifest
principles, would be a very great step.
- Isaac Newton, Optics
We do not ask for what useful purpose the birds do sing, for song is their pleasuresince they were created for singing. Similarly, we ought not to ask why the humanmind troubles to fathom the secrets of the heavens . . . The diversity of thephenomena of Nature is so great, and the treasures hidden in the heavens so rich,precisely in order that the human mind shall never be lacking in fresh nourishment.
- Johannes Kepler, Mysterium Cosmographicum
If we lived on a planet where nothing ever changed, there would be little to do. There
would be nothing to figure out. There would be no impetus for science. And if we lived in
an unpredictable world, where things changed in random or very complex ways, we would
not be able to figure things out. Again, there would be no such thing as science. But we live
in an in-between universe, where things change, but according to patterns, rules, or, as wecall them, laws of nature. If I throw a stick up in the air, it always falls down. If the sun setsin the west, it always rises again the next morning in the east. And so it becomes possible tofigure things out. We can do science, and with it we can improve our lives.
Human beings are good at understanding the world. We always have been. We wereable to hunt game or build fires only because we had figured something out. There was atime before television, before motion pictures, before radio, before books. The greatest partof human existence was spent in such a time. Over the dying embers of the campfire, on a
moonless night, we watched the stars.
The night sky is interesting. There are patterns there. Without even trying, you canimagine pictures. In the northern sky, for example, there is a pattern, or constellation, thatlooks a little ursine. Some cultures call it the Great Bear. Others see quite different images.

These pictures are not, of course, really in the night sky; we put them there ourselves. We
were hunter folk, and we saw hunters and dogs, bears and young women, all manner ofthings of interest to us. When seventeenth-century European sailors first saw the southernskies they put objects of seventeenth century interest in the heavens - toucans and peacocks,
telescopes and microscopes, compasses and the sterns of ships. If the constellations had
been named in the twentieth century, I suppose we would see bicycles and refrigerators inthe sky, rock-and-roll ‘stars’ and perhaps even mushroom clouds - a new set of humanhopes and fears placed among the stars.
Occasionally our ancestors would see a very bright star with a tail, glimpsed for justa moment, hurtling across the sky. They called it a falling star, but it is not a good name:the old stars are still there after the falling star falls. In some seasons there are many fallingstars; in others very few. There is a kind of regularity here as well.
Like the Sun and the Moon, stars always rise in the east and set in the west, taking
the whole night to cross the sky if they pass overhead. There are different constellations indifferent seasons. The same constellations always rise at the beginning of autumn, say. Itnever happens that a new constellation suddenly rises out of the east. There is an order, apredictability, a permanence about the stars. In a way, they are almost comforting.
Certain stars rise just before or set just after the Sun - and at times and positions thatvary with the seasons. If you made careful observations of the stars and recorded them overmany years, you could predict the seasons. You could also measure the time of year bynoting where on the horizon the Sun rose each day. In the skies was a great calendar,
available to anyone with dedication and ability and the means to keep records.
Our ancestors built devices to measure the passing of the seasons. In Chaco Canyon,
in New Mexico, there is a great roofless ceremonial kiva or temple, dating from theeleventh century. On June 21, the longest day of the year, a shaft of sunlight enters awindow at dawn and slowly moves so that it covers a special niche. But this happens onlyaround June 21. I imagine the proud Anasazi people, who described themselves as ‘The
Ancient Ones,’ gathered in their pews every June 21, dressed in feathers and rattles andturquoise to celebrate the power of the Sun. They also monitored the apparent motion of theMoon: the twenty-eight higher niches in the kiva may represent the number of days for theMoon to return to the same position among the constellations. These people paid close
attention to the Sun and the Moon and the stars. Other devices based on similar ideas arefound at Angkor Wat in Cambodia; Stonehenge in England; Abu Simbel in Egypt; Chichén
Itzá in Mexico; and the Great Plains in North America.
Some alleged calendrical devices may just possibly be due to chance - an accidentalalignment of window and niche on June 21, say. But there are other devices wonderfully
different. At one locale in the American Southwest is a set of three upright slabs whichwere moved from their original position about 1,000 years ago. A spiral a little like agalaxy has been carved in the rock. On June 21, the first day of summer, a dagger ofsunlight pouring through an opening between the slabs bisects the spiral; and on December
21, the first day of winter, there are two daggers of sunlight that flank the spiral, a uniqueapplication of the midday sun to read the calendar in the sky.
Why did people all over the world make such an effort to learn astronomy? We
hunted gazelles and antelope and buffalo whose migrations ebbed and flowed with theseasons. Fruits and nuts were ready to be picked in some times but not in others. When we
invented agriculture, we had to take care to plant and harvest our crops in the right season.
Annual meetings of far-flung nomadic tribes were set for prescribed times. The ability toread the calendar in the skies was literally a matter of life and death. The reappearance ofthe crescent moon after the new moon; the return of the Sun after a total eclipse; the risingof the Sun in the morning after its troublesome absence at night were noted by people
around the world: these phenomena spoke to our ancestors of the possibility of surviving
death. Up there in the skies was also a metaphor of immortality.
The wind whips through the canyons in the American Southwest, and there is no
one to hear it but us - a reminder of the 40,000 generations of thinking men and women
who preceded us, about whom we know almost nothing, upon whom our civilization isbased.
As ages passed, people learned from their ancestors. The more accurately you knew
the position and movements of the Sun and Moon and stars, the more reliably you could
predict when to hunt, when to sow and reap, when to gather the tribes. As precision of
measurement improved, records had to be kept, so astronomy encouraged observation andmathematics and the development of writing.
But then, much later, another rather curious idea arose, an assault by mysticism andsuperstition into what had been largely an empirical science. The Sun and stars controlled
the seasons, food, warmth. The Moon controlled the tides, the life cycles of many animals,
and perhaps the human menstrual* period - of central importance for a passionate species
devoted to having children. There was another kind of object in the sky, the wandering or
vagabond stars called planets. Our nomadic ancestors must have felt an affinity for theplanets. Not counting the Sun and the Moon, you could see only five of them. They moved
against the background of more distant stars. If you followed their apparent motion overmany months, they would leave one constellation, enter another, occasionally even do akind of slow loop-the-loop in the sky. Everything else in the sky had some real effect onhuman life. What must the influence of the planets be?
* The root of the word means ‘Moon.’

In contemporary Western society, buying a magazine on astrology - at a newsstand,say - is easy; it is much harder to find one on astronomy. Virtually every newspaper in
America has a daily column on astrology; there are hardly any that have even a weekly
column on astronomy. There are ten times more astrologers in the United States thanastronomers. At parties, when I meet people who do not know I am a scientist, I am
sometimes asked, ‘Are you a Gemini?’ (chances of success, one in twelve), or ‘What sign
are you?’ Much more rarely am I asked, ‘Have you heard that gold is made in supernova
explosions?’ or ‘When do you think Congress will approve a Mars Rover?’Astrology contends that which constellation the planets are in at the moment of yourbirth profoundly influences your future. A few thousand years ago, the idea developed thatthe motions of the planets determined the fates of kings, dynasties, empires. Astrologers
studied the motions of the planets and asked themselves what had happened the last time
that, say, Venus was rising in the Constellation of the Goat; perhaps something similar
would happen this time as well. It was a subtle and risky business. Astrologers came to be
employed only by the State. In many countries it was a capital offense for anyone but theofficial astrologer to read the portents in the skies: a good way to overthrow a regime wasto predict its downfall. Chinese court astrologers who made inaccurate predictions were
executed. Others simply doctored the records so that afterwards they were in perfectconformity with events. Astrology developed into a strange combination of observations,mathematics and careful record-keeping with fuzzy thinking and pious fraud.
But if the planets could determine the destinies of nations, how could they avoid
influencing what will happen to me tomorrow? The notion of a personal astrology
developed in Alexandrian Egypt and spread through the Greek and Roman worlds about
2,000 years ago. We today can recognize the antiquity of astrology in words such as
disaster, which is Greek for ‘bad star,’ influenza, Italian for (astral) ‘influence’; mazeltov,
Hebrew - and, ultimately, Babylonian - for ‘good constellation,’ or the Yiddish word
shlamazel, applied to someone plagued by relentless ill-fortune, which again traces to the
Babylonian astronomical lexicon. According to Pliny, there were Romans consideredsideratio, ‘planetstruck.’ Planets were widely thought to be a direct cause of death. Or
consider consider: it means ‘with the planets,’ evidently the prerequisite for serious
reflection. John Graunt compiled the mortality statistics in the City of London in 1632.
Among the terrible losses from infant and childhood diseases and such exotic illnesses as‘the rising of the lights’ and ‘the King’s evil,’ we find that, of 9,535 deaths, 13 people
succumbed to ‘planet,’ more than died of cancer. I wonder what the symptoms were.
And personal astrology is with us still: consider two different newspaper astrology
columns published in the same city on the same day. For example, we can examine the
New York Post and the New York Daily News on September 21, 1979. Suppose you are a
Libra - that is, born between September 23 and October 22. According to the astrologer for
the Post, ‘a compromise will help ease tension’; useful, perhaps, but somewhat vague.
According to the Daily News’s astrologer, you must ‘demand more of yourself,’ anadmonition that is also vague but also different. These ‘predictions’ are not predictions;rather they are pieces of advice - they tell what to do, not what will happen. Deliberately,
they are phrased so generally that they could apply to anyone. And they display major
mutual inconsistencies. Why are they published as unapologetically as sports statistics andstock market reports?
Astrology can be tested by the lives of twins. There are many cases in which one
twin is killed in childhood, in a riding accident, say, or is struck by lightning, while theother lives to a prosperous old age. Each was born in precisely the same place and withinminutes of the other. Exactly the same planets were rising at their births. If astrology werevalid, how could two such twins have such profoundly different fates? It also turns out thatastrologers cannot even agree among themselves on what a given horoscope means. In
careful tests, they are unable to predict the character and future of people they knew
nothing about except their time and place of birth.** Skepticism about astrology and related doctrines is neither new nor exclusive to the West. For
example, in the Essays on Idleness, written in 1332 by Tsurezuregusa Kenko, we read:The Yin-Yang teachings (in Japan] have nothing to say on the subject of the Red Tongue Days.
Formerly people did not avoid these days, but of late - I wonder who is responsible for starting thiscustom - people have taken to saying things such as, ‘An enterprise begun on a Red Tongue Day will
never see an end,’ or, ‘Anything you say or do on a Red Tongue Day is bound to come to naught:
you lose what you’ve won, your plans are undone.’ What nonsense! If one counted the projects
begun on carefully selected ‘lucky days’ which came to nothing in the end, they would probably bequite as many as the fruitless enterprises begun on the Red Tongue days.

There is something curious about the national flags of the planet Earth. The flag of
the United States has fifty stars; the Soviet Union and Israel, one each; Burma, fourteen;
Grenada and Venezuela, seven; China, five; Iraq, three; São Tomé a Príncipe, two; Japan,

Uruguay, Malawi, Bangladesh and Taiwan, the Sun; Brazil, a celestial sphere; Australia,
Western Samoa, New Zealand and Papua New Guinea, the constellation of the Southern
Cross; Bhutan, the dragon pearl, symbol of the Earth; Cambodia, the Angkor Wat
astronomical observatory; India, South Korea and the Mongolian Peoples’ Republic,
cosmological symbols. Many socialist nations display stars. Many Islamic countries display
crescent moons. Almost half of our national flags exhibit astronomical symbols. The
phenomenon is transcultural, non-sectarian, worldwide. It is also not restricted to our time:
Sumerian cylinder seals from the third millennium B.C. and Taoist flags inprerevolutionary China displayed constellations. Nations, I do not doubt, wish to embracesomething of the power and credibility of the heavens. We seek a connection with the
Cosmos. We want to count in the grand scale of things. And it turns out we are connected not in the personal, small-scale unimaginative fashion that the astrologers pretend, but inthe deepest ways, involving the origin of matter, the habitability of the Earth, the evolution
and destiny of the human species, themes to which we will return.
Modern popular astrology runs directly back to Claudius Ptolemaeus, whom we call
Ptolemy, although he was unrelated to the kings of the same name. He worked in the
Library of Alexandria in the second century. All that arcane business about planets
ascendant in this or that solar or lunar ‘house’ or the ‘Age of Aquarius’ comes fromPtolemy, who codified the Babylonian astrological tradition. Here is a typical horoscopefrom Ptolemy’s time, written in Greek on papyrus, for a little girl born in the year 150: ‘The
birth of Philoe. The 10th year of Antoninus Caesar the lord, Phamenoth 15 to 16, first hourof the night. Sun in Pisces, Jupiter and Mercury in Aries, Saturn in Cancer, Mars in Leo,
Venus and the Moon in Aquarius, horoscopus Capricorn.’ The method of enumerating themonths and the years has changed much more over the intervening centuries than have theastrological niceties. A typical excerpt from Ptolemy’s astrological book, the Tetrabiblos,
reads: ‘Saturn, if he is in the orient, makes his subjects in appearance dark-skinned, robust,
black-haired, curly-haired, hairy-chested, with eyes of moderate size, of middling stature,and in temperament having an excess of the moist and cold.’ Ptolemy believed not only thatbehavior patterns were influenced by the planet’s and the stars but also that questions ofstature, complexion, national character and even congenital physical abnormalities were
determined by the stars. On this point modern astrologers seem to have adopted a morecautious position.
But modern astrologers have forgotten about the precession of the equinoxes, which
Ptolemy understood. They ignore atmospheric refraction, about which Ptolemy wrote. They
pay almost no attention to all the moons and planets, asteroids and comets, quasars andpulsars, exploding galaxies, symbiotic stars, cataclysmic variables and X-ray sources thathave been discovered since Ptolemy’s time. Astronomy is a science - the study of theuniverse as it is. Astrology is a pseudoscience - a claim, in the absence of good evidence,that the other planets affect our everyday lives. In Ptolemy’s time the distinction between
astronomy and astrology was not clear. Today it is.
As an astronomer, Ptolemy named the stars, listed their brightnesses, gave goodreasons for believing that the Earth is a sphere, set down rules for predicting eclipses and,perhaps most important, tried to understand why planets exhibit that strange, wandering
motion against the background of distant constellations. He developed a predictive modelto understand planetary motions and decode the message in the skies. The study of theheavens brought Ptolemy a kind of ecstasy. ‘Mortal as I am,’ he wrote, ‘I know that I amborn for a day. But when I follow at my pleasure the serried multitude of the stars in theircircular course, my feet no longer touch the Earth. . .’Ptolemy believed that the Earth was at the center of the universe; that the Sun,
Moon, planets and stars went around the Earth. This is the most natural idea in the world.
The Earth seems steady, solid, immobile, while we can see the heavenly bodies rising andsetting each day. Every culture has leaped to the geocentric hypothesis. As Johannes Kepler
wrote, ‘It is therefore impossible that reason not previously instructed should imagine
anything other than that the Earth is a kind of vast house with the vault of the sky placed on
top of it; it is motionless and within it the Sun being so small passes from one region toanother, like a bird wandering through the air.’ But how do we explain the apparent motionof the planets - Mars, for example, which had been known for thousands of years before
Ptolemy’s time? (One of the epithets given Mars by the ancient Egyptians was sekded-ef
em khetkhet, which means ‘who travels backwards,’ a clear reference to its retrograde or
loop-the-loop apparent motion.)
Ptolemy’s model of planetary motion can be represented by a little machine, likethose that, serving a similar purpose, existed in Ptolemy’s time.* The problem was to figureout a ‘real’ motion of the planets, as seen from up there, on the ‘outside,’ which would
reproduce with great accuracy the apparent motion of the planets, as seen from down here,on the ‘inside.’* Four centuries earlier, such a device was constructed by Archimedes and examined and describedby Cicero in Rome, where it had been carried by the Roman general Marcellus, one of whose soldiers had,gratuitously and against orders, killed the septuagenarian scientist during the conquest of Syracuse.

The planets were imagined to go around the Earth affixed to perfect transparent
spheres. But they were not attached directly to the spheres, but indirectly, through a kind ofoff-center wheel. The sphere turns, the little wheel rotates, and, as seen from the Earth,
Mars does its loop-the-loop. This model permitted reasonably accurate predictions ofplanetary motion, certainly good enough for the precision of measurement available in
Ptolemy’s day, and even many centuries later.
Ptolemy’s aetherial spheres, imagined in medieval times to be made of crystal, are
why we still talk about the music of the spheres and a seventh heaven (there was a‘heaven,’ or sphere for the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn, andone more for the stars). With the Earth the center of the Universe, with creation pivoted
about terrestrial events, with the heavens imagined constructed on utterly unearthly
principles, there was little motivation for astronomical observations. Supported by the
Church through the Dark Ages, Ptolemy’s model helped prevent the advance of astronomy
for a millennium. Finally, in 1543, a quite different hypothesis to explain the apparent
motion of the planets was published by a Polish Catholic cleric named Nicholas
Copernicus. Its most daring feature was the proposition that the Sun, not the Earth, was at
the center of the universe. The Earth was demoted to just one of the planets, third from theSun, moving in a perfect circular orbit. (Ptolemy had considered such a heliocentric model
but rejected it immediately; from the physics of Aristotle, the implied violent rotation of the
Earth seemed contrary to observation.)
It worked at least as well as Ptolemy’s spheres in explaining the apparent motion of
the planets. But it annoyed many people. In 1616 the Catholic Church placed Copernicus’
work on its list of forbidden books ‘until corrected’ by local ecclesiastical censors, where it
remained until 1835.* Martin Luther described him as ‘an upstart astrologer . . . This fool

wishes to reverse the entire science of astronomy. But Sacred Scripture tells us that Joshua
commanded the Sun to stand still, and not the Earth.’ Even some of Copernicus’ admirers
argued that he had not really believed in a Sun-centered universe but had merely proposed
it as a convenience for calculating the motions of the planets.
* In a recent inventory of nearly every sixteenth-century copy of Copernicus’ book, Owen Gingerichhas found the censorship to have been ineffective: only 60 percent of the copies in Italy were ‘corrected,’ andnot one in Iberia.

The epochal confrontation between the two views of the Cosmos - Earth-centered
and Sun-centered - reached a climax in the sixteenth and seventeenth centuries in the
person of a man who was, like Ptolemy, both astrologer and astronomer. He lived in a time
when the human spirit was fettered and the mind chained; when the ecclesiasticalpronouncements of a millennium or two earlier on scientific matters were considered morereliable than contemporary findings made with techniques unavailable to the ancients;
when deviations, even on arcane theological matters, from the prevailing doxologicalpreferences, Catholic or Protestant, were punished by humiliation, taxation, exile, torture ordeath. The heavens were inhabited by angels, demons and the Hand of God, turning theplanetary crystal spheres. Science was barren of the idea that underlying the phenomena of
Nature might be the laws of physics. But the brave and lonely struggle of this man was toignite the modern scientific revolution.
Johannes Kepler was born in Germany in 1571 and sent as a boy to the Protestantseminary school in the provincial town of Maulbronn to be educated for the clergy. It was akind of boot camp, training young minds in the use of theological weaponry against thefortress of Roman Catholicism. Kepler, stubborn, intelligent and fiercely independent,
suffered two friendless years in bleak Maulbronn, becoming isolated and withdrawn, histhoughts devoted to his imagined unworthiness in the eyes of God. He repented a thousandsins no more wicked than another’s and despaired of ever attaining salvation.
But God became for him more than a divine wrath craving propitiation. Kepler’s God was
the creative power of the Cosmos. The boy’s curiosity conquered his fear. He wished tolearn the eschatology of the world; he dared to contemplate the Mind of God. These
dangerous visions, at first insubstantial as a memory, became a lifelong obsession. The
hubristic longings of a child seminarian were to carry Europe out of the cloister of medieval
thought.
The sciences of classical antiquity had been silenced more than a thousand years
before, but in the late Middle Ages some faint echoes of those voices, preserved by Arabscholars, began to insinuate themselves into the European educational curriculum. In
Maulbronn, Kepler heard their reverberations, studying, besides theology, Greek and Latin,music and mathematics. In the geometry of Euclid he thought he glimpsed an image of
perfection and cosmic glory. He was later to write: ‘Geometry existed before the Creation.
It is co-eternal with the mind of God . . . Geometry provided God with a model for theCreation . . . Geometry is God Himself.’
In the midst of Kepler’s mathematical raptures, and despite his sequestered life, theimperfections of the outside world must also have molded his character. Superstition was a
widely available nostrum for people powerless against the miseries of famine, pestilenceand deadly doctrinal conflict. For many, the only certainty was the stars, and the ancientastrological conceit prospered in the courtyards and taverns of fear-haunted Europe. Kepler,whose attitude toward astrology remained ambiguous all his life, wondered whether there
might be hidden patterns underlying the apparent chaos of daily life. If the world wascrafted by God, should it not be examined closely? Was not all of creation an expression of
the harmonies in the mind of God? The book of Nature had waited more than a millennium
for a reader.
In 1589, Kepler left Maulbronn to study for the clergy at the great university in
Tübingen and found it a liberation. Confronted by the most vital intellectual currents of thetime, his genius was immediately recognized by his teachers - one of whom introduced the
young man to the dangerous mysteries of the Copernican hypothesis. A heliocentric
universe resonated with Kepler’s religious sense, and he embraced it with fervor. The Sun
was a metaphor for God, around Whom all else revolves. Before he was to be ordained, he
was made an attractive offer of secular employment, which - perhaps because he felt
himself indifferently suited to an ecclesiastical career - he found himself accepting. He wassummoned to Graz, in Austria, to teach secondary school mathematics, and began a little
later to prepare astronomical and meteorological almanacs and to cast horoscopes. ‘God
provides for every animal his means of sustenance,’ he wrote. ‘For the astronomer, He has
provided astrology.’
Kepler was a brilliant thinker and a lucid writer, but he was a disaster as aclassroom teacher. He mumbled. He digressed. He was at times utterly incomprehensible.
He drew only a handful of students his first year at Graz; the next year there were none. He
was distracted by an incessant interior clamor of associations and speculations vying for his
attention. And one pleasant summer afternoon, deep in the interstices of one of his
interminable lectures, he was visited by a revelation that was to alter radically the future ofastronomy. Perhaps he stopped in mid-sentence. His inattentive students, longing for theend of the day, took little notice, I suspect, of the historic moment.
There were only six planets known in Kepler’s time: Mercury, Venus, Earth, Mars,
Jupiter and Saturn. Kepler wondered why only six? Why not twenty, or a hundred? Why
did they have the spacing between their orbits that Copernicus had deduced? No one had
ever asked such questions before. There were known to be five regular or ‘platonic’ solids,
whose sides were regular polygons, as known to the ancient Greek mathematicians after thetime of Pythagoras. Kepler thought the two numbers were connected, that the reason there
were only six planets was because there were only five regular solids, and that these solids,
inscribed or nested one within another, would specify the distances of the planets from theSun. In these perfect forms, he believed he had recognized the invisible supporting
structures for the spheres of the six planets. He called his revelation The Cosmic Mystery.
The connection between the solids of Pythagoras and the disposition of the planets could
admit but one explanation: the Hand of God, Geometer.
Kepler was amazed that he - immersed, so he thought, in sin - should have been
divinely chosen to make this great discovery. He submitted a proposal for a research grantto the Duke of Württemberg, offering to supervise the construction of his nested solids as athree-dimensional model so that others could glimpse the beauty of the holy geometry. It
might, he added, be contrived of silver and precious stones and serve incidentally as a ducal
chalice. The proposal was rejected with the kindly advice that he first construct a lessexpensive version out of paper, which he promptly attempted to do: ‘The intense pleasure I
have received from this discovery can never be told in words . . . I shunned no calculation
no matter how difficult. Days and nights I spent in mathematical labors, until I could see
whether my hypothesis would agree with the orbits of Copernicus or whether my joy was tovanish into thin air.’ But no matter how hard he tried, the solids and the planetary orbits did
not agree well. The elegance and grandeur of the theory, however, persuaded him that theobservations must be in error, a conclusion drawn when the observations are unobliging by
many other theorists in the history of science. There was then only one man in the world
who had access to more accurate observations of apparent planetary positions, a self-exiled
Danish nobleman who had accepted the post of Imperial Mathematician in the Court of theHoly Roman Emperor, Rudolf II. That man was Tycho Brahe. By chance, at Rudolf’s
suggestion, he had just invited Kepler, whose mathematical fame was growing, to join him
in Prague.
A provincial schoolteacher of humble origins, unknown to all but a few
mathematicians, Kepler was diffident about Tycho’s offer. But the decision was made forhim. In 1598, one of the many premonitory tremors of the coming Thirty Years’ War
engulfed him. The local Catholic archduke, steadfast in dogmatic certainty, vowed hewould rather ‘make a desert of the country than rule over heretics.’* Protestants were
excluded from economic and political power, Kepler’s school was closed, and prayers,books and hymns deemed heretical were forbidden. Finally the townspeople were
summoned to individual examinations on the soundness of their private religious
convictions, those refusing to profess the Roman Catholic faith being fined a tenth of theirincome and, upon pain of death, exiled forever from Graz. Kepler chose exile: ‘Hypocrisy I
have never learned. I am in earnest about faith. I do not play with it.’
* By no means the most extreme such remark in medieval or Reformation Europe. Upon being asked
how to distinguish the faithful from the infidel in the siege of a largely Albigensian city, Domingo de
Guzmán, later known as Saint Dominic, allegedly replied: ‘Kill them all. God will know his own.’

Leaving Graz, Kepler, his wife and stepdaughter set out on the difficult journey toPrague. Theirs was not a happy marriage. Chronically ill, having recently lost two young
children, his wife was described as ‘stupid, sulking, lonely, melancholy.’ She had nounderstanding of her husband’s work and, having been raised among the minor rural
gentry, she despised his impecunious profession. He for his part alternately admonishedand ignored her, ‘for my studies sometimes made me thoughtless; but I learned my lesson, I
learned to have patience with her. When I saw that she took my words to heart, I would
rather have bitten my own finger than to give her further offense.’ But Kepler remained
preoccupied with his work.
He envisioned Tycho’s domain as a refuge from the evils of the time, as the place
where his Cosmic Mystery would be confirmed. He aspired to become a colleague of thegreat Tycho Brahe, who for thirty-five years had devoted himself, before the invention of
the telescope, to the measurement of a clockwork universe, ordered and precise. Kepler’s
expectations were to be unfulfilled. Tycho himself was a flamboyant figure, festooned witha golden nose, the original having been lost in a student duel fought over who was thesuperior mathematician. Around him was a raucous entourage of assistants, sycophants,
distant relatives and assorted hangers-on. Their endless revelry, their innuendoes andintrigues, their cruel mockery of the pious and scholarly country bumpkin depressed andsaddened Kepler: ‘Tycho . . . is superlatively rich but knows not how to make use of it. Any
single instrument of his costs more than my and my whole family’s fortunes put together.’
Impatient to see Tycho’s astronomical data, Kepler would be thrown only a few
scraps at a time: ‘Tycho gave me no opportunity to share in his experiences. He would

only, in the course of a meal and, in between other matters, mention, as if in passing, today
the figure of the apogee of one planet, tomorrow the nodes of another . . . Tycho posesses
the best observations . . . He also has collaborators. He lacks only the architect who would
put all this to use.’ Tycho was the greatest observational genius of the age, and Kepler thegreatest theoretician. Each knew that, alone, he would be unable to achieve the synthesis ofan accurate and coherent world system, which they both felt to be imminent. But Tycho
was not about to make a gift of his life’s work to a much younger potential rival. Joint
authorship of the results, if any, of the collaboration was for some reason unacceptable. The
birth of modern science - the offspring of theory and observation - teetered on the precipiceof their mutual mistrust. In the remaining eighteen months that Tycho was to live, the twoquarreled and were reconciled repeatedly. At a dinner given by the Baron of Rosenberg,
Tycho, having robustly drunk much wine, ‘placed civility ahead of health,’ and resisted his
body’s urgings to leave, even if briefly, before the baron. The consequent urinary infection
worsened when Tycho resolutely rejected advice to temper his eating and drinking. On his
deathbed, Tycho bequeathed his observations to Kepler, and ‘on the last night of his gentledelirium, he repeated over and over again these words, like someone composing a poem:“Let me not seem to have lived in vain . . . Let me not seem to have lived in vain.” ’After Tycho’s death, Kepler, now the new Imperial Mathematician, managed to
extract the observations from Tycho’s recalcitrant family. His conjecture that the orbits ofthe planets are circumscribed by the five platonic solids was no more supported by Tycho’s
data than by Copernicus’. His ‘Cosmic Mystery’ was disproved entirely by the much later
discoveries of the planets Uranus, Neptune and Pluto - there are no additional platonic
solids* that would determine their distances from the Sun. The nested Pythagorean solids
also made no allowance for the existence of the Earth’s moon, and Galileo’s discovery of
the four large moons of Jupiter was also discomfiting. But far from becoming morose,
Kepler wished to find additional satellites and wondered how many satellites each planet
should have. He wrote to Galileo: ‘I immediately began to think how there could be any
addition to the number of the planets without overturning my Mysterium Cosmographicum,
according to which Euclid’s five regular solids do not allow more than six planets aroundthe Sun . . . I am so far from disbelieving the existence of the four circumjovial planets that
I long for a telescope, to anticipate you, if possible, in discovering two around Mars, as the
proportion seems to require, six or eight round Saturn, and perhaps one each round
Mercury and Venus.’ Mars does have two small moons, and a major geological feature on
the larger of them is today called the Kepler Ridge in honor of this guess. But he wasentirely mistaken about Saturn, Mercury and Venus, and Jupiter has many more moons
than Galileo discovered. We still do not really know why there are only nine planets, moreor less, and why they have the relative distances from the Sun that they do. (See Chapter 8.)* The proof of this statement can be found in Appendix 2.

Tycho’s observations of the apparent motion of Mars and other planets through theconstellations were made over a period of many years. These data, from the last few
decades before the telescope was invented, were the most accurate that had yet beenobtained. Kepler worked with a passionate intensity to understand them: What real motionof the Earth and Mars about the Sun could explain, to the precision of measurement, theapparent motion of Mars in the sky, including its retrograde loops through the backgroundconstellations? Tycho had commended Mars to Kepler because its apparent motion seemedmost anomalous, most difficult to reconcile with an orbit made of circles. (To the reader
who might be bored by his many calculations, he later wrote: ‘If you are wearied by thistedious procedure, take pity on me who carried out at least seventy trials.’)
Pythagoras, in the sixth century B.C., Plato, Ptolemy and all the Christian
astronomers before Kepler had assumed that the planets moved in circular paths. The circle
was thought to be a ‘perfect’ geometrical shape and the planets, placed high in the heavens,away from earthly ‘corruption,’ were also thought to be in some mystical sense ‘perfect’
Galileo, Tycho and Copernicus were all committed to uniform circular planetary motion,
the latter asserting that ‘the mind shudders’ at the alternative, because ‘it would be
unworthy to suppose such a thing in a Creation constituted in the best possible way.’ So at
first Kepler tried to explain the observations by imagining that the Earth and Mars moved
in circular orbits about the Sun.
After three years of calculation, he believed he had found the correct values for a
Martian circular orbit, which matched ten of Tycho’s observations within two minutes ofarc. Now, there are 60 minutes of arc in an angular degree, and 90 degrees, a right angle,
from the horizon to the zenith. So a few minutes of arc is a very small quantity to measure especially without a telescope. It is one-fifteenth the angular diameter of the full Moon as
seen from Earth. But Kepler’s replenishable ecstasy soon crumbled into gloom - because
two of Tycho’s further observations were inconsistent with Kepler’s orbit, by as much as
eight minutes of arc:
Divine Providence granted us such a diligent observer in Tycho Brahe that his
observations convicted this . . . calculation of an error of eight minutes; it is only
right that we should accept God’s gift with a grateful mind . . . If I had believed that
we could ignore these eight minutes, I would have patched up my hypothesis
accordingly. But, since it was not permissible to ignore, those eight minutes pointed
the road to a complete reformation in astronomy.
The difference between a circular orbit and the true orbit could be distinguishedonly by precise measurement and a courageous acceptance of the facts: ‘The universe isstamped with the adornment of harmonic proportions, but harmonies must accommodate
experience.’ Kepler was shaken at being compelled to abandon a circular orbit and toquestion his faith in the Divine Geometer. Having cleared the stable of astronomy of circles
and spirals, he was left, he said, with ‘only a single cartful of dung,’ a stretched-out circle
something like an oval.
Eventually, Kepler came to feel that his fascination with the circle had been adelusion. The Earth was a planet, as Copernicus had said, and it was entirely obvious to
Kepler that the Earth, wracked by wars, pestilence, famine and unhappiness, fell short ofperfection. Kepler was one of the first people since antiquity to propose that the planets
were material objects made of imperfect stuff like the Earth. And if planets were
‘imperfect,’ why not their orbits as well? He tried various oval-like curves, calculatedaway, made some arithmetical mistakes (which caused him at first to reject the correct
answer) and months later in some desperation tried the formula for an ellipse, first codified
in the Alexandrian Library by Apollonius of Perga. He found that it matched Tycho’s
observations beautifully: ‘The truth of nature, which I had rejected and chased away,
returned by stealth through the back door, disguising itself to be accepted . . . Ah, what afoolish bird I have been!’Kepler had found that Mars moves about the Sun not in a circle, but in an ellipse.
The other planets have orbits much less elliptical than that of Mars, and if Tycho had urged
him to study the motion of, say, Venus, Kepler might never have discovered the true orbitsof the planets. In such an orbit the Sun is not at the center but is offset, at the focus of theellipse. When a given planet is at its nearest to the Sun, it speeds up. When it is at its
farthest, it slows down. Such motion is why we describe the planets as forever falling
toward, but never reaching, the Sun. Kepler’s first law of planetary motion is simply this: A
planet moves in an ellipse with the Sun at one focus.
In uniform circular motion, an equal angle or fraction of the arc of a circle iscovered in equal times. So, for example, it takes twice as long to go two-thirds of the way
around a circle as it does to go one-third of the way around. Kepler found something
different for elliptical orbits: As the planet moves along its orbit, it sweeps out a little
wedge-shaped area within the ellipse. When it is close to the Sun, in a given period of timeit traces out a large arc in its orbit, but the area represented by that arc is not very large
because the planet is then near the Sun. When the planet is far from the Sun, it covers amuch smaller arc in the same period of time, but that arc corresponds to a bigger area
because the Sun is now more distant. Kepler found that these two areas were precisely thesame no matter how elliptical the orbit: the long skinny area, corresponding to the planet
far from the Sun, and the shorter, squatter area, when the planet is close to the Sun, areexactly equal. This was Kepler’s second law of planetary motion: Planets sweep out equalareas in equal times.
Kepler’s first two laws may seem a little remote and abstract: planets move in
ellipses, and sweep out equal areas in equal times. Well, so what? Circular motion is easierto grasp. We might have a tendency to dismiss these laws as mere mathematical tinkering,something removed from everyday life. But these are the laws our planet obeys as weourselves, glued by gravity to the surface of the Earth, hurtle through interplanetary space.
We move in accord with laws of nature that Kepler first discovered. When we send
spacecraft to the planets, when we observe double stars, when we examine the motion of
distant galaxies; we find that throughout the universe Kepler’s laws are obeyed.
Many years later, Kepler came upon his third and last law of planetary motion, alaw that relates the motion of various planets to one another, that lays out correctly theclockwork of the solar system. He described it in a book called The Harmonies of theWorld. Kepler understood many things by the word harmony: the order and beauty ofplanetary motion, the existence of mathematical laws explaining that motion - an idea that
goes back to Pythagoras - and even harmony in the musical sense, the ‘harmony of thespheres.’ Unlike the orbits of Mercury and Mars, the orbits of other planets depart so little
from circularity that we cannot make out their true shapes even in an extremely accurate
diagram. The Earth is our moving platform from which we observe the motion of the otherplanets against the backdrop of distant constellations. The inner planets move rapidly intheir orbits - that is why Mercury has the name it does: Mercury was the messenger of thegods. Venus, Earth and Mars move progressively less rapidly about the Sun. The outerplanets, such as Jupiter and Saturn, move stately and slow, as befits the kings of the gods.
Kepler’s third or harmonic law states that the squares of the periods of the planets(the times for them to complete one orbit) are proportional to the cubes of their average
distance from the Sun; the more distant the planet, the more slowly it moves, but according
to a precise mathematical law: P2 = a3, where P represents the period of revolution of theplanet about the Sun, measured in years, and a the distance of the planet from the Sunmeasured in ‘astronomical units.’ An astronomical unit is the distance of the Earth from theSun. Jupiter, for example, is five astronomical units from the Sun, and a3 = 5 x 5 x 5 =125.
What number times itself equals 125? Why, 11, close enough. And 11 years is the period
for Jupiter to go once around the Sun. A similar argument applies for every planet andasteroid and comet. Not content merely to have extracted from Nature the laws of planetary
motion, Kepler endeavored to find some still more fundamental underlying cause, someinfluence of the Sun on the kinematics of worlds. The planets sped up on approaching the
Sun and slowed down on retreating from it. Somehow the distant planets sensed the Sun’spresence. Magnetism also was an influence felt at a distance, and in a stunning anticipationof the idea of universal gravitation, Kepler suggested that the underlying cause was akin tomagnetism:
My aim in this is to show that the celestial machine is to be likened not to a divineorganism but rather to a clockwork . . . , insofar as nearly all the manifold
movements are carried out by means of a single, quite simple magnetic force, as in
the case of a clockwork [where] all motions [are caused] by a simple weight.
Magnetism is, of course, not the same as gravity, but Kepler’s fundamental
innovation here is nothing short of breathtaking: he proposed that quantitative physicallaws that apply to the Earth are also the underpinnings of quantitative physical laws thatgovern the heavens. It was the first nonmystical explanation of motion in the heavens; it
made the Earth a province of the Cosmos. ‘Astronomy,’ he said ‘is part of physics.’ Kepler
stood at a cusp in history; the last scientific astrologer was the first astrophysicist.
Not given to quiet understatement, Kepler assessed his discoveries in these words:
With this symphony of voices man can play through the eternity of time in less thanan hour, and can taste in small measure the delight of God, the Supreme Artist . . . I
yield freely to the sacred frenzy . . . the die is cast, and I am writing the book - to beread either now or by posterity, it matters not. It can wait a century for a reader, as
God Himself has waited 6,000 years for a witness.
Within the ‘symphony of voices,’ Kepler believed that the speed of each planet corresponds
to certain notes in the Latinate musical scale popular in his day - do, re, mi, fa, sol, la, ti,do. He claimed that in the harmony of the spheres, the tones of Earth are fa and mi, that the
Earth is forever humming fa and mi, and that they stand in a straightforward way for theLatin word for famine. He argued, not unsuccessfully, that the Earth was best described bythat single doleful word.
Exactly eight days after Kepler’s discovery of his third law, the incident that
unleashed the Thirty Years’ War transpired in Prague. The war’s convulsions shattered thelives of millions, Kepler among them. He lost his wife and son to an epidemic carried by
the soldiery, his royal patron was deposed, and he was excommunicated by the Lutheran
Church for his uncompromising individualism on matters of doctrine. Kepler was a refugee
once again. The conflict, portrayed by both the Catholics and the Protestants as a holy war,
was more an exploitation of religious fanaticism by those hungry for land and power. In thepast, wars had tended to be resolved when the belligerent princes had exhausted theirresources. But now organized pillage was introduced as a means of keeping armies in thefield. The savaged population of Europe stood helpless as plowshares and pruning hooks
were literally beaten into swords and spears.** Some examples are still to be seen in the Graz armory.

Waves of rumor and paranoia swept through the countryside, enveloping especiallythe powerless. Among the many scapegoats chosen were elderly women living alone, whowere charged with witchcraft. Kepler’s mother was carried away in the middle of the nightin a laundry chest. In Kepler’s little hometown of Weil der Stadt, roughly three women
were tortured and killed as witches every year between 1615 and 1629. And Katharina
Kepler was a cantankerous old woman. She engaged in disputes that annoyed the local
nobility, and she sold soporific and perhaps hallucinogenic drugs as do contemporary
Mexican curanderas. Poor Kepler believed that he himself had contributed to her arrest.
It came about because Kepler wrote one of the first works of science fiction,
intended to explain and popularize science. It was called the Somnium, ‘The Dream.’ Heimagined a journey to the Moon, the space travelers standing on the lunar surface andobserving the lovely planet Earth rotating slowly in the sky above them. By changing ourperspective we can figure out how worlds work. In Kepler’s time one of the chief
objections to the idea that the Earth turns was the fact that people do not feel the motion. In
the Somnium he tried to make the rotation of the Earth plausible, dramatic, comprehensible:‘As long as the multitude does not err .... I want to be on the side of the many. Therefore, I
take great pains to explain to as many people as possible.’ (On another occasion he wrote ina letter, ‘Do not sentence me completely to the treadmill of mathematical calculations leave me time for philosophical speculations, my sole delight.’*)* Brahe, like Kepler, was far from hostile to astrology, although he carefully distinguished his own
secret version of astrology from the more common variants of his time, which he thought conducive tosuperstition. In his book Astronomiae Instauratae Mechonica, published in 1598, he argued that astrology is‘really more reliable than one would think’ if charts of the position of the stars were properly improved. Brahewrote: ‘I have been occupied in alchemy, as much as by the celestial studies, from my 23rd year.’ But both ofthese pseudosciences, he felt, had secrets far too dangerous for the general populace (although entirely safe,
he thought, in the hands of those princes and kings from whom he sought support). Brahe continued the longand truly dangerous tradition of some scientists who believe that only they and the temporal and ecclesiasticalpowers can be trusted with arcane knowledge: ‘It serves no useful purpose and is unreasonable, to make suchthings generally known.’ Kepler, on the other hand, lectured on astronomy in schools, published extensively
and often at his own expense, and wrote science fiction, which was certainly not intended primarily for his
scientific peers. He may not have been a popular writer of science in the modern sense, but the transition in
attitudes in the single generation that separated Tycho and Kepler is telling.

With the invention of the telescope, what Kepler called ‘lunar geography’ wasbecoming possible. In the Somnium, he described the Moon as filled with mountains andvalleys and as ‘porous, as though dug through with hollows and continuous caves,’ areference to the lunar craters Galileo had recently discovered with the first astronomical
telescope. He also imagined that the Moon had its inhabitants, well adapted to theinclemencies of the local environment. He describes the slowly rotating Earth viewed fromthe lunar surface and imagines the continents and oceans of our planet to produce someassociative image like the Man in the Moon. He pictures the near contact of southern Spain
with North Africa at the Straits of Gibraltar as a young woman in a flowing dress about tokiss her lover - although rubbing noses looks more like it to me.

Because of the length of the lunar day and night Kepler described ‘the great
intemperateness of climate and the most violent alternation of extreme heat and cold on theMoon,’ which is entirely correct. Of course, he did not get everything right. He believed,for example, that there was a substantial lunar atmosphere and oceans and inhabitants. Most
curious is his view of the origin of the lunar craters, which make the Moon, he says, ‘notdissimilar to the face of a boy disfigured with smallpox.’ He argued correctly that thecraters are depressions rather than mounds. From his own observations he noted the
ramparts surrounding many craters and the existence of central peaks. But he thought that
their regular circular shape implied such a degree of order that only intelligent life could
explain them. He did not realize that great rocks falling out of the sky would produce a
local explosion, perfectly symmetric in all directions, that would carve out a circular cavity- the origin of the bulk of the craters on the Moon and the other terrestrial planets. He
deduced instead ‘the existence of some race rationally capable of constructing thosehollows on the surface of the Moon. This race must have many individuals, so that one
group puts one hollow to use while another group constructs another hollow.’ Against theview that such great construction projects were unlikely, Kepler offered as counterexamples
the pyramids of Egypt and the Great Wall of China, which can, in fact, be seen today from
Earth orbit. The idea that geometrical order reveals an underlying intelligence was central
to Kepler’s life. His argument on the lunar craters is a clear foreshadowing of the Martiancanal controversy (Chapter 5). It is striking that the observational search for extraterrestrial
life began in the same generation as the invention of the telescope, and with the greatest
theoretician of the age.
Parts of the Somnium were clearly autobiographical. The hero, for example, visits
Tycho Brahe. He has parents who sell drugs. His mother consorts with spirits and daemons,
one of whom eventually provides the means to travel to the moon. The Somnium makes
clear to us, although it did not to all of Kepler’s contemporaries, that ‘in a dream one mustbe allowed the liberty of imagining occasionally that which never existed in the world ofsense perception.’ Science fiction was a new idea at the time of the Thirty Years’ War, andKepler’s book was used as evidence that his mother was a witch.
In the midst of other grave personal problems. Kepler rushed to Württemberg tofind his seventy-four-year-old mother chained in a Protestant secular dungeon andthreatened, like Galileo in a Catholic dungeon, with torture. He set about, as a scientist
naturally would, to find natural explanations for the various events that had precipitated theaccusations of witchcraft, including minor physical ailments that the burghers ofWürttemberg had attributed to her spells. The research was successful, a triumph, as wasmuch of the rest of his life, of reason over superstition. His mother was exiled, with asentence of death passed on her should she ever return to Württemberg; and Kepler’s
spirited defense apparently led to a decree by the Duke forbidding further trials for
witchcraft on such slender evidence.
The upheavals of the war deprived Kepler of much of his financial support, and theend of his life was spent fitfully, pleading for money and sponsors. He cast horoscopes forthe Duke of Wallenstein, as he had done for Rudolf II, and spent his final years in a
Silesian town controlled by Wallenstein and called Sagan. His epitaph, which he himself
composed, was: ‘I measured the skies, now the shadows I measure. Sky-bound was themind, Earth-bound the body rests.’ But the Thirty Years’ War obliterated his grave. If amarker were to be erected today, it might read, in homage to his scientific courage: ‘Hepreferred the hard truth to his dearest illusions.’

Johannes Kepler believed that there would one day be ‘celestial ships with sails
adapted to the winds of heaven’ navigating the sky, filled with explorers ‘who would not
fear the vastness’ of space. And today those explorers, human and robot, employ as
unerring guides on their voyages through the vastness of space the three laws of planetary
motion that Kepler uncovered during a lifetime of personal travail and ecstatic discovery.
The lifelong quest of Johannes Kepler, to understand the motions of the planets, to seek aharmony in the heavens, culminated thirty-six years after his death, in the work of Isaac
Newton. Newton was born on Christmas Day, 1642, so tiny that, as his mother told him
years later, he would have fit into a quart mug. Sickly, feeling abandoned by his parents,
quarrelsome, unsociable, a virgin to the day he died, Isaac Newton was perhaps the greatest
scientific genius who ever lived.
Even as a young man, Newton was impatient with insubstantial questions, such aswhether light was ‘a substance or an accident,’ or how gravitation could act over anintervening vacuum. He early decided that the conventional Christian belief in the Trinity
was a misreading of Scripture. According to his biographer, John Maynard Keynes,
He was rather a Judaic Monotheist of the school of Maimonides. He arrived at thisconclusion, not on so-to-speak rational or sceptical grounds, but entirely on theinterpretation of ancient authority. He was persuaded that the revealed documents
gave no support to the Trinitarian doctrines which were due to late falsifications.
The revealed God was one God. But this was a dreadful secret which Newton was at
desperate pains to conceal all his life.
Like Kepler, he was not immune to the superstitions of his day and had many encounterswith mysticism. Indeed, much of Newton’s intellectual development can be attributed tothis tension between rationalism and mysticism. At the Stourbridge Fair in 1663, at age
twenty, he purchased a book on astrology, ‘out of a curiosity to see what there was in it.’He read it until he came to an illustration which he could not understand, because he wasignorant of trigonometry. So he purchased a book on trigonometry but soon found himself
unable to follow the geometrical arguments. So he found a copy of Euclid’s Elements ofGeometry, and began to read. Two years later he invented the differential calculus.
As a student, Newton was fascinated by light and transfixed by the Sun. He took to
the dangerous practice of staring at the Sun’s image in a looking glass:In a few hours I had brought my eyes to such a pass that I could look upon no brightobject with neither eye but I saw the Sun before me, so that I durst neither write nor
read but to recover the use of my eyes shut my self up in my chamber made dark
three days together & used all means to divert my imagination from the Sun. For if I
thought upon him I presently saw his picture though I was in the dark.
In 1666, at the age of twenty-three, Newton was an undergraduate at Cambridge University
when an outbreak of plague forced him to spend a year in idleness in the isolated village ofWoolsthorpe, where he had been born. He occupied himself by inventing the differential
and integral calculus, making fundamental discoveries on the nature of light and laying thefoundation for the theory of universal gravitation. The only other year like it in the history

of physics was Einstein’s ‘Miracle Year’ of 1905. When asked how he accomplished his
astonishing discoveries, Newton replied unhelpfully, ‘By thinking upon them.’ His work
was so significant that his teacher at Cambridge, Isaac Barrow, resigned his chair ofmathematics in favor of Newton five years after the young student returned to college.
Newton, in his mid-forties, was described by his servant as follows:
I never knew him to take any recreation or pastime either in riding out to take theair, walking, bowling, or any other exercise whatever, thinking all hours lost thatwere not spent in his studies, to which he kept so close that he seldom left his
chamber unless [to lecture] at term time . . . where so few went to hear him, andfewer understood him, that ofttimes he did in a manner, for want of hearers, read to
the walls.
Students both of Kepler and of Newton never knew what they were missing.
Newton discovered the law of inertia, the tendency of a moving object to continue
moving in a straight line unless something influences it and moves it out of its path. The
Moon, it seemed to Newton, would fly off in a straight line, tangential to its orbit, unlessthere were some other force constantly diverting the path into a near circle, pulling it in the
direction of the Earth. This force Newton called gravity, and believed that it acted at a
distance. There is nothing physically connecting the Earth and the Moon. And yet the Earthis constantly pulling the Moon toward us. Using Kepler’s third law, Newton
mathematically deduced the nature of the gravitational force.* He showed that the sameforce that pulls an apple down to Earth keeps the Moon in its orbit and accounts for the
revolutions of the then recently discovered moons of Jupiter in their orbits about that
distant planet.
* Sadly, Newton does not acknowledge his debt to Kepler in his masterpiece the Principia. But in a
1686 letter to Edmund Halley, he says of his law of gravitation: ‘I can affirm that I gathered it from Kepler’s
theorem about twenty years ago.’

Things had been falling down since the beginning of time. That the Moon wentaround the Earth had been believed for all of human history. Newton was the first person
ever to figure out that these two phenomena were due to the same force. This is themeaning of the word ‘universal’ as applied to Newtonian gravitation. The same law ofgravity applies everywhere in the universe.
It is a law of the inverse square. The force declines inversely as the square of
distance. If two objects are moved twice as far away, the gravity now pulling them together
is only one-quarter as strong. If they are moved ten times farther away, the gravity is ten
squared, 102 = 100 times smaller. Clearly, the force must in some sense be inverse - that is,
declining with distance. If the force were direct, increasing with distance, then the strongest
force would work on the most distant objects, and I suppose all the matter in the universe
would find itself careering together into a single cosmic lump. No, gravity must decrease
with distance, which is why a comet or a planet moves slowly when far from the Sun andfaster when close to the Sun - the gravity it feels is weaker the farther from the Sun it is.
All three of Kepler’s laws of planetary motion can be derived from Newtonian
principles. Kepler’s laws were empirical, based upon the painstaking observations of Tycho
Brahe. Newton’s laws were theoretical, rather simple mathematical abstractions from which

all of Tycho’s measurements could ultimately be derived. From these laws, Newton wrote
with undisguised pride in the Principia, ‘I now demonstrate the frame of the System of theWorld.’
Later in his life, Newton presided over the Royal Society, a fellowship of scientists,
and was Master of the Mint, where he devoted his energies to the suppression of counterfeit
coinage. His natural moodiness and reclusivity grew; he resolved to abandon thosescientific endeavors that brought him into quarrelsome disputes with other scientists,
chiefly on issues of priority; and there were those who spread tales that he had experienced
the seventeenth-century equivalent of a ‘nervous breakdown.’ However, Newton continued
his lifelong experiments on the border between alchemy and chemistry, and some recent
evidence suggests that what he was suffering from was not so much a psychogenic ailment
as heavy metal poisoning, induced by systematic ingestion of small quantities of arsenic
and mercury. It was a common practice for chemists of the time to use the sense of taste asan analytic tool.
Nevertheless his prodigious intellectual powers persisted unabated. In 1696, theSwiss mathematician Johann Bernoulli challenged his colleagues to solve an unresolved
issue called the brachistochrone problem, specifying the curve connecting two points
displaced from each other laterally, along which a body, acted upon by gravity, would fallin the shortest time. Bernoulli originally specified a deadline of six months, but extended it
to a year and a half at the request of Leibniz, one of the leading scholars of the time, and theman who had, independently of Newton, invented the differential and integral calculus. The
challenge was delivered to Newton at four P.M. on January 29, 1697. Before leaving forwork the next morning, he had invented an entire new branch of mathematics called thecalculus of variations, used it to solve the brachistochrone problem and sent off thesolution, which was published, at Newton’s request, anonymously. But the brilliance andoriginality of the work betrayed the identity of its author. When Bernoulli saw the solution,
he commented. ‘We recognize the lion by his claw.’ Newton was then in his fifty-fifth year.
The major intellectual pursuit of his last years was a concordance and calibration of
the chronologies of ancient civilizations, very much in the tradition of the ancient historians
Manetho, Strabo and Eratosthenes. In his last, posthumous work, ‘The Chronology ofAncient Kingdoms Amended,’ we find repeated astronomical calibrations of historicalevents; an architectural reconstruction of the Temple of Solomon; a provocative claim that
all the Northern Hemisphere constellations are named after the personages, artifacts andevents in the Greek story of Jason and the Argonauts, and the consistent assumption thatthe gods of all civilizations, with the single exception of Newton’s own, were merely
ancient kings and heroes deified by later generations.
Kepler and Newton represent a critical transition in human history, the discovery
that fairly simple mathematical laws pervade all of Nature; that the same rules apply on
Earth as in the skies; and that there is a resonance between the way we think and the way
the world works. They unflinchingly respected the accuracy of observational data, and theirpredictions of the motion of the planets to high precision provided compelling evidence
that, at an unexpectedly deep level, humans can understand the Cosmos. Our modern global
civilization, our view of the world and our present exploration of the Universe areprofoundly indebted to their insights.
Newton was guarded about his discoveries and fiercely competitive with hisscientific colleagues. He thought nothing of waiting a decade or two after its discovery to
publish the inverse square law. But before the grandeur and intricacy of Nature, he was,

like Ptolemy and Kepler, exhilarated as well as disarmingly modest. Just before his death
he wrote: ‘I do not know what I may appear to the world; but to myself I seem to have been
only like a boy, playing on the seashore, and diverting myself, in now and then finding asmoother pebble or a prettier shell than ordinary, while the great ocean of truth lay all
undiscovered before me.’

CHAPTER IV
Heaven and HellNine worlds I remember.
- The Icelandic Edda of Snorri Sturluson, 1200
I am become death, the shatterer of worlds.
- Bhagavad Gita
The doors of heaven and hell are adjacent and identical.
- Nikos Kazantzakis, The Last Temptation of Christ
The Earth is a lovely and more or less placid place. Things change, but slowly. We can leada full life and never personally encounter a natural disaster more violent than a storm. And
so we become complacent, relaxed, unconcerned. But in the history of Nature, the record isclear. Worlds have been devastated. Even we humans have achieved the dubious technicaldistinction of being able to make our own disasters, both intentional and inadvertent. On
the landscapes of other planets where the records of the past have been preserved, there isabundant evidence of major catastrophes. It is all a matter of time scale. An event thatwould be unthinkable in a hundred years may be inevitable in a hundred million. Even on
the Earth, even in our own century, bizarre natural events have occurred.
In the early morning hours of June 30, 1908, in Central Siberia, a giant fireball wasseen moving rapidly across the sky. Where it touched the horizon, an enormous explosion
took place. It leveled some 2,000 square kilometers of forest and burned thousands of trees
in a flash fire near the impact site. It produced an atmospheric shock wave that twice
circled the Earth. For two days afterwards, there was so much fine dust in the atmospherethat one could read a newspaper at night by scattered light in the streets of London, 10,000
kilometers away.
The government of Russia under the Czars could not be bothered to investigate sotrivial an event, which, after all, had occurred far away, among the backward Tungus
people of Siberia. It was ten years after the Revolution before an expedition arrived to
examine the ground and interview the witnesses. These are some of the accounts theybrought back:
Early in the morning when everyone was asleep in the tent, it was blown up into theair, together with the occupants. When they fell back to Earth, the whole family
suffered slight bruises, but Akulina and Ivan actually lost consciousness. When theyregained consciousness they heard a great deal of noise and saw the forest blazing
round them and much of it devastated.
I was sitting in the porch of the house at the trading station of Vanovara at breakfast
time and looking towards the north. I had just raised my axe to hoop a cask, whensuddenly . . . the sky was split in two, and high above the forest the whole northernpart of the sky appeared to be covered with fire. At that moment I felt a great heat as
if my shirt had caught fire . . . I wanted to pull off my shirt and throw it away, but atthat moment there was a bang in the sky, and a mighty crash was heard. I was
thrown on the ground about three sajenes away from the porch and for a moment I
lost consciousness. My wife ran out and carried me into the hut. The crash was
followed by a noise like stones falling from the sky, or guns firing. The Earthtrembled, and when I lay on the ground I covered my head because I was afraid thatstones might hit it. At that moment when the sky opened, a hot wind, as from acannon, blew past the huts from the north. It left its mark on the ground . . .
When I sat down to have my breakfast beside my plough, I heard sudden bangs, as
if from gun-fire. My horse fell to its knees. From the north side above the forest aflame shot up . . . Then I saw that the fir forest had been bent over by the wind and I
thought of a hurricane. I seized hold of my plough with both hands, so that it would
not be carried away. The wind was so strong that it carried off some of the soil fromthe surface of the ground, and then the hurricane drove a wall of water up theAngara. I saw it all quite clearly, because my land was on a hillside.
The roar frightened the horses to such an extent that some galloped off in panic,dragging the ploughs in different directions, and others collapsed.
The carpenters, after the first and second crashes, had crossed themselves instupefaction, and when the third crash resounded they fell backwards from thebuilding onto the chips of wood. Some of them were so stunned and utterly terrified
that I had to calm them down and reassure them. We all abandoned work and went
into the village. There, whole crowds of local inhabitants were gathered in thestreets in terror, talking about this phenomenon.
I was in the fields . . . and had only just got one horse harnessed to the harrow andbegun to attach another when suddenly I heard what sounded like a single loud shot
to the right. I immediately turned round and saw an elongated flaming object flying
through the sky. The front part was much broader than the tail end and its color was
like fire in the day-time. It was many times bigger than the sun but much dimmer,
so that it was possible to look at it with the naked eye. Behind the flames trailed
what looked like dust. It was wreathed in little puffs, and blue streamers were left
behind from the flames . . . As soon as the flame had disappeared, bangs louder thanshots from a gun were heard, the ground could be felt to tremble, and the windowpanes in the cabin were shattered.
. . . I was washing wool on the bank of the River Kan. Suddenly a noise like thefluttering of the wings of a frightened bird was heard . . . and a kind of swell cameup the river. After this came a single sharp bang so loud that one of the workmen . .
. fell into the water.
This remarkable occurrence is called the Tunguska Event. Some scientists have suggested
that it was caused by a piece of hurtling antimatter, annihilated on contact with the ordinary
matter of the Earth, disappearing in a flash of gamma rays. But the absence of radioactivity
at the impact site gives no support to this explanation. Others postulate that a mini blackhole passed through the Earth in Siberia and out the other side. But the records ofatmospheric shock waves show no hint of an object booming out of the North Atlantic laterthat day. Perhaps it was a spaceship of some unimaginably advanced extraterrestrial
civilization in desperate mechanical trouble, crashing in a remote region of an obscureplanet. But at the site of the impact there is no trace of such a ship. Each of these ideas has
been proposed, some of them more or less seriously. Not one of them is strongly supportedby the evidence. The key point of the Tunguska Event is that there was a tremendous
explosion, a great shock wave, an enormous forest fire, and yet there is no impact crater at
the site. There seems to be only one explanation consistent with all the facts: In 1908 apiece of comet hit the Earth.
In the vast spaces between the planets there are many objects, some rocky, somemetallic, some icy, some composed partly of organic molecules. They range from grains ofdust to irregular blocks the size of Nicaragua or Bhutan. And sometimes, by accident, there
is a planet in the way. The Tunguska Event was probably caused by an icy cometaryfragment about a hundred meters across - the size of a football field - weighing a million
tons, moving at about 30 kilometers per second, 70,000 miles per hour.
If such an impact occurred today it might be mistaken, especially in the panic of the
moment, for a nuclear explosion. The cometary impact and fireball would simulate all
effects of a one-megaton nuclear burst, including the mushroom cloud, with two
exceptions: there would be no gamma radiation or radioactive fallout. Could a rare but
natural event, the impact of a sizable cometary fragment, trigger a nuclear war? A strange
scenario: a small comet hits the Earth, as millions of them have, and the response of ourcivilization is promptly to self-destruct. It might be a good idea for us to understand comets
and collisions and catastrophes a little better than we do. For example, an American Velasatellite detected an intense double flash of light from the vicinity of the South Atlantic andWestern Indian Ocean on September 22, 1979. Early speculation held that it was aclandestine test of a low yield (two kilotons, about a sixth the energy of the Hiroshimabomb) nuclear weapon by South Africa or Israel. The political consequences wereconsidered serious around the world. But what if the flashes were instead caused by the
impact of a small asteroid or a piece of a comet? Since airborne over-flights in the vicinityof the flashes showed not a trace of unusual radioactivity in the air, this is a real possibility
and underscores the dangers in an age of nuclear weapons of not monitoring impacts fromspace better than we do.
A comet is made mostly of ice - water (H2O) ice, with a little methane (CH4) ice,and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometaryfragment would produce a great radiant fireball and a mighty blast wave, which would burntrees, level forests and be heard around the world. But it might not make much of a crater inthe ground. The ices would all be melted during entry. There would be few recognizablepieces of the comet left - perhaps only a smattering of small grains from the non-icy partsof the cometary nucleus. Recently, the Soviet scientist E. Sobotovich has identified a large
number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known
to exist in meteorites that have survived impact, and that may originate ultimately fromcomets.
On many a clear night, if you look patiently up at the sky, you will see a solitary
meteor blazing briefly overhead. On some nights you can see a shower of meteors, always
on the same few days of every year - a natural fireworks display, an entertainment in theheavens. These meteors are made by tiny grains, smaller than a mustard seed. They are lessshooting stars than falling fluff. Momentarily brilliant as they enter the Earth’s atmosphere,they are heated and destroyed by friction at a height of about 100 kilometers. Meteors are
the remnants of comets.* Old comets, heated by repeated passages near the Sun, break up,
evaporate and disintegrate. The debris spreads to fill the full cometary orbit. Where thatorbit intersects the orbit of the Earth, there is a swarm of meteors waiting for us. Some partof the swarm is always at the same position in the Earth’s orbit, so the meteor shower isalways observed on the same day of every year. June 30, 1908 was the day of the Beta
Taurid meteor shower, connected with the orbit of Comet Encke. The Tunguska Event
seems to have been caused by a chunk of Comet Encke, a piece substantially larger than thetiny fragments that cause those glittering, harmless meteor showers.
* That meteors and meteorites are connected with the comets was first proposed by Alexander von
Humboldt in his broad-gauge popularization of all of science, published in the years 1845 to 1862, a work
called Kosmos. It was reading Humboldt’s earlier work that fired the young Charles Darwin to embark on acareer combining geographical exploration and natural history. Shortly thereafter he accepted a position as
naturalist aboard the ship HMS Beagle, the event that led to The Origin of Species.

Comets have always evoked fear and awe and superstition. Their occasional
apparitions disturbingly challenged the notion of an unalterable and divinely ordered
Cosmos. It seemed inconceivable that a spectacular streak of milk-white flame, rising andsetting with the stars night after night, was not there for a reason, did not hold some portentfor human affairs. So the idea arose that comets were harbingers of disaster, auguries of
divine wrath - that they foretold the deaths of princes, the fall of kingdoms. The
Babylonians thought that comets were celestial beards. The Greeks thought of flowing hair,
the Arabs of flaming swords. In Ptolemy’s time comets were elaborately classified as‘beams,’ ‘trumpets,’ ‘jars’ and so on, according to their shapes. Ptolemy thought thatcomets bring wars, hot weather and ‘disturbed conditions.’ Some medieval depictions ofcomets resemble unidentified flying crucifixes. A Lutheran ‘Superintendent’ or Bishop of
Magdeburg named Andreas Celichius published in 1578 a ‘Theological Reminder of the
New Comet,’ which offered the inspired view that a comet is ‘the thick smoke of humansins, rising every day, every hour, every moment, full of stench and horror before the faceof God, and becoming gradually so thick as to form a comet, with curled and plaitedtresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge.’
But others countered that if comets were the smoke of sin, the skies would be continuallyablaze with them.
The most ancient record of an apparition of Halley’s (or any other) Comet appears
in the Chinese Book of Prince Huai Nan, attendant to the march of King Wu against Zhou
of Yin. The year was 1057 B.C. The approach to Earth of Halley’s Comet in the year 66 is
the probable explanation of the account by Josephus of a sword that hung over Jerusalem
for a whole year. In 1066 the Normans witnessed another return of Halley’s Comet. Since it
must, they thought, presage the fall of some kingdom, the comet encouraged, in some sense
precipitated, the invasion of England by William the Conqueror. The comet was duly noted
in a newspaper of the time, the Bayeux Tapestry. In 1301, Giotto, one of the founders ofmodern realistic painting, witnessed another apparition of Comet Halley and inserted it intoa nativity scene. The Great Comet of 1466 - yet another return of Halley’s Comet panicked Christian Europe; the Christians feared that God, who sends comets, might be on
the side of the Turks, who had just captured Constantinople.
The leading astronomers of the sixteenth and seventeenth centuries were fascinatedby comets, and even Newton became a little giddy over them. Kepler described comets as
darting through space ‘as the fishes in the sea,’ but being dissipated by sunlight, as thecometary tail always points away from the sun. David Hume, in many cases anuncompromising rationalist, at least toyed with the notion that comets were thereproductive cells - the eggs or sperm - of planetary systems, that planets are produced by akind of interstellar sex. As an undergraduate, before his invention of the reflecting
telescope, Newton spent many consecutive sleepless nights searching the sky for comets
with his naked eye, pursuing them with such fervor that he felt ill from exhaustion.
Following Tycho and Kepler, Newton concluded that the comets seen from Earth do not
move within our atmosphere, as Aristotle and others had thought, but rather are moredistant than the Moon, although closer than Saturn. Comets shine, as the planets do, byreflected sunlight, ‘and they are much mistaken who remove them almost as far as the fixedstars; for if it were so, the comets could receive no more light from our Sun than our planets
do from the fixed stars.’ He showed that comets, like planets, move in ellipses: ‘Comets area sort of planets revolved in very eccentric orbits about the Sun.’ This demystification, thisprediction of regular cometary orbits, led his friend Edmund Halley in 1707 to calculatethat the comets of 1531, 1607 and 1682 were apparitions at 76-year intervals of the samecomet, and predicted its return in 1758. The comet duly arrived and was named for him
posthumously. Comet Halley has played an interesting role in human history, and may be
the target of the first space vehicle probe of a comet, during its return in 1986.
Modern planetary scientists sometimes argue that the collision of a comet with aplanet might make a significant contribution to the planetary atmosphere. For example, all
the water in the atmosphere of Mars today could be accounted for by a recent impact of asmall comet. Newton noted that the matter in the tails of comets is dissipated ininterplanetary space, lost to the comet and little by little attracted gravitationally to nearby
planets. He believed that the water on the Earth is gradually being lost, ‘spent upon
vegetation and putrefaction, and converted into dry earth . . . The fluids, if they are notsupplied from without, must be in a continual decrease, and quite fail at last.’ Newton
seems to have believed that the Earth’s oceans are of cometary origin, and that life ispossible only because cometary matter falls upon our planet. In a mystical reverie, he went
still further: ‘I suspect, moreover, that it is chiefly from the comets that spirit comes, whichis indeed the smallest but the most subtle and useful part of our air, and so much required tosustain the life of all things with us.’As early as 1868 the astronomer William Huggins found an identity between somefeatures in the spectrum of a comet and the spectrum of natural or ‘olefiant’ gas. Huggins
had found organic matter in the comets; in subsequent years cyanogen, CN, consisting of acarbon and a nitrogen atom, the molecular fragment that makes cyanides, was identified in
the tails of comets. When the Earth was about to pass through the tail of Halley’s Comet in
1910, many people panicked. They overlooked the fact that the tail of a comet is
extravagantly diffuse: the actual danger from the poison in a comet’s tail is far less than thedanger, even in 1910, from industrial pollution in large cities.
But that reassured almost no one. For example, headlines in the San Francisco
Chronicle for May 15, 1910, include ‘Comet Camera as Big as a House,’ ‘Comet Comesand Husband Reforms,’ ‘Comet Parties Now Fad in New York.’ The Los Angeles
Examiner adopted a light mood: ‘Say! Has That Comet Cyanogened You Yet? . . . Entire
Human Race Due for Free Gaseous Bath,’ ‘Expect “High Jinks,”’ ‘Many Feel CyanogenTang,’ ‘Victim Climbs Trees, Tries to Phone Comet.’ In 1910 there were parties, making

merry before the world ended of cyanogen pollution. Entrepreneurs hawked anti-cometpills and gas masks, the latter an eerie premonition of the battlefields of World War 1.
Some confusion about comets continues to our own time. In 1957, I was a graduate
student at the University of Chicago’s Yerkes Observatory. Alone in the observatory late
one night, I heard the telephone ring persistently. When I answered, a voice, betraying awell-advanced state of inebriation, said, ‘Lemme talk to a shtrominer.’ ‘Can I help you?’‘Well, see, we’re havin’ this garden party out here in Wilmette, and there’s somethin’ in thesky. The funny part is, though, if you look straight at it, it goes away. But if you don’t lookat it, there it is.’ The most sensitive part of the retina is not at the center of the field of view.
You can see faint stars and other objects by averting your vision slightly. I knew that,barely visible in the sky at this time, was a newly discovered comet called Arend-Roland.
So I told him that he was probably looking at a comet. There was a long pause, followed by
the query: ‘Wash’a comet?’ ‘A comet,’ I replied, ‘is a snowball one mile across.’ There wasa longer pause, after which the caller requested, ‘Lemme talk to a real shtrominer.’ When
Halley’s Comet reappears in 1986, I wonder what political leaders will fear the apparition,
what other silliness will then be upon us.
While the planets move in elliptical orbits around the Sun, their orbits are not very
elliptical. At first glance they are, by and large, indistinguishable from circles. It is thecomets - especially the long-period comets - that have dramatically elliptical orbits. The
planets are the old-timers in the inner solar system; the comets are the newcomers. Why are
the planetary orbits nearly circular and neatly separated one from the other? Because if
planets had very elliptical orbits, so that their paths intersected, sooner or later there would
be a collision. In the early history of the solar system, there were probably many planets inthe process of formation. Those with elliptical crossing orbits tended to collide and destroy
themselves. Those with circular orbits tended to grow and survive. The orbits of the present
planets are the orbits of the survivors of this collisional natural selection, the stable middle
age of a solar system dominated by early catastrophic impacts.
In the outermost solar system, in the gloom far beyond the planets, there is a vastspherical cloud of a trillion cometary nuclei, orbiting the Sun no faster than a racing car at
the Indianapolis 500.* A fairly typical comet would look like a giant tumbling snowballabout 1 kilometer across. Most never penetrate the border marked by the orbit of Pluto. But
occasionally a passing star makes a gravitational flurry and commotion in the cometary
cloud, and a group of comets finds itself in highly elliptical orbits, plunging toward theSun. After its path is further changed by gravitational encounters with Jupiter or Saturn, it
tends to find itself, once every century or so, careering toward the inner solar system.
Somewhere between the orbits of Jupiter and Mars it would begin heating and evaporating.
Matter blown outwards from the Sun’s atmosphere, the solar wind, carries fragments ofdust and ice back behind the comet, making an incipient tail. If Jupiter were a meter across,our comet would be smaller than a speck of dust, but when fully developed, its tail would
be as great as the distances between the worlds. When within sight of the Earth on each of
its orbits, it would stimulate outpourings of superstitious fervor among the Earthlings. But
eventually they would understand that it lived not in their atmosphere, but out among theplanets. They would calculate its orbit. And perhaps one day soon they would launch asmall space vehicle devoted to exploring this visitor from the realm of the stars.
* The Earth is r = 1 astronomical unit = 150,000,000 kilometers from the Sun. Its roughly circularorbit then has a circumference of 2 r 109 km. Our planet circulates once along this path every year. One year

= 3 x 107 seconds. So the Earth’s orbital speed is 109 km/3 x 107 sec 30 km/sec. Now consider the sphericalshell of orbiting comets that many astronomers believe surrounds the solar system at a distance 100,000
astronomical units, almost halfway to the nearest star. From Kepler’s third law it immediately follows that theorbital period about the Sun of any one of them is about (105)3/2 = 1075 3 x 107 or 30 million years. Oncearound the Sun is a long time if you live in the outer reaches of the solar system. The cometary orbit is 2 a =2 x 105 x 1.5 x 108 km 1014 km around, and its speed is therefore only 1014 km/ 1015 sec = 0.1 km/sec 220
miles per hour.

Sooner or later comets will collide with planets. The Earth and its companion theMoon must be bombarded by comets and small asteroids, debris left over from theformation of the solar system. Since there are more small objects than large ones, there
should be more impacts by small objects than by large ones. An impact of a small cometaryfragment with the Earth, as at Tunguska, should occur about once every thousand years.
But an impact with a large comet, such as Halley’s Comet, whose nucleus is perhaps
twenty kilometers across, should occur only about once every billion years.
When a small, icy object collides with a planet or a moon, it may not produce a very
major scar. But if the impacting object is larger or made primarily of rock, there is an
explosion on impact that carves out a hemispherical bowl called an impact crater. And if noprocess rubs out or fills in the crater, it may last for billions of years. Almost no erosion
occurs on the Moon and when we examine its surface, we find it covered with impact
craters, many more than can be accounted for by the rather sparse population of cometary
and asteroidal debris that now fills the inner solar system. The lunar surface offers eloquenttestimony of a previous age of the destruction of worlds, now billions of years gone.
Impact craters are not restricted to the Moon. We find them throughout the innersolar system - from Mercury, closest to the Sun, to cloud-covered Venus to Mars and its
tiny moons, Phobos and Deimos. These are the terrestrial planets, our family of worlds, theplanets more or less like the Earth. They have solid surfaces, interiors made of rock andiron, and atmospheres ranging from near-vacuum to pressures ninety times higher than the
Earth’s. They huddle around the Sun, the source of light and heat, like campers around afire. The planets are all about 4.6 billion years old. Like the Moon, they all bear witness toan age of impact catastrophism in the early history of the solar system.
As we move out past Mars we enter a very different regime - the realm of Jupiterand the other giant or Jovian planets. These are great worlds, composed largely of hydrogenand helium, with smaller amounts of hydrogen-rich gases such as methane, ammonia andwater. We do not see solid surfaces here, only the atmosphere and the multicolored clouds.
These are serious planets, not fragmentary worldlets like the Earth. A thousand Earths
could fit inside Jupiter. If a comet or an asteroid dropped into the atmosphere of Jupiter, we
would not expect a visible crater, only a momentary break in the clouds. Nevertheless, weknow there has been a many-billion-year history of collisions in the outer solar system aswell - because Jupiter has a great system of more than a dozen moons, five of which were
examined close up by the Voyager spacecraft. Here again we find evidence of pastcatastrophes. When the solar system is all explored, we will probably have evidence for
impact catastrophism on all nine worlds, from Mercury to Pluto, and on all the smaller
moons, comets and asteroids.
There are about 10,000 craters on the near side of the Moon, visible to telescopes on
Earth. Most of them are in the ancient lunar highlands and date from the time of the final
accretion of the Moon from interplanetary debris. There are about a thousand craters largerthan a kilometer across in the maria (Latin for ‘seas’), the lowland regions that were
flooded, perhaps by lava, shortly after the formation of the Moon, covering over the preexisting craters. Thus, very roughly, craters on the Moon should be formed today at the rateof about 109 years/104 craters, = 105 years/crater, a hundred thousand years between
cratering events. Since there may have been more interplanetary debris a few billion years
ago than there is today, we might have to wait even longer than a hundred thousand years
to see a crater form on the Moon. Because the Earth has a larger area than the Moon, we
might have to wait something like ten thousand years between collisions that would makecraters as big as a kilometer across on our planet. And since Meteor Crater, Arizona, an
impact crater about a kilometer across, has been found to be twenty or thirty thousand years
old, the observations on the Earth are in agreement with such crude calculations.
The actual impact of a small comet or asteroid with the Moon might make amomentary explosion sufficiently bright to be visible from the Earth. We can imagine ourancestors gazing idly up on some night a hundred thousand years ago and noting a strange
cloud arising from the unilluminated part of the Moon, suddenly struck by the Sun’s rays.
But we would not expect such an event to have happened in historical times. The odds
against it must be something like a hundred to one. Nevertheless, there is an historicalaccount which may in fact describe an impact on the Moon seen from Earth with the naked
eye: On the evening of June 25, 1178, five British monks reported something extraordinary,
which was later recorded in the chronicle of Gervase of Canterbury, generally considered a
reliable reporter on the political and cultural events of his time, after he had interviewed theeyewitnesses who asserted, under oath, the truth of their story. The chronicle reads:
There was a bright New Moon, and as usual in that phase its horns were tilted
towards the east. Suddenly, the upper horn split in two. From the midpoint of thedivision, a flaming torch sprang up, spewing out fire, hot coals, and sparks.
The astronomers Derral Mulholland and Odile Calame have calculated that a lunar
impact would produce a dust cloud rising off the surface of the Moon with an appearance
corresponding rather closely to the report of the Canterbury monks.
If such an impact were made only 800 years ago, the crater should still be visible.
Erosion on the Moon is so inefficient, because of the absence of air and water, that evensmall craters a few billion years old are still comparatively well preserved. From thedescription recorded by Gervase, it is possible to pinpoint the sector of the Moon to which
the observations refer. Impacts produce rays, linear trails of fine powder spewed out during
the explosion. Such rays are associated with the very youngest craters on the Moon - forexample, those named after Aristarchus and Copernicus and Kepler. But while the craters
may withstand erosion on the Moon, the rays, being exceptionally thin, do not. As timegoes on, even the arrival of micrometeorites - fine dust from space stirs up and covers over
the rays, and they gradually disappear. Thus rays are a signature of a recent impact.
The meteoriticist Jack Hartung has pointed out that a very recent, very fresh-looking
small crater with a prominent ray system lies exactly in the region of the Moon referred toby the Canterbury monks. It is called Giordano Bruno after the sixteenth-century Roman
Catholic scholar who held that there are an infinity of worlds and that many are inhabited.
For this and other crimes he was burned at the stake in the year 1600.
Another line of evidence consistent with this interpretation has been provided byCalame and Mulholland. When an object impacts the Moon at high speed, it sets the Moonslightly wobbling. Eventually the vibrations die down but not in so short a period as eight
hundred years. Such a quivering can be studied by laser reflection techniques. The Apollo
astronauts emplaced in several locales on the Moon special mirrors called laser retroreflectors. When a laser beam from Earth strikes the mirror and bounces back, the roundtrip travel time can be measured with remarkable precision. This time multiplied by the
speed of light gives us the distance to the Moon at that moment to equally remarkable
precision. Such measurements, performed over a period of years, reveal the Moon to belibrating, or quivering with a period (about three years) and amplitude (about three meters),
consistent with the idea that the crater Giordano Bruno was gouged out less than a thousand
years ago.
All this evidence is inferential and indirect. The odds, as I have said, are against
such an event happening in historical times. But the evidence is at least suggestive. As the
Tunguska Event and Meteor Crater, Arizona, also remind us, not all impact catastrophes
occurred in the early history of the solar system. But the fact that only a few of the lunar
craters have extensive ray systems also reminds us that, even on the Moon, some erosion
occurs.* By noting which craters overlap which and other signs of lunar stratigraphy, we
can reconstruct the sequence of impact and flooding events of which the production ofcrater Bruno is perhaps the most recent example.
* On Mars, where erosion is much more efficient, although there are many craters there are virtuallyno ray craters, as we would expect.

The Earth is very near the Moon. If the Moon is so severely cratered by impacts,
how has the Earth avoided them? Why is Meteor Crater so rare? Do the comets andasteroids think it inadvisable to impact an inhabited planet? This is an unlikely forbearance.
The only possible explanation is that impact craters are formed at very similar rates on boththe Earth and the Moon, but that on the airless, waterless Moon they are preserved for
immense periods of time, while on the Earth slow erosion wipes them out or fills them in.
Running water, windblown sand and mountain-building are very slow processes. But overmillions or billions of years, they are capable of utterly erasing even very large impact
scars.
On the surface of any moon or planet, there will be external processes, such as
impacts from space, and internal processes, such as earthquakes; there will be fast,catastrophic events, such as volcanic explosions, and processes of excruciating slowness,such as the pitting of a surface by tiny airborne sand grains. There is no general answer tothe question of which processes dominate, the outside ones or the inside ones; the rare but
violent events, or the common and inconspicuous occurrences. On the Moon, the outside,catastrophic events hold sway; on Earth, the inside, slow processes dominate. Mars is anintermediate case.
Between the orbits of Mars and Jupiter are countless asteroids, tiny terrestrial
planets. The largest are a few hundred kilometers across. Many have oblong shapes and aretumbling through space. In some cases there seem to be two or more asteroids in tightmutual orbits. Collisions among the asteroids happen frequently, and occasionally a piece ischipped off and accidentally intercepts the Earth, falling to the ground as a meteorite. In the
exhibits, on the shelves of our museums are the fragments of distant worlds. The asteroid
belt is a great grinding mill, producing smaller and smaller pieces down to motes of dust.
The bigger asteroidal pieces, along with the comets, are mainly responsible for the recent

craters on planetary surfaces. The asteroid belt may be a place where a planet was once
prevented from forming because of the gravitational tides of the giant nearby planet Jupiter;or it may be the shattered remains of a planet that blew itself up. This seems improbablebecause no scientist on Earth knows how a planet might blow itself up, which is probably
just as well.
The rings of Saturn bear some resemblance to the asteroid belt: trillions of tiny icy
moonlets orbiting the planet. They may represent debris prevented by the gravity of Saturnfrom accreting into a nearby moon, or they may be the remains of a moon that wandered
too close and was torn apart by the gravitational tides. Alternatively, they may be the steadystate equilibrium between material ejected from a moon of Saturn, such as Titan, andmaterial falling into the atmosphere of the planet. Jupiter and Uranus also have ringsystems, discovered only recently, and almost invisible from the Earth. Whether Neptune
has a ring is a problem high on the agenda of planetary scientists. Rings may be a typicaladornment of Jovian-type planets throughout the cosmos.
Major recent collisions from Saturn to Venus were alleged in a popular book,
Worlds in Collision, published in 1950 by a psychiatrist named Immanuel Velikovsky. He
proposed that an object of planetary mass, which he called a comet, was somehow
generated in the Jupiter system. Some 3,500 years ago, it careered in toward the inner solar
system and made repeated encounters with the Earth and Mars, having as incidental
consequences the parting of the Red Sea, allowing Moses and the Israelites to escape from
Pharaoh, and the stopping of the Earth from rotating on Joshua’s command. It also caused,
he said, extensive vulcanism and floods.* Velikovsky imagined the comet, after acomplicated game of interplanetary billiards, to settle down into a stable, nearly circularorbit, becoming the planet Venus - which he claimed never existed before then.
* As far as I know, the first essentially nonmystical attempt to explain a historical event by cometaryintervention was Edmund Halley’s proposal that the Noachic flood was ‘the casual Choc [shock] of a Comet.’

As I have discussed at some length elsewhere, these ideas are almost certainly
wrong. Astronomers do not object to the idea of major collisions, only to major recent
collisions. In any model of the solar system it is impossible to show the sizes of the planetson the same scale as their orbits, because the planets would then be almost too small to see.
If the planets were really shown to scale, as grains of dust, we would easily note that the
chance of collision of a particular comet with the Earth in a few thousand years is
extraordinarily low. Moreover, Venus is a rocky and metallic, hydrogen-poor planet,
whereas Jupiter - where Velikovsky supposed it comes from - is made almost entirely ofhydrogen. There are no energy sources for comets or planets to be ejected by Jupiter. If one
passed by the Earth, it could not ‘stop’ the Earth’s rotation, much less start it up again attwenty-four hours a day. No geological evidence supports the idea of an unusual frequency
of vulcanism or floods 3,500 years ago. There are Mesopotamian inscriptions referring toVenus that predate the time when Velikovsky says Venus changed from a comet into aplanet.* It is very unlikely that an object in such a highly elliptical orbit could be rapidly
moved into the nearly perfectly circular orbit of present-day Venus. And so on.
* The Adda cylinder seal, dating from the middle of the third millennium B.C., prominently displays
Inanna, the goddess of Venus, the morning star, and precursor of the Babylonian Ishtar.

Many hypotheses proposed by scientists as well as by non-scientists turn out to be
wrong. But science is a self-correcting enterprise. To be accepted, all new ideas must
survive rigorous standards of evidence. The worst aspect of the Velikovsky affair is not that
his hypotheses were wrong or in contradiction to firmly established facts, but that somewho called themselves scientists attempted to suppress Velikovsky’s work. Science isgenerated by and devoted to free inquiry: the idea that any hypothesis, no matter how
strange, deserves to be considered on its merits. The suppression of uncomfortable ideas
may be common in religion and politics, but it is not the path to knowledge; it has no placein the endeavor of science. We do not know in advance who will discover fundamental new
insights.
Venus has almost the same mass,* size, and density as the Earth. As the nearest
planet, it has for centuries been thought of as the Earth’s sister. What is our sister planet
really like? Might it be a balmy, summer planet, a little warmer than the Earth because it isa little closer to the Sun? Does it have impact craters, or have they all eroded away? Are
there volcanoes? Mountains? Oceans? Life?
* It is, incidentally, some 30 million times more massive than the most massive comet known.

The first person to look at Venus through the telescope was Galileo in 1609. He saw
an absolutely featureless disc. Galileo noted that it went through phases, like the Moon,
from a thin crescent to a full disc, and for the same reason: we are sometimes looking
mostly at the night side of Venus and sometimes mostly at the day side, a finding thatincidentally reinforced the view that the Earth went around the Sun and not vice versa. As
optical telescopes became larger and their resolution (or ability to discriminate fine detail)improved, they were systematically turned toward Venus. But they did no better than
Galileo’s. Venus was evidently covered by a dense layer of obscuring cloud. When we lookat the planet in the morning or evening skies, we are seeing sunlight reflected off the cloudsof Venus. But for centuries after their discovery, the composition of those clouds remained
entirely unknown.
The absence of anything to see on Venus led some scientists to the curious
conclusion that the surface was a swamp, like the Earth in the Carboniferous Period. The
argument - if we can dignify it by such a word - went something like this:‘I can’t see a thing on Venus.’‘Why not?’‘Because it’s totally covered with clouds.’‘What are clouds made of?’‘Water, of course.’‘Then why are the clouds of Venus thicker than the clouds on Earth?’‘Because there’s more water there.’‘But if there is more water in the clouds, there must be more water on the surface.
What kind of surfaces are very wet?’‘Swamps.’
And if there are swamps, why not cyacads and dragonflies and perhaps even dinosaurs on
Venus? Observation: There was absolutely nothing to see on Venus. Conclusion: It must becovered with life. The featureless clouds of Venus reflected our own predispositions. Weare alive, and we resonate with the idea of life elsewhere. But only careful accumulation
and assessment of the evidence can tell us whether a given world is inhabited. Venus turns
out not to oblige our predispositions.
The first real clue to the nature of Venus came from work with a prism made ofglass or a flat surface, called a diffraction grating, covered with fine, regularly spaced, ruled
lines. When an intense beam of ordinary white light passes through a narrow slit and thenthrough a prism or grating, it is spread into a rainbow of colors called a spectrum. The
spectrum runs from high frequencies* of visible light to low ones - violet, blue, green,
yellow, orange and red. Since we see these colors, it is called the spectrum of visible light.
But there is far more light than the small segment of the spectrum we can see. At higher
frequencies, beyond the violet, is a part of the spectrum called the ultraviolet: a perfectly
real kind of light, carrying death to the microbes. It is invisible to us, but readily detectableby bumblebees and photoelectric cells. There is much more to the world than we can see.
Beyond the ultraviolet is the X-ray part of the spectrum, and beyond the X-rays are the
gamma rays. At lower frequencies, on the other side of red, is the infrared part of thespectrum. It was first discovered by placing a sensitive thermometer in what to our eyes isthe dark beyond the red. The temperature rose. There was light falling on the thermometereven though it was invisible to our eyes. Rattlesnakes and doped semiconductors detect
infrared radiation perfectly well. Beyond the infrared is the vast spectral region of the radio
waves. From gamma rays to radio waves, all are equally respectable brands of light. All areuseful in astronomy. But because of the limitations of our eyes, we have a prejudice, a bias,
toward that tiny rainbow band we call the spectrum of visible light.
* Light is a wave motion; its frequency is the number of wave crests, say, entering a detection
instrument, such as a retina, in a given unit of time, such as a second. The higher the frequency, the moreenergetic the radiation.

Schematic diagram of the electromagnetic spectrum, ranging from the shortest wavelengths(gamma rays) to the longest (radio waves). The wavelength of light is measured inÅngstroms (Å), micrometers (µm), centimeters (cm) and meters (m).

In 1844, the philosopher Auguste Comte was searching for an example of a sort ofknowledge that would be always hidden. He chose the composition of distant stars andplanets. We would never physically visit them, he thought, and with no sample in hand it
seemed we would forever be denied knowledge of their composition. But only three years
after Comte’s death, it was discovered that a spectrum can be used to determine thechemistry of distant objects. Different molecules and chemical elements absorb different
frequencies or colors of light, sometimes in the visible and sometimes elsewhere in the
spectrum. In the spectrum of a planetary atmosphere, a single dark line represents an imageof the slit in which light is missing, the absorption of sunlight during its brief passage
through the air of another world. Each such line is made by a particular kind of molecule oratom. Every substance has its characteristic spectral signature. The gases on Venus can be
identified from the Earth, 60 million kilometers away. We can divine the composition of
the Sun (in which helium, named after the Greek sun god Helios, was first found); of
magnetic A stars rich in europium; of distant galaxies analyzed through the collective lightof a hundred billion constituent stars. Astronomical spectroscopy is an almost magical
technique. It amazes me still. Auguste Comte picked a particularly unfortunate example.
If Venus were soaking wet, it should be easy to see the water vapor lines in its
spectrum. But the first spectroscopic searches, attempted at Mount Wilson Observatory
around 1920, found not a hint, not a trace, of water vapor above the clouds of Venus,
suggesting an arid, desert-like surface, surmounted by clouds of fine drifting silicate dust.
Further study revealed enormous quantities of carbon dioxide in the atmosphere, implying
to some scientists that all the water on the planet had combined with hydrocarbons to form
carbon dioxide, and that therefore the surface of Venus was a global oil field, a planet-wide
sea of petroleum. Others concluded that there was no water vapor above the clouds because
the clouds were very cold, that all the water had condensed out into water droplets, which
do not have the same pattern of spectral lines as water vapor. They suggested that the planet
was totally covered with water - except perhaps for an occasional limestone-encrusted
island, like the cliffs of Dover. But because of the vast quantities of carbon dioxide in theatmosphere, the sea could not be ordinary water; physical chemistry required carbonatedwater. Venus, they proposed, had a vast ocean of seltzer.
The first hint of the true situation came not from spectroscopic studies in the visibleor near-infrared parts of the spectrum, but rather from the radio region. A radio telescope
works more like a light meter than a camera. You point it toward some fairly broad regionof the sky, and it records how much energy, in a particular radio frequency, is coming down
to Earth. We are used to radio signals transmitted by some varieties of intelligent life namely, those who run radio and television stations. But there are many other reasons fornatural objects to give off radio waves. One is that they are hot. And when, in 1956, anearly radio telescope was turned toward Venus, it was discovered to be emitting radio
waves as if it were at an extremely high temperature. But the real demonstration that thesurface of Venus is astonishingly hot came when the Soviet spacecraft of the Venera series
first penetrated the obscuring clouds and landed on the mysterious and inaccessible surfaceof the nearest planet. Venus, it turns out, is broiling hot. There are no swamps, no oil fields,
no seltzer oceans. With insufficient data, it is easy to go wrong.
When I greet a friend, I am seeing her in reflected visible light, generated by theSun, say, or by an incandescent lamp. The light rays bounce off my friend and into my eye.
But the ancients, including no less a figure than Euclid, believed that we see by virtue ofrays somehow emitted by the eye and tangibly, actively contacting the object observed.
This is a natural notion and can still be encountered, although it does not account for theinvisibility of objects in a darkened room. Today we combine a laser and a photocell, or aradar transmitter and a radio telescope, and in this way make active contact by light with
distant objects. In radar astronomy, radio waves are transmitted by a telescope on Earth,strike, say, that hemisphere of Venus that happens to be facing the Earth, and bounce back.
At many wavelengths the clouds and atmosphere of Venus are entirely transparent to radio
waves. Some places on the surface will absorb them or, if they are very rough, will scatterthem sideways and so will appear dark to radio waves. By following the surface features
moving with Venus as it rotates, it was possible for the first time to determine reliably thelength of its day - how long it takes Venus to spin once on its axis. It turns out that, withrespect to the stars, Venus turns once every 243 Earth days, but backwards, in the opposite
direction from all other planets in the inner solar system. As a result, the Sun rises in thewest and sets in the east, taking 118 Earth days from sunrise to sunrise. What is more, it
presents almost exactly the same face to the Earth each time it is closest to our planet.
However the Earth’s gravity has managed to nudge Venus into this Earth-locked rotation
rate, it cannot have happened rapidly. Venus could not be a mere few thousand years oldbut, rather, it must be as old as all the other objects in the inner solar system.
Radar pictures of Venus have been obtained, some from ground-based radar
telescopes, some from the Pioneer Venus vehicle in orbit around the planet. They show
provocative evidence of impact craters. There are just as many craters that are not too big
or too small on Venus as there are in the lunar highlands, so many that Venus is again
telling us that it is very old. But the craters of Venus are remarkably shallow, almost as if
the high surface temperatures have produced a kind of rock that flows over long periods oftime, like taffy or putty, gradually softening the relief. There are great mesas here, twice ashigh as the Tibetan plateau, an immense rift valley, possibly giant volcanoes and amountain as high as Everest. We now see before us a world previously hidden entirely by
clouds - its features first explored by radar and by space vehicles.
The surface temperatures on Venus, as deduced from radio astronomy andconfirmed by direct spacecraft measurements are around 480°C or 900°F, hotter than thehottest household oven. The corresponding surface pressure is 90 atmospheres, 90 times thepressure we feel from the Earth’s atmosphere, the equivalent of the weight of water 1
kilometer below the surface of the oceans. To survive for long on Venus, a space vehicle
would have to be refrigerated as well as built like a deep submersible.
Something like a dozen space vehicles from the Soviet Union and United States
have entered the dense Venus atmosphere, and penetrated the clouds; a few of them haveactually survived for an hour or so on the surface.* Two spacecraft in the Soviet Venera
series have taken pictures down there. Let us follow in the footsteps of these pioneering
missions, and visit another world.
* Pioneer Venus was a successful US mission in 1978-79, combining an orbiter and fouratmospheric entry probes, two of which briefly survived the inclemencies of the Venus surface. There aremany unexpected developments in mustering spacecraft to explore the planets. This is one of them: Amongthe instruments aboard one of the Pioneer Venus entry probes was a net flux radiometer, designed to measure
simultaneously the amount of infrared energy flowing upwards and downwards at each position in the Venus
atmosphere. The instrument required a sturdy window that was also transparent to infrared radiation. A 13.5karat diamond was imported and milled into the desired window. However, the contractor was required to pay
a $12,000 import duty. Eventually, the US Customs service decided that after the diamond was launched toVenus it was unavailable for trade on Earth and refunded the money to the manufacturer.

In ordinary visible light, the faintly yellowish clouds of Venus can be made out, butthey show, as Galileo first noted, virtually no features at all. If the cameras look in theultraviolet, however, we see a graceful, complex swirling weather system in the highatmosphere, where the winds are around 100 meters per second, some 220 miles per hour.
The atmosphere of Venus is composed of 96 percent carbon dioxide. There are small tracesof nitrogen, water vapor, argon, carbon monoxide and other gases, but the onlyhydrocarbons or carbohydrates present are there in less than 0.1 parts per million. The
clouds of Venus turn out to be chiefly a concentrated solution of sulfuric acid. Smallquantities of hydrochloric acid and hydrofluoric acid are also present. Even at its high, coolclouds, Venus turns out to be a thoroughly nasty place.
High above the visible cloud deck, at about 70 kilometers altitude, there is acontinuous haze of small particles. At 60 kilometers, we plunge into the clouds, and findourselves surrounded by droplets of concentrated sulfuric acid. As we go deeper, the cloudparticles tend to get bigger. The pungent gas, sulfur dioxide, SO2, is present in trace
amounts in the lower atmosphere. It is circulated up above the clouds, broken down by
ultraviolet light from the Sun and recombined with water there to form sulfuric acid - whichcondenses into droplets, settles, and at lower altitudes is broken down by heat into SO2 andwater again, completing the cycle. It is always raining sulfuric acid on Venus, all over theplanet, and not a drop ever reaches the surface.
The sulfur-colored mist extends downwards to some 45 kilometers above thesurface of Venus, where we emerge into a dense but crystal-clear atmosphere. The
atmospheric pressure is so high, however, that we cannot see the surface. Sunlight isbounced about by atmospheric molecules until we lose all images from the surface. There isno dust here, no clouds, just an atmosphere getting palpably denser. Plenty of sunlight istransmitted by the overlying clouds, about as much as on an overcast day on the Earth.
With searing heat, crushing pressures, noxious gases and everything suffused in aneerie, reddish glow, Venus seems less the goddess of love than the incarnation of hell. As
nearly as we can make out, at least some places on the surface are strewn fields of jumbled,softened irregular rocks, a hostile, barren landscape relieved only here and there by theeroded remnants of a derelict spacecraft from a distant planet, utterly invisible through the
thick, cloudy, poisonous atmosphere.** In this stifling landscape, there is not likely to be anything alive, even creatures very different fromus. Organic and other conceivable biological molecules would simply fall to pieces. But, as an indulgence, let
us imagine that intelligent life once evolved on such a planet. Would it then invent science? The developmentof science on Earth was spurred fundamentally by observations of the regularities of the stars and planets. But
Venus is completely cloud-covered. The night is pleasingly long - about 59 Earth days long - but nothing of
the astronomical universe would be visible if you looked up into the night sky of Venus. Even the Sun would
be invisible in the daytime; its light would be scattered and diffused over the whole sky - just as scuba divers
see only a uniform enveloping radiance beneath the sea. If a radio telescope were built on Venus, it could
detect the Sun, the Earth and other distant objects. If astrophysics developed, the existence of stars could
eventually be deduced from the principles of physics, but they would be theoretical constructs only. I
sometimes wonder what their reaction would be if intelligent beings on Venus one day learned to fly, to sailin the dense air, to penetrate the mysterious cloud veil 45 kilometers above them and eventually to emerge out
the top of the clouds, to look up and for the first time witness that glorious universe of Sun and planets andstars.

Venus is a kind of planet-wide catastrophe. It now seems reasonably clear that thehigh surface temperature comes about through a massive greenhouse effect. Sunlight passes
through the atmosphere and clouds of Venus, which are semi-transparent to visible light,and reaches the surface. The surface being heated endeavors to radiate back into space. But
because Venus is much cooler than the Sun, it emits radiation chiefly in the infrared rather
than the visible region of the spectrum. However, the carbon dioxide and water vapor* in
the Venus atmosphere are almost perfectly opaque to infrared radiation, the heat of the Sun
is efficiently trapped, and the surface temperature rises - until the little amount of infrared
radiation that trickles out of this massive atmosphere just balances the sunlight absorbed in
the lower atmosphere and surface.
* At the present time there is still a little uncertainty about the abundance of water vapor on Venus.
The gas chromatograph on the Pioneer Venus entry probes gave an abundance of water in the lower
atmosphere of a few tenths of a percent. On the other hand, infrared measurements by the Soviet entry
vehicles, Veneras 11 and 12, gave an abundance of about a hundredth of a percent. If the former value
applies, then carbon dioxide and water vapor alone are adequate to seal in almost all the heat radiation fromthe surface and keep the Venus ground temperature at about 480°C. If the latter number applies - and myguess is that it is the more reliable estimate - then carbon dioxide and water vapor alone are adequate to keep
the surface temperature only at about 380°C, and some other atmospheric constituent is necessary to close theremaining infrared frequency windows in the atmospheric greenhouse. However, the small quantities of SO2,
CO and HCl, all of which have been detected in the Venus atmosphere, seem adequate for this purpose. Thus
recent American and Soviet missions to Venus seem to have provided verification that the greenhouse effectis indeed the reason for the high surface temperature.

Our neighboring world turns out to be a dismally unpleasant place. But we will go back to
Venus. It is fascinating in its own right. Many mythic heroes in Greek and Norse
mythology, after all, made celebrated efforts to visit Hell. There is also much to be learned
about our planet, a comparative Heaven, by comparing it with Hell.
The Sphinx, half human, half lion, was constructed more than 5,500 years ago. Its
face was once crisp and cleanly rendered. It is now softened and blurred by thousands ofyears of Egyptian desert sandblasting and by occasional rains. In New York City there is anobelisk called Cleopatra’s Needle, which came from Egypt. In only about a hundred years
in that city’s Central Park, its inscriptions have been almost totally obliterated, because ofsmog and industrial pollution - chemical erosion like that in the atmosphere of Venus.
Erosion on Earth slowly wipes out information, but because they are gradual - the patter ofa raindrop, the sting of a sand grain - those processes can be missed. Big structures, such as
mountain ranges, survive tens of millions of years; smaller impact craters, perhaps ahundred thousand*; and large-scale human artifacts only some thousands. In addition tosuch slow and uniform erosion, destruction also occurs through catastrophes large andsmall. The Sphinx is missing a nose. Someone shot it off in a moment of idle desecration some say it was Mameluke Turks, others, Napoleonic soldiers.
* More precisely, an impact crater 10 kilometers in diameter is produced on the Earth about once
every 500,000 years; it would survive erosion for about 300 million years in areas that are geologically stable,
such as Europe and North America. Smaller craters are produced more frequently and destroyed more rapidly,
especially in geologically active regions.

On Venus, on Earth and elsewhere in the solar system, there is evidence forcatastrophic destruction, tempered or overwhelmed by slower, more uniform processes: on
the Earth, for example, rainfall, coursing into rivulets, streams and rivers of running water,
creating huge alluvial basins; on Mars, the remnants of ancient rivers, perhaps arising frombeneath the ground; on Io, a moon of Jupiter, what seem to be broad channels made byflowing liquid sulfur. There are mighty weather systems on the Earth - and in the highatmosphere of Venus and on Jupiter. There are sandstorms on the Earth and on Mars;
lightning on Jupiter and Venus and Earth. Volcanoes inject debris into the atmospheres of
the Earth and Io. Internal geological processes slowly deform the surfaces of Venus, Mars,
Ganymede and Europa, as well as Earth. Glaciers, proverbial for their slowness, produce
major reworkings of landscapes on the Earth and probably also on Mars. These processes
need not be constant in time. Most of Europe was once covered with ice. A few million
years ago, the present site of the city of Chicago was buried under three kilometers of frost.
On Mars, and elsewhere in the solar system, we see features that could not be produced
today, landscapes carved hundreds of millions or billions of years ago when the planetaryclimate was probably very different.
There is an additional factor that can alter the landscape and the climate of Earth:intelligent life, able to make major environmental changes. Like Venus, the Earth also has agreenhouse effect due to its carbon dioxide and water vapor. The global temperature of the
Earth would be below the freezing point of water if not for the greenhouse effect. It keeps
the oceans liquid and life possible. A little greenhouse is a good thing. Like Venus, theEarth also has about 90 atmospheres of carbon dioxide; but it resides in the crust as
limestone and other carbonates, not in the atmosphere. If the Earth were moved only a little
closer to the Sun, the temperature would increase slightly. This would drive some of theCO2 out of the surface rocks, generating a stronger greenhouse effect, which would in turnincrementally heat the surface further. A hotter surface would vaporize still morecarbonates into CO2, and there would be the possibility of a runaway greenhouse effect tovery high temperatures. This is just what we think happened in the early history of Venus,
because of Venus’s proximity to the Sun. The surface environment of Venus is a warning:something disastrous can happen to a planet rather like our own.
The principal energy sources of our present industrial civilization are the so-calledfossil fuels. We burn wood and oil, coal and natural gas, and, in the process, release waste
gases, principally CO2, into the air. Consequently, the carbon dioxide content of the Earth’s
atmosphere is increasing dramatically. The possibility of a runaway greenhouse effectsuggests that we have to be careful: Even a one- or two-degree rise in the global
temperature can have catastrophic consequences. In the burning of coal and oil andgasoline, we are also putting sulfuric acid into the atmosphere. Like Venus, ourstratosphere even now has a substantial mist of tiny sulfuric acid droplets. Our major cities
are polluted with noxious molecules. We do not understand the long-term effects of ourcourse of action.
But we have also been perturbing the climate in the opposite sense. For hundreds ofthousands of years human beings have been burning and cutting down forests andencouraging domestic animals to graze on and destroy grasslands. Slash-and-burn
agriculture, industrial tropical deforestation and overgrazing are rampant today. But forests
are darker than grasslands, and grasslands are darker than deserts. As a consequence, theamount of sunlight that is absorbed by the ground has been declining, and by changes in theland use we are lowering the surface temperature of our planet. Might this cooling increase
the size of the polar ice cap, which, because it is bright, will reflect still more sunlight fromthe Earth, further cooling the planet, driving a runaway albedo* effect?
* The albedo is the fraction of the sunlight striking a planet that is reflected back to space. The
albedo of the Earth is some 30 to 35 percent. The rest of the sunlight is absorbed by the ground and isresponsible for the average surface temperature.

Our lovely blue planet, the Earth, is the only home we know. Venus is too hot. Mars
is too cold. But the Earth is just right, a heaven for humans. After all, we evolved here. But
our congenial climate may be unstable. We are perturbing our poor planet in serious andcontradictory ways. Is there any danger of driving the environment of the Earth toward theplanetary Hell of Venus or the global ice age of Mars? The simple answer is that nobody
knows. The study of the global climate, the comparison of the Earth with other worlds, aresubjects in their earliest stages of development. They are fields that are poorly andgrudgingly funded. In our ignorance, we continue to push and pull, to pollute theatmosphere and brighten the land, oblivious of the fact that the long-term consequences arelargely unknown.
A few million years ago, when human beings first evolved on Earth, it was alreadya middle-aged world, 4.6 billion years along from the catastrophes and impetuosities of its
youth. But we humans now represent a new and perhaps decisive factor. Our intelligenceand our technology have given us the power to affect the climate. How will we use thispower? Are we willing to tolerate ignorance and complacency in matters that affect theentire human family? Do we value short-term advantages above the welfare of the Earth?
Or will we think on longer time scales, with concern for our children and our
grandchildren, to understand and protect the complex life-support systems of our planet?
The Earth is a tiny and fragile world. It needs to be cherished.

CHAPTER V
Blues for a Red Planet
In the orchards of the gods, he watches the canals . . .
- Enuma Elish, Sumer, c. 2500 B.C.
A man that is of Copernicus’ Opinion, that this Earth of ours is a Planet, carry’dround and enlightn’d by the Sun, like the rest of them, cannot but sometimes have a
fancy . . . that the rest of the Planets have their Dress and Furniture, nay and their
Inhabitants too as well as this Earth of ours . . . But we were always apt to conclude,that ’twas in vain to enquire after what Nature had been pleased to do there, seeing
there was no likelihood of ever coming to an end of the Enquiry . . . but a while ago,
thinking somewhat seriously on this matter (not that I count my self quicker sighted
than those great Men [of the past], but that I had the happiness to live after most ofthem) me thoughts the Enquiry was not so impracticable nor the way so stopt up
with Difficulties, but that there was very good room left for probable Conjectures.
- Christian Huygens, New Conjectures Concerning the Planetary Worlds, Their
Inhabitants and Productions, c. 1690
A time would come when Men should be able to stretch out their Eyes . . . theyshould see the Planets like our Earth.
- Christopher Wren, Inauguration Speech, Gresham College, 1657
Many years ago, so the story goes, a celebrated newspaper publisher sent a telegram to anoted astronomer: WIRE COLLECT IMMEDIATELY FIVE HUNDRED WORDS ON
WHETHER THERE IS LIFE ON MARS. The astronomer dutifully replied: NOBODYKNOWS, NOBODY KNOWS, NOBODY KNOWS . . . 250 times. But despite thisconfession of ignorance, asserted with dogged persistence by an expert, no one paid any
heed, and from that time to this, we hear authoritative pronouncements by those who thinkthey have deduced life on Mars, and by those who think they have excluded it. Somepeople very much want there to be life on Mars; others very much want there to be no life
on Mars. There have been excesses in both camps. These strong passions have somewhat
frayed the tolerance for ambiguity that is essential to science. There seem to be many
people who simply wish to be told an answer, any answer, and thereby avoid the burden of
keeping two mutually exclusive possibilities in their heads at the same time. Some
scientists have believed that Mars is inhabited on what has later proved to be the flimsiest
evidence. Others have concluded the planet is lifeless because a preliminary search for aparticular manifestation of life has been unsuccessful or ambiguous. The blues have been
played more than once for the red planet.
Why Martians? Why so many eager speculations and ardent fantasies about
Martians, rather than, say, Saturnians or Plutonians? Because Mars seems, at first glance,very Earthlike. It is the nearest planet whose surface we can see. There are polar ice caps,
drifting white clouds, raging dust storms, seasonally changing patterns on its red surface,even a twenty-four-hour day. It is tempting to think of it as an inhabited world. Mars has
become a kind of mythic arena onto which we have projected our earthly hopes and fears.

But our psychological predispositions pro or con must not mislead us. All that matters isthe evidence, and the evidence is not yet in. The real Mars is a world of wonders. Its future
prospects are far more intriguing than our past apprehensions about it. In our time we havesifted the sands of Mars, we have established a presence there, we have fulfilled a century
of dreams!
No one would have believed in the last years of the nineteenth century that thisworld was being watched keenly and closely by intelligences greater than man’s
and yet as mortal as his own; that as men busied themselves about their various
concerns, they were scrutinized and studied, perhaps almost as narrowly as a manwith a microscope might scrutinize the transient creatures that swarm and multiply
in a drop of water. With infinite complacency, men went to and fro over this globe
about their little affairs, serene in their assurances of their empire over matter. It is
possible that the infusoria under the microscope do the same. No one gave a thought
to the older worlds of space as sources of human danger, or thought of them only todismiss the idea of life upon them as impossible or improbable. It is curious torecall some of the mental habits of those departed days. At most, terrestrial men
fancied there might be other men upon Mars, perhaps inferior to themselves andready to welcome a missionary enterprise. Yet across the gulf of space, minds thatare to our minds as ours are to those of the beasts that perish, intellects vast and cooland unsympathetic, regarded this Earth with envious eyes, and slowly and surely
drew their plans against us.
These opening lines of H. G. Wells’ 1897 science fiction classic The War of the Worlds
maintain their haunting power to this day.* For all of our history, there has been the fear, or
hope, that there might be life beyond the Earth. In the last hundred years, that premonition
has focused on a bright red point of light in the night sky. Three years before The War of
the Worlds was published, a Bostonian named Percival Lowell founded a major
observatory where the most elaborate claims in support of life on Mars were developed.
Lowell dabbled in astronomy as a young man, went to Harvard, secured a semi-official
diplomatic appointment to Korea, and otherwise engaged in the usual pursuits of thewealthy. Before he died in 1916, he had made major contributions to our knowledge of thenature and evolution of the planets, to the deduction of the expanding universe and, in adecisive way, to the discovery of the planet Pluto, which is named after him. The first two
letters of the name Pluto are the initials of Percival Lowell. Its symbol is , a planetarymonogram.
* In 1938, a radio version, produced by Orson Welles, transposed the Martian invasion from Englandto the eastern United States, and frightened millions in war-jittery America into believing that the Martians
were in fact attacking.

But Lowell’s lifelong love was the planet Mars. He was electrified by theannouncement in 1877 by an Italian astronomer, Giovanni Schiaparelli, of canali on Mars.
Schiaparelli had reported during a close approach of Mars to Earth an intricate network ofsingle and double straight lines crisscrossing the bright areas of the planet. Canali in Italianmeans channels or grooves, but was promptly translated into English as canals, a word that
implies intelligent design. A Mars mania coursed through Europe and America, and Lowellfound himself swept up with it.
In 1892, his eyesight failing, Schiaparelli announced he was giving up observing
Mars. Lowell resolved to continue the work. He wanted a first-rate observing site,undisturbed by clouds or city lights and marked by good ‘seeing,’ the astronomer’s term fora steady atmosphere through which the shimmering of an astronomical image in thetelescope is minimized. Bad seeing is produced by small-scale turbulence in the atmosphereabove the telescope and is the reason the stars twinkle. Lowell built his observatory far
away from home, on Mars Hill in Flagstaff, Arizona.* He sketched the surface features ofMars, particularly the canals, which mesmerized him. Observations of this sort are not easy.
You put in long hours at the telescope in the chill of the early morning. Often the seeing ispoor and the image of Mars blurs and distorts. Then you must ignore what you have seen.
Occasionally the image steadies and the features of the planet flash out momentarily,
marvelously. You must then remember what has been vouchsafed to you and accurately
commit it to paper. You must put your preconceptions aside and with an open mind set
down the wonders of Mars.
* Isaac Newton had written ‘If the Theory of making Telescopes could at length be fully brought into
practice, yet there would be certain Bounds beyond which Telescopes could not perform. For the Air throughwhich we look upon the Stars, is in perpetual tremor . . . The only remedy is the most serene and quiet Air,such as may perhaps be found on the tops of the highest mountains above the grosser Clouds.’

Percival Lowell’s notebooks are full of what he thought he saw: bright and darkareas, a hint of polar cap, and canals, a planet festooned with canals. Lowell believed hewas seeing a globe girdling network of great irrigation ditches, carrying water from themelting polar caps to the thirsty inhabitants of the equatorial cities. He believed the planet
to be inhabited by an older and wiser race, perhaps very different from us. He believed thatthe seasonal changes in the dark areas were due to the growth and decay of vegetation. He
believed that Mars was, very closely, Earth-like. All in all, he believed too much.
Lowell conjured up a Mars that was ancient, arid, withered, a desert world. Still, it
was an Earth-like desert. Lowell’s Mars had many features in common with the American
Southwest, where the Lowell Observatory was located. He imagined the Martian
temperatures a little on the chilly side but still as comfortable as ‘the South of England.’The air was thin, but there was enough oxygen to be breathable. Water was rare, but theelegant network of canals carried the life-giving fluid all over the planet.
What was in retrospect the most serious contemporary challenge to Lowell’s ideas
came from an unlikely source. In 1907, Alfred Russel Wallace, co-discoverer of evolution
by natural selection, was asked to review one of Lowell’s books. He had been an engineer
in his youth and, while somewhat credulous on such issues as extrasensory perception, wasadmirably skeptical on the habitability of Mars. Wallace showed that Lowell had erred in
his calculation of the average temperatures on Mars; instead of being as temperate as theSouth of England, they were, with few exceptions, everywhere below the freezing point ofwater. There should be permafrost, a perpetually frozen subsurface. The air was muchthinner than Lowell had calculated. Craters should be as abundant as on the Moon. And as
for the water in the canals:
Any attempt to make that scanty surplus [of water], by means of overflowing
canals, travel across the equator into the opposite hemisphere, through such terrible

desert regions and exposed to such a cloudless sky as Mr Lowell describes, would
be the work of a body of madmen rather than of intelligent beings. It may be safely
asserted that not one drop of water would escape evaporation or insoak at even ahundred miles from its source.
This devastating and largely correct physical analysis was written in Wallace’s eightyfourth year. His conclusion was that life on Mars - by this he meant civil engineers with aninterest in hydraulics - was impossible. He offered no opinion on microorganisms.
Despite Wallace’s critique, despite the fact that other astronomers with telescopes
and observing sites as good as Lowell’s could find no sign of the fabled canals, Lowell’svision of Mars gained popular acceptance. It had a mythic quality as old as Genesis. Part ofits appeal was the fact that the nineteenth century was an age of engineering marvels,
including the construction of enormous canals: the Suez Canal, completed in 1869; theCorinth Canal, in 1893; the Panama Canal, in 1914; and, closer to home, the Great Lake
locks, the barge canals of upper New York State, and the irrigation canals of the American
Southwest. If Europeans and Americans could perform such feats, why not Martians?
Might there not be an even more elaborate effort by an older and wiser species,
courageously battling the advance of desiccation on the red planet?
We have now sent reconnaissance satellites into orbit around Mars. The entire
planet has been mapped. We have landed two automated laboratories on its surface. The
mysteries of Mars have, if anything, deepened since Lowell’s day. However, with pictures
far more detailed than any view of Mars that Lowell could have glimpsed, we have found
not a tributary of the vaunted canal network, not one lock. Lowell and Schiaparelli andothers, doing visual observations under difficult seeing conditions, were misled - in part
perhaps because of a predisposition to believe in life on Mars.
The observing notebooks of Percival Lowell reflect a sustained effort at thetelescope over many years. They show Lowell to have been well aware of the skepticism
expressed by other astronomers about the reality of the canals. They reveal a man
convinced that he has made an important discovery and distressed that others have not yet
understood its significance. In his notebook for 1905, for example, there is an entry on
January 21: ‘Double canals came out by flashes, convincing of reality.’ In reading Lowell’s
notebooks I have the distinct but uncomfortable feeling that he was really seeing
something. But what?
When Paul Fox of Cornell and I compared Lowell’s maps of Mars with the Mariner
9 orbital imagery sometimes with a resolution a thousand times superior to that of Lowell’s
Earthbound twenty-four-inch refracting telescope - we found virtually no correlation at all.
It was not that Lowell’s eye had strung up disconnected fine detail on the Martian surface
into illusory straight lines. There was no dark mottling or crater chains in the position ofmost of his canals. There were no features there at all. Then how could he have drawn thesame canals year after year? How could other astronomers some of whom said they had not
examined Lowell’s maps closely until after their own observations - have drawn the same
canals? One of the great findings of the Mariner 9 mission to Mars was that there are timevariable streaks and splotches on the Martian surface - many connected with the rampartsof impact craters - which change with the seasons. They are due to windblown dust, thepatterns varying with the seasonal winds. But the streaks do not have the character of thecanals, they are not in the position of the canals, and none of them is large enoughindividually to be seen from the Earth in the first place. It is unlikely that there were realfeatures on Mars even slightly resembling Lowell’s canals in the first few decades of thiscentury that have disappeared without a trace as soon as close-up spacecraft investigations
became possible.
The canals of Mars seem to be some malfunction, under difficult seeing conditions,of the human hand/eye/brain combination (or at least for some humans; many otherastronomers, observing with equally good instruments in Lowell’s time and after, claimed
there were no canals whatever). But this is hardly a comprehensive explanation, and I havethe nagging suspicion that some essential feature of the Martian canal problem still remains
undiscovered. Lowell always said that the regularity of the canals was an unmistakable signthat they were of intelligent origin. This is certainly true. The only unresolved question was
which side of the telescope the intelligence was on.
Lowell’s Martians were benign and hopeful, even a little god-like, very different
from the malevolent menace posed by Wells and Welles in The War of the Worlds. Bothsets of ideas passed into the public imagination through Sunday supplements and science
fiction. I can remember as a child reading with breathless fascination the Mars novels of
Edgar Rice Burroughs. I journeyed with John Carter, gentleman adventurer from Virginia,
to ‘Barsoom,’ as Mars was known to its inhabitants. I followed herds of eight-legged beastsof burden, the thoats. I won the hand of the lovely Dejah Thoris, Princess of Helium. I
befriended a four-meter-high green fighting man named Tars Tarkas. I wandered within thespired cities and domed pumping stations of Barsoom, and along the verdant banks of the
Nilosyrtis and Nepenthes canals.
Might it really be possible - in fact and not fancy - to venture with John Carter tothe Kingdom of Helium on the planet Mars? Could we venture out on a summer evening,
our way illuminated by the two hurtling moons of Barsoom, for a journey of high scientificadventure? Even if all Lowell’s conclusions about Mars, including the existence of thefabled canals, turned out to be bankrupt, his depiction of the planet had at least this virtue:
it aroused generations of eight-year-olds, myself among them, to consider the explorationof the planets as a real possibility, to wonder if we ourselves might one day voyage toMars. John Carter got there by standing in an open field, spreading his hands and wishing. I
can remember spending many an hour in my boyhood, arms resolutely outstretched in an
empty field, imploring what I believed to be Mars to transport me there. It never worked.
There had to be some other way.
Like organisms, machines also have their evolutions. The rocket began, like thegunpowder that first powered it, in China where it was used for ceremonial and aesthetic
purposes. Imported to Europe around the fourteenth century, it was applied to warfare,discussed in the late nineteenth century as a means of transportation to the planets by the
Russian schoolteacher Konstantin Tsiolkovsky, and first developed seriously for highaltitude flight by the American scientist Robert Goddard. The German V-2 military rocketof World War II employed virtually all of Goddard’s innovations and culminated in 1948 in
the two-stage launching of the V-2/WAC Corporal combination to the then-unprecedented
altitude of 400 kilometers. In the 1959s, engineering advances organized by Sergei Korolov
in the Soviet Union and Wernher von Braun in the United States, funded as delivery
systems for weapons of mass destruction, led to the first artificial satellites. The pace ofprogress has continued to be brisk: manned orbital flight; humans orbiting, then landing on
the moon; and unmanned spacecraft outward bound throughout the solar system. Manyother nations have now launched spacecraft, including Britain, France, Canada, Japan and
China, the society that invented the rocket in the first place.

Among the early applications of the space rocket, as Tsiolkovsky and Goddard(who as a young man had read Wells and had been stimulated by the lectures of Percival
Lowell) delighted in imagining, were an orbiting scientific station to monitor the Earthfrom a great height and a probe to search for life on Mars. Both these dreams have now
been fulfilled.
Imagine yourself a visitor from some other and quite alien planet, approaching
Earth with no preconceptions. Your view of the planet improves as you come closer andmore and more fine detail stands out. Is the planet inhabited? At what point can you
decide? If there are intelligent beings, perhaps they have created engineering structures thathave high-contrast components on a scale of a few kilometers, structures detectable when
our optical systems and distance from the Earth provide kilometer resolution. Yet at thislevel of detail, the earth seems utterly barren. There is no sign of life, intelligent orotherwise, in places we call Washington, New York, Boston, Moscow, London, Paris,
Berlin, Tokyo and Peking. If there are intelligent beings on Earth, they have not muchmodified the landscape into regular geometrical patterns at kilometer resolution.
But when we improve the resolution tenfold, when we begin to see detail as small
as a hundred meters across, the situation changes. Many places on Earth seem suddenly to
crystallize out, revealing an intricate pattern of squares and rectangles, straight lines andcircles. These are, in fact, the engineering artifacts of intelligent beings: roads, highways,
canals, farmland, city streets - a pattern disclosing the twin human passions for Euclidean
geometry and territoriality. On this scale, intelligent life can be discerned in Boston andWashington and New York. And at ten-meter resolution, the degree to which the landscapehas been reworked first really becomes evident. Humans have been very busy. These
photos have been taken in daylight. But at twilight or during the night, other things arevisible: oil-well fires in Libya and the Persian Gulf; deepwater illumination by the Japanesesquid fishing fleet; the bright lights of large cities. And if, in daylight, we improve ourresolution so we can make out things that are a meter across, then we begin to detect for the
first time individual organisms - whales, cows, flamingos, people.
Intelligent life on Earth first reveals itself through the geometric regularity of its
constructions. If Lowell’s canal network really existed, the conclusion that intelligent
beings inhabit Mars might be similarly compelling. For life to be detected on Mars
photographically, even from Mars orbit, it must likewise have accomplished a major
reworking of the surface. Technical civilizations, canal builders, might be easy to detect.
But except for one or two enigmatic features, nothing of the sort is apparent in the exquisite
profusion of Martian surface detail uncovered by unmanned spacecraft. However, there aremany other possibilities, ranging from large plants and animals to microorganisms, toextinct forms, to a planet that is now and was always lifeless. Because Mars is farther fromthe Sun than is the Earth, its temperatures are considerably lower. Its air is thin, containing
mostly carbon dioxide but also some molecular nitrogen and argon and very small
quantities of water vapor, oxygen and ozone. Open bodies of liquid water are impossible
today because the atmospheric pressure on Mars is too low to keep even cold water fromrapidly boiling. There may be minute quantities of liquid water in pores and capillaries inthe soil. The amount of oxygen is far too little for a human being to breathe. The ozone
abundance is so small that germicidal ultraviolet radiation from the Sun strikes the Martiansurface unimpeded. Could any organism survive in such an environment?
To test this question, many years ago my colleagues and I prepared chambers thatsimulated the Martian environment as it was then known, inoculated them with terrestrial

microorganisms and waited to see if anybody survived. Such chambers are called, ofcourse, Mars Jars. The Mars Jars cycled the temperatures within a typical Martian range
from a little above the freezing point around noon to about -80°C just before dawn, in an
anoxic atmosphere composed chiefly of CO2 and N2. Ultraviolet lamps reproduced thefierce solar flux. No liquid water was present except for very thin films wetting individual
sand grains. Some microbes froze to death after the first night and were never heard fromagain. Others gasped and perished from lack of oxygen. Some died of thirst, and some werefried by the ultraviolet light. But there were always a fair number of varieties of terrestrial
microbes that did not need oxygen; that temporarily closed up shop when the temperatures
dropped too low; that hid from the ultraviolet light under pebbles or thin layers of sand. In
other experiments, when small quantities of liquid water were present, the microbes
actually grew. If terrestrial microbes can survive the Martian environment, how much better
Martian microbes, if they exist, must do on Mars. But first we must get there.
The Soviet Union maintains an active program of unmanned planetary exploration.
Every year or two the relative positions of the planets and the physics of Kepler and
Newton permit the launch of a spacecraft to Mars or Venus with a minimum expenditure ofenergy. Since the early 1960’s the U.S.S.R. has missed few such opportunities. Soviet
persistence and engineering skills have eventually paid off handsomely. Five Soviet
spacecraft - Venera 8 through 12 - have landed on Venus and successfully returned datafrom the surface, no insignificant feat in so hot, dense and corrosive a planetaryatmosphere. Yet despite many attempts, the Soviet Union has never landed successfully on
Mars - a place that, at least at first sight, seems more hospitable, with chilly temperatures, amuch thinner atmosphere and more benign gases; with polar ice caps, clear pink skies,
great sand dunes, ancient river beds, a vast rift valley, the largest volcanic construct, so far
as we know, in the solar system, and balmy equatorial summer afternoons. It is a far more
Earth-like world than Venus.
In 1971, the Soviet Mars 3 spacecraft entered the Martian atmosphere. According to
the information automatically radioed back, it successfully deployed its landing systems
during entry, correctly oriented its ablation shield downward, properly unfurled its great
parachute and fired its retro-rockets near the end of its descent path. According to the data
returned by Mars 3, it should have landed successfully on the red planet. But after landing,the spacecraft returned a twenty-second fragment of a featureless television picture to Earthand then mysteriously failed. In 1973, a quite similar sequence of events occurred with theMars 6 lander, in that case the failure occurring within one second of touchdown. Whatwent wrong?
The first illustration I ever saw of Mars 3 was on a Soviet postage stamp(denomination, 16 kopecks), which depicted the spacecraft descending through a kind of
purple muck. The artist was trying, I think, to illustrate dust and high winds: Mars 3 hadentered the Martian atmosphere during an enormous global dust storm. We have evidence
from the U.S. Mariner 9 mission that near-surface winds of more than 140 meters per
second - faster than half the speed of sound on Mars - arose in that storm. Both our Soviet
colleagues and we think it likely that these high winds caught the Mars 3 spacecraft withparachute unfurled, so that it landed gently in the vertical direction but with breakneck
speed in the horizontal direction. A spacecraft descending on the shrouds of a large
parachute is particularly vulnerable to horizontal winds. After landing, Mars 3 may havemade a few bounces, hit a boulder or other example of Martian relief, tipped over, lost theradio link with its carrier ‘bus’ and failed.

But why did Mars 3 enter in the midst of a great dust storm? The Mars 3 mission
was rigidly organized before launch. Every step it was to perform was loaded into the onboard computer before it left earth. There was no opportunity to change the computer
program, even as the extent of the great 1971 dust storm became clear. In the jargon of
space exploration, the Mars 3 mission was preprogrammed, not adaptive. The failure ofMars 6 is more mysterious. There was no planet-wide storm when this spacecraft entered
the Martian atmosphere, and no reason to suspect a local storm, as sometimes happens, atthe landing site. Perhaps there was an engineering failure just at the moment of touchdown.
Or perhaps there is something particularly dangerous about the Martian surface.
The combination of Soviet successes in landing on Venus and Soviet failures inlanding on Mars naturally caused us some concern about the U.S. Viking mission, which
had been informally scheduled to set one of its two descent craft gently down on theMartian surface on the Bicentennial of the United States, July 4, 1976. Like its Soviet
predecessors, the Viking landing maneuver involved an ablation shield, a parachute andretro-rockets. Because the Martian atmosphere is only 1 percent as dense as the Earth’s, avery large parachute, eighteen meters in diameter, was deployed to slow the spacecraft as itentered the thin air of Mars. The atmosphere is so thin that if Viking had landed at a highelevation there would not have been enough atmosphere to brake the descent adequately: it
would have crashed. One requirement, therefore, was for a landing site in a low-lying
region. From Mariner 9 results and ground-based radar studies, we knew many such areas.
To avoid the probable fate of Mars 3, we wanted Viking to land in a place and timeat which the winds were low. Winds that would make the lander crash were probably
strong enough to lift dust off the surface. If we could check that the candidate landing site
was not covered with shifting, drifting dust, we would have at least a fair chance ofguaranteeing that the winds were not intolerably high. This was one reason that each
Viking lander was carried into Mars orbit with its orbiter, and descent delayed until theorbiter surveyed the landing site. We had discovered with Mariner 9 that characteristicchanges in the bright and dark patterns on the Martian surface occur during times of highwinds. We certainly would not have certified a Viking landing site as safe if orbital
photographs had shown such shifting patterns. But our guarantees could not be 100 percent
reliable. For example, we could imagine a landing site at which the winds were so strongthat all mobile dust had already been blown away. We would then have had no indicationof the high winds that might have been there. Detailed weather predictions for Mars were,of course, much less reliable than for Earth. (Indeed one of the many objectives of theViking mission was to improve our understanding of the weather on both planets.)
Because of communication and temperature constraints, Viking could not land athigh Martian latitudes. Farther poleward than about 45 or 50 degrees in both hemispheres,
either the time of useful communication of the spacecraft with the Earth or the period
during which the spacecraft would avoid dangerously low temperatures would have been
awkwardly short.
We did not wish to land in too rough a place. The spacecraft might have tipped overand crashed, or at the least its mechanical arm, intended to acquire Martian soil samples,might have become wedged or been left waving helplessly a meter too high above thesurface. Likewise, we did not want to land in places that were too soft. If the spacecraft’s
three landing pods had sunk deeply into a loosely packed soil, various undesirable
consequences would have followed, including immobilization of the sample arm. But we
did not want to land in a place that was too hard either - had we landed in a vitreous lava

field, for example, with no powdery surface material, the mechanical arm would have been
unable to acquire the sample vital to the projected chemistry and biology experiments.
The best photographs then available of Mars - from the Mariner 9 orbiter - showedfeatures no smaller than 90 meters (100 yards) across. The Viking orbiter pictures
improved this figure only slightly. Boulders one meter (three feet) in size were entirely
invisible in such photographs, and could have had disastrous consequences for the Viking
lander. Likewise, a deep, soft powder might have been indetectable photographically.
Fortunately, there was a technique that enabled us to determine the roughness or softness ofa candidate landing site: radar. A very rough place would scatter radar from Earth off to thesides of the beam and therefore appear poorly reflective, or radar-dark. A very soft place
would also appear poorly reflective because of the many interstices between individual
sand grains. While we were unable to distinguish between rough places and soft places, we
did not need to make such distinctions for landing-site selection. Both, we knew, weredangerous. Preliminary radar surveys suggested that as much as a quarter to a third of thesurface area of Mars might be radar-dark, and therefore dangerous for Viking. But not all ofMars can be viewed by Earth-based radar - only a swath between about 25° N and about
25° S. The Viking orbiter carried no radar system of its own to map the surface.
There were many constraints - perhaps, we feared, too many. Our landing sites had
to be not too high, too windy, too hard, too soft, too rough or too close to the pole. It was
remarkable that there were any places at all on Mars that simultaneously satisfied all oursafety criteria. But it was also clear that our search for safe harbors had led us to landingsites that were, by and large, dull.
When each of the two Viking orbiter-lander combinations was inserted into Martian
orbit, it was unalterably committed to landing at a certain latitude on Mars. If the low pointin the orbit was at 21° Martian north latitude, the lander would touch down at 21° N,
although, by waiting for the planet to turn beneath it, it could land at any longitude
whatever. Thus the Viking science teams selected candidate latitudes for which there wasmore than one promising site. Viking 1 was targeted for 21° N. The prime site was in aregion called Chryse (Greek for ‘the land of gold’), near the confluence of four sinuous
channels thought to have been carved in previous epochs of Martian history by runningwater. The Chryse site seemed to satisfy all safety criteria. But the radar observations had
been made nearby, not in the Chryse landing site itself. Radar observations of Chryse weremade for the first time - because of the geometry of Earth and Mars - only a few weeks
before the nominal landing date.
The candidate landing latitude for Viking 2 was 44° N; the prime site, a locale
called Cydonia, chosen because, according to some theoretical arguments, there was asignificant chance of small quantities of liquid water there, at least at some time during theMartian year. Since the Viking biology experiments were strongly oriented toward
organisms that are comfortable in liquid water, some scientists held that the chance ofViking finding life would be substantially improved in Cydonia. On the other hand, it wasargued that, on so windy a planet as Mars, microorganisms should be everywhere if they
are anywhere. There seemed to be merit to both positions; and it was difficult to decide
between them. What was quite clear, however, was that 44° N was completely inaccessible
to radar site-certification; we had to accept a significant risk of failure with Viking 2 if it
was committed to high northern latitudes. It was sometimes argued that if Viking 1 wasdown and working well we could afford to accept a greater risk with Viking 2. I foundmyself making very conservative recommendations on the fate of a billion-dollar mission. Icould imagine, for example, a key instrument failure in Chryse just after an unfortunate
crash landing in Cydonia. To improve the Viking options, additional landing sites,
geologically very different from Chryse and Cydonia, were selected in the radar-certified
region near 4° S latitude. A decision on whether Viking 2 would set down at high or at low
latitude was not made until virtually the last minute, when a place with the hopeful name of
Utopia, at the same latitude as Cydonia, was chosen.
For Viking 1, the original landing site seemed, after we examined orbiter
photographs and late-breaking Earth-based radar data, unacceptably risky. For a while I
worried that Viking 1 had been condemned, like the legendary Flying Dutchman, to wander
the skies of Mars forever, never to find safe haven. Eventually we found a suitable spot,
still in Chryse but far from the confluence of the four ancient channels. The delay prevented
us from setting down on July 4, 1976, but it was generally agreed that a crash landing onthat date would have been an unsatisfactory two hundredth birthday present for the UnitedStates. We deboosted from orbit and entered the Martian atmosphere sixteen days later.
After an interplanetary voyage of a year and a half, covering a hundred million
kilometers the long way round the Sun, each orbiter/lander combination was inserted into
its proper orbit about Mars; the orbiters surveyed candidate landing sites; the landers
entered the Martian atmosphere on radio command and correctly oriented ablation shields,
deployed parachutes, divested coverings, and fired retro-rockets. In Chryse and Utopia, forthe first time in human history, spacecraft had touched down, gently and safely, on the redplanet. These triumphant landings were due in considerable part to the great skill invested
in their design, fabrication and testing, and to the abilities of the spacecraft controllers. But
for so dangerous and mysterious a planet as Mars, there was also at least an element ofluck.
Immediately after landing, the first pictures were to be returned. We knew we hadchosen dull places. But we could hope. The first picture taken by the Viking 1 lander wasof one of its own footpads - in case it were to sink into Martian quicksand, we wanted toknow about it before the spacecraft disappeared. The picture built up, line by line, untilwith enormous relief we saw the footpad sitting high and dry above the Martian surface.
Soon other pictures came into being, each picture element radioed individually back to
Earth.
I remember being transfixed by the first lander image to show the horizon of Mars.
This was not an alien world, I thought. I knew places like it in Colorado and Arizona and
Nevada. There were rocks and sand drifts and a distant eminence, as natural andunselfconscious as any landscape on Earth. Mars was a place. I would, of course, have beensurprised to see a grizzled prospector emerge from behind a dune leading his mule, but at
the same time the idea seemed appropriate. Nothing remotely like it ever entered my mind
in all the hours I spent examining the Venera 9 and 10 images of the Venus surface. One
way or another, I knew, this was a world to which we would return.
The landscape is stark and red and lovely: boulders thrown out in the creation of acrater somewhere over the horizon, small sand dunes, rocks that have been repeatedly
covered and uncovered by drifting dust, plumes of fine-grained material blown about by thewinds. Where did the rocks come from? How much sand has been blown by wind? What
must the previous history of the planet have been to create sheared rocks, buried boulders,
polygonal gouges in the ground? What are the rocks made of? The same materials as the
sand? Is the sand merely pulverized rock or something else? Why is the sky pink? What is
the air made of? How fast does the wind blow? Are there marsquakes? How does theatmospheric pressure and the appearance of the landscape change with the seasons?
For every one of these questions Viking has provided definitive or at least plausible
answers. The Mars revealed by the Viking mission is of enormous interest - particularly
when we remember that the landing sites were chosen for their dullness. But the cameras
revealed no sign of canal builders, no Barsoomian aircars or short swords, no princesses orfighting men, no thoats, no footprints, not even a cactus or a kangaroo rat. For as far as we
could see, there was not a sign of life.** There was a brief flurry when the uppercase letter B, a putative Martian graffito, seemed to bevisible on a small boulder in Chryse. But later analysis showed it to be a trick of light and shadow and thehuman talent for pattern recognition. It also seemed remarkable that the Martians should have tumbled
independently to the Latin alphabet. But there was just a moment when resounding in my head was the distantecho of a word from my boyhood - Barsoom.

Perhaps there are large lifeforms on Mars, but not in our two landing sites. Perhaps
there are smaller forms in every rock and sand grain. For most of its history, those regionsof the Earth not covered by water looked rather like Mars today - with an atmosphere richin carbon dioxide, with ultraviolet light shining fiercely down on the surface through an
atmosphere devoid of ozone. Large plants and animals did not colonize the land until thelast 10 percent of Earth history. And yet for three billion years there were microorganisms
everywhere on Earth. To look for life on Mars, we must look for microbes.
The Viking lander extends human capabilities to other and alien landscapes. By
some standards, it is about as smart as a grasshopper; by others, only as intelligent as abacterium. There is nothing demeaning in these comparisons. It took nature hundreds ofmillions of years to evolve a bacterium, and billions to make a grasshopper. With only alittle experience in this sort of business, we are becoming fairly skillful at it. Viking has
two eyes as we do, but they also work in the infrared, as ours do not; a sample arm that canpush rocks, dig and acquire soil samples; a kind of finger that it puts up to measure windspeed and direction; a nose and taste buds, of a sort, with which it senses, to a much higher
precision than we can, the presence of trace molecules; an interior ear with which it can
detect the rumbling of marsquakes and the gentler wind-driven jiggling of the spacecraft;
and a means of detecting microbes. The spacecraft has its own self-contained radioactive
power source. It radios all the scientific information it acquires back to Earth. It receives
instructions from Earth, so human beings can ponder the significance of the Viking results
and tell the spacecraft to do something new.
But what is the optimum way, given severe constraints on size, cost and power
requirements, to search for microbes on Mars? We cannot - at least as yet - send
microbiologists there. I once had a friend, an extraordinary microbiologist named Wolf
Vishniac, of the University of Rochester, in New York. In the late 1950’s, when we were
just beginning to think seriously about looking for life on Mars, he found himself at ascientific meeting where an astronomer expressed amazement that the biologists had nosimple, reliable, automated instrument capable of looking for microorganisms. Vishniac
decided he would do something about the matter.
He developed a small device to be sent to the planets. His friends called it the Wolf
Trap. It would carry a little vial of nutrient organic matter to Mars, arrange for a sample ofMartian soil to be mixed in with it, and observe the changing turbidity or cloudiness ofliquid as the Martian bugs (if there were any) grew (if they would). The Wolf Trap wasselected along with three other microbiology experiments to go aboard the Viking landers.
Two of the other three experiments also chose to send food to the Martians. The success ofthe Wolf Trap required that Martian bugs like liquid water. There were those who thoughtthat Vishniac would only drown the little Martians. But the advantage of the Wolf Trap wasthat it laid no requirements on what the Martian microbes must do with their food. They
had only to grow. All the other experiments made specific assumptions about gases thatwould be given off or taken in by the microbes, assumptions that were little more thanguesses.
The National Aeronautics and Space Administration, which runs the United Statesplanetary space program, is subject to frequent and unpredictable budget cuts. Only rarely
are there unanticipated budget increases. NASA scientific activities have very little
effective support in the government, and so science is most often the target when money
needs to be taken away from NASA. In 1971 it was decided that one of the four
microbiology experiments must be removed, and the Wolf Trap was offloaded. It was a
crushing disappointment for Vishniac, who had invested twelve years in its development.
Many others in his place might have stalked off the Viking Biology Team. But
Vishniac was a gentle and dedicated man. He decided instead that he could best serve thesearch for life on Mars by voyaging to the most Mars-like environment on Earth - the dryvalleys of Antarctica. Some previous investigators had examined Antarctic soil and decided
that the few microbes they were able to find were not really natives of the dry valleys, but
had been blown there from other, more clement environments. Recalling the Mars Jars
experiments, Vishniac believed that life was tenacious and that Antarctica was perfectly
consistent with microbiology. If terrestrial bugs could live on Mars, he thought, why not in
Antarctica - which was by and large warmer, wetter, and had more oxygen and much lessultraviolet light. Conversely, finding life in Antarctic dry valleys would correspondingly
improve, he thought, the chances of life on Mars. Vishniac believed that the experimental
techniques previously used to deduce no indigenous microbes in Antarctica were flawed.
The nutrients, while suitable for the comfortable environment of a university microbiology
laboratory, were not designed for the arid polar wasteland.
So on November 8, 1973, Vishniac, his new microbiology equipment and ageologist companion were transported by helicopter from McMurdo Station to an area near
Mount Balder, a dry valley in the Asgard range. His practice was to implant the little
microbiology stations in the Antarctic soil and return about a month later to retrieve them.
On December 10, 1973, he left to gather samples on Mount Balder; his departure wasphotographed from about three kilometers away. It was the last time anyone saw him alive.
Eighteen hours later, his body was discovered at the base of a cliff of ice. He had wandered
into an area not previously explored, had apparently slipped on the ice and tumbled andbounced for a distance of 150 meters. Perhaps something had caught his eye, a likely
habitat for microbes, say, or a patch of green where none should be. We will never know.
In the small brown notebook he was carrying that day, the last entry reads; ‘Station 202
retrieved. 10 December, 1973. 2230 hours. Soil temperature, -10°. Air temperature -16°.’ It
had been a typical summer temperature for Mars.
Many of Vishniac’s microbiology stations are still sitting in Antarctica. But the
samples that were returned were examined, using his methods, by his professional
colleagues and friends. A wide variety of microbes, which would have been indetectablewith conventional scoring techniques, was found in essentially every site examined. A new
species of yeast, apparently unique to Antarctica, was discovered in his samples by hiswidow, Helen Simpson Vishniac. Large rocks returned from Antarctica in that expedition,
examined by Imre Friedmann, turn out to have a fascinating microbiology - one or two
millimeters inside the rock, algae have colonized a tiny world in which small quantities ofwater are trapped and made liquid. On Mars such a place would be even more interesting,because while the visible light necessary for photosynthesis would penetrate to that depth,
the germicidal ultraviolet light would be at least partially attenuated.
Because the design of space missions is finalized many years before launch, andbecause of Vishniac’s death, the results of his Antarctic experiments did not influence theViking design for seeking Martian life. In general, the microbiology experiments were notcarried out at the low ambient Martian temperatures, and most did not provide longincubation times. They all made fairly strong assumptions about what Martian metabolism
had to be like. There was no way to look for life inside the rocks.
Each Viking lander was equipped with a sample arm to acquire material from thesurface and then slowly withdraw it into the innards of the spacecraft, transporting theparticles on little hoppers like an electric train to five different experiments: one on the
inorganic chemistry of the soil, another to look for organic molecules in the sand and dust,
and three to look for microbial life. When we look for life on a planet, we are making
certain assumptions. We try, as well as we can, not to assume that life elsewhere will bejust like life here. But there are limits to what we can do. We know in detail only about life
here. While the Viking biology experiments are a pioneering first effort, they hardly
represent a definitive search for life on Mars. The results have been tantalizing, annoying,provocative, stimulating, and, at least until recently, substantially inconclusive.
Each of the three microbiology experiments asked a different kind of question, but
in all cases a question about Martian metabolism. If there are microorganisms in theMartian soil, they must take in food and give off waste gases; or they must take in gases
from the atmosphere and, perhaps with the aid of sunlight, convert them into usefulmaterials. So we bring food to Mars and hope that the Martians, if there are any, will find it
tasty. Then we see if any interesting new gases come out of the soil. Or we provide our own
radioactively labeled gases and see if they are converted into organic matter, in which casesmall Martians are inferred.
By criteria established before launch, two of the three Viking microbiology
experiments seem to have yielded positive results. First, when Martian soil was mixed witha sterile organic soup from Earth, something in the soil chemically broke down the soup almost as if there were respiring microbes metabolizing a food package from Earth.
Second, when gases from Earth were introduced into the Martian soil sample, the gases
became chemically combined with the soil - almost as if there were photosynthesizing
microbes, generating organic matter from atmospheric gases. Positive results in Martian
microbiology were achieved in seven different samplings- in two locales on Mars separated
by 5,000 kilometers.
But the situation is complex, and the criteria of experimental success may have been
inadequate. Enormous efforts were made to build the Viking microbiology experiments andtest them with a variety of microbes. Very little effort was made to calibrate theexperiments with plausible inorganic Martian surface materials. Mars is not the Earth. As
the legacy of Percival Lowell reminds us, we can be fooled. Perhaps there is an exotic
inorganic chemistry in the Martian soil that is able by itself, in the absence of Martian
microbes, to oxidize foodstuffs. Perhaps there is some special inorganic, nonliving catalyst
in the soil that is able to fix atmospheric gases and convert them into organic molecules.

Recent experiments suggest that this may indeed be the case. In the great Martiandust storm of 1971, spectral features of the dust were obtained by the Mariner 9 infrared
spectrometer. In analyzing these spectra, O. B. Toon, J. B. Pollack and I found that certain
features seem best accounted for by montmorillonite and other kinds of clay. Subsequent
observations by the Viking lander support the identification of windblown clays on Mars.
Now, A. Banin and J. Rishpon have found that they can reproduce some of the key features- those resembling photosynthesis as well as those resembling respiration of the‘successful’ Viking microbiology experiments if in laboratory experiments they substitutesuch clays for the Martian soil. The clays have a complex active surface, given to absorbing
and releasing gases and to catalyzing chemical reactions. It is too soon to say that all the
Viking microbiology results can be explained by inorganic chemistry, but such a result
would no longer be surprising. The clay hypothesis hardly excludes life on Mars, but it
certainly carries us far enough to say that there is no compelling evidence for microbiology
on Mars.
Even so, the results of Banin and Rishpon are of great biological importancebecause they show that in the absence of life there can be a kind of soil chemistry that doessome of the same things life does. On the Earth before life, there may already have beenchemical processes resembling respiration and photosynthesis cycling in the soil, perhaps
to be incorporated by life once it arose. In addition, we know that montmorillonite clays area potent catalyst for combining amino acids into longer chain molecules resembling
proteins. The clays of the primitive Earth may have been the forge of life, and thechemistry of contemporary Mars may provide essential clues to the origin and early historyof life on our planet.
The Martian surface exhibits many impact craters, each named after a person,
usually a scientist. Crater Vishniac lies appropriately in the Antarctic region of Mars.
Vishniac did not claim that there had to be life on Mars, merely that it was possible, andthat it was extraordinarily important to know if it was there. If life on Mars exists, we will
have a unique opportunity to test the generality of our form of fife. And if there is no life on
Mars, a planet rather like the Earth, we must understand why - because in that case, as
Vishniac stressed, we have the classic scientific confrontation of the experiment and thecontrol.
The finding that the Viking microbiology results can be explained by clays, thatthey need not imply life, helps to resolve another mystery: the Viking organic chemistry
experiment showed not a hint of organic matter in the Martian soil. If there is life on Mars,
where are the dead bodies? No organic molecules could be found - no building blocks ofproteins and nucleic acids, no simple hydrocarbons, nothing of the stuff of life on Earth.
This is not necessarily a contradiction, because the Viking microbiology experiments are athousand times more sensitive (per equivalent carbon atom) than the Viking chemistry
experiments, and seem to detect organic matter synthesized in the Martian soil. But thisdoes not leave much margin. Terrestrial soil is loaded with the organic remains of onceliving organisms; Martian soil has less organic matter than the surface of the Moon. If weheld to the life hypothesis, we might suppose that the dead bodies have been destroyed by
the chemically reactive, oxidizing surface of Mars - like a germ in a bottle of hydrogen
peroxide; or that there is life, but of a kind in which organic chemistry plays a less central
role than it does in life on Earth.
But this last alternative seems to me to be special pleading: I am, reluctantly, a selfconfessed carbon chauvinist. Carbon is abundant in the Cosmos. It makes marvelously

complex molecules, good for life. I am also a water chauvinist. Water makes an ideal
solvent system for organic chemistry to work in and stays liquid over a wide range oftemperatures. But sometimes I wonder. Could my fondness for these materials havesomething to do with the fact that I am made chiefly of them? Are we carbon- and waterbased because those materials were abundant on the Earth at the time of the origin of life?
Could life elsewhere - on Mars, say - be built of different stuff?
I am a collection of water, calcium and organic molecules called Carl Sagan. You
are a collection of almost identical molecules with a different collective label. But is thatall? Is there nothing in here but molecules? Some people find this idea somehow
demeaning to human dignity. For myself, I find it elevating that our universe permits theevolution of molecular machines as intricate and subtle as we.
But the essence of life is not so much the atoms and simple molecules that make us
up as the way in which they are put together. Every now and then we read that thechemicals which constitute the human body cost ninety-seven cents or ten dollars or somesuch figure; it is a little depressing to find our bodies valued so little. However, theseestimates are for human beings reduced to our simplest possible components. We are mademostly of water, which costs almost nothing; the carbon is costed in the form of coal; thecalcium in our bones as chalk; the nitrogen in our proteins as air (cheap also); the iron in
our blood as rusty nails. If we did not know better, we might be tempted to take all theatoms that make us up, mix them together in a big container and stir. We can do this as
much as we want. But in the end all we have is a tedious mixture of atoms. How could wehave expected anything else?
Harold Morowitz has calculated what it would cost to put together the correct
molecular constituents that make up a human being by buying the molecules from chemical
supply houses. The answer turns out to be about ten million dollars, which should make us
all feel a little better. But even then we could not mix those chemicals together and have ahuman being emerge from the jar. That is far beyond our capability and will probably be sofor a very long period of time. Fortunately, there are other less expensive but still highly
reliable methods of making human beings.
I think the lifeforms on many worlds will consist, by and large, of the same atoms
we have here, perhaps even many of the same basic molecules, such as proteins and nucleic
acids - but put together in unfamiliar ways. Perhaps organisms that float in dense planetaryatmospheres will be very much like us in their atomic composition, except they might nothave bones and therefore not need much calcium. Perhaps elsewhere some solvent otherthan water is used. Hydrofluoric acid might serve rather well, although there is not a great
deal of fluorine in the Cosmos; hydrofluoric acid would do a great deal of damage to thekind of molecules that make us up, but other organic molecules, paraffin waxes, forexample, are perfectly stable in its presence. Liquid ammonia would make an even better
solvent system, because ammonia is very abundant in the Cosmos. But it is liquid only on
worlds much colder than the Earth or Mars. Ammonia is ordinarily a gas on Earth, as water
is on Venus. Or perhaps there are living things that do not have a solvent system at all solid-state life, where there are electrical signals propagating rather than molecules floating
about.
But these ideas do not rescue the notion that the Viking lander experiments indicate
life on Mars. On that rather Earth-like world, with abundant carbon and water, life, if itexists, should be based on organic chemistry. The organic chemistry results, like theimaging and microbiology results, are all consistent with no life in the fine particles ofChryse and Utopia in the late 1970’s. Perhaps some millimeters beneath the rocks (as in the
Antarctic dry valleys), or elsewhere on the planet, or in some earlier, more clement time.
But not where and when we looked.
The Viking exploration of Mars is a mission of major historical importance, the firstserious search for what other kinds of life may be, the first survival of a functioning
spacecraft for more than an hour or so on any other planet (Viking 1 has survived foryears), the source of a rich harvest of data on the geology, seismology, mineralogy,
meteorology and half a dozen other sciences of another world. How should we follow up
on these spectacular advances? Some scientists want to send an automatic device thatwould land, acquire soil samples, and return them to Earth, where they could be examinedin great detail in the large sophisticated laboratories of Earth rather than in the limited
microminiaturized laboratories that we are able to send to Mars. In this way most of theambiguities of the Viking microbiology experiments could be resolved. The chemistry andmineralogy of the soil could be determined; rocks could be broken open to search forsubsurface life; hundreds of tests for organic chemistry and life could be performed,
including direct microscopic examination, under a wide range of conditions. We could even
use Vishniac’s scoring techniques. Although it would be fairly expensive, such a mission isprobably within our technological capability.
However, it carries with it a novel danger: back-contamination. If we wish on Earth
to examine samples of Martian soil for microbes, we must, of course, not sterilize thesamples beforehand. The point of the expedition is to bring them back alive. But what then?
Might Martian microorganisms returned to Earth pose a public health hazard? The Martiansof H. G. Wells and Orson Welles, preoccupied with the suppression of Bournemouth andJersey City, never noticed until too late that their immunological defenses were unavailingagainst the microbes of Earth. Is the converse possible? This is a serious and difficult issue.
There may be no micromartians. If they exist, perhaps we can eat a kilogram of them withno ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized
Martian samples to Earth, we must have a containment procedure that is stupefyingly
reliable. There are nations that develop and stockpile bacteriological weapons. They seemto have an occasional accident, but they have not yet, so far as I know, produced global
pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want tobe very sure before considering a returned-sample mission.
There is another way to investigate Mars and the full range of delights anddiscoveries this heterogeneous planet holds for us. My most persistent emotion in working
with the Viking lander pictures was frustration at our immobility. I found myself
unconsciously urging the spacecraft at least to stand on its tiptoes, as if this laboratory,
designed for immobility, were perversely refusing to manage even a little hop. How we
longed to poke that dune with the sample arm, look for life beneath that rock, see if that
distant ridge was a crater rampart. And not so very far to the southeast, I knew, were thefour sinuous channels of Chryse. For all the tantalizing and provocative character of theViking results, I know a hundred places on Mars which are far more interesting than ourlanding sites. The ideal tool is a roving vehicle carrying on advanced experiments,
particularly in imaging, chemistry and biology. Prototypes of such rovers are underdevelopment by NASA. They know on their own how to go over rocks, how not to fall
down ravines, how to get out of tight spots. It is within our capability to land a rover onMars that could scan its surroundings, see the most interesting place in its field of view and,by the same time tomorrow, be there. Every day a new place, a complex, winding traverse

over the varied topography of this appealing planet.
Such a mission would reap enormous scientific benefits, even if there is no life on
Mars. We could wander down the ancient river valleys, up the slopes of one of the great
volcanic mountains, along the strange stepped terrain of the icy polar terraces, or muster aclose approach to the beckoning pyramids of Mars.* Public interest in such a mission
would be sizable. Every day a new set of vistas would arrive on our home televisionscreens. We could trace the route, ponder the findings, suggest new destinations. The
journey would be long, the rover obedient to radio commands from Earth. There would beplenty of time for good new ideas to be incorporated into the mission plan. A billion people
could participate in the exploration of another world.
* The largest are 3 kilometers across at the base, and 1 kilometer high - much larger than the
pyramids of Sumer, Egypt or Mexico on Earth. They seem eroded and ancient, and are, perhaps, only small
mountains, sandblasted for ages. But they warrant, I think, a careful look.

The surface area of Mars is exactly as large as the land area of the Earth. A
thorough reconnaissance will clearly occupy us for centuries. But there will be a time when
Mars is all explored; a time after robot aircraft have mapped it from aloft, a time after
rovers have combed the surface, a time after samples have been returned safely to Earth, atime after human beings have walked the sands of Mars. What then? What shall we do with
Mars?
There are so many examples of human misuse of the Earth that even phrasing thisquestion chills me. If there is life on Mars, I believe we should do nothing with Mars. Mars
then belongs to the Martians, even if the Martians are only microbes. The existence of an
independent biology on a nearby planet is a treasure beyond assessing, and the preservationof that life must, I think, supersede any other possible use of Mars. However, suppose Marsis lifeless. It is not a plausible source of raw materials: the freightage from Mars to Earthwould be too expensive for many centuries to come. But might we be able to live on Mars?
Could we in some sense make Mars habitable?
A lovely world, surely, but there is - from our parochial point of view - much wrong
with Mars, chiefly the low oxygen abundance, the absence of liquid water, and the highultraviolet flux. (The low temperatures do not pose an insuperable obstacle, as the yearround scientific stations in Antarctica demonstrate.) All of these problems could be solved
if we could make more air. With higher atmospheric pressures, liquid water would bepossible. With more oxygen we might breathe the atmosphere, and ozone would form toshield the surface from solar ultraviolet radiation. The sinuous channels, stacked polar
plates and other evidence suggest that Mars once had such a denser atmosphere. Those
gases are unlikely to have escaped from Mars. They are, therefore, on the planet
somewhere. Some are chemically combined with the surface rocks. Some are in subsurface
ice. But most may be in the present polar ice caps.
To vaporize the caps, we must heat them; perhaps we could dust them with a dark
powder, heating them by absorbing more sunlight, the opposite of what we do to the Earthwhen we destroy forests and grasslands. But the surface area of the caps is very large. The
necessary dust would require 1,200 Saturn 5 rocket boosters to be transported from Earth toMars; even then, the winds might blow the dust off the polar caps. A better way would beto devise some dark material able to make copies of itself, a little dusky machine which wedeliver to Mars and which then goes about reproducing itself from indigenous materials allover the polar caps. There is a category of such machines. We call them plants. Some arevery hardy and resilient. We know that at least some terrestrial microbes can survive on.
Mars. What is necessary is a program of artificial selection and genetic engineering of darkplants - perhaps lichens - that could survive the much more severe Martian environment. Ifsuch plants could be bred, we might imagine them being seeded on the vast expanse of theMartian polar ice caps, taking root, spreading, blackening the ice caps, absorbing sunlight,
heating the ice, and releasing the ancient Martian atmosphere from its long captivity. We
might even imagine a kind of Martian Johnny Appleseed, robot or human, roaming thefrozen polar wastes in an endeavor that benefits only the generations of humans to come.
The general concept is called terraforming: the changing of an alien landscape into
one more suitable for human beings. In thousands of years humans have managed toperturb the global temperature of the Earth by only about one degree through greenhouseand albedo changes, although at the present rate of burning fossil fuels and destroying
forests and grasslands we can now change the global temperature by another degree in only
a century or two. These and other considerations suggest that a time scale for a significant
terraforming of Mars is probably hundreds to thousands of years. In a future time of greatly
advanced technology we might wish not only to increase the total atmospheric pressure andmake liquid water possible but also to carry liquid water from the melting polar caps to thewarmer equatorial regions. There is, of course, a way to do it. We would build canals.
The melting surface and subsurface ice would be transported by a great canal
network. But this is precisely what Percival Lowell, not a hundred years ago, mistakenly
proposed was in fact happening on Mars. Lowell and Wallace both understood that thecomparative inhospitability of Mars was due to the scarcity of water. If only a network ofcanals existed, the lack would be remedied, the habitability of Mars would becomeplausible. Lowell’s observations were made under extremely difficult seeing conditions.
Others, like Schiaparelli, had already observed something like the canals; they were called
canali before Lowell began his lifelong love affair with Mars. Human beings have ademonstrated talent for self-deception when their emotions are stirred, and there are few
notions more stirring than the idea of a neighboring planet inhabited by intelligent beings.
The power of Lowell’s idea may, just possibly, make it a kind of premonition. His
canal network was built by Martians. Even this may be an accurate prophecy: If the planet
ever is terraformed, it will be done by human beings whose permanent residence andplanetary affiliation is Mars. The Martians will be us.

CHAPTER VI
Travelers’ TalesDo there exist many worlds, or is there but a single world? This is one of the mostnoble and exalted questions in the study of Nature.
- Albertus Magnus, thirteenth century
In the first ages of the world, the islanders either thought themselves to be the only
dwellers upon the earth, or else if there were any other, yet they could not possibly
conceive how they might have any commerce with them, being severed by the deep
and broad sea, but the aftertimes found out the invention of ships . . . So, perhaps,
there may be some other means invented for a conveyance to the Moone . . . We
have not now any Drake or Columbus to undertake this voyage, or any Daedalus toinvent a conveyance through the aire. However I doubt not but that time who is still
the father of new truths, and hath revealed unto us many things which our ancestors
were ignorant of, will also manifest to our posterity that which we now desire but
cannot know.
- John Wilkins, The Discovery of a World in the Moone, 1638
We may mount from this dull Earth; and viewing it from on high, consider whether
Nature has laid out all her cost and finery upon this small speck of Dirt. So, like
Travellers into other distant countries, we shall be better able to judge of what’sdone at home, know how to make a true estimate of, and set its own value uponevery thing. We shall be less apt to admire what this World calls great, shall nobly
despise those Trifles the generality of Men set their Affections on, when we knowthat there are a multitude of such Earths inhabited and adorn’d as well as our own.
- Christiaan Huygens, The Celestial Worlds Discovered, c. 1690
This is the time when humans have begun to sail the sea of space. The modern ships thatply the Keplerian trajectories to the planets are unmanned. They are beautifully constructed,
semi-intelligent robots exploring unknown worlds. Voyages to the outer solar system arecontrolled from a single place on the planet Earth, the Jet Propulsion Laboratory (JPL) of
the National Aeronautics and Space Administration in Pasadena, California.
On July 9, 1979, a spacecraft called Voyager 2 encountered the Jupiter system. It
had been almost two years sailing through interplanetary space. The ship is made ofmillions of separate parts assembled redundantly, so that if some component fails, others
will take over its responsibilities. The spacecraft weighs 0.9 tons and would fill a large
living room. Its mission takes it so far from the Sun that it cannot be powered by solar
energy, as other spacecraft are. Instead, Voyager relies on a small nuclear power plant,
drawing hundreds of watts from the radioactive decay of a pellet of plutonium. Its three
integrated computers and most of its housekeeping functions - for example, its temperaturecontrol system - are localized in its middle. It receives commands from Earth and radios its
findings back to Earth through a large antenna, 3.7 meters in diameter. Most of its scientificinstruments are on a scan platform, which tracks Jupiter or one of its moons as thespacecraft hurtles past. There are many scientific instruments - ultraviolet and infraredspectrometers, devices to measure charged particles and magnetic fields and the radio
emission from Jupiter - but the most productive have been the two television cameras,
designed to take tens of thousands of pictures of the planetary islands in the outer solar
system.
Jupiter is surrounded by a shell of invisible but extremely dangerous high-energy
charged particles. The spacecraft must pass through the outer edge of this radiation belt to
examine Jupiter and its moons close up, and to continue its mission to Saturn and beyond.
But the charged particles can damage the delicate instruments and fry the electronics.
Jupiter is also surrounded by a ring of solid debris, discovered four months earlier by
Voyager 1, which Voyager 2 had to traverse. A collision with a small boulder could havesent the spacecraft tumbling wildly out of control, its antenna unable to lock on the Earth,its data lost forever. Just before Encounter, the mission controllers were restive. There weresome alarms and emergencies, but the combined intelligence of the humans on Earth andthe robot in space circumvented disaster.
Launched on August 20, 1977, it moved on an arcing trajectory past the orbit ofMars, through the asteroid belt, to approach the Jupiter system and thread its way past theplanet and among its fourteen or so moons. Voyager’s passage by Jupiter accelerated it
towards a close encounter with Saturn. Saturn’s gravity will propel it on to Uranus. After
Uranus it will plunge on past Neptune, leaving the solar system, becoming an interstellar
spacecraft, fated to roam forever the great ocean between the stars.
These voyages of exploration and discovery are the latest in a long series that havecharacterized and distinguished human history. In the fifteenth and sixteenth centuries you
could travel from Spain to the Azores in a few days, the same time it takes us now to crossthe channel from the Earth to the Moon. It took then a few months to traverse the Atlantic
Ocean and reach what was called the New World, the Americas. Today it takes a few
months to cross the ocean of the inner solar system and make planet-fall on Mars or Venus,
which are truly and literally now worlds awaiting us. In the seventeenth and eighteenth
centuries you could travel from Holland to China in a year or two, the time it has taken
Voyager to travel from Earth to Jupiter.* The annual costs were, relatively, more then thannow, but in both cases less than 1 percent of the appropriate Gross National Product. Ourpresent spaceships, with their robot crews, are the harbingers, the vanguards of future
human expeditions to the planets. We have traveled this way before.
* Or, to make a different comparison, a fertilized egg takes as long to wander from the Fallopian
tubes and implant itself in the uterus as Apollo 11 took to journey to the Moon; and as long to develop into afull-term infant as Viking took on its trip to Mars. The normal human lifetime is longer than Voyager willtake to venture beyond the orbit of Pluto.

The fifteenth through seventeenth centuries represent a major turning point in our
history. It then became clear that we could venture to all parts of our planet. Plucky sailing
vessels from half a dozen European nations dispersed to every ocean. There were many
motivations for these journeys: ambition, greed, national pride, religious fanaticism, prison
pardons, scientific curiosity, the thirst for adventure and the unavailability of suitable
employment in Estremadura. These voyages worked much evil as well as much good. But
the net result has been to bind the Earth together, to decrease provincialism, to unify thehuman species and to advance powerfully our knowledge of our planet and ourselves.
Emblematic of the epoch of sailing-ship exploration and discovery is therevolutionary Dutch Republic of the seventeenth century. Having recently declared its
independence from the powerful Spanish Empire, it embraced more fully than any othernation of its time the European Enlightenment. It was a rational, orderly, creative society.
But because Spanish ports and vessels were closed to Dutch shipping, the economic
survival of the tiny republic depended on its ability to construct, man and deploy a great
fleet of commercial sailing vessels.
The Dutch East India Company, a joint governmental and private enterprise, sent
ships to the far corners of the world to acquire rare commodities and resell them at a profit
in Europe. Such voyages were the life blood of the Republic. Navigational charts and maps
were classified as state secrets. Ships often embarked with sealed orders. Suddenly the
Dutch were present all over the planet. The Barents Sea in the Arctic Ocean and Tasmania
in Australia are named after Dutch sea captains. These expeditions were not merelycommercial exploitations, although there was plenty of that. There were powerful elementsof scientific adventure and the zest for discovery of new lands, new plants and animals,
new people; the pursuit of knowledge for its own sake.
The Amsterdam Town Hall reflects the confident and secular self-image ofseventeenth-century Holland. It took shiploads of marble to build. Constantijn Huygens, apoet and diplomat of the time, remarked that the Town Hall dispelled ‘the Gothic squint
and squalor.’ In the Town Hall to this day, there is a statue of Atlas supporting the heavens,festooned with constellations. Beneath is Justice, brandishing a golden sword and scales,
standing between Death and Punishment, and treading underfoot Avarice and Envy, thegods of the merchants. The Dutch, whose economy was based on private profit,
nevertheless understood that the unrestrained pursuit of profit posed a threat to the nation’ssoul.
A less allegorical symbol may be found under Atlas and Justice, on the floor of the
Town Hall. It is a great inlaid map, dating from the late seventeenth or early eighteenth
centuries, reaching from West Africa to the Pacific Ocean. The whole world was Holland’s
arena. And on this map, with disarming modesty the Dutch omitted themselves, using only
the old Latin name Belgium for their part of Europe.
In a typical year many ships set sail halfway around the world. Down the west coastof Africa, through what they called the Ethiopian Sea, around the south coast of Africa,
within the Straits of Madagascar, and on past the southern tip of India they sailed, to one
major focus of their interests, the Spice Islands, present-day Indonesia. Some expeditions
journeyed from there to a land named New Holland, and today called Australia. A few
ventured through the Straits of Malacca, past the Philippines, to China. We know from a
mid-seventeenth-century account of an ‘Embassy from the East India Company of theUnited Provinces of the Netherlands, to the Grand Tartar, Cham, Emperor of China.’ The
Dutch burgers, ambassadors and sea captains stood wide-eyed in amazement, face to facewith another civilization in the Imperial City of Peking.** We even know what gifts they brought the Court. The Empress was presented with ‘six little chestsof divers pictures.’ And the Emperor received ‘two fardels of cinnamon.’Never before or since has Holland been the world power it was then. A small
country, forced to live by its wits, its foreign policy contained a strong pacifist element.
Because of its tolerance for unorthodox opinions, it was a haven for intellectuals who wererefugees from censorship and through control elsewhere in Europe - much as the United

States benefited enormously in the 1930’s by the exodus of intellectuals from Nazidominated Europe. So seventeenth-century Holland was the home of the great Jewish
philosopher Spinoza, whom Einstein admired; of Descartes, a pivotal figure in the historyof mathematics and philosophy; and of John Locke, a political scientist who influenced agroup of philosophically inclined revolutionaries named Paine, Hamilton, Adams, Franklin
and Jefferson. Never before or since has Holland been graced by such a galaxy of artists
and scientists, philosophers and mathematicians. This was the time of the master painters
Rembrandt and Vermeer and Frans Hals; of Leeuwenhoek, the inventor of the microscope;of Grotius, the founder of international law; of Willebrord Snellius, who discovered the lawof the refraction of light.
In the Dutch tradition of encouraging freedom of thought, the University of Leiden
offered a professorship to an Italian scientist named Galileo, who had been forced by the
Catholic Church under threat of torture to recant his heretical view that the Earth moved
about the Sun and not vice versa.* Galileo had close ties with Holland, and his first
astronomical telescope was an improvement of a spyglass of Dutch design. With it hediscovered sunspots, the phases of Venus, the craters of the Moon, and the four large
moons of Jupiter now called, after him, the Galilean satellites. Galileo’s own description of
his ecclesiastical travails is contained in a letter he wrote in the year 1615 to the Grand
Duchess Christina:
Some years ago as Your Serene Highness well knows, I discovered in the heavens
many things that had not been seen before our own age. The novelty of these things,
as well as some consequences which followed from them in contradiction to thephysical notions commonly held among academic philosophers, stirred up against
me no small number of professors [many of them ecclesiastics] - as if I had placedthese things in the sky with my own hands in order to upset Nature and overturn thesciences. They seemed to forget that the increase of known truths stimulates theinvestigation, establishment, and growth of the arts.*** In 1979 Pope John Paul II cautiously proposed reversing the condemnation of Galileo done 346
years earlier by the ‘Holy Inquisition.’** The courage of Galileo (and Kepler) in promoting the heliocentric hypothesis was not evident in
the actions of others, even those residing in less fanatically doctrinal parts of Europe. For example, in a letter
dated April 1634, René Descartes, then living in Holland, wrote:
Doubtless you know that Galileo was recently censured by the Inquisitors of the Faith, and that his
views about the movement of the Earth were condemned as heretical. I must tell you that all thethings I explained in my treatise, which included the doctrine of the movement of the Earth, were sointerdependent that it is enough to discover that one of them is false to know that all the arguments I
was using are unsound. Though I thought they were based on very certain and evident proofs, I
would not wish, for anything in the world, to maintain them against the authority of the Church . . . I
desire to live in peace and to continue the life I have begun under the motto to live well you must live
unseen.

The connection between Holland as an exploratory power and Holland as an intellectualand cultural center was very strong. The improvement of sailing ships encouraged
technology of all kinds. People enjoyed working with their hands. Inventions were prized.
Technological advance required the freest possible pursuit of knowledge, so Hollandbecame the leading publisher and bookseller in Europe, translating works written in otherlanguages and permitting the publication of works proscribed elsewhere. Adventures into
exotic lands and encounters with strange societies shook complacency, challenged thinkers
to reconsider the prevailing wisdom and showed that ideas that had been accepted forthousands of years - for example, on geography - were fundamentally in error. In a time
when kings and emperors ruled much of the world, the Dutch Republic was governed, morethan any other nation, by the people. The openness of the society and its encouragement of
the life of the mind, its material well-being and its commitment to the exploration andutilization of new worlds generated a joyful confidence in the human enterprise.** This exploratory tradition may account for the fact that Holland has, to this day, produced far morethan its per capita share of distinguished astronomers, among them Gerard Peter Kuiper, who in the 1940’s
and 1950’s was the world’s only full-time planetary astrophysicist. The subject was then considered by mostprofessional astronomers to be at least slightly disreputable, tainted with Lowellian excesses. I am grateful tohave been Kuiper’s student.

In Italy, Galileo had announced other worlds, and Giordano Bruno had speculated
on other lifeforms. For this they had been made to suffer brutally. But in Holland, theastronomer Christiaan Huygens, who believed in both, was showered with honors. His
father was Constantijn Huygens, a master diplomat of the age, a litterateur, poet, composer,
musician, close friend and translator of the English poet John Donne, and the head of anarchetypical great family. Constantijn admired the painter Rubens, and ‘discovered’ ayoung artist named Rembrandt van Rijn, in several of whose works he subsequently
appears. After their first meeting, Descartes wrote of him: ‘I could not believe that a single
mind could occupy itself with so many things, and equip itself so well in all of them.’ The
Huygens home was filled with goods from all over the world. Distinguished thinkers fromother nations were frequent guests. Growing up in this environment, the young Christiaan
Huygens became simultaneously adept in languages, drawing, law, science, engineering,mathematics and music. His interests and allegiances were broad. ‘The world is mycountry,’ he said, ‘science my religion.’Light was a motif of the age: the symbolic enlightenment of freedom of thought andreligion, of geographical discovery; the light that permeated the paintings of the time,
particularly the exquisite work of Vermeer; and light as an object of scientific inquiry, as inSnell’s study of refraction, Leeuwenhoek’s invention of the microscope and Huygens’ own
wave theory of light.* These were all connected activities, and their practitioners mingled
freely. Vermeer’s interiors are characteristically filled with nautical artifacts and wall maps.
Microscopes were drawing-room curiosities. Leeuwenhoek was the executor of Vermeer’s
estate and a frequent visitor at the Huygens home in Hofwijck.
* Isaac Newton admired Christiaan Huygens and thought him ‘the most elegant mathematician’ of
their time, and the truest follower of the mathematical tradition of the ancient Greeks - then, as now, a greatcompliment. Newton believed, in part because shadows had sharp edges, that light behaved as if it were a
stream of tiny particles. He thought that red light was composed of the largest particles and violet thesmallest. Huygens argued that instead light behaved as if it were a wave propagating in a vacuum, as an ocean
wave does in the sea - which is why we talk about the wavelength and frequency of light. Many properties oflight, including diffraction, are naturally explained by the wave theory, and in subsequent years Huygens’
view carried the day. But in 1905, Einstein showed that the particle theory of light could explain thephotoelectric effect, the ejection of electrons from a metal upon exposure to a beam of light. Modern quantum
mechanics combines both ideas, and it is customary today to think of light as behaving in some circumstancesas a beam of particles and in others as a wave. This wave-particle dualism may not correspond readily to ourcommon-sense notions, but it is in excellent accord with what experiments have shown light really does.
There is something mysterious and stirring in this marriage of opposites, and it is fitting that Newton andHuygens, bachelors both, were the parents of our modern understanding of the nature of light.

Leeuwenhoek’s microscope evolved from the magnifying glasses employed bydrapers to examine the quality of cloth. With it he discovered a universe in a drop of water:
the microbes, which he described as ‘animalcules’ and thought ‘cute’. Huygens had
contributed to the design of the first microscopes and himself made many discoveries withthem. Leeuwenhoek and Huygens were among the first people ever to see human sperm
cells, a prerequisite for understanding human reproduction. To explain how
microorganisms slowly develop in water previously sterilized by boiling, Huygens
proposed that they were small enough to float through the air and reproduced on alighting
in water. Thus he established an alternative to spontaneous generation - the notion that life
could arise, in fermenting grape juice or rotting meat, entirely independent of preexisting
life. It was not until the time of Louis Pasteur, two centuries later, that Huygens’
speculation was proved correct. The Viking search for life on Mars can be traced in more
ways than one back to Leeuwenhoek and Huygens. They are also the grandfathers of thegerm theory of disease, and therefore of much of modern medicine. But they had nopractical motives in mind. They were merely tinkering in a technological society.
The microscope and telescope, both developed in early seventeenth-century
Holland, represent an extension of human vision to the realms of the very small and thevery large. Our observations of atoms and galaxies were launched in this time and place.
Christiaan Huygens loved to grind and polish lenses for astronomical telescopes andconstructed one five meters long. His discoveries with the telescope would by themselves
have ensured his place in the history of human accomplishment. In the footsteps of
Eratosthenes, he was the first person to measure the size of another planet. He was also thefirst to speculate that Venus is completely covered with clouds; the first to draw a surface
feature on the planet Mars (a vast dark windswept slope called Syrtis Major); and byobserving the appearance and disappearance of such features as the planet rotated, the firstto determine that the Martian day was, like ours, roughly twenty-four hours long. He was
the first to recognize that Saturn was surrounded by a system of rings which nowhere
touches the planet.* And he was the discoverer of Titan, the largest moon of Saturn and, as
we now know, the largest moon in the solar system - a world of extraordinary interest andpromise. Most of these discoveries he made in his twenties. He also thought astrology was
nonsense.
* Galileo discovered the rings, but had no idea what to make of them. Through his early
astronomical telescope, they seemed to be two projections symmetrically attached to Saturn, resembling, hesaid in some bafflement, ears.

Huygens did much more. A key problem for marine navigation in this age was thedetermination of longitude. Latitude could easily be determined by the stars - the farther
south you were, the more southern constellations you could see. But longitude required
precise timekeeping. An accurate shipboard clock would tell the time in your home port;
the rising and setting of the Sun and stars would specify the local shipboard time; and thedifference between the two would yield your longitude. Huygens invented the pendulum
clock (its principle had been discovered earlier by Galileo), which was then employed,although not fully successfully, to calculate position in the midst of the great ocean. His
efforts introduced an unprecedented accuracy in astronomical and other scientificobservations and stimulated further advances in nautical clocks. He invented the spiralbalance spring still used in some watches today; made fundamental contributions tomechanics - e.g., the calculation of centrifugal force - and, from a study of the game ofdice, to the theory of probability. He improved the air pump, which was later torevolutionize the mining industry, and the ‘magic lantern,’ the ancestor of the slide
projector. He also invented something called the ‘gunpowder engine,’ which influenced thedevelopment of another machine, the steam engine.
Huygens was delighted that the Copernican view of the Earth as a planet in motion
around the Sun was widely accepted even by the ordinary people in Holland. Indeed, hesaid, Copernicus was acknowledged by all astronomers except those who ‘were a bit slowwitted or under the superstitions imposed by merely human authority.’ In the Middle Ages,
Christian philosophers were fond of arguing that, since the heavens circle the Earth once
every day, they can hardly be infinite in extent; and therefore an infinite number of worlds,
or even a large number of them (or even one other of them), is impossible. The discovery
that the Earth is turning rather than the sky moving had important implications for theuniqueness of the Earth and the possibility of life elsewhere. Copernicus held that not justthe solar system but the entire universe was heliocentric, and Kepler denied that the stars
have planetary systems. The first person to make explicit the idea of a large - indeed, an
infinite - number of other worlds in orbit about other suns seems to have been Giordano
Bruno. But others thought that the plurality of worlds followed immediately from the ideasof Copernicus and Kepler and found themselves aghast. In the early seventeenth century,
Robert Merton contended that the heliocentric hypothesis implied a multitude of otherplanetary systems, and that this was an argument of the sort called reductio ad absurdum(Appendix 1), demonstrating the error of the initial assumption. He wrote, in an argument
which may once have seemed withering,
For if the firmament be of such an incomparable bigness, as these Copernical giants
will have it . . . , so vast and full of innumerable stars, as being infinite in extent . . .
why may we not suppose . . . those infinite stars visible in the firmament to be somany suns, with particular fixed centers; to have likewise their subordinate planets,
as the sun hath his dancing still around him? . . . And so, in consequence, there areinfinite habitable worlds; what hinders? . . . these and suchlike insolent and boldattempts, prodigious paradoxes, inferences must needs follow, if it once be granted
which . . . Kepler . . . and others maintain of the Earth’s motion.
But the Earth does move. Merton, if he lived today, would be obliged to deduce ‘infinite,
habitable worlds.’ Huygens did not shrink from this conclusion; he embraced it gladly:
Across the sea of space the stars are other suns. By analogy with our solar system, Huygens
reasoned that those stars should have their own planetary systems and that many of theseplanets might be inhabited: ‘Should we allow the planets nothing but vast deserts . . . anddeprive them of all those creatures that more plainly bespeak their divine architect, weshould sink them below the Earth in beauty and dignity, a thing very unreasonable.’** A few others had held similar opinions. In his Harmonice Mundi Kepler remarked ‘it was Tycho
Brahe’s opinion concerning that bare wilderness of globes that it does not exist fruitlessly but is filled with

inhabitants.’

These ideas were set forth in an extraordinary book bearing the triumphant title The
Celestial Worlds Discover’d: Conjectures Concerning the Inhabitants, Plants andProductions of the Worlds in the Planets. Composed shortly before Huygens died in 1690,
the work was admired by many, including Czar Peter the Great, who made it the first
product of Western science to be published in Russia. The book is in large part about thenature or environments of the planets. Among the figures in the finely rendered first editionis one in which we see, to scale, the Sun and the giant planets Jupiter and Saturn. They are,comparatively, rather small. There is also an etching of Saturn next to the Earth: Our planet
is a tiny circle.
By and large Huygens imagined the environments and inhabitants of other planets
to be rather like those of seventeenth-century Earth. He conceived of ‘planetarians’ whose‘whole Bodies, and every part of them, may be quite distinct and different from ours . . . ‘tisa very ridiculous opinion . . . that it is impossible a rational Soul should dwell in any othershape than ours.’ You could be smart, he was saying, even if you looked peculiar. But hethen went on to argue that they would not look very peculiar - that they must have hands
and feet and walk upright, that they would have writing and geometry, and that Jupiter has
its four Galilean satellites to provide a navigational aid for the sailors in the Jovian oceans.
Huygens was, of course, a citizen of his time. Who of us is not? He claimed science as his
religion and then argued that the planets must be inhabited because otherwise God had
made worlds for nothing. Because he lived before Darwin, his speculations about
extraterrestrial life are innocent of the evolutionary perspective. But he was able to develop
on observational grounds something akin to the modern cosmic perspective:
What a wonderful and Amazing scheme have we here of the magnificent vastnessof the universe . . . So many Suns, so many Earths . . . and every one of themstock’d with so many Herbs, Trees, and Animals, adorn’d with so many Seas andMountains! . . . And how must our Wonder and Admiration be increased when we
consider the prodigious Distance and Multitude of the Stars.
The Voyager spacecraft are the lineal descendants of those sailing-ship voyages of
exploration, and of the scientific and speculative tradition of Christiaan Huygens. The
Voyagers are caravels bound for the stars, and on the way exploring those worlds that
Huygens knew and loved so well.
One of the main commodities returned on those voyages of centuries ago weretravelers’ tales,* stories of alien lands and exotic creatures that evoked our sense of wonder
and stimulated future exploration. There had been accounts of mountains that reached thesky; of dragons and sea monsters; of everyday eating utensils made of gold; of a beast withan arm for a nose; of people who thought the doctrinal disputes among Protestants,
Catholics, Jews and Muslims to be silly; of a black stone that burned; of headless humans
with mouths in their chests; of sheep that grew on trees. Some of these stories were true;
some were lies. Others had a kernel of truth, misunderstood or exaggerated by the explorers
or their informants. In the hands of Voltaire, say, or Jonathan Swift, these accounts
stimulated a new perspective on European society, forcing a reconsideration of that insularworld.

* Such tales are an ancient human tradition; many of them have had, from the beginning of
exploration, a cosmic motif. For example, the fifteenth-century explorations of Indonesia, Sri Lanka, India,
Arabia and Africa by the Ming Dynasty Chinese were described by Fei Hsin, one of the participants, in a
picture book prepared for the Emperor, as ‘The Triumphant Visions of the Starry Raft.’ Unfortunately, thepictures - although not the text - have been lost.

Modern Voyagers also return travelers’ tales, tales of a world shattered like a crystal
sphere; a globe where the ground is covered, pole to pole, with what looks like a network ofcobwebs; tiny moons shaped like potatoes; a world with an underground ocean; a land thatsmells of rotten eggs and looks like a pizza pie, with lakes of molten sulfur and volcanic
eruptions ejecting smoke directly into space; a planet called Jupiter that dwarfs our own so large that 1,000 Earths would fit within it.
The Galilean satellites of Jupiter are each almost as big as the planet Mercury. We
can measure their sizes and masses and so calculate their density, which tells us something
about the composition of their interiors. We find that the inner two, lo and Europa, have adensity as high as rock. The outer two, Ganymede and Callisto, have a much lower density,
halfway between rock and ice. But the mixture of ice and rocks within these outer moons
must contain, as do rocks on Earth, traces of radioactive minerals, which heat theirsurroundings. There is no effective way for this heat, accumulated over billions of years, toreach the surface and be lost to space, and the radioactivity inside Ganymede and Callisto
must therefore melt their icy interiors. We anticipate underground oceans of slush andwater in these moons, a hint, before we have ever seen the surfaces of the Galilean satellites
close up, that they may be very different one from another. When we do look closely,
through the eyes of Voyager, this prediction is confirmed. They do not resemble each other.
They are different from any worlds we have ever seen before.
The Voyager 2 spacecraft will never return to Earth. But its scientific findings, its
epic discoveries, its travelers’ tales, do return. Take July 9, 1979, for instance. At 8:04
Pacific Standard Time on this morning, the first pictures of a new world, called Europa
after an old one, were received on Earth.
How does a picture from the outer solar system get to us? Sunlight shines on Europa
in its orbit around Jupiter and is reflected back to space, where some of it strikes thephosphors of the Voyager television cameras, generating an image. The image is read by
the Voyager computers, radioed back across the immense intervening distance of half abillion kilometers to a radio telescope, a ground station on the Earth. There is one in Spain,
one in the Mojave Desert of Southern California and one in Australia. (On that Julymorning in 1979 it was the one in Australia that was pointed toward Jupiter and Europa.) It
then passes the information via a communications satellite in Earth orbit to Southern
California, where it is transmitted by a set of microwave relay towers to a computer at the
Jet Propulsion Laboratory, where it is processed. The picture is fundamentally like anewspaper wirephoto, made of perhaps a million individual dots, each a different shade ofgray, so fine and close together that at a distance the constituent dots are invisible. We see
only their cumulative effect. The information from the spacecraft specifies how bright or
dark each dot is to be. After processing, the dots are then stored on a magnetic disc,something like a phonograph record. There are some eighteen thousand photographs takenin the Jupiter system by Voyager 1 that are stored on such magnetic discs, and an
equivalent number for Voyager 2. Finally, the end product of this remarkable set of links
and relays is a thin piece of glossy paper, in this case showing the wonders of Europa,

recorded, processed and examined for the first time in human history on July 9, 1979.
What we saw on such pictures was absolutely astonishing. Voyager 1 obtained
excellent imagery of the other three Galilean satellites of Jupiter. But not Europa. It was left
for Voyager 2 to acquire the first close-up pictures of Europa, where we see things that are
only a few kilometers across. At first glance, the place looks like nothing so much as thecanal network that Percival Lowell imagined to adorn Mars, and that, we now know fromspace vehicle exploration, does not exist at all. We see on Europa an amazing, intricate
network of intersecting straight and curved lines. Are they ridges - that is, raised? Are they
troughs - that is, depressed? How are they made? Are they part of a global tectonic system,
produced perhaps by fracturing of an expanding or contracting planet? Are they connectedwith plate tectonics on the Earth? What light do they shed on the other satellites of the
Jovian system? At the moment of discovery, the vaunted technology has produced
something astonishing. But it remains for another device, the human brain, to figure it out.
Europa turns out to be as smooth as a billiard ball despite the network of lineations. The
absence of impact craters may be due to the heating and flow of surface ice upon impact.
The lines are grooves or cracks, their origin still being debated long after the mission.
If the Voyager missions were manned, the captain would keep a ship’s log, and thelog, a combination of the events of Voyagers 1 and 2, might read something like this:Day 1 After much concern about provisions and instruments, which seemed to be
malfunctioning, we successfully lifted off from Cape Canaveral on our long journeyto the planets and the stars.
Day 2 A problem in the deployment of the boom that supports the science scan
platform. If the problem is not solved, we will lose most of our pictures and otherscientific data.
Day 13 We have looked back and taken the first photograph ever obtained of the
Earth and Moon as worlds together in space. A pretty pair.
Day 150 Engines fired nominally for a mid-course trajectory correction.
Day 170 Routine housekeeping functions. An uneventful few months.
Day 185 Successful calibration images taken of Jupiter.
Day 207 Boom problem solved, but failure of main radio transmitter. We have
moved to back-up transmitter. If it fails, no one on Earth will ever hear from us
again.
Day 215 We cross the orbit of Mars. The planet itself is on the other side of the Sun.
Day 295 We enter the asteroid belt. There are many large, tumbling boulders here,
the shoals and reefs of space. Most of them are uncharted. Lookouts posted. We
hope to avoid a collision.
Day 475 We safely emerge from the main asteroid belt, happy to have survived.

Day 570 Jupiter is becoming prominent in the sky. We can now make out finer
detail on it than the largest telescopes on Earth have ever obtained.
Day 615 The colossal weather systems and changing clouds of Jupiter, spinning inspace before us, have us hypnotized. The planet is immense. It is more than twice as
massive as all the other planets put together. There are no mountains, valleys,
volcanoes, rivers; no boundaries between land and air; just a vast ocean of dense gas
and floating clouds - a world without a surface. Everything we can see on Jupiter is
floating in its sky.
Day 630 The weather on Jupiter continues to be spectacular. This ponderous world
spins on its axis in less than ten hours. Its atmospheric motions are driven by the
rapid rotation, by sunlight and by the heat bubbling and welling up from its interior.
Day 640 The cloud patterns are distinctive and gorgeous. They remind us a little of
Van Gogh’s Starry Night, or works by William Blake or Edvard Munch. But only alittle. No artist ever painted like this because none of them ever left our planet. No
painter trapped on Earth ever imagined a world so strange and lovely.
We observe the multicolored belts and bands of Jupiter close up. The whitebands are thought to be high clouds, probably ammonia crystals; the brownishcolored belts, deeper and hotter places where the atmosphere is sinking. The blue
places are apparently deep holes in the overlying clouds through which we see clearsky.
We do not know the reason for the reddish-brown color of Jupiter. Perhaps itis due to the chemistry of phosphorus or sulfur. Perhaps it is due to complex
brightly colored organic molecules produced when ultraviolet light from the Sun
breaks down the methane, ammonia, and water in the Jovian atmosphere and themolecular fragments recombine. In that case, the colors of Jupiter speak to us ofchemical events that four billion years ago back on Earth led to the origin of life.
Day 647 The Great Red Spot. A great column of gas reaching high above theadjacent clouds, so large that it could hold half a dozen Earths. Perhaps it is red
because it is carrying up to view the complex molecules produced or concentrated at
greater depth. It may be a great storm system a million years old.
Day 650 Encounter. A day of wonders. We successfully negotiate the treacherous
radiation belts of Jupiter with only one instrument, the photopolarimeter, damaged.
We accomplish the ring plane crossing and suffer no collisions with the particles
and boulders of the newly discovered rings of Jupiter. And wonderful images ofAmalthea, a tiny, red, oblong world that lives in the heart of the radiation belt; ofmulticolored Io; of the linear markings on Europa; the cobwebby features ofGanymede; the great multi-ringed basin on Callisto. We round Callisto and pass theorbit of Jupiter 13, the outermost of the planet’s known moons. We are outward
bound.
Day 662 Our particle and field detectors indicate that we have left the Jovian

radiation belts. The planet’s gravity has boosted our speed. We are free of Jupiter atlast and sail again the sea of space.
Day 874 A loss of the ship’s lock on the star Canopus - in the lore of constellations
the rudder of a sailing vessel. It is our rudder too, essential for the ship’s orientationin the dark of space, to find our way through this unexplored part of the cosmic
ocean. Canopus lock reacquired. The optical sensors seem to have mistaken Alphaand Beta Centauri for Canopus. Next port of call, two years hence: the Saturnsystem.
Of all the travelers’ tales returned by Voyager, my favorites concern the discoveries
made on the innermost Galilean satellite, Io.* Before Voyager, we were aware ofsomething strange about Io. We could resolve few features on its surface, but we knew it
was red - extremely red, redder than Mars, perhaps the reddest object in the solar system.
Over a period of years something seemed to be changing on it, in infrared light and perhaps
in its radar reflection properties. We also know that partially surrounding Jupiter in theorbital position of Io was a great doughnut-shaped tube of atoms, sulfur and sodium andpotassium, material somehow lost from Io.
* Frequently pronounced ‘eye-oh’ by Americans, because this is the preferred enunciation in the
Oxford English Dictionary. But the British have no special wisdom here. The word is of Eastern
Mediterranean origin and is pronounced throughout the rest of Europe, correctly, as ‘ee-oh.’

When Voyager approached this giant moon we found a strange multicolored surface
unlike any other in the solar system. Io is near the asteroid belt. It must have beenthoroughly pummeled throughout its history by falling boulders. Impact craters must havebeen made. Yet there were none to be seen. Accordingly, there had to be some process on
Io that was extremely efficient in rubbing craters out or filling them in. The process could
not be atmospheric, since Io’s atmosphere has mostly escaped to space because of its low
gravity. It could not be running water; Io’s surface is far too cold. There were a few places
that resembled the summits of volcanoes. But it was hard to be sure.
Linda Morabito, a member of the Voyager Navigation Team responsible forkeeping Voyager precisely on its trajectory, was routinely ordering a computer to enhance
an image of the edge of Io, to bring out the stars behind it. To her astonishment, she saw abright plume standing off in the darkness from the satellite’s surface and soon determinedthat the plume was in exactly the position of one of the suspected volcanoes. Voyager had
discovered the first active volcano beyond the Earth. We know now of nine large
volcanoes, spewing out gas and debris, and hundreds - perhaps thousands - of extinct
volcanoes on Io. The debris, rolling and flowing down the sides of the volcanic mountains,arching in great jets over the polychrome landscape, is more than enough to cover the
impact craters. We are looking at a fresh planetary landscape, a surface newly hatched.
How Galileo and Huygens would have marveled.
The volcanoes of Io were predicted, before they were discovered, by Stanton Pealeand his co-workers, who calculated the tides that would be raised in the solid interior of Io
by the combined pulls of the nearby moon Europa and the giant planet Jupiter. They foundthat the rocks inside Io should have been melted, not by radioactivity but by tides; thatmuch of the interior of Io should be liquid. It now seems likely that the volcanoes of Io aretapping an underground ocean of liquid sulfur, melted and concentrated near the surface.
When solid sulfur is heated a little past the normal boiling point of water, to about 115°C, it
melts and changes color. The higher the temperature, the deeper the color. If the molten
sulfur is quickly cooled, it retains its color. The pattern of colors that we see on Io
resembles closely what we would expect if rivers and torrents and sheets of molten sulfur
were pouring out of the mouths of the volcanoes: black sulfur, the hottest, near the top of
the volcano; red and orange, including the rivers, nearby; and great plains covered byyellow sulfur at a greater remove. The surface of Io is changing on a time scale of months.
Maps will have to be issued regularly, like weather reports on Earth. Those future explorers
on Io will have to keep their wits about them.
The very thin and tenuous atmosphere of Io was found by Voyager to be composed
mainly of sulfur dioxide. But this thin atmosphere can serve a useful purpose, because it
may be just thick enough to protect the surface from the intense charged particles in the
Jupiter radiation belt in which Io is embedded. At night the temperature drops so low thatthe sulfur dioxide should condense out as a kind of white frost; the charged particles would
then immolate the surface, and it would probably be wise to spend the nights just slightly
underground.
The great volcanic plumes of Io reach so high that they are close to injecting theiratoms directly into the space around Jupiter. The volcanoes are the probable source of thegreat doughnut-shaped ring of atoms that surrounds Jupiter in the position of Io’s orbit.
These atoms, gradually spiraling in toward Jupiter, should coat the inner moon Amalthea
and may be responsible for its reddish coloration. It is even possible that the material
outgassed from Io contributes, after many collisions and condensations, to the ring system
of Jupiter.
A substantial human presence on Jupiter itself is much more difficult to imagine although I suppose great balloon cities permanently floating in its atmosphere are atechnological possibility for the remote future. As seen from the near sides of Io or Europa,that immense and variable world fills much of the sky, hanging aloft, never to rise or set,because almost every satellite in the solar system keeps a constant face to its planet, as theMoon does to the Earth. Jupiter will be a source of continuing provocation and excitement
for the future human explorers of the Jovian moons.
As the solar system condensed out of interstellar gas and dust, Jupiter acquired mostof the matter that was not ejected into interstellar space and did not fall inward to form theSun. Had Jupiter been several dozen times more massive, the matter in its interior would
have undergone thermonuclear reactions, and Jupiter would have begun to shine by its own
light. The largest planet is a star that failed. Even so, its interior temperatures are
sufficiently high that it gives off about twice as much energy as it receives from the Sun. In
the infrared part of the spectrum, it might even be correct to consider Jupiter a star. Had it
become a star in visible light, we would today inhabit a binary or double-star system, with
two suns in our sky, and the nights would come more rarely - a commonplace, I believe, incountless solar systems throughout the Milky Way Galaxy. We would doubtless think thecircumstances natural and lovely.
Deep below the clouds of Jupiter the weight of the overlying layers of atmosphere
produces pressures much higher than any found on Earth, pressures so great that electrons
are squeezed off hydrogen atoms, producing a remarkable substance, liquid metallic
hydrogen - a physical state that has never been observed in terrestrial laboratories, because
the requisite pressures have never been achieved on Earth. (There is some hope thatmetallic hydrogen is a superconductor at moderate temperatures. If it could bemanufactured on Earth, it would work a revolution in electronics.) In the interior of Jupiter,where the pressures are about three million times the atmospheric pressure at the surface of
the Earth, there is almost nothing but a great dark sloshing ocean of metallic hydrogen. But
at the very core of Jupiter there may be a lump of rock and iron, an Earth-like world in apressure vise, hidden forever at the center of the largest planet.
The electrical currents in the liquid metal interior of Jupiter may be the source of theplanet’s enormous magnetic field, the largest in the solar system, and of its associated beltof trapped electrons and protons. These charged particles are ejected from the Sun in thesolar wind and captured and accelerated by Jupiter’s magnetic field. Vast numbers of them
are trapped far above the clouds and are condemned to bounce from pole to pole until bychance they encounter some high-altitude atmospheric molecule and are removed from theradiation belt. Io moves in an orbit so close to Jupiter that it plows through the midst of thisintense radiation, creating cascades of charged particles, which in turn generate violent
bursts of radio energy. (They may also influence eruptive processes on the surface of Io.) It
is possible to predict radio bursts from Jupiter with better reliability than weather forecasts
on Earth, by computing the position of Io.
That Jupiter is a source of radio emission was discovered accidentally in the 1950’s,
the early days of radio astronomy. Two young Americans, Bernard Burke and Kenneth
Franklin, were examining the sky with a newly constructed and for that time very sensitive
radio telescope. They were searching the cosmic radio background - that is, radio sources
far beyond our solar system. To their surprise, they found an intense and previously
unreported source that seemed to correspond to no prominent star, nebula or galaxy. Whatis more, it gradually moved, with respect to the distant stars, much faster than any remoteobject could.* After finding no likely explanation of all this in their charts of the distantCosmos, they one day stepped outside the observatory and looked up at the sky with the
naked eye to see if anything interesting happened to be there. Bemusedly they noted an
exceptionally bright object in the right place, which they soon identified as the planet
Jupiter. This accidental discovery is, incidentally, entirely typical of the history of science.
* Because the speed of light is finite (see Chapter 8).

Every evening before Voyager 1’s encounter with Jupiter, I could see that giant
planet twinkling in the sky, a sight our ancestors have enjoyed and wondered at for amillion years. And on the evening of Encounter, on my way to study the Voyager data
arriving at JPL, I thought that Jupiter would never be the same, never again just a point oflight in the night sky, but would forever after be a place to be explored and known. Jupiterand its moons are a kind of miniature solar system of diverse and exquisite worlds withmuch to teach us.
In composition and in many other respects Saturn is similar to Jupiter, althoughsmaller. Rotating once every ten hours, it exhibits colorful equatorial banding, which is,
however, not so prominent as Jupiter’s. It has a weaker magnetic field and radiation belt
than Jupiter and a more spectacular set of circumplanetary rings. And it also is surrounded
by a dozen or more satellites.
The most interesting of the moons of Saturn seems to be Titan, the largest moon in
the solar system and the only one with a substantial atmosphere. Prior to the encounter of
Voyager 1 with Titan in November 1980, our information about Titan was scanty andtantalizing. The only gas known unambiguously to be present was methane, CH4,
discovered by G. P. Kuiper. Ultraviolet light from the sun converts methane to more
complex hydrocarbon molecules and hydrogen gas. The hydrocarbons should remain on
Titan, covering the surface with a brownish tarry organic sludge, something like that
produced in experiments on the origin of life on Earth. The lightweight hydrogen gas
should, because of Titan’s low gravity, rapidly escape to space by a violent process known
as ‘blowoff,’ which should carry the methane and other atmospheric constituents with it.
But Titan has an atmospheric pressure at least as great as that of the planet Mars. Blowoff
does not seem to be happening. Perhaps there is some major and as yet undiscovered
atmospheric constituent - nitrogen, for example - which keeps the average molecularweight of the atmosphere high and prevents blowoff. Or perhaps blowoff is happening, but
the gases lost to space are being replenished by others released from the satellite’s interior.
The bulk density of Titan is so low that there must be a vast supply of water and other ices,
probably including methane, which are at unknown rates being released to the surface byinternal heating.
When we examine Titan through the telescope we see a barely perceptible reddish
disc. Some observers have reported variable white clouds above that disc - most likely,
clouds of methane crystals. But what is responsible for the reddish coloration? Most
students of Titan agree that complex organic molecules are the most likely explanation. The
surface temperature and atmospheric thickness are still under debate. There have been somehints of an enhanced surface temperature due to an atmospheric greenhouse effect. Withabundant organic molecules on its surface and in its atmosphere, Titan is a remarkable andunique denizen of the solar system. The history of our past voyages of discovery suggeststhat Voyager and other spacecraft reconnaissance missions will revolutionize ourknowledge of this place.
Through a break in the clouds of Titan, you might glimpse Saturn and its rings, theirpale yellow color diffused by the intervening atmosphere. Because the Saturn system is ten
times farther from the Sun than is the Earth, the sunshine on Titan is only 1 percent as
intense as we are accustomed to, and the temperatures should be far below the freezing
point of water even with a sizable atmospheric greenhouse effect. But with abundantorganic matter, sunlight and perhaps volcanic hot spots, the possibility of life on Titan*cannot be readily dismissed. In that very different environment, it would, of course, have tobe very different from life on Earth. There is no strong evidence either for or against life on
Titan. It is merely possible. We are unlikely to determine the answer to this question
without landing instrumented space vehicles on the Titanian surface.
* The view of Huygens, who discovered Titan in 1655, was: ‘Now can any one look upon, andcompare these Systems [of Jupiter and Saturn] together, without being amazed at the vast Magnitude andnoble Attendants of these two Planets, in respect of this little pitiful Earth of ours? Or can they force
themselves to think, that the wise Creator has disposed of all his Animals and Plants here, has furnished andadorn’d this Spot only, and has left all those Worlds bare and destitute of Inhabitants, who might adore andworship Him; or that all those prodigious Bodies were made only to twinkle to, and be studied by some few
perhaps of us poor Fellows?’ Since Saturn moves around the Sun once every thirty years, the length of theseasons on Saturn and its moons is much longer than on Earth. Of the presumed inhabitants of the moons ofSaturn, Huygens therefore wrote: ‘It is impossible but that their way of living must be very different fromours, having such tedious Winters.’

To examine the individual particles composing the rings of Saturn, we must

approach them closely, for the particles are small - snowballs and ice chips and tiny
tumbling bonsai glaciers, a meter or so across. We know they are composed of water ice,
because the spectral properties of sunlight reflected off the rings match those of ice in thelaboratory measurements. To approach the particles in a space vehicle, we must slow down,
so that we move along with them as they circle Saturn at some 45,000 miles per hour; thatis, we must be in orbit around Saturn ourselves, moving at the same speed as the particles.
Only then will we be able to see them individually and not as smears or streaks.
Why is there not a single large satellite instead of a ring system around Saturn? The
closer a ring particle is to Saturn, the faster its orbital speed (the faster it is ‘falling’ aroundthe planet - Kepler’s third law); the inner particles are streaming past the outer ones (the‘passing lane’ as we see it is always to the left). Although the whole assemblage is tearing
around the planet itself at some 20 kilometers per second, the relative speed of two adjacentparticles is very low, only some few centimeters per minute. Because of this relative
motion, the particles can never stick together by their mutual gravity. As soon as they try,
their slightly different orbital speeds pull them apart. If the rings were not so close to
Saturn, this effect would not be so strong, and the particles could accrete, making small
snowballs and eventually growing into satellites. So it is probably no coincidence thatoutside the rings of Saturn there is a system of satellites varying in size from a few hundred
kilometers across to Titan, a giant moon nearly as large as the planet Mars. The matter inall the satellites and the planets themselves may have been originally distributed in the formof rings, which condensed and accumulated to form the present moons and planets.
For Saturn as for Jupiter, the magnetic field captures and accelerates the charged
particles of the solar wind. When a charged particle bounces from one magnetic pole to theother, it must cross the equatorial plane of Saturn. If there is a ring particle in the way, theproton or electron is absorbed by this small snowball. As a result, for both planets, the rings
clear out the radiation belts, which exist only interior and exterior to the particle rings. A
close moon of Jupiter or Saturn will likewise gobble up radiation belt particles, and in fact
one of the new moons of Saturn was discovered in just this way: Pioneer 11 found an
unexpected gap in the radiation belts, caused by the sweeping up of charged particles by apreviously unknown moon.
The solar wind trickles into the outer solar system far beyond the orbit of Saturn.
When Voyager reaches Uranus and the orbits of Neptune and Pluto, if the instruments arestill functioning, they will almost certainly sense its presence, the wind between the worlds,
the top of the Sun’s atmosphere blown outward toward the realm of the stars. Some two or
three times farther from the Sun than Pluto is, the pressure of the interstellar protons andelectrons becomes greater than the minuscule pressure there exerted by the solar wind. That
place, called the heliopause, is one definition of the outer boundary of the Empire of theSun. But the Voyager spacecraft will plunge on, penetrating the heliopause sometime in the
middle of the twenty-first century, skimming through the ocean of space, never to enter
another solar system, destined to wander through eternity far from the stellar islands and tocomplete its first circumnavigation of the massive center of the Milky Way a few hundred
million years from now. We have embarked on epic voyages.

CHAPTER VII
The Backbone of Night
They came to a round hole in the sky . . . glowing like fire. This, the Raven said,
was a star.
- Eskimo creation myth
I would rather understand one cause than be King of Persia.
- Democritus of Abdera
Bur Aristarchus of Samos brought out a book consisting of some hypotheses, inwhich the premises lead to the result that the universe is many times greater thanthat now so called. His hypotheses are that the fixed stars and the Sun remain
unmoved, that the Earth revolves about the Sun in the circumference of a circle, theSun lying in the middle of the orbit, and that the sphere of the fixed stars, situatedabout the same center as the Sun, is so great that the circle in which he supposes the
Earth to revolve bears such a proportion to the distance of the fixed stars as thecenter of the sphere bears to its surface.
- Archimedes, The Sand Reckoner
If a faithful account was rendered of Man’s ideas upon Divinity, he would be
obliged to acknowledge, that for the most part the word ‘gods’ has been used toexpress the concealed, remote, unknown causes of the effects he witnessed; that heapplies this term when the spring of the natural, the source of known causes, ceases
to be visible: as soon as he loses the thread of these causes, or as soon as his mind
can no longer follow the chain, he solves the difficulty, terminates his research, byascribing it to his gods . . . When, therefore, he ascribes to his gods the productionof some phenomenon . . . does he, in fact, do any thing more than substitute for the
darkness of his own mind, a sound to which he has been accustomed to listen withreverential awe?
- Paul Heinrich Dietrich, Baron von Holbach, Système de la Nature, London, 1770
When I was little, I lived in the Bensonhurst section of Brooklyn in the City of New York. I
knew my immediate neighborhood intimately, every apartment building, pigeon coop,backyard, front stoop, empty lot, elm tree, ornamental railing, coal chute and wall forplaying Chinese handball, among which the brick exterior of a theater called the Loew’s
Stillwell was of superior quality. I knew where many people lived: Bruno and Dino, Ronald
and Harvey, Sandy, Bernie, Danny, Jackie and Myra. But more than a few blocks away,
north of the raucous automobile traffic and elevated railway on 86th Street, was a strange
unknown territory, off-limits to my wanderings. It could have been Mars for all I knew.
Even with an early bedtime, in winter you could sometimes see the stars. I would
look at them, twinkling and remote, and wonder what they were. I would ask older children
and adults, who would only reply, ‘They’re lights in the sky, kid.’ I could see they werelights in the sky. But what were they? Just small hovering lamps? Whatever for? I felt akind of sorrow for them: a commonplace whose strangeness remained somehow hidden

from my incurious fellows. There had to be some deeper answer.
As soon as I was old enough, my parents gave me my first library card. I think thelibrary was on 85th Street, an alien land. Immediately, I asked the librarian for something
on stars. She returned with a picture book displaying portraits of men and women withnames like Clark Gable and Jean Harlow. I complained, and for some reason then obscure
to me, she smiled and found another book - the right kind of book. I opened it breathlessly
and read until I found it. The book said something astonishing, a very big thought. It said
that the stars were suns, only very far away. The Sun was a star, but close up.
Imagine that you took the Sun and moved it so far away that it was just a tiny
twinkling point of light. How far away would you have to move it? I was innocent of thenotion of angular size. I was ignorant of the inverse square law for light propagation. I had
not a ghost of a chance of calculating the distance to the stars. But I could tell that if thestars were suns, they had to be very far away - farther away than 85th Street, farther away
than Manhattan, farther away, probably, than New Jersey. The Cosmos was much bigger
than I had guessed.
Later I read another astonishing fact. The Earth, which includes Brooklyn, is aplanet, and it goes around the Sun. There are other planets. They also go around the Sun;
some are closer to it and some are farther away. But the planets do not shine by their own
light, as the Sun does. They merely reflect light from the Sun. If you were a great distanceaway, you would not see the Earth and the other planets at all; they would be only faintluminous points, lost in the glare of the Sun. Well, then, I thought, it stood to reason thatthe other stars must have planets too, ones we have not yet detected, and some of thoseother planets should have life (why not?), a kind of life probably different from life as weknow it, life in Brooklyn. So I decided I would be an astronomer, learn about the stars andplanets and, if I could, go and visit them.
It has been my immense good fortune to have parents and some teachers who
encouraged this odd ambition and to live in this time, the first moment in human history
when we are, in fact, visiting other worlds and engaging in a deep reconnaissance of the
Cosmos. If I had been born in a much earlier age, no matter how great my dedication, I
would not have understood what the stars and planets are. I would not have known that
there were other suns and other worlds. This is one of the great secrets, wrested fromNature through a million years of patient observation and courageous thinking by ourancestors.
What are the stars? Such questions are as natural as an infant’s smile. We havealways asked them. What is different about our time is that at last we know some of theanswers. Books and libraries provide a ready means for finding out what those answers are.
In biology there is a principle of powerful if imperfect applicability called recapitulation: in
our individual embryonic development we retrace the evolutionary history of the species.
There is, I think, a kind of recapitulation that occurs in our individual intellectual
developments as well. We unconsciously retrace the thoughts of our remote ancestors.
Imagine a time before science, a time before libraries. Imagine a time hundreds ofthousands of years ago. We were then just about as smart, just as curious, just as involvedin things social and sexual. But the experiments had not yet been done, the inventions had
not yet been made. It was the childhood of genus Homo. Imagine the time when fire was
first discovered. What were human lives like then? What did out ancestors believe the stars
were? Sometimes, in my fantasies, I imagine there was someone who thought like this:We eat berries and roots. Nuts and leaves. And dead animals. Some animals wefind. Some we kill. We know which foods are good and which are dangerous. If we taste
some foods we are struck down, in punishment for eating them. We did not mean to dosomething bad. But foxglove or hemlock can kill you. We love our children and our friends.
We warn them of such foods.
When we hunt animals, then also can we be killed. We can be gored. Or trampled.
Or eaten. What animals do means life and death for us: how they behave, what tracks theyleave, their times for mating and giving birth, their times for wandering. We must know
these things. We tell our children. They will tell their children.
We depend on animals. We follow them - especially in winter when there are few
plants to eat. We are wandering hunters and gatherers. We call ourselves the hunterfolk.
Most of us fall asleep under the sky or under a tree or in its branches. We use
animal skins for clothing: to keep us warm, to cover our nakedness and sometimes as ahammock. When we wear the animal skins we feel the animal’s power. We leap with thegazelle. We hunt with the bear. There is a bond between us and the animals. We hunt andeat the animals. They hunt and eat us. We are part of one another.
We make tools and stay alive. Some of us are experts at splitting, flaking,sharpening and polishing, as well as finding, rocks. Some rocks we tie with animal sinew toa wooden handle and make an ax. With the ax we strike plants and animals. Other rocks
are tied to long sticks. If we are quiet and watchful, we can sometimes come close to an
animal and stick it with the spear.
Meat spoils. Sometimes we are hungry and try not to notice. Sometimes we mix
herbs with the bad meat to hide the taste. We fold foods that will not spoil into pieces ofanimal skin. Or big leaves. Or the shell of a large nut. It is wise to put food aside and carryit. If we eat this food too early, some of us will starve later. So we must help one another.
For this and many other reasons we have rules. Everyone must obey the rules. We havealways had rules. Rules are sacred.
One day there was a storm, with much lightning and thunder and rain. The little
ones are afraid of storms. And sometimes so am I. The secret of the storm is hidden. The
thunder is deep and loud; the lightning is brief and bright. Maybe someone very powerful isvery angry. It must be someone in the sky, I think.
After the storm there was a flickering and crackling in the forest nearby. We went tosee. There was a bright, hot, leaping thing, yellow and red. We had never seen such a thing
before. We now call it ‘flame’. It has a special smell. In a way it is alive: It eats food. It eats
plants and tree limbs and even whole trees, if you let it. It is strong. But it is not very smart.
If all the food is gone, it dies. It will not walk a spear’s throw from one tree to another if
there a no food along the way. It cannot walk without eating. But where there is much food,it grows and makes many flame children.
One of us had a brave and fearful thought: to capture the flame, feed it a little, andmake it our friend. We found some long branches of hard wood. The flame was eating them,but slowly. We could pick them up by the end that had no flame. If you run fast with a small
flame, it dies. Their children are weak. We did not run. We walked, shouting good wishes.
‘Do not die,’ we said to the flame. The other hunterfolk looked with wide eyes.
Ever after, we have carried it with us. We have a flame mother to feed the flame
slowly so it does not die of hunger.* Flame is a wonder, and useful too; surely a gift frompowerful beings. Are they the same as the angry beings in the storm?
* This sense of fire as a living thing, to be protected and cared for, should not be dismissed as a‘primitive’ notion. It is to be found near the root of many modern civilizations. Every home in ancient Greece
and Rome and among the Brahmans of ancient India had a hearth and a set of prescribed rules for caring forthe flame. At night the coals were covered with ashes for insulation; in the morning twigs were added torevive the flame. The death of the flame in the hearth was considered synonymous with the death of thefamily. In all three cultures, the hearth ritual was connected with the worship of ancestors. This is the originof the eternal flame, a symbol still widely employed in religious, memorial, political and athletic ceremonials
throughout the world.

The flame keeps us warm on cold nights. It gives us light. It makes holes in thedarkness when the Moon is new. We can fix spears at night for tomorrow’s hunt. And if weare not tired, even in the darkness we can see each other and talk. Also - a good thing! fire keeps animals away. We can be hurt at night. Sometimes we have been eaten, even bysmall animals, hyenas and wolves. Now it is different. Now the flame keeps the animals
back. We see them baying softly in the dark, prowling, their eyes glowing in the light of theflame. They are frightened of the flame. But we are not frightened. The flame is ours. We
take care of the flame. The flame takes care of us.
The sky is important. It covers us. It speaks to us. Before the time we found theflame, we would lie back in the dark and look up at all the points of light. Some points
would come together to make a picture in the sky. One of us could see the pictures better
than the rest. She taught us the star pictures and what names to call them. We would sitaround late at night and make up stories about the pictures in the sky: lions, dogs, bears,
hunterfolk. Other, stranger things. Could they be the pictures of the powerful beings in thesky, the ones who make the storms when angry?
Mostly, the sky does not change. The same star pictures are there year after year.
The Moon grows from nothing to a thin sliver to a round ball, and then back again tonothing. When the Moon changes, the women bleed. Some tribes have rules against sex at
certain times in the growing and shrinking of the Moon. Some tribes scratch the days of theMoon or the days that the women bleed on antler bones. They can plan ahead and obey
their rules. Rules are sacred.
The stars are very far away. When we climb a hill or a tree they are no closer. And
clouds come between us and the stars: the stars must be behind the clouds. The Moon, as it
slowly moves, passes in front of stars. Later you can see that the stars are not harmed. The
Moon does not eat stars. The stars must be behind the Moon. They flicker. A strange, cold,
white, faraway light. Many of them. All over the sky. But only at night. I wonder what theyare.
After we found the flame, I was sitting near the campfire wondering about the stars.
Slowly a thought came: The stars are flame, I thought. Then I had another thought: Thestars are campfires that other hunterfolk light at night. The stars give a smaller light thancampfires. So the stars must be campfires very far away. ‘But,’ they ask me, ‘how can there
be campfires in the sky? Why do the campfires and the hunter people around those flames
not fall down at our feet? Why don’t strange tribes drop from the sky?’Those are good questions. They trouble me. Sometimes I think the sky is half of a
big eggshell or a big nutshell. I think the people around those faraway campfires look downat us - except for them it seems up - and say that we are in their sky, and wonder why we do
not fall up to them, if you see what I mean. But hunterfolk say, ‘Down is down and up is
up.’ That is a good answer, too.
There is another thought that one of us had. His thought is that night is a great
black animal skin, thrown up over the sky. There are holes in the skin. We look through theholes. And we see flame. His thought is not just that there is flame in a few places where wesee stars. He thinks there is flame everywhere. He thinks flame covers the whole sky. But
the skin hides the flame. Except where there are holes.
Some stars wander. Like the animals we hunt. Like us. If you watch with care overmany months, you find they move. There are only five of them, like the fingers on a hand.
They wander slowly among the stars. If the campfire thought is true, those stars must betribes of wandering hunterfolk, carrying big fires. But I don’t see how wandering stars canbe holes in a skin. When you make a hole, there it is. A hole is a hole. Holes do not wander.
Also, I don’t want to be surrounded by a sky of flame. If the skin fell, the night sky would bebright - too bright - like seeing flame everywhere. I think a sky of flame would eat us all.
Maybe there are two kinds of powerful beings in the sky. Bad ones, who wish the flame toeat us. And good ones who put up the skin to keep the flame away. We must find some way
to thank the good ones.
I don’t know if the stars are campfires in the sky. Or holes in a skin through which
the flame of power looks down on us. Sometimes I think one way. Sometimes 1 think adifferent way. Once I thought there are no campfires and no holes but something else, too
hard for me to understand.
Rest your neck on a log. Your head goes back. Then you can see only the sky. No
hills, no trees, no hunterfolk, no campfire. Just sky. Sometimes I feel I may fall up into thesky. If the stars are campfires, I would like to visit those other hunterfolk - the ones who
wander. Then I feel good about falling up. But if the stars are holes in a skin, I become
afraid. 1 don’t want to fall up through a hole and into the flame of power.
I wish I knew which was true. I don’t like not knowing.
I do not imagine that many members of a hunter/gatherer group had thoughts likethese about the stars. Perhaps, over the ages, a few did, but never all these thoughts in thesame person. Yet, sophisticated ideas are common in such communities. For example, the!Kung* Bushmen of the Kalahari Desert in Botswana have an explanation for the MilkyWay, which at their latitude is often overhead. They call it ‘the backbone of night,’ as if thesky were some great beast inside which we live. Their explanation makes the Milky Way
useful as well as understandable. The !Kung believe the Milky Way holds up the night; that
if it were not for the Milky Way, fragments of darkness would come crashing down at ourfeet. It is an elegant idea.
* The exclamation point is a click, made by touching the tongue against the inside of the incisors,and simultaneously pronouncing the K.

Metaphors like those about celestial campfires or galactic backbones were
eventually replaced in most human cultures by another idea: The powerful beings in the sky
were promoted to gods. They were given names and relatives, and special responsibilities
for the cosmic services they were expected to perform. There was a god or goddess forevery human concern. Gods ran Nature. Nothing could happen without their directintervention. If they were happy, there was plenty of food, and humans were happy. But if
something displeased the gods - and sometimes it took very little - the consequences were
awesome: droughts, storms, wars, earthquakes, volcanoes, epidemics. The gods had to bepropitiated, and a vast industry of priests and oracles arose to make the gods less angry. But
because the gods were capricious, you could not be sure what they would do. Nature was amystery. It was hard to understand the world.

Little remains of the Heraion on the Aegean isle of Samos, one of the wonders of
the ancient world, a great temple dedicated to Hera, who began her career as goddess of thesky. She was the patron deity of Samos, playing the same role there as Athena did in
Athens. Much later she married Zeus, the chief of the Olympian gods. They honeymooned
on Samos, the old stories tell us. The Greek religion explained that diffuse band of light in
the night sky as the milk of Hera, squirted from her breast across the heavens, a legend thatis the origin of the phrase Westerners still use - the Milky Way. Perhaps it originally
represented the important insight that the sky nurtures the Earth; if so, that meaning seems
to have been forgotten millennia ago.
We are, almost all of us, descended from people who responded to the dangers ofexistence by inventing stories about unpredictable or disgruntled deities. For a long time
the human instinct to understand was thwarted by facile religious explanations, as inancient Greece in the time of Homer, where there were gods of the sky and the Earth, thethunderstorm, the oceans and the underworld, fire and time and love and war; where every
tree and meadow had its dryad and maenad.
For thousands of years humans were oppressed - as some of us still are - by thenotion that the universe is a marionette whose strings are pulled by a god or gods, unseen
and inscrutable. Then, 2,500 years ago, there was a glorious awakening in Ionia: on Samosand the other nearby Greek colonies that grew up among the islands and inlets of the busy
eastern Aegean Sea.* Suddenly there were people who believed that everything was madeof atoms; that human beings and other animals had sprung from simpler forms; thatdiseases were not caused by demons or the gods; that the Earth was only a planet goingaround the Sun. And that the stars were very far away.
* As an aid to confusion, Ionia is not in the Ionian Sea; it was named by colonists from the coast of
the Ionian Sea.

This revolution made Cosmos and Chaos. The early Greeks had believed that thefirst being was Chaos, corresponding to the phrase in Genesis in the same context, ‘without
form’. Chaos created and then mated with a goddess called Night, and their offspring
eventually produced all the gods and men. A universe created from Chaos was in perfectkeeping with the Greek belief in an unpredictable Nature run by capricious gods. But in thesixth century B.C., in Ionia, a new concept developed, one of the great ideas of the humanspecies. The universe is knowable, the ancient Ionians argued, because it exhibits aninternal order: there are regularities in Nature that permit its secrets to be uncovered. Nature
is not entirely unpredictable; there are rules even she must obey. This ordered andadmirable character of the universe was called Cosmos.
But why Ionia, why in these unassuming and pastoral landscapes, these remoteislands and inlets of the Eastern Mediterranean? Why not in the great cities of India or
Egypt, Babylonia, China or Mesoamerica? China had an astronomical tradition millennia
old; it invented paper and printing, rockets, clocks, silk, porcelain, and ocean-going navies.
Some historians argue it was nevertheless too traditionalist a society, too unwilling to adopt
innovations. Why not India, an extremely rich, mathematically gifted culture? Because,some historians maintain, of a rigid fascination with the idea of an infinitely old universe
condemned to an endless cycle of deaths and rebirths, of souls and universes, in whichnothing fundamentally new could ever happen. Why not Mayan and Aztec societies, whichwere accomplished in astronomy and captivated, as the Indians were, by large numbers?

Because, some historians declare, they lacked the aptitude or impetus for mechanicalinvention. The Mayans and the Aztecs did not even - except for children’s toys - invent thewheel.
The Ionians had several advantages. Ionia is an island realm. Isolation, even if
incomplete, breeds diversity. With many different islands, there was a variety of politicalsystems. No single concentration of power could enforce social and intellectual conformity
in all the islands. Free inquiry became possible. The promotion of superstition was notconsidered a political necessity. Unlike many other cultures, the Ionians were at thecrossroads of civilizations, not at one of the centers. In Ionia, the Phoenician alphabet was
first adapted to Greek usage and widespread literacy became possible. Writing was nolonger a monopoly of the priests and scribes. The thoughts of many were available forconsideration and debate. Political power was in the hands of the merchants, who actively
promoted the technology on which their prosperity depended. It was in the Eastern
Mediterranean that African, Asian, and European civilizations, including the great culturesof Egypt and Mesopotamia, met and cross-fertilized in a vigorous and heady confrontationof prejudices, languages, ideas and gods. What do you do when you are faced with several
different gods each claiming the same territory? The Babylonian Marduk and the Greek
Zeus was each considered master of the sky and king of the gods. You might decide that
Marduk and Zeus were really the same. You might also decide, since they had quite
different attributes, that one of them was merely invented by the priests. But if one, why not
both?
And so it was that the great idea arose, the realization that there might be a way toknow the world without the god hypothesis; that there might be principles, forces, laws ofnature, through which the world could be understood without attributing the fall of every
sparrow to the direct intervention of Zeus.
China and India and Mesoamerica would, I think, have tumbled to science too, if
only they had been given a little more time. Cultures do not develop with identical rhythms
or evolve in lockstep. They arise at different times and progress at different rates. The
scientific world view works so well, explains so much and resonates so harmoniously withthe most advanced parts of our brains that in time, I think, virtually every culture on the
Earth, left to its own devices, would have discovered science. Some culture had to be first.
As it turned out, Ionia was the place where science was born.
Between 600 and 400 B.C., this great revolution in human thought began. The key
to the revolution was the hand. Some of the brilliant Ionian thinkers were the sons of sailors
and farmers and weavers. They were accustomed to poking and fixing, unlike the priests
and scribes of other nations, who, raised in luxury, were reluctant to dirty their hands. They
rejected superstition, and they worked wonders. In many cases we have only fragmentaryor secondhand accounts of what happened. The metaphors used then may be obscure to us
now. There was almost certainly a conscious effort a few centuries later to suppress the new
insights. The leading figures in this revolution were men with Greek names, largely
unfamiliar to us today, but the truest pioneers in the development of our civilization and ourhumanity.
The first Ionian scientist was Thales of Miletus, a city in Asia across a narrow
channel of water from the island of Samos. He had traveled in Egypt and was conversant
with the knowledge of Babylon. It is said that he predicted a solar eclipse. He learned how
to measure the height of a pyramid from the length of its shadow and the angle of the Sun
above the horizon, a method employed today to determine the heights of the mountains of

the Moon. He was the first to prove geometric theorems of the sort codified by Euclid threecenturies later - for example, the proposition that the angles at the base of an isosceles
triangle are equal. There is a clear continuity of intellectual effort from Thales to Euclid to
Isaac Newton’s purchase of the Elements of Geometry at Stourbridge Fair in 1663 (Chapter3), the event that precipitated modern science and technology.
Thales attempted to understand the world without invoking the intervention of thegods. Like the Babylonians, he believed the world to have once been water. To explain the
dry land, the Babylonians added that Marduk had placed a mat on the face of the waters andpiled dirt upon it.* Thales held a similar view, but, as Benjamin Farrington said, ‘left
Marduk out.’ Yes, everything was once water, but the Earth formed out of the oceans by anatural process - similar, he thought, to the silting he had observed at the delta of the Nile.
Indeed, he thought that water was a common principle underlying all of matter, just as
today we might say the same of electrons, protons and neutrons, or of quarks. Whether
Thales’ conclusion was correct is not as important as his approach: The world was notmade by the gods, but instead was the work of material forces interacting in Nature. Thales
brought back from Babylon and Egypt the seeds of the new sciences of astronomy andgeometry, sciences that would sprout and grow in the fertile soil of Ionia.
* There is some evidence that the antecedent, early Sumerian creation myths were largely naturalistic
explanations, later codified around 1000 B.C. in the Enuma elish (‘When on high,’ the first words of thepoem); but by then the gods had replaced Nature, and the myth offers a theogony, not a cosmogony. The
Enuma elish is reminiscent of the Japanese and Ainu myths in which an originally muddy cosmos is beaten by
the wings of a bird, separating the land from the water. A Fijian creation myth says: ‘Rokomautu created theland. He scooped it up out of the bottom of the ocean in great handfuls and accumulated it in piles here andthere. These are the Fiji Islands.’ The distillation of land from water is a natural enough idea for island andseafaring peoples.

Very little is known about the personal life of Thales, but one revealing anecdote istold by Aristotle in his Politics:[Thales] was reproached for his poverty, which was supposed to show thatphilosophy is of no use. According to the story, he knew by his skill [in interpretingthe heavens] while it was yet winter that there would be a great harvest of olives in
the coming year; so, having a little money, he gave deposits for the use of all theolive-presses in Chios and Miletus, which he hired at a low price because no one
bid against him. When the harvest time came, and many were wanted all at once, he
let them out at any rate which he pleased and made a quantity of money. Thus heshowed the world philosophers can easily be rich if they like, but that their ambition
is of another sort.
He was also famous as a political sage, successfully urging the Milesians to resist
assimilation by Croesus, King of Lydia, and unsuccessfully urging a federation of all theisland states of Ionia to oppose the Lydians.
Anaximander of Miletus was a friend and colleague of Thales, one of the firstpeople we know of to do an experiment. By examining the moving shadow cast by avertical stick he determined accurately the length of the year and the seasons. For ages men
had used sticks to club and spear one another. Anaximander used one to measure time. He
was the first person in Greece to make a sundial, a map of the known world and a celestialglobe that showed the patterns of the constellations. He believed the Sun, the Moon and thestars to be made of fire seen through moving holes in the dome of the sky, probably a mucholder idea. He held the remarkable view that the Earth is not suspended or supported fromthe heavens, but that it remains by itself at the center of the universe; since it was
equidistant from all places on the ‘celestial sphere,’ there was no force that could move it.
He argued that we are so helpless at birth that, if the first human infants had been
put into the world on their own, they would immediately have died. From thisAnaximander concluded that human beings arose from other animals with more self-reliant
newborns: He proposed the spontaneous origin of life in mud, the first animals being fishcovered with spines. Some descendants of these fishes eventually abandoned the water andmoved to dry land, where they evolved into other animals by the transmutation of one form
into another. He believed in an infinite number of worlds, all inhabited, and all subject tocycles of dissolution and regeneration. ‘Nor’, as Saint Augustine ruefully complained, ‘didhe, any more than Thales, attribute the cause of all this ceaseless activity to a divine mind.’
In the year 540 B.C. or thereabouts, on the island of Samos, there came to power atyrant named Polycrates. He seems to have started as a caterer and then gone on tointernational piracy. Polycrates was a generous patron of the arts, sciences and engineering.
But he oppressed his own people; he made war on his neighbors; he quite rightly feared
invasion. So he surrounded his capital city with a massive wall, about six kilometers long,whose remains stand to this day. To carry water from a distant spring through thefortifications, he ordered a great tunnel built. A kilometer long, it pierces a mountain. Two
cuttings were dug from either end which met almost perfectly in the middle. The project
took about fifteen years to complete, a testament to the civil engineering of the day and an
indication of the extraordinary practical capability of the Ionians. But there is another andmore ominous side to the enterprise: it was built in part by slaves in chains, many capturedby the pirate ships of Polycrates.
This was the time of Theodorus, the master engineer of the age, credited among the
Greeks with the invention of the key, the ruler, the carpenter’s square, the level, the lathe,bronze casting and central heating. Why are there no monuments to this man? Those who
dreamed and speculated about the laws of Nature talked with the technologists and theengineers. They were often the same people. The theoretical and the practical were one.
About the same time, on the nearby island of Cos, Hippocrates was establishing his
famous medical tradition, now barely remembered because of the Hippocratic oath. It was apractical and effective school of medicine, which Hippocrates insisted had to be based on
the contemporary equivalent of physics and chemistry.* But it also had its theoretical side.
In his book On Ancient Medicine, Hippocrates wrote: ‘Men think epilepsy divine, merely
because they do not understand it. But if they called everything divine which they do notunderstand, why, there would be no end of divine things.’* And astrology, which was then widely regarded as a science. In a typical passage, Hippocrates
writes: ‘One must also guard against the risings of the stars, especially of the Dog Star [Sirius], then of
Arcturus, and also of the setting of the Pleiades.’In time, the Ionian influence and the experimental method spread to the mainland of
Greece, to Italy, to Sicily. There was once a time when hardly anyone believed in air. They
knew about breathing, of course, and they thought the wind was the breath of the gods. But
the idea of air as a static, material but invisible substance was unimagined. The firstrecorded experiment on air was performed by a physician* named Empedocles, whoflourished around 450 B.C. Some accounts claim he identified himself as a god. But
perhaps it was only that he was so clever that others thought him a god. He believed thatlight travels very fast, but not infinitely fast. He taught that there was once a much greater
variety of living things on the Earth, but that many races of beings ‘must have been unable
to beget and continue their kind. For in the case of every species that exists, either craft or
courage or speed has from the beginning of its existence protected and preserved it.’ In thisattempt to explain the lovely adaptation of organisms to their environments, Empedocles,
like Anaximander and Democritus (see below), clearly anticipated some aspects of
Darwin’s great idea of evolution by natural selection.
* The experiment was performed in support of a totally erroneous theory of the circulation of theblood, but the idea of performing any experiment to probe Nature is the important innovation.

Empedocles performed his experiment with a household implement people had used
for centuries, the so-called clepsydra or ‘water thief’, which was used as a kitchen ladle. A
brazen sphere with an open neck and small holes in the bottom, it is filled by immersing it
in water. If you pull it out with the neck uncovered, the water pours out of the holes,
making a little shower. But if you pull it out properly, with your thumb covering the neck,
the water is retained within the sphere until you lift your thumb. If you try to fill it with theneck covered, nothing happens. Some material substance must be in the way of the water.
We cannot see such a substance. What could it be? Empedocles argued that it could only beair. A thing we cannot see can exert pressure, can frustrate my wish to fill a vessel withwater if I were dumb enough to leave my finger on the neck. Empedocles had discovered
the invisible. Air, he thought, must be matter in a form so finely divided that it could not be
seen.
Empedocles is said to have died in an apotheotic fit by leaping into the hot lava at
the summit caldera of the great volcano of Aetna. But I sometimes imagine that he merelyslipped during a courageous and pioneering venture in observational geophysics.
This hint, this whiff, of the existence of atoms was carried much further by a man
named Democritus, who came from the Ionian colony of Abdera in northern Greece.
Abdera was a kind of joke town. If in 430 B.C. you told a story about someone from
Abdera, you were guaranteed a laugh. It was in a way the Brooklyn of its time. For
Democritus all of life was to be enjoyed and understood; understanding and enjoyment
were the same thing. He said that ‘a life without festivity is a long road without an inn.’
Democritus may have come from Abdera, but he was no dummy. He believed that a large
number of worlds had formed spontaneously out of diffuse matter in space, evolved andthen decayed. At a time when no one knew about impact craters, Democritus thought thatworlds on occasion collide; he believed that some worlds wandered alone through the
darkness of space, while others were accompanied by several suns and moons; that someworlds were inhabited, while others had no plants or animals or even water; that thesimplest forms of life arose from a kind of primeval ooze. He taught that perception - thereason, say, I think there is a pen in my hand - was a purely physical and mechanistic
process; that thinking and feeling were attributes of matter put together in a sufficiently fineand complex way and not due to some spirit infused into matter by the gods.
Democritus invented the word atom, Greek for ‘unable to be cut.’ Atoms were theultimate particles, forever frustrating our attempts to break them into smaller pieces.

Everything, he said, is a collection of atoms, intricately assembled. Even we. ‘Nothing
exists,’ he said, ‘but atoms and the void.’
When we cut an apple, the knife must pass through empty spaces between theatoms, Democritus argued. If there were no such empty spaces, no void, the knife would
encounter the impenetrable atoms, and the apple could not be cut. Having cut a slice from acone, say, let us compare the cross sections of the two pieces. Are the exposed areas equal?
No, said Democritus. The slope of the cone forces one side of the slice to have a slightly
smaller cross section than the other. If the two areas were exactly equal, we would have acylinder, not a cone. No matter how sharp the knife, the two pieces have unequal crosssections. Why? Because, on the scale of the very small, matter exhibits some irreducibleroughness. This fine scale of roughness Democritus identified with the world of the atoms.
His arguments were not those we use today, but they were subtle and elegant, derived fromeveryday life. And his conclusions were fundamentally correct.
In related exercise. Democritus imagined calculating the volume of a cone or apyramid by a very large number of extremely small stacked plates tapering in size from thebase to the apex. He had stated the problem that, in mathematics, is called the theory oflimits. He was knocking at the door of the differential and integral calculus, that
fundamental tool for understanding the world that was not, so far as we know from written
records, in fact discovered until the time of Isaac Newton. Perhaps if Democritus’ work had
not been almost completely destroyed, there would have been calculus by the time ofChrist.** The frontiers of the calculus were also later breached by Eudoxus and Archimedes.

Thomas Wright marveled in 1750 that Democritus had believed the Milky Way to
be composed mainly of unresolved stars: ‘long before astronomy reaped any benefit fromthe improved sciences of optics; [he] saw, as we may say, through the eye of reason, full as
far into infinity as the most able astronomers in more advantageous times have done since.’
Beyond the Milk of Hera, past the Backbone of Night, the mind of Democritus soared.
As a person, Democritus seems to have been somewhat unusual. Women, children
and sex discomfited him, in part because they took time away from thinking. But he valued
friendship, held cheerfulness to be the goal of life and devoted a major philosophicalinquiry to the origin and nature of enthusiasm. He journeyed to Athens to visit Socrates andthen found himself too shy to introduce himself. He was a close friend of Hippocrates. He
was awed by the beauty and elegance of the physical world. He felt that poverty in a
democracy was preferable to wealth in a tyranny. He believed that the prevailing religionsof his time were evil and that neither immortal souls nor immortal gods exist: ‘Nothing
exists, but atoms and the void.’
There is no record of Democritus having been persecuted for his opinions - but then,he came from Abdera. However, in his time the brief tradition of tolerance forunconventional views began to erode and then to shatter. People came to be punished forhaving unusual ideas. A portrait of Democritus is now on the Greek hundred-drachma bill.
But his insights were suppressed, his influence on history made minor. The mystics werebeginning to win.
Anaxagoras was an Ionian experimentalist who flourished around 450 B.C. andlived in Athens. He was a rich man, indifferent to his wealth but passionate about science.
Asked what was the purpose of life, he replied, ‘the investigation of the Sun, the Moon, andthe heavens,’ the reply of a true astronomer. He performed a clever experiment in which asingle drop of white liquid, like cream, was shown not to lighten perceptibly the contents ofa great pitcher of dark liquid, like wine. There must, he concluded, be changes deducible byexperiment that are too subtle to be perceived directly by the senses.
Anaxagoras was not nearly so radical as Democritus. Both were thoroughgoingmaterialists, not in prizing possessions but in holding that matter alone provided theunderpinnings of the world. Anaxagoras believed in a special mind substance anddisbelieved in the existence of atoms. He thought humans were more intelligent than otheranimals because of our hands, a very Ionian idea.
He was the first person to state clearly that the Moon shines by reflected light, andhe accordingly devised a theory of the phases of the Moon. This doctrine was so dangerous
that the manuscript describing it had to be circulated in secret, an Athenian samizdat. It was
not in keeping with the prejudices of the time to explain the phases or eclipses of the Moonby the relative geometry of the Earth, the Moon and the self-luminous Sun. Aristotle, twogenerations later, was content to argue that those things happened because it was the natureof the Moon to have phases and eclipses - mere verbal juggling, an explanation that
explains nothing.
The prevailing belief was that the Sun and Moon were gods. Anaxagoras held that
the Sun and stars are fiery stones. We do not feel the heat of the stars because they are too
far away. He also thought that the Moon has mountains (right) and inhabitants (wrong). He
held that the Sun was so huge that it was probably larger than the Peloponnesus, roughly
the southern third of Greece. His critics thought this estimate excessive and absurd.
Anaxagoras was brought to Athens by Pericles, its leader in its time of greatest
glory, but also the man whose actions led to the Peloponnesian War, which destroyed
Athenian democracy. Pericles delighted in philosophy and science, and Anaxagoras was
one of his principal confidants. There are those who think that in this role Anaxagoras
contributed significantly to the greatness of Athens. But Pericles had political problems. He
was too powerful to be attacked directly, so his enemies attacked those close to him.
Anaxagoras was convicted and imprisoned for the religious crime of impiety - because he
had taught that the Moon was made of ordinary matter, that it was a place, and that the Sun
was a red-hot stone in the sky. Bishop John Wilkins commented in 1638 on these
Athenians: ‘Those zealous idolators [counted] it a great blasphemy to make their God astone, whereas notwithstanding they were so senseless in their adoration of idols as to makea stone their God.’ Pericles seems to have engineered Anaxagoras’ release from prison, but
it was too late. In Greece the tide was turning, although the Ionian tradition continued in
Alexandrian Egypt two hundred years later.
The great scientists from Thales to Democritus and Anaxagoras have usually been
described in history or philosophy books as ‘Presocratics’, as if their main function was tohold the philosophical fort until the advent of Socrates, Plato, and Aristotle and perhaps
influence them a little. Instead, the old Ionians represent a different and largely
contradictory tradition, one in much better accord with modern science. That their influence
was felt powerfully for only two or three centuries is an irreparable loss for all those humanbeings who lived between the Ionian Awakening and the Italian Renaissance.
Perhaps the most influential person ever associated with Samos was Pythagoras,* acontemporary of Polycrates in the sixth century B.C. According to local tradition, he lived
for a time in a cave on the Samian Mount Kerkis, and was the first person in the history of
the world to deduce that the Earth is a sphere. Perhaps he argued by analogy with the Moon

and the Sun, or noticed the curved shadow of the Earth on the Moon during a lunar eclipse,
or recognized that when ships leave Samos and recede over the horizon, their masts
disappear last.
* The sixth century B.C. was a time of remarkable intellectual and spiritual ferment across theplanet. Not only was it the time of Thales, Anaximander, Pythagoras and others in Ionia, but also the time of
the Egyptian Pharaoh Necho who caused Africa to be circumnavigated, of Zoroaster in Persia, Confucius andLao-tse in China, the Jewish prophets in Israel, Egypt and Babylon, and Gautama Buddha in India. It is hardto think these activities altogether unrelated.

He or his disciples discovered the Pythagorean theorem: the sum of the squares ofthe shorter sides of a right triangle equals the square of the longer side. Pythagoras did not
simply enumerate examples of this theorem; he developed a method of mathematicaldeduction to prove the thing generally. The modern tradition of mathematical argument,
essential to all of science, owes much to Pythagoras. It was he who first used the word
Cosmos to denote a well-ordered and harmonious universe, a world amenable to humanunderstanding.
Many Ionians believed the underlying harmony of the universe to be accessible
through observation and experiment, the method that dominates science today. However,Pythagoras employed a very different method. He taught that the laws of Nature could be
deduced by pure thoughts. He and his followers were not fundamentally experimentalists.*
They were mathematicians. And they were thoroughgoing mystics. According to Bertrand
Russell, in a perhaps uncharitable passage, Pythagoras ‘founded a religion, of which themain tenets were the transmigration of souls and the sinfulness of eating beans. His religion
was embodied in a religious order, which, here and there, acquired control of the State andestablished a rule of the saints. But the unregenerate hankered after beans, and sooner orlater rebelled.’* Although there were a few welcome exceptions. The Pythagorean fascination with whole-number
ratios in musical harmonies seems clearly to be based on observation, or even experiment on the sounds
issued from plucked strings. Empedocles was, at least in part, a Pythagorean. One of Pythagoras’ students,
Alcmaeon, is the first person known to have dissected a human body; he distinguished between arteries andveins, was the first to discover the optic nerve and the eustachian tubes, and identified the brain as the seat of
the intellect (a contention later denied by Aristotle, who placed intelligence in the heart, and then revived by
Herophilus of Chalcedon). He also founded the science of embryology. But Alcmaeon’s zest for the impure
was not shared by most of his Pythagorean colleagues in later times.

The Pythagoreans delighted in the certainty of mathematical demonstration, thesense of a pure and unsullied world accessible to the human intellect, a Cosmos in which
the sides of right triangles perfectly obey simple mathematical relationships. It was instriking contrast to the messy reality of the workaday world. They believed that in theirmathematics they had glimpsed a perfect reality, a realm of the gods, of which our familiar
world is but an imperfect reflection. In Plato’s famous parable of the cave, prisoners wereimagined tied in such a way that they saw only the shadows of passersby and believed the
shadows to be real - never guessing the complex reality that was accessible if they would
but turn their heads. The Pythagoreans would powerfully influence Plato and, later,
Christianity.
They did not advocate the free confrontation of conflicting points of view. Instead,like all orthodox religions, they practiced a rigidity that prevented them from correcting

their errors. Cicero wrote:
In discussion it is not so much weight of authority as force of argument that should
be demanded. Indeed, the authority of those who profess to teach is often a positive
hindrance to those who desire to learn; they cease to employ their own judgment,
and take what they perceive to be the verdict of their chosen master as settling thequestion. In fact I am not disposed to approve the practice traditionally ascribed to
the Pythagoreans, who, when questioned as to the grounds of any assertion that they
advanced in debate, are said to have been accustomed to reply ‘The Master said so,’‘the Master’ being Pythagoras. So potent was an opinion already decided, making
authority prevail unsupported by reason.
The Pythagoreans were fascinated by the regular solids, symmetrical three-dimensional
objects all of whose sides are the same regular polygon. The cube is the simplest example,
having six squares as sides. There are an infinite number of regular polygons, but only fiveregular solids. (The proof of this statement, a famous example of mathematical reasoning,is given in Appendix 2.) For some reason, knowledge of a solid called the dodecahedronhaving twelve pentagons as sides seemed to them dangerous. It was mystically associatedwith the Cosmos. The other four regular solids were identified, somehow, with the four‘elements’ then imagined to constitute the world: earth, fire, air and water. The fifth regularsolid must then, they thought, correspond to some fifth element that could only be thesubstance of the heavenly bodies. (This notion of a fifth essence is the origin of our word
quintessence.) Ordinary people were to be kept ignorant of the dodecahedron.
In love with whole numbers, the Pythagoreans believed all things could be derived
from them, certainly all other numbers. A crisis in doctrine arose when they discovered thatthe square root of two (the ratio of the diagonal to the side of a square) was irrational, that it
cannot be expressed accurately as the ratio of any two whole numbers, no matter how big
these numbers are. Ironically this discovery (reproduced in Appendix 1) was made with the
Pythagorean theorem as a tool. ‘Irrational’ originally meant only that a number could not be
expressed as a ratio. But for the Pythagoreans it came to mean something threatening, a hintthat their world view might not make sense, which is today the other meaning of‘irrational.’ Instead of sharing these important mathematical discoveries, the Pythagoreanssuppressed knowledge of the square root of two and the dodecahedron. The outside world
was not to know.* Even today there are scientists opposed to the popularization of science:the sacred knowledge is to be kept within the cult, unsullied by public understanding.
* A Pythagorean named Hippasus published the secret of the ‘sphere with twelve pentagons’, thedodecahedron. When he later died in a shipwreck, we are told, his fellow Pythagoreans remarked on thejustice of the punishment. His book has not survived.

The Pythagoreans believed the sphere to be ‘perfect’, all points on its surface beingat the same distance from its center. Circles were also perfect. And the Pythagoreansinsisted that planets moved in circular paths at constant speeds. They seemed to believe thatmoving slower or faster at different places in the orbit would be unseemly; noncircularmotion was somehow flawed, unsuitable for the planets, which, being free of the Earth,
were also deemed ‘perfect.’
The pros and cons of the Pythagorean tradition can be seen clearly in the life’s work

of Johannes Kepler (Chapter 3). The Pythagorean idea of a perfect and mystical world,
unseen by the senses, was readily accepted by the early Christians and was an integral
component of Kepler’s early training. On the one hand, Kepler was convinced thatmathematical harmonies exist in nature (he wrote that ‘the universe was stamped with theadornment of harmonic proportions’); that simple numerical relationships must determine
the motion of the planets. On the other hand, again following the Pythagoreans, he long
believed that only uniform circular motion was admissible. He repeatedly found theobserved planetary motions could not be explained in this way, and repeatedly tried again.
But unlike many Pythagoreans, he believed in observations and experiment in the realworld. Eventually the detailed observations of the apparent motion of the planets forced
him to abandon the idea of circular paths and to realize that planets travel in ellipses.
Kepler was both inspired in his search for the harmony of planetary motion and delayed formore than a decade by the attractions of Pythagorean doctrine.
A disdain for the practical swept the ancient world. Plato urged astronomers to think
about the heavens, but not to waste their time observing them. Aristotle believed that: ‘The
lower sort are by nature slaves, and it is better for them as for all inferiors that they should
be under the rule of a master .... The slave shares in his master’s life; the artisan is lessclosely connected with him, and only attains excellence in proportion as he becomes aslave. The meaner sort of mechanic has a special and separate slavery.’ Plutarch wrote: ‘It
does not of necessity follow that, if the work delight you with its grace, the one who
wrought it is worthy of esteem.’ Xenophon’s opinion was: ‘What are called the mechanicalarts carry a social stigma and are rightly dishonoured in our cities.’ As a result of such
attitudes, the brilliant and promising Ionian experimental method was largely abandoned
for two thousand years. Without experiment, there is no way to choose among contending
hypotheses, no way for science to advance. The antiempirical taint of the Pythagoreanssurvives to this day. But why? Where did this distaste for experiment come from?
An explanation for the decline of ancient science has been put forward by thehistorian of science, Benjamin Farrington: The mercantile tradition, which led to Ionian
science, also led to a slave economy. The owning of slaves was the road to wealth andpower. Polycrates’ fortifications were built by slaves. Athens in the time of Pericles, Platoand Aristotle had a vast slave population. All the brave Athenian talk about democracy
applied only to a privileged few. What slaves characteristically perform is manual labor.
But scientific experimentation is manual labor, from which the slaveholders are
preferentially distanced; while it is only the slaveholders - politely called ‘gentle-men’ insome societies - who have the leisure to do science. Accordingly, almost no one didscience. The Ionians were perfectly able to make machines of some elegance. But theavailability of slaves undermined the economic motive for the development of technology.
Thus the mercantile tradition contributed to the great Ionian awakening around 600 B.C.,
and, through slavery, may have been the cause of its decline some two centuries later.
There are great ironies here.
Similar trends are apparent throughout the world. The high point in indigenous
Chinese astronomy occurred around 1280, with the work of Kuo Shou-ching, who used an
observational baseline of 1,500 years and improved both astronomical instruments andmathematical techniques for computation. It is generally thought that Chinese astronomy
thereafter underwent a steep decline. Nathan Sivin believes that the reason lies at least
partly ‘in increasing rigidity of elite attitudes, so that the educated were less inclined to be
curious about techniques and less willing to value science as an appropriate pursuit for agentleman.’ The occupation of astronomer became a hereditary office, a practiceinconsistent with the advance of the subject. Additionally, ‘the responsibility for theevolution of astronomy remained centered in the Imperial Court and was largely abandoned
to foreign technicians,’ chiefly the Jesuits, who had introduced Euclid and Copernicus to
the astonished Chinese, but who, after the censorship of the latter’s book, had a vested
interest in disguising and suppressing heliocentric cosmology. Perhaps science wasstillborn in Indian, Mayan and Aztec civilizations for the same reason it declined in Ionia,
the pervasiveness of the slave economy. A major problem in the contemporary (political)
Third World is that the educated classes tend to be the children of the wealthy, with avested interest in the status quo, and are unaccustomed either to working with their hands
or to challenging conventional wisdom. Science has been very slow to take root.
Plato and Aristotle were comfortable in a slave society. They offered justifications
for oppression. They served tyrants. They taught the alienation of the body from the mind
(a natural enough ideal in a slave society); they separated matter from thought; they
divorced the Earth from the heavens - divisions that were to dominate Western thinking for
more than twenty centuries. Plato, who believed that ‘all things are full of gods,’ actuallyused the metaphor of slavery to connect his politics with his cosmology. He is said to haveurged the burning of all the books of Democritus (he had a similar recommendation for thebooks of Homer), perhaps because Democritus did not acknowledge immortal souls or
immortal gods or Pythagorean mysticism, or because he believed in an infinite number ofworlds. Of the seventy-three books Democritus is said to have written, covering all of
human knowledge, not a single work survives. All we know is from fragments, chiefly on
ethics, and secondhand accounts. The same is true of almost all the other ancient Ionian
scientists.
In the recognition by Pythagoras and Plato that the Cosmos is knowable, that there
is a mathematical underpinning to nature, they greatly advanced the cause of science. But
in the suppression of disquieting facts, the sense that science should be kept for a small
elite, the distaste for experiment, the embrace of mysticism and the easy acceptance ofslave societies, they set back the human enterprise. After a long mystical sleep in which thetools of scientific inquiry lay moldering, the Ionian approach, in some cases transmitted
through scholars at the Alexandrian Library, was finally rediscovered. The Western world
reawakened. Experiment and open inquiry became once more respectable. Forgotten books
and fragments were again read. Leonardo and Columbus and Copernicus were inspired byor independently retraced parts of this ancient Greek tradition. There is in our time much
Ionian science, although not in politics and religion, and a fair amount of courageous freeinquiry. But there are also appalling superstitions and deadly ethical ambiguities. We areflawed by ancient contradictions.
The Platonists and their Christian successors held the peculiar notion that the Earth
was tainted and somehow nasty, while the heavens were perfect and divine. The
fundamental idea that the Earth is a planet, that we are citizens of the Universe, wasrejected and forgotten. This idea was first argued by Aristarchus, born on Samos threecenturies after Pythagoras. Aristarchus was one of the last of the Ionian scientists. By thistime, the center of intellectual enlightenment had moved to the great Library of Alexandria.
Aristarchus was the first person to hold that the Sun rather than the Earth is at the center ofthe planetary system, that all the planets go around the Sun rather than the Earth. Typically,
his writings on this matter are lost. From the size of the Earth’s shadow on the Moon during
a lunar eclipse, he deduced that the Sun had to be much larger than the Earth, as well asvery far away. He may then have reasoned that it is absurd for so large a body as the Sun to
revolve around so small a body as the Earth. He put the Sun at the center, made the Earthrotate on its axis once a day and orbit the Sun once a year.
It is the same idea we associate with the name of Copernicus, whom Galileo
described as the ‘restorer and confirmer’, not the inventor, of the heliocentric hypothesis.*
For most of the 1,800 years between Aristarchus and Copernicus nobody knew the correct
disposition of the planets, even though it had been laid out perfectly clearly around 280
B.C. The idea outraged some of Aristarchus’ contemporaries. There were cries, like thosevoiced about Anaxagoras and Bruno and Galileo, that he be condemned for impiety. The
resistance to Aristarchus and Copernicus, a kind of geocentrism in everyday life, remains
with us: we still talk about the Sun ‘rising’ and the Sun ‘setting’. It is 2,200 years since
Aristarchus, and our language still pretends that the Earth does not turn.
* Copernicus may have gotten the idea from reading about Aristarchus. Recently discoveredclassical texts were a source of great excitement in Italian universities when Copernicus went to medical
school there. In the manuscript of his book, Copernicus mentioned Aristarchus’ priority, but he omitted thecitation before the book saw print. Copernicus wrote in a letter to Pope Paul III: ‘According to Cicero, Nicetas
had thought the Earth was moved . . . According to Plutarch [who discusses Aristarchusl... certain others had
held the same opinion. When from this, therefore, I had conceived its possibility, I myself also began to
meditate upon the mobility of the Earth.’

The separation of the planets from one another - forty million kilometers from Earthto Venus at closest approach, six billion kilometers to Pluto - would have stunned those
Greeks who were outraged by the contention that the Sun might be as large as the
Peloponnesus. It was natural to think of the solar system as much more compact and local.
If I hold my finger before my eyes and examine it first with my left and then with my righteye, it seems to move against the distant background. The closer my finger is, the more it
seems to move. I can estimate the distance to my finger from the amount of this apparent
motion, or parallax. If my eyes were farther apart, my finger would seem to move
substantially more. The longer the baseline from which we make our two observations, thegreater the parallax and the better we can measure the distance to remote objects. But welive on a moving platform, the Earth, which every six months has progressed from one endof its orbit to the other, a distance of 300,000,000 kilometers. If we look at the sameunmoving celestial object six months apart, we should be able to measure very great
distances. Aristarchus suspected the stars to be distant suns. He placed the Sun ‘among’ thefixed stars. The absence of detectable stellar parallax as the Earth moved suggested that thestars were much farther away than the Sun. Before the invention of the telescope, theparallax of even the nearest stars was too small to detect. Not until the nineteenth century
was the parallax of a star first measured. It then became clear, from straightforward Greek
geometry, that the stars were light-years away.
There is another way to measure the distance to the stars which the Ionians were
fully capable of discovering, although, so far as we know, they did not employ it. Everyone
knows that the farther away an object is, the smaller it seems. This inverse proportionality
between apparent size and distance is the basis of perspective in art and photography. So
the farther away we are from the Sun, the smaller and dimmer it appears. How far would
we have to be from the Sun for it to appear as small and as dim as a star? Or, equivalently,
how small a piece of the Sun would be as bright as a star?
An early experiment to answer this question was performed by Christiaan Huygens,very much in the Ionian tradition. Huygens drilled small holes in a brass plate, held theplate up to the Sun and asked himself which hole seemed as bright as he remembered thebright star Sirius to have been the night before. The hole was effectively* 1/28,000 the
apparent size of the Sun. So Sirius, he reasoned, must be 28,000 times farther from us thanthe Sun, or about half a light-year away. It is hard to remember just how bright a star is
many hours after you look at it, but Huygens remembered very well. If he had known that
Sirius was intrinsically brighter than the Sun, he would have come up with almost exactly
the right answer: Sirius is 8.8 light-years away. The fact that Aristarchus and Huygens used
imprecise data and derived imperfect answers hardly matters. They explained their methods
so clearly that, when better observations were available, more accurate answers could bederived.
* Huygens actually used a glass bead to reduce the amount of light passed by the hole.

Between the times of Aristarchus and Huygens, humans answered the question that
had so excited me as a boy growing up in Brooklyn: What are the stars? The answer is thatthe stars are mighty suns, light-years away in the vastness of interstellar space.
The great legacy of Aristarchus is this: neither we nor our planet enjoys a privileged
position in Nature. This insight has since been applied upward to the stars, and sideways to
many subsets of the human family, with great success and invariable opposition. It has been
responsible for major advances in astronomy, physics, biology, anthropology, economics
and politics. I wonder if its social extrapolation is a major reason for attempts at its
suppression.
The legacy of Aristarchus has been extended far beyond the realm of the stars. At
the end of the eighteenth century, William Herschel, musician and astronomer to George III
of England, completed a project to map the starry skies and found apparently equal
numbers of stars in all directions in the plane or band of the Milky Way; from this,reasonably enough, he deduced that we were at the center of the Galaxy.* Just before
World War I, Harlow Shapley of Missouri devised a technique for measuring the distances
to the globular clusters, those lovely spherical arrays of stars which resemble a swarm ofbees. Shapley had found a stellar standard candle, a star noticeable because of its
variability, but which had always the same average intrinsic brightness. By comparing thefaintness of such stars when found in globular clusters with their real brightness, as
determined from nearby representatives, Shapley could calculate how far away they are just as, in a field, we can estimate the distance of a lantern of known intrinsic brightness
from the feeble light that reaches us - essentially, the method of Huygens. Shapley
discovered that the globular clusters were not centered around the solar neighborhood but
rather about a distant region of the Milky Way, in the direction of the constellation
Sagittarius, the Archer. It seemed to him very likely that the globular clusters used in thisinvestigation, nearly a hundred of them, would be orbiting about, paying homage to, themassive center of the Milky Way.
* This supposed privileged position of the Earth, at the center of what was then considered the
known universe, led A. R. Wallace to the anti-Aristarchian position, in his book Man’s Place in the Universe(1903), that ours may be the only inhabited planet.

Shapley had in 1915 the courage to propose that the solar system was in theoutskirts and not near the core of our galaxy. Herschel had been misled because of thecopious amount of obscuring dust in the direction of Sagittarius; he had no way to know ofthe enormous numbers of stars beyond. It is now very clear that we live some 30,000 lightyears from the galactic core, on the fringes of a spiral arm, where the local density of stars
is relatively sparse. There may be those who live on a planet that orbits a central star in oneof Shapley’s globular clusters, or one located in the core. Such beings may pity us for ourhandful of naked-eye stars, because their skies will be ablaze with them. Near the center ofthe Milky Way, millions of brilliant stars would be visible to the naked eye, compared toour paltry few thousand. Our Sun or suns might set, but the night would never come.
Well into the twentieth century, astronomers believed that there was only one
galaxy in the Cosmos, the Milky Way - although in the eighteenth century Thomas Wrightof Durban and Immanuel Kant of Königsberg each had a premonition that the exquisite
luminous spiral forms, viewed through the telescope, were other galaxies. Kant suggested
explicitly that M31 in the constellation Andromeda was another Milky Way, composed ofenormous numbers of stars, and proposed calling such objects by the evocative andhaunting phrase ‘island universes.’ Some scientists toyed with the idea that the spiralnebulae were not distant island universes but rather nearby condensing clouds of interstellargas, perhaps on their way to make solar systems. To test the distance of the spiral nebula aclass of intrinsically much brighter variable stars was needed to furnish a new standard
candle. Such stars, identified in M31 by Edwin Hubble in 1924, were discovered to bealarmingly dim, and it became apparent that M31 was a prodigious distance away, anumber now estimated at a little more than two million light-years. But if M31 were at sucha distance, it could not be a cloud of mere interstellar dimensions; it had to be much larger an immense galaxy in its own right. And the other, fainter galaxies must be more distant
still, a hundred billion of them, sprinkled through the dark to the frontiers of the known
Cosmos.
As long as there have been humans, we have searched for our place in the Cosmos. In thechildhood of our species (when our ancestors gazed a little idly at the stars), among the
Ionian scientists of ancient Greece, and in our own age, we have been transfixed by thisquestion: Where are we? Who are we? We find that we live on an insignificant planet of a
humdrum star lost between two spiral arms in the outskirts of a galaxy which is a memberof a sparse cluster of galaxies, tucked away in some forgotten corner of a universe in which
there are far more galaxies than people. This perspective is a courageous continuation of
our penchant for constructing and testing mental models of the skies; the Sun as a red-hot
stone, the stars as celestial flame, the Galaxy as the backbone of night.
Since Aristarchus, every step in our quest has moved us farther from center stage inthe cosmic drama. There has not been much time to assimilate these new findings. The
discoveries of Shapley and Hubble were made within the lifetimes of many people still
alive today. There are those who secretly deplore these great discoveries, who consider
every step a demotion, who in their heart of hearts still pine for a universe whose center,focus and fulcrum is the Earth. But if we are to deal with the Cosmos we must firstunderstand it, even if our hopes for some unearned preferential status are, in the process,contravened. Understanding where we live is an essential precondition for improving theneighborhood. Knowing what other neighborhoods are like also helps. If we long for ourplanet to be important, there is something we can do about it. We make our world
significant by the courage of our questions and by the depth of our answers.

We embarked on our cosmic voyage with a question first framed in the childhood of
our species and in each generation asked anew with undiminished wonder: What are thestars? Exploration is in our nature. We began as wanderers, and we are wanderers still. We
have lingered long enough on the shores of the cosmic ocean. We are ready at last to set
sail for the stars.

CHAPTER VIII
Travels in Space and Time
No one has lived longer than a dead child, and Methusula* died young. Heaven andEarth are as old as I, and the ten thousand things are one.
- Chuang Tzu, about 300 B.C., China* Actually, P’eng Tsu, the Chinese equivalent.

We have loved the stars too fondly to be fearful of the night.
- Tombstone epitaph of two amateur astronomers
Stars scribble in our eyes the frosty sagas, The gleaming cantos of unvanquishedspace.
- Hart Crane, The Bridge
The rising and falling of the surf is produced in part by tides. The Moon and the Sun are far
away. But their gravitational influence is very real and noticeable back here on Earth. The
beach reminds us of space. Fine sand grains, all more or less uniform in size, have been
produced from larger rocks through ages of jostling and rubbing, abrasion and erosion,
again driven through waves and weather by the distant Moon and Sun. The beach also
reminds us of time. The world is much older than the human species.
A handful of sand contains about 10,000 grains, more than the number of stars we
can see with the naked eye on a clear night. But the number of stars we can see is only thetiniest fraction of the number of stars that are. What we see at night is the merest
smattering of the nearest stars. Meanwhile the Cosmos is rich beyond measure: the total
number of stars in the universe is greater than all the grains of sand on all the beaches of theplanet Earth.
Despite the efforts of ancient astronomers and astrologers to put pictures in theskies, a constellation is nothing more than an arbitrary grouping of stars composed of
intrinsically dim stars that seem to us bright because they are nearby, and intrinsicallybrighter stars that are somewhat more distant. All places on Earth are, to high precision, thesame distance from any star. This is why the star patterns in a given constellation do notchange as we go from, say, Soviet Central Asia to the American Midwest. Astronomically,
the U.S.S.R. and the United States are the same place. The stars in any constellation are allso far away that we cannot recognize them as a three-dimensional configuration as long as
we are tied to Earth. The average distance between the stars is a few light-years, a lightyear being, we remember, about ten trillion kilometers. For the patterns of theconstellations to change, we must travel over distances comparable to those that separate
the stars; we must venture across the light-years. Then some nearby stars will seem to moveout of the constellation, others will enter it, and its configuration will alter dramatically.
Our technology is, so far, utterly incapable of such grand interstellar voyages, atleast in reasonable transit times. But our computers can be taught the three-dimensional
positions of all the nearby stars, and we can ask to be taken on a little trip - acircumnavigation of the collection of bright stars that constitute the Big Dipper, say - andwatch the constellations change. We connect the stars in typical constellations, in the usual
celestial follow-the-dots drawings. As we change our perspective, we see their apparent
shapes distort severely. The inhabitants of the planets of distant stars witness quite different
constellations in their night skies than we do in ours - other Rorschach tests for otherminds. Perhaps sometime in the next few centuries a spaceship from Earth will actually
travel such distances at some remarkable speed and see new constellations that no humanhas ever viewed before - except with such a computer.
The appearance of the constellations changes not only in space but also in time; notonly if we alter our position but also if we merely wait sufficiently long. Sometimes stars
move together in a group or cluster; other times a single star may move very rapidly withrespect to its fellows. Eventually such stars leave an old constellation and enter a new one.
Occasionally, one member of a double-star system explodes, breaking the gravitational
shackles that bound its companion, which then leaps into space at its former orbital
velocity, a slingshot in the sky. In addition, stars are born, stars evolve, and stars die. If wewait long enough, new stars appear and old stars vanish. The patterns in the sky slowly
melt and alter.
Even over the lifetime of the human species - a few million years - constellations
have been changing. Consider the present configuration of the Big Dipper, or Great Bear.
Our computer can carry us in time as well as in space. As we run the Big Dipper backwards
into the past, allowing for the motion of its stars, we find quite a different appearance a
million years ago. The Big Dipper then looked quite a bit like a spear. If a time machine
dropped you precipitously in some unknown age in the distant past, you could in principle
determine the epoch by the configuration of the stars: If the Big Dipper is a spear, this must
be the Middle Pleistocene.
We can also ask the computer to run a constellation forward into time. Consider Leo
the Lion. The zodiac is a band of twelve constellations seemingly wrapped around the sky
in the apparent annual path of the Sun through the heavens. The root of the word is that forzoo, because the zodiacal constellations, like Leo, are mainly fancied to be animals. A
million years from now, Leo will look still less like a lion than it does today. Perhaps our
remote descendants will call it the constellation of the radio telescope - although I suspect amillion years from now the radio telescope will have become more obsolete than the stone
spear is now.
The (nonzodiacal) constellation of Orion, the hunter, is outlined by four bright stars
and bisected by a diagonal line of- three stars, which represent the belt of the hunter. Three
dimmer stars hanging from the belt are, according to the conventional astronomical
projective test, Orion’s sword. The middle star in the sword is not actually a star but a great
cloud of gas called the Orion Nebula, in which stars are being born. Many of the stars inOrion are hot and young, evolving rapidly and ending their lives in colossal cosmic
explosions called supernovae. They are born and die in periods of tens of millions of years.
If, on our computer, we were to run Orion rapidly into the far future, we would see a
startling effect, the births and spectacular deaths of many of its stars, flashing on andwinking off like fireflies in the night.
The solar neighborhood, the immediate environs of the Sun in space, includes thenearest star system, Alpha Centauri. It is really a triple system, two stars revolving aroundeach other, and a third, Proxima Centauri, orbiting the pair at a discreet distance. At somepositions in its orbit, Proxima is the closest known star to the Sun - hence its name. Moststars in the sky are members of double or multiple star systems. Our solitary Sun issomething of an anomaly.
The second brightest star in the constellation Andromeda, called Beta Andromedae,
is seventy-five light-years away. The light by which we see it now has spent seventy-fiveyears traversing the dark of interstellar space on its long journey to Earth. In the unlikely
event that Beta Andromedae blew itself up last Tuesday, we would not know it for anotherseventy-five years, as this interesting information, traveling at the speed of light, would
require seventy-five years to cross the enormous interstellar distances. When the light bywhich we now see this star set out on its long voyage, the young Albert Einstein, working
as a Swiss patent clerk, had just published his epochal special theory of relativity here on
Earth.
Space and time are interwoven. We cannot look out into space without looking backinto time. Light travels very fast. But space is very empty, and the stars are far apart.
Distances of seventy-five light-years or less are very small compared to other distances inastronomy. From the Sun to the center of the Milky Way Galaxy is 30,000 light-years.
From our galaxy to the nearest spiral galaxy, M31, also in the constellation Andromeda, is
2,000,000 light-years. When the light we see today from M31 left for Earth, there were no
humans on our planet, although our ancestors were evolving rapidly to our present form.
The distance from the Earth to the most remote quasars is eight or ten billion light-years.
We see them today as they were before the Earth accumulated, before the Milky Way wasformed.
This is not a situation restricted to astronomical objects, but only astronomical
objects are so far away that the finite speed of light becomes important. If you are looking
at a friend three meters (ten feet) away, at the other end of the room, you are not seeing her
as she is ‘now’; but rather as she ‘was’ a hundred millionth of a second ago. [(3 m) / (3 x
108 m/sec) = 1/(108/sec) =10-8 sec, or a hundredth of a microsecond. In this calculation we
have merely divided the distance by the speed to get the travel time.] But the difference
between your friend ‘now’ and now minus a hundred-millionth of a second is too small to
notice. On the other hand, when we look at a quasar eight billion light-years away, the factthat we are seeing it as it was eight billion years ago may be very important. (For example,
there are those who think that quasars are explosive events likely to happen only in theearly history of galaxies. In that case, the more distant the galaxy, the earlier in its history
we are observing it, and the more likely it is that we should see it as a quasar. Indeed, thenumber of quasars increases as we look to distances of more than about five billion lightyears).
The two Voyager interstellar spacecraft, the fastest machines ever launched from
Earth, are now traveling at one ten-thousandth the speed of light. They would need 40,000
years to go the distance to the nearest star. Do we have any hope of leaving Earth andtraversing the immense distances even to Proxima Centauri in convenient periods of time?
Can we do something to approach the speed of light? What is magic about the speed oflight? Might we someday be able to go faster than that?
If you had walked through the pleasant Tuscan countryside in the 1890’s, you might
have come upon a somewhat long-haired teenage high school dropout on the road to Pavia.
His teachers in Germany had told him that he would never amount to anything, that hisquestions destroyed classroom discipline, that he would be better off out of school. So heleft and wandered, delighting in the freedom of Northern Italy, where he could ruminate on
matters remote from the subjects he had been force-fed in his highly disciplined Prussian
schoolroom. His name was Albert Einstein, and his ruminations changed the world.

Einstein had been fascinated by Bernstein’s People’s Book of Natural Science, a
popularization of science that described on its very first page the astonishing speed ofelectricity through wires and light through space. He wondered what the world would look
like if you could travel on a wave of light. To travel at the speed of light! What an engaging
and magical thought for a boy on the road in a countryside dappled and rippling in sunlight.
You could not tell you were on a light wave if you traveled with it. If you started on a wave
crest, you would stay on the crest and lose all notion of it being a wave. Something strange
happens at the speed of light. The more Einstein thought about such questions, the moretroubling they became. Paradoxes seemed to emerge everywhere if you could travel at the
speed of light. Certain ideas had been accepted as true without sufficiently careful thought.
Einstein posed simple questions that could have been asked centuries earlier. For example,
what do we mean when we say that two events are simultaneous?
Imagine that I am riding a bicycle toward you. As I approach an intersection I
nearly collide, so it seems to me, with a horse-drawn cart. I swerve and barely avoid being
run over. Now think of the event again, and imagine that the cart and the bicycle are bothtraveling close to the speed of light. If you are standing down the road, the cart is travelingat right angles to your line of sight. You see me, by reflected sunlight, traveling toward
you. Would not my speed be added to the speed of light, so that my image would get to you
considerably before the image of the cart? Should you not see me swerve before you see thecart arrive? Can the cart and I approach the intersection simultaneously from my point ofview, but not from yours? Could I experience a near collision with the cart while you
perhaps see me swerve around nothing and pedal cheerfully on toward the town of Vinci?
These are curious and subtle questions. They challenge the obvious. There is a reason thatno one thought of them before Einstein. From such elementary questions, Einstein
produced a fundamental rethinking of the world, a revolution in physics.
If the world is to be understood, if we are to avoid such logical paradoxes whentraveling at high speeds, there are some rules, commandments of Nature, that must beobeyed. Einstein codified these rules in the special theory of relativity. Light (reflected oremitted) from an object travels at the same velocity whether the object is moving or
stationary: Thou shalt not add thy speed to the speed of light. Also, no material object may
move faster than light: Thou shalt not travel at or beyond the speed of light. Nothing inphysics prevents you from traveling as close to the speed of light as you like; 99.9 percentof the speed of light would be just fine. But no matter how hard you try, you can never gainthat last decimal point. For the world to be logically consistent, there must be a cosmic
speed limit. Otherwise, you could get to any speed you wanted by adding velocities on amoving platform.
Europeans around the turn of the century generally believed in privileged frames ofreference: that German, or French, or British culture and political organization were better
than those of other countries; that Europeans were superior to other peoples who were
fortunate enough to be colonized. The social and political application of the ideas ofAristarchus and Copernicus was rejected or ignored. The young Einstein rebelled against
the notion of privileged frames of reference in physics as much as he did in politics. In a
universe filled with stars rushing helter-skelter in all directions, there was no place that was‘at rest,’ no framework from which to view the universe that was superior to any otherframework. This is what the word relativity means. The idea is very simple, despite its
magical trappings: in viewing the universe, every place is as good as every other place. The
laws of Nature must be identical no matter who is describing them. If this is to be true - andit would be stunning if there were something special about our insignificant location in the
Cosmos - then it follows that no one may travel faster than light.
We hear the crack of a bullwhip because its tip is moving faster than the speed ofsound, creating a shock wave, a small sonic boom. A thunderclap has a similar origin. It
was once thought that airplanes could not travel faster than sound. Today supersonic flightis commonplace. But the light barrier is different from the sound barrier. It is not merely anengineering problem like the one the supersonic airplane solves. It is a fundamental law of
Nature, as basic as gravity. And there are no phenomena in our experience - like the crackof the bullwhips or the clap of thunder for sound - to suggest the possibility of traveling in a
vacuum faster than light. On the contrary, there is an extremely wide range of experience with nuclear accelerators and atomic clocks, for example - in precise quantitative
agreement with special relativity.
The problems of simultaneity do not apply to sound as they do to light because
sound is propagated through some material medium, usually air. The sound wave thatreaches you when a friend is talking is the motion of molecules in the air. Light, however,travels in a vacuum. There are restrictions on how molecules of air can move which do not
apply to a vacuum. Light from the Sun reaches us across the intervening empty space, but
no matter how carefully we listen, we do not hear the crackle of sunspots or the thunder ofthe solar flares. It was once thought, in the days before relativity, that light did propagate
through a special medium that permeated all of space, called ‘the luminiferous aether.’ But
the famous Michelson-Morley experiment demonstrated that such an aether does not exist.
We sometimes hear of things that can travel faster than light. Something called ‘thespeed of thought’ is occasionally proffered. This is an exceptionally silly notion especiallysince the speed of impulses through the neurons in our brains is about the same as the speedof a donkey cart. That human beings have been clever enough to devise relativity showsthat we think well, but I do not think we can boast about thinking fast. The electricalimpulses in modern computers do, however, travel nearly at the speed of light.
Special relativity, fully worked out by Einstein in his middle twenties, is supported
by every experiment performed to check it. Perhaps tomorrow someone will invent a theory
consistent with everything else we know that circumvents paradoxes on such matters as
simultaneity, avoids privileged reference frames and still permits travel faster than light.
But I doubt it very much. Einstein’s prohibition against traveling faster than light may clash
with our common sense. But on this question, why should we trust common sense? Why
should our experience at 10 kilometers an hour constrain the laws of nature at 300,000
kilometers per second? Relativity does set limits on what humans can ultimately do. But theuniverse is not required to be in perfect harmony with human ambition. Special relativity
removes from our grasp one way of reaching the stars, the ship that can go faster than light.
Tantalizingly, it suggests another and quite unexpected method.
Following George Gamow, let us imagine a place where the speed of light is not itstrue value of 300,000 kilometers per second, but something very modest: 40 kilometers perhour, say - and strictly enforced. (There are no penalties for breaking laws of Nature,
because there are no crimes: Nature is self-regulating and merely arranges things so that its
prohibitions are impossible to transgress.) Imagine that you are approaching the speed oflight on a motor scooter. (Relativity is rich in sentences beginning ‘Imagine . . .’ Einsteincalled such an exercise a Gedankenexperiment, a thought experiment.) As your speed
increases, you begin to see around the corners of passing objects. While you are rigidly
facing forward, things that are behind you appear within your forward field of vision. Closeto the speed of light, from your point of view, the world looks very odd - ultimately
everything is squeezed into a tiny circular window, which stays just ahead of you. From thestandpoint of a stationary observer, light reflected off you is reddened as you depart andblued as you return. If you travel toward the observer at almost the speed of light, you will
become enveloped in an eerie chromatic radiance: your usually invisible infrared emission
will be shifted to the shorter visible wavelengths. You become compressed in the directionof motion, your mass increases, and time, as you experience it, slows down, a breathtaking
consequence of traveling close to the speed of light called time dilation. But from thestandpoint of an observer moving with you - perhaps the scooter has a second seat - none ofthese effects occur.
These peculiar and at first perplexing predictions of special relativity are true in the
deepest sense that anything in science is true. They depend on your relative motion. But
they are real, not optical illusions. They can be demonstrated by simple mathematics,
mainly first-year algebra and therefore understandable to any educated person. They arealso consistent with many experiments. Very accurate clocks carried in airplanes slow
down a little compared to stationary clocks. Nuclear accelerators are designed to allow for
the increase of mass with increasing speed; if they were not designed in this way,
accelerated particles would all smash into the walls of the apparatus, and there would be
little to do in experimental nuclear physics. A speed is a distance divided by a time. Sincenear the velocity of light we cannot simply add speeds, as we are used to doing in theworkaday world, the familiar notions of absolute space and absolute time - independent of
your relative motion - must give way. That is why you shrink. That is the reason for time
dilation.
Traveling close to the speed of light you would hardly age at all, but your friends
and your relatives back home would be aging at the usual rate. When you returned fromyour relativistic journey, what a difference there would be between your friends and you,they having aged decades, say, and you having aged hardly at all! Traveling close to the
speed of light is a kind of elixir of life. Because time slows down close to the speed of light,
special relativity provides us with a means of going to the stars. But is it possible, in terms
of practical engineering, to travel close to the speed of light? Is a starship feasible?
Tuscany was not only the caldron of some of the thinking of the young Albert
Einstein; it was also the home of another great genius who lived 400 years earlier,
Leonardo da Vinci, who delighted in climbing the Tuscan hills and viewing the groundfrom a great height, as if he were soaring like a bird. He drew the first aerial perspectives oflandscapes, towns and fortifications. Among Leonardo’s many interests andaccomplishments - in painting, sculpture, anatomy, geology, natural history, military andcivil engineering - he had a great passion: to devise and fabricate a machine that could fly.
He drew pictures, constructed models, built full-size prototypes and not one of themworked. No sufficiently powerful and lightweight engine then existed. The designs,
however, were brilliant and encouraged the engineers of future times. Leonardo himself
was depressed by these failures. But it was hardly his fault. He was trapped in the fifteenth
century.
A similar case occurred in 1939 when a group of engineers calling themselves the
British Interplanetary Society designed a ship to take people to the Moon - using 1939
technology. It was by no means identical to the design of the Apollo spacecraft, whichaccomplished exactly this mission three decades later, but it suggested that a mission to themoon might one day be a practical engineering possibility.

Today we have preliminary designs for ships to take people to the stars. None ofthese spacecraft is imagined to leave the Earth directly. Rather, they are constructed in
Earth orbit from where they are launched on their long interstellar journeys. One of themwas called Project Orion after the constellation, a reminder that the ship’s ultimate
objective was the stars. Orion was designed to utilize explosions of hydrogen bombs,
nuclear weapons, against an inertial plate, each explosion providing a kind of ‘putt-putt,’ avast nuclear motorboat in space. Orion seems entirely practical from an engineering pointof view. By its very nature it would have produced vast quantities of radioactive debris, but
for conscientious mission profiles only in the emptiness of interplanetary or interstellar
space. Orion was under serious development in the United States until the signing of theinternational treaty that forbids the detonation of nuclear weapons in space. This seems tome a great pity. The Orion starship is the best use of nuclear weapons I can think of.
Project Daedalus is a recent design of the British Interplanetary Society. It assumes
the existence of a nuclear fusion reactor - something much safer as well as more efficientthan existing fission power plants. We do not have fusion reactors yet, but they are
confidently expected in the next few decades. Orion and Daedalus might travel at 10
percent the speed of light. A trip to Alpha Centauri, 4.3 light-years away, would then takeforty-three years, less than a human lifetime. Such ships could not travel close enough tothe speed of light for special relativistic time dilation to become important. Even with
optimistic projections on the development of our technology, it does not seem likely that
Orion, Daedalus or their ilk will be built before the middle of the twenty-first century,
although if we wished we could build Orion now.
For voyages beyond the nearest stars, something else must be done. Perhaps Orionand Daedalus could be used as multigeneration ships, so those arriving at a planet ofanother star would be the remote descendants of those who had set out some centuries
before. Or perhaps a safe means of hibernation for humans will be found, so that the spacetravelers could be frozen and then reawakened centuries later. These nonrelativisticstarships, enormously expensive as they would be, look relatively easy to design and build
and use compared to starships that travel close to the speed of light. Other star systems areaccessible to the human species, but only after great effort.
Fast interstellar spaceflight - with the ship velocity approaching the speed of light is an objective not for a hundred years but for a thousand or ten thousand. But it is inprinciple possible. A kind of interstellar ramjet has been proposed by R. W. Bussard which
scoops up the diffuse matter, mostly hydrogen atoms, that floats between the stars,
accelerates it into a fusion engine and ejects it out the back. The hydrogen would be usedboth as fuel and as reaction mass. But in deep space there is only about one atom in every
ten cubic centimeters, a volume the size of a grape. For the ramjet to work, it needs afrontal scoop hundreds of kilometers across. When the ship reaches relativistic velocities,
the hydrogen atoms will be moving with respect to the spaceship at close to the speed oflight. If adequate precautions are not taken, the spaceship and its passengers will be fried bythese induced cosmic rays. One proposed solution uses a laser to strip the electrons off theinterstellar atoms and make them electrically charged while they are still some distanceaway, and an extremely strong magnetic field to deflect the charged atoms into the scoop
and away from the rest of the spacecraft. This is engineering on a scale so far
unprecedented on Earth. We are talking of engines the size of small worlds.
But let us spend a moment thinking about such a ship. The Earth gravitationallyattracts us with a certain force, which if we are falling we experience as an acceleration.

Were we to fall out of a tree - and many of our proto-human ancestors must have done so we would plummet faster and faster, increasing our fall speed by ten meters (or thirty-twofeet) per second, every second. This acceleration, which characterizes the force of gravity
holding us to the Earth’s surface, is called 1 g, g for Earth gravity. We are comfortable withaccelerations of 1 g; we have grown up with 1 g. If we lived in an interstellar spacecraftthat could accelerate at 1 g, we would find ourselves in a perfectly natural environment. In
fact, the equivalence between gravitational forces and the forces we would feel in an
accelerating spaceship is a major feature of Einstein’s later general theory of relativity.
With a continuous 1 g acceleration, after one year in space we would be traveling very
close to the speed of light [(0.01 km/sec2) x (3 x 107 sec) = 3 x 105 km/sec].
Suppose that such a spacecraft accelerates at 1 g, approaching more and moreclosely to the speed of light until the midpoint of the journey; and then is turned around anddecelerates at 1 g until arriving at its destination. For most of the trip the velocity would bevery close to the speed of light and time would slow down enormously. A nearby mission
objective, a sun that may have planets, is Barnard’s Star, about six light-years away. It
could be reached in about eight years as measured by clocks aboard the ship; the center ofthe Milky Way, in twenty-one years; M31, the Andromeda galaxy, in twenty-eight years.
Of course, people left behind on Earth would see things differently. Instead of twenty-one
years to the center of the Galaxy, they would measure an elapsed time of 30,000 years.
When we got home, few of our friends would be left to greet us. In principle, such ajourney, mounting the decimal points ever closer to the speed of light, would even permit
us to circumnavigate the known universe in some fifty-six years ship time. We would
return tens of billions of years in our future - to find the Earth a charred cinder and the Sun
dead. Relativistic spaceflight makes the universe accessible to advanced civilizations, but
only to those who go on the journey. There seems to be no way for information to travelback to those left behind any faster than the speed of light.
The designs for Orion, Daedalus and the Bussard Ramjet are probably farther fromthe actual interstellar spacecraft we will one day build than Leonardo’s models are fromtoday’s supersonic transports. But if we do not destroy ourselves, I believe that we will one
day venture to the stars. When our solar system is all explored, the planets of other stars
will beckon.
Space travel and time travel are connected. We can travel fast into space only by
traveling fast into the future. But what of the past? Could we return to the past and change
it? Could we make events turn out differently from what the history books assert? We
travel slowly into the future all the time, at the rate of one day every day. With relativistic
spaceflight we could travel fast into the future. But many physicists believe that a voyage
into the past is impossible. Even if you had a device that could travel backwards in time,they say, you would be unable to do anything that would make any difference. If you
journeyed into the past and prevented your parents from meeting, then you would never
have been born - which is something of a contradiction, since you clearly exist. Like theproof of the irrationality of the square root of two, like the discussion of simultaneity in
special relativity, this is an argument in which the premise is challenged because theconclusion seems absurd.
But other physicists propose that two alternative histories, two equally valid
realities, could exist side by side - the one you know and the one in which you were neverborn. Perhaps time itself has many potential dimensions, despite the fact that we arecondemned to experience only one of them. Suppose you could go back into the past andchange it - by persuading Queen Isabella not to support Christopher Columbus, forexample. Then, it is argued, you would have set into motion a different sequence ofhistorical events, which those you left behind in our time line would never know about. If
that kind of time travel were possible, then every imaginable alternative history might insome sense really exist.
History consists for the most part of a complex bundle of deeply interwoven
threads, social, cultural and economic forces that are not easily unraveled. The countless
small, unpredictable and random events that flow on continually often have no long-range
consequences. But some, those occurring at critical junctures or branch points, may change
the pattern of history. There may be cases where profound changes can be made byrelatively trivial adjustments. The farther in the past such an event is, the more powerful
may be its influence - because the longer the lever arm of time becomes.
A polio virus is a tiny microorganmism. We encounter many of them every day. But
only rarely, fortunately, does one of them infect one of us and cause this dread disease.
Franklin D. Roosevelt, the thirty-second President of the United States, had polio. Because
the disease was crippling, it may have provided Roosevelt with a greater compassion for
the underdog; or perhaps it improved his striving for success. If Roosevelt’s personality had
been different, or if he had never had the ambition to be President of the United States, thegreat depression of the 1930’s, World War II and the development of nuclear weapons
might just possibly have turned out differently. The future of the world might have beenaltered. But a virus is an insignificant thing, only a millionth of a centimeter across. It is
hardly anything at all.
On the other hand, suppose our time traveler had persuaded Queen Isabella that
Columbus’ geography was faulty, that from Eratosthenes’ estimate of the circumference of
the Earth, Columbus could never reach Asia. Almost certainly some other European would
have come along within a few decades and sailed west to the New World. Improvements innavigation, the lure of the spice trade and competition among rival European powers madethe discovery of America around 1500 more or less inevitable. Of course, there would
today be no nation of Colombia, or District of Columbia or Columbus, Ohio, or Columbia
University in the Americas. But the overall course of history might have turned out more orless the same. In order to affect the future profoundly, a time traveler would probably haveto intervene in a number of carefully chosen events, to change the weave of history.
It is a lovely fantasy, to explore those worlds that never were. By visiting them we
could truly understand how history works; history could become an experimental science.
If an apparently pivotal person had never lived - Plato, say, or Paul, or Peter the Great how different would the world be? What if the scientific tradition of the ancient Ionian
Greeks had survived and flourished? That would have required many of the social forces of
the time to have been different - including the prevailing belief that slavery was natural andright. But what if that light that dawned in the eastern Mediterranean 2,500 years ago had
not flickered out? What if science and the experimental method and the dignity of crafts
and mechanical arts had been vigorously pursued 2,000 years before the Industrial
Revolution? What if the power of this new mode of thought had been more generally
appreciated? I sometimes think we might then have saved ten or twenty centuries. Perhaps
the contributions of Leonardo would have been made a thousand years ago and those ofAlbert Einstein five hundred years ago. In such an alternate Earth, Leonardo and Einsteinwould, of course, never have been born. Too many things would have been different. In
every ejaculation there are hundreds of millions of sperm cells, only one of which canfertilize an egg and produce a member of the next generation of human beings. But which
sperm succeeds in fertilizing an egg must depend on the most minor and insignificant offactors, both internal and external. If even a little thing had gone differently 2,500 years
ago, none of us would be here today. There would be billions of others living in our place.
If the Ionian spirit had won, I think we - a different ‘we,’ of course - might by now
be venturing to the stars. Our first survey ships to Alpha Centauri and Barnard’s Star, Sirius
and Tau Ceti would have returned long ago. Great fleets of interstellar transports would beunder construction in Earth orbit - unmanned survey ships, liners for immigrants, immense
trading ships to plow the seas of space. On all these ships there would be symbols andwriting. If we looked closely, we might see that the language was Greek. And perhaps the
symbol on the bow of one of the first starships would be a dodecahedron, with theinscription ‘Starship Theodorus of the Planet Earth.’
In the time line of our world, things have gone somewhat more slowly. We are notyet ready for the stars. But perhaps in another century or two, when the solar system is all
explored, we will also have put our planet in order. We will have the will and the resources
and the technical knowledge to go to the stars. We will have examined from great distances
the diversity of other planetary systems, some very much like our own and some extremely
different. We will know which stars to visit. Our machines and our descendants will thenskim the light years, the children of Thales and Aristarchus, Leonardo and Einstein.
We are not yet certain how many planetary systems there are, but there seem to be agreat abundance. In our immediate vicinity, there is not just one, but in a sense four:
Jupiter, Saturn and Uranus each has a satellite system that, in the relative sizes and spacingsof the moons, resembles closely the planets about the Sun. Extrapolation of the statistics ofdouble stars which are greatly disparate in mass suggests that almost all single stars like theSun should have planetary companions.
We cannot yet directly see the planets of other stars, tiny points of light swamped in
the brilliance of their local suns. But we are becoming able to detect the gravitational
influence of an unseen planet on an observed star. Imagine such a star with a large ‘propermotion,’ moving over decades against the backdrop of more distant constellations; and witha large planet, the mass of Jupiter, say, whose orbital plane is by chance aligned at rightangles to our line of sight. When the dark planet is, from our perspective, to the right of thestar, the star will be pulled a little to the right, and conversely when the planet is to the left.
Consequently, the path of the star will be altered, or perturbed, from a straight line to awavy one. The nearest star for which this gravitational perturbation method can be applied
is Barnard’s Star, the nearest single star. The complex interactions of the three stars in theAlpha Centauri system would make the search for a low-mass companion there very
difficult. Even for Barnard’s Star, the investigation must be painstaking, a search formicroscopic displacements of position on photographic plates exposed at the telescope overa period of decades. Two such quests have been performed for planets around Barnard’sStar, and both have been by some criteria successful, implying the presence of two or moreplanets of Jovian mass moving in an orbit (calculated by Kepler’s third law) somewhat
closer to their star than Jupiter and Saturn are to the Sun. But unfortunately the two sets ofobservations seem mutually incompatible. A planetary system around Barnard’s Star may
well have been discovered, but an unambiguous demonstration awaits further study.
Other methods of detecting planets around the stars are under development,
including one where the obscuring light from the star is artificially occulted - with a disk infront of a space telescope, or by using the dark edge of the moon as such a disk - and thereflected light from the planet, no longer hidden by the brightness of the nearby star,emerges. In the next few decades we should have definitive answers to which of thehundred nearest stars have large planetary companions.
In recent years, infrared observations have revealed a number of likely preplanetarydisk-shaped clouds of gas and dust around some of the nearby stars. Meanwhile, someprovocative theoretical studies have suggested that planetary systems are a galactic
commonplace. A set of computer investigations has examined the evolution of a flat,
condensing disk of gas and dust of the sort that is thought to lead to stars and planets. Smalllumps of matter - the first condensations in the disk - are injected at random times into thecloud. The lumps accrete dust particles as they move. When they become sizable, they also
gravitationally attract gas, mainly hydrogen, in the cloud. When two moving lumps collide,the computer program makes them stick. The process continues until all the gas and dust
has been in this way used up. The results depend on the initial conditions, particularly on
the distribution of gas and dust density with distance from the center of the cloud. But for arange of plausible initial conditions, planetary systems - about ten planets, terrestrials close
to the star, Jovians on the exterior - recognizably like ours are generated. Under othercircumstances, there are no planets - just a smattering of asteroids; or there may be Jovian
planets near the star; or a Jovian planet may accrete so much gas and dust as to become astar, the origin of a binary star system. It is still too early to be sure, but it seems that asplendid variety of planetary systems is to be found throughout the Galaxy, and with highfrequency - all stars must come, we think, from such clouds of gas and dust. There may be
a hundred billion planetary systems in the Galaxy awaiting exploration.
Not one of those worlds will be identical to Earth. A few will be hospitable; mostwill appear hostile. Many will be achingly beautiful. In some worlds there will be manysuns in the daytime sky, many moons in the heavens at night, or great particle ring systems
soaring from horizon to horizon. Some moons will be so close that their planet will loom
high in the heavens, covering half the sky. And some worlds will look out onto a vast
gaseous nebula, the remains of an ordinary star that once was and is no longer. In all thoseskies, rich in distant and exotic constellations, there will be a faint yellow star - perhaps
barely seen by the naked eye, perhaps visible only through the telescope - the home star of
the fleet of interstellar transports exploring this tiny region of the great Milky Way Galaxy.
The themes of space and time are, as we have seen, intertwined. Worlds and stars,
like people, are born, live and die. The lifetime of a human being is measured in decades;
the lifetime of the Sun is a hundred million times longer. Compared to a star, we are like
mayflies, fleeting ephemeral creatures who live out their whole lives in the course of asingle day. From the point of view of a mayfly, human beings are stolid, boring, almost
entirely immovable, offering hardly a hint that they ever do anything. From the point ofview of a star, a human being is a tiny flash, one of billions of brief lives flickering
tenuously on the surface of a strangely cold, anomalously solid, exotically remote sphere ofsilicate and iron.
In all those other worlds in space there are events in progress, occurrences that will
determine their futures. And on our small planet, this moment in history is a historicalbranch point as profound as the confrontation of the Ionian scientists with the mystics 2,500
years ago. What we do with our world in this time will propagate down through thecenturies and powerfully determine the destiny of our descendants and their fate, if any,
among the stars.

CHAPTER IX
The Lives of the Stars
Opening his two eyes, [Ra, the Sun god] cast light on Egypt, he separated nightfrom day. The gods came forth from his mouth and mankind from his eyes. All
things took their birth from him, the child who shines in the lotus and whose rayscause all beings to live.
- An incantation from Ptolemaic Egypt
God is able to create particles of matter of several sizes and figures . . . and perhapsof different densities and forces, and thereby to vary the laws of Nature, and makeworlds of several sorts in several parts of the Universe. At least, I see nothing ofcontradiction in all this.
- Isaac Newton, Optics
We had the sky, up there, all speckled with stars, and we used to lay on our backs
and look up at them, and discuss about whether they was made, or only justhappened.
- Mark Twain, Huckleberry Finn
I have . . . a terrible need . . . shall I say the word? . . . of religion. Then I go out atnight and paint the stars.
- Vincent van Gogh
To make an apple pie, you need wheat, apples, a pinch of this and that, and the heat of theoven. The ingredients are made of molecules - sugar, say, or water. The molecules, in turn,are made of atoms - carbon, oxygen, hydrogen and a few others. Where do these atoms
come from? Except for hydrogen, they are all made in stars. A star is a kind of cosmic
kitchen inside which atoms of hydrogen are cooked into heavier atoms. Stars condensefrom interstellar gas and dust, which are composed mostly of hydrogen. But the hydrogen
was made in the Big Bang, the explosion that began the Cosmos. If you wish to make anapple pie from scratch, you must first invent the universe.
Suppose you take an apple pie and cut it in half; take one of the two pieces, cut it in
half; and, in the spirit of Democritus, continue. How many cuts before you are down to asingle atom? The answer is about ninety successive cuts. Of course, no knife could be sharp
enough, the pie is too crumbly, and the atom would in any case be too small to see unaided.
But there is a way to do it.
At Cambridge University in England, in the forty-five years centered on 1910, thenature of the atom was first understood - partly by shooting pieces of atoms at atoms andwatching how they bounce off. A typical atom has a kind of cloud of electrons on theoutside. Electrons are electrically charged, as their name suggests. The charge is arbitrarily
called negative. Electrons determine the chemical properties of the atom - the glitter ofgold, the cold feel of iron, the crystal structure of the carbon diamond. Deep inside theatom, hidden far beneath the electron cloud, is the nucleus, generally composed of
positively charged protons and electrically neutral neutrons. Atoms are very small - onehundred million of them end to end would be as large as the tip of your little finger. But thenucleus is a hundred thousand times smaller still, which is part of the reason it took so long
to be discovered.* Nevertheless, most of the mass of an atom is in its nucleus; the electrons
are by comparison just clouds of moving fluff. Atoms are mainly empty space. Matter iscomposed chiefly of nothing.
* It had previously been thought that the protons were uniformly distributed throughout the electron
cloud, rather than being concentrated in a nucleus of positive charge at the center. The nucleus was discoveredby Ernest Rutherford at Cambridge when some of the bombarding particles were bounced back in the
direction from which they had come. Rutherford commented: ‘It was quite the most incredible event that has
ever happened to me in my life. It was almost as incredible as if you fired a 15-inch [cannon] shell at a pieceof tissue paper and it came back and hit you.’

I am made of atoms. My elbow, which is resting on the table before me, is made ofatoms. The table is made of atoms. But if atoms are so small and empty and the nuclei
smaller still, why does the table hold me up? Why, as Arthur Eddington liked to ask, do the
nuclei that comprise my elbow not slide effortlessly through the nuclei that comprise thetable? Why don’t I wind up on the floor? Or fall straight through the Earth?
The answer is the electron cloud. The outside of an atom in my elbow has anegative electrical charge. So does every atom in the table. But negative charges repel eachother. My elbow does not slither through the table because atoms have electrons aroundtheir nuclei and because electrical forces are strong. Everyday life depends on the structureof the atom. Turn off the electrical charges and everything crumbles to an invisible finedust. Without electrical forces, there would no longer be things in the universe - merely
diffuse clouds of electrons, protons and neutrons, and gravitating spheres of elementary
particles, the featureless remnants of worlds.
When we consider cutting an apple pie, continuing down beyond a single atom, we
confront an infinity of the very small. And when we look up at the night sky, we confront
an infinity of the very large. These infinities represent an unending regress that goes on notjust very far, but forever. If you stand between two mirrors - in a barber shop, say - you seea large number of images of yourself, each the reflection of another. You cannot see an
infinity of images because the mirrors are not perfectly flat and aligned, because light does
not travel infinitely fast, and because you are in the way. When we talk about infinity weare talking about a quantity greater than any number, no matter how large.
The American mathematician Edward Kasner once asked his nine-year-old nephew
to invent a name for an extremely large number - ten to the power one hundred (10100), a
one followed by a hundred zeroes. The boy called it a googol. Here it is: 10, 000, 000, 000,
000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000,
000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000, 000. You, too, can make up yourown very large numbers and give them strange names. Try it. It has a certain charm,
especially if you happen to be nine.
If a googol seems large, consider a googolplex. It is ten to the power of a googol that is, a one followed by a googol zeros. By comparison, the total number of atoms in yourbody is about 1028, and the total number of elementary particles - protons and neutrons andelectrons - in the observable universe is about 1080. If the universe were packed solid* withneutrons, say, so there was no empty space anywhere, there would still be only about 10128
particles in it, quite a bit more than a googol but trivially small compared to a googolplex.
And yet these numbers, the googol and the googolplex, do not approach, they come

nowhere near, the idea of infinity. A googolplex is precisely as far from infinity as is thenumber one. We could try to write out a googolplex, but it is a forlorn ambition. A piece of
paper large enough to have all the zeroes in a googolplex written out explicitly could not be
stuffed into the known universe. Happily, there is a simpler and very concise way ofwriting a googolplex: 101010; and even infinity: (pronounced ‘infinity’).
* The spirit of this calculation is very old. The opening sentences of Archimedes’ The Sand
Reckoner are: ‘There are some, King Gelon, who think that the number of the sand is infinite in multitude:
and I mean by the sand not only that which exists about Syracuse and the rest of Sicily, but also that which isfound in every region, whether inhabited or uninhabited. And again, there are some who, without regarding it
as infinite, yet think that no number has been named which is great enough to exceed its multitude.’Archimedes then went on not only to name the number but to calculate it. Later he asked how many grains ofsand would fit, side by side, into the universe that he knew. His estimate; 1063, which corresponds, by acurious coincidence, to 1083 or so atoms.

In a burnt apple pie, the char is mostly carbon. Ninety cuts and you come to acarbon atom, with six protons and six neutrons in its nucleus and six electrons in theexterior cloud. If we were to pull a chunk out of the nucleus - say, one with two protons andtwo neutrons - it would be not the nucleus of a carbon atom, but the nucleus of a helium
atom. Such a cutting or fission of atomic nuclei occurs in nuclear weapons andconventional nuclear power plants, although it is not carbon that is split. If you make theninety-first cut of the apple pie, if you slice a carbon nucleus, you make not a smaller pieceof carbon, but something else - an atom with completely different chemical properties. If
you cut an atom, you transmute the elements.
But suppose we go farther. Atoms are made of protons, neutrons and electrons. Can
we cut a proton? If we bombard protons at high energies with other elementary particles other protons, say - we begin to glimpse more fundamental units hiding inside the proton.
Physicists now propose that so-called elementary particles such as protons and neutrons arein fact made of still more elementary particles called quarks, which come in a variety of‘colors’ and ‘flavors’, as their properties have been termed in a poignant attempt to makethe subnuclear world a little more like home. Are quarks the ultimate constituents of matter,or are they too composed of still smaller and more elementary particles? Will we ever cometo an end in our understanding of the nature of matter, or is there an infinite regression into
more and more fundamental particles? This is one of the great unsolved problems inscience.
The transmutation of the elements was pursued in medieval laboratories in a quest
called alchemy. Many alchemists believed that all matter was a mixture of four elementary
substances: water, air, earth and fire, an ancient Ionian speculation. By altering the relative
proportions of earth and fire, say, you would be able, they thought, to change copper into
gold. The field swarmed with charming frauds and con men, such as Cagliostro and theCount of Saint-Germain, who pretended not only to transmute the elements but also to holdthe secret of immortality. Sometimes gold was hidden in a wand with a false bottom, toappear miraculously in a crucible at the end of some arduous experimental demonstration.
With wealth and immortality the bait, the European nobility found itself transferring large
sums to the practitioners of this dubious art. But there were more serious alchemists such as
Paracelsus and even Isaac Newton. The money was not altogether wasted - new chemicalelements, such as phosphorous, antimony and mercury, were discovered. In fact, the originof modern chemistry can be traced directly to these experiments.

There are ninety-two chemically distinct kinds of naturally occurring atoms. They
are called the chemical elements and until recently constituted everything on our planet,although they are mainly found combined into molecules. Water is a molecule made ofhydrogen and oxygen atoms. Air is made mostly of the atoms nitrogen (N), oxygen (O),carbon (C), hydrogen (H) and argon (Ar), in the molecular forms N2, O2, CO2, H2O and Ar.
The Earth itself is a very rich mixture of atoms, mostly silicon,* oxygen, aluminum,magnesium and iron. Fire is not made of chemical elements at all. It is a radiating plasma inwhich the high temperature has stripped some of the electrons from their nuclei. Not one of
the four ancient Ionian and alchemical ‘elements’ is in the modern sense an element at all:one is a molecule, two are mixtures of molecules, and the last is a plasma.
* Silicon is an atom. Silicone is a molecule, one of billions of different varieties containing silicon.
Silicon and silicone have different properties and applications.

Since the time of the alchemists, more and more elements have been discovered, thelatest to be found tending to be the rarest. Many are familiar - those that primarily make up
the Earth; or those fundamental to life. Some are solids, some gases, and two (bromine andmercury) are liquids at room temperature. Scientists conventionally arrange them in orderof complexity. The simplest, hydrogen, is element 1; the most complex, uranium is element
92. Other elements are less familiar - hafnium, erbium, dysprosium and praseodymium,say, which we do not much bump into in everyday life. By and large, the more familiar anelement is, the more abundant it is. The Earth contains a great deal of iron and rather little
yttrium. There are, of course, exceptions to this rule, such as gold or uranium, elements
prized because of arbitrary economic conventions or aesthetic judgments, or because they
have remarkable practical applications.
The fact that atoms are composed of three kinds of elementary particles - protons,
neutrons and electrons is a comparatively recent finding. The neutron was not discovered
until 1932. Modern physics and chemistry have reduced the complexity of the sensible
world to an astonishing simplicity: three units put together in various patterns make,essentially, everything.
The neutrons, as we have said and as their name suggests, carry no electrical charge.
The protons have a positive charge and the electrons an equal negative charge. The
attraction between the unlike charges of electrons and protons is what holds the atom
together. Since each atom is electrically neutral, the number of protons in the nucleus must
exactly equal the number of electrons in the electron cloud. The chemistry of an atom
depends only on the number of electrons, which equals the number of protons, and which iscalled the atomic number. Chemistry is simply numbers, an idea Pythagoras would have
liked. If you are an atom with one proton, you are hydrogen; two, helium; three, lithium;four, beryllium; five, boron; six, carbon; seven, nitrogen; eight, oxygen; and so on, up to 92
protons, in which case your name is uranium.
Like charges, charges of the same sign, strongly repel one another. We can think of
it as a dedicated mutual aversion to their own kind, a little as if the world were densely
populated by anchorites and misanthropes. Electrons repel electrons. Protons repel protons.
So how can a nucleus stick together? Why does it not instantly fly apart? Because there isanother force of nature: not gravity, not electricity, but the short-range nuclear force, which,like a set of hooks that engage only when protons and neutrons come very close together,
thereby overcomes the electrical repulsion among the protons. The neutrons, whichcontribute nuclear forces of attraction and no electrical forces of repulsion, provide a kindof glue that helps to hold the nucleus together. Longing for solitude, the hermits have been
chained to their grumpy fellows and set among others given to indiscriminate and voluble
amiability.
Two protons and two neutrons are the nucleus of a helium atom, which turns out tobe very stable. Three helium nuclei make a carbon nucleus; four, oxygen; five, neon; six,magnesium; seven, silicon; eight, sulfur; and so on. Every time we add one or more protons
and enough neutrons to keep the nucleus together, we make a new chemical element. If we
subtract one proton and three neutrons from mercury, we make gold, the dream of theancient alchemists. Beyond uranium there are other elements that do not naturally occur on
Earth. They are synthesized by human beings and in most cases promptly fall to pieces.
One of them, Element 94, is called plutonium and is one of the most toxic substances
known. Unfortunately, it falls to pieces rather slowly.
Where do the naturally occurring elements come from? We might contemplate aseparate creation of each atomic species. But the universe, all of it, almost everywhere, is99 percent hydrogen and helium,* the two simplest elements. Helium, in fact, was detectedon the Sun before it was found on the Earth - hence its name (from Helios, one of the Greeksun gods). Might the other chemical elements have somehow evolved from hydrogen andhelium? To balance the electrical repulsion, pieces of nuclear matter would have to bebrought very close together so that the short-range nuclear forces are engaged. This canhappen only at very high temperatures where the particles are moving so fast that therepulsive force does not have time to act - temperatures of tens of millions of degrees. In
nature, such high temperatures and attendant high pressures are common only in the insidesof the stars.
* The Earth is an exception, because our primordial hydrogen, only weakly bound by our planet’s
comparatively feeble gravitational attraction, has by now largely escaped to space. Jupiter, with its more
massive gravity, has retained at least much of its original complement of the lightest element.

We have examined our Sun, the nearest star, in various wavelengths from radio
waves to ordinary visible light to X-rays, all of which arise only from its outermost layers.
It is not exactly a red-hot stone, as Anaxagoras thought, but rather a great ball of hydrogenand helium gas, glowing because of its high temperatures, in the same way that a poker
glows when it is brought to red heat. Anaxagoras was at least partly right. Violent solar
storms produce brilliant flares that disrupt radio communications on Earth; and immense
arching plumes of hot gas, guided by the Sun’s magnetic field, the solar prominences,which dwarf the Earth. The sunspots, sometimes visible to the naked eye at sunset, arecooler regions of enhanced magnetic field strength. All this incessant, roiling, turbulent
activity is in the comparatively cool visible surface. We see only to temperatures of about
6,000 degrees. But the hidden interior of the Sun, where sunlight is being generated, is at
40 million degrees.
Stars and their accompanying planets are born in the gravitational collapse of acloud of interstellar gas and dust. The collision of the gas molecules in the interior of thecloud heats it, eventually to the point where hydrogen begins to fuse into helium: fourhydrogen nuclei combine to form a helium nucleus, with an attendant release of a gammaray photon. Suffering alternate absorption and emission by the overlying matter, graduallyworking its way toward the surface of the star, losing energy at every step, the photon’sepic journey takes a million years until, as visible light, it reaches the surface and isradiated to space. The star has turned on. The gravitational collapse of the prestellar cloudhas been halted. The weight of the outer layers of the star is now supported by the hightemperatures and pressures generated in the interior nuclear reactions. The Sun has been insuch a stable situation for the past five billion years. Thermonuclear reactions like those ina hydrogen bomb are powering the Sun in a contained and continuous explosion,
converting some four hundred million tons (4 x 1014 grams) of hydrogen into helium every
second. When we look up at night and view the stars, everything we see is shining becauseof distant nuclear fusion.
In the direction of the star Deneb, in the constellation of Cygnus the Swan, is anenormous glowing superbubble of extremely hot gas, probably produced by supernova
explosions, the deaths of stars, near the center of the bubble. At the periphery, interstellar
matter is compressed by the supernova shock wave, triggering new generations of cloud
collapse and star formation. In this sense, stars have parents; and, as is sometimes also truefor humans, a parent may die in the birth of the child.
Stars like the Sun are born in batches, in great compressed cloud complexes such as
the Orion Nebula. Seen from the outside, such clouds seem dark and gloomy. But inside,
they are brilliantly illuminated by the hot newborn stars. Later, the stars wander out of theirnursery to seek their fortunes in the Milky Way, stellar adolescents still surrounded by tufts
of glowing nebulosity, residues still gravitationally attached of their amniotic gas. The
Pleiades are a nearby example. As in the families of humans, the maturing stars journey far
from home, and the siblings see little of each other. Somewhere in the Galaxy there arestars - perhaps dozens of them - that are the brothers and sisters of the Sun, formed from thesame cloud complex, some 5 billion years ago. But we do not know which stars they are.
They may, for all we know, be on the other side of the Milky Way.
The conversion of hydrogen into helium in the center of the Sun not only accounts
for the Sun’s brightness in photons of visible light; it also produces a radiance of a moremysterious and ghostly kind: The Sun glows faintly in neutrinos, which, like photons,
weigh nothing and travel at the speed of light. But neutrinos are not photons. They are not akind of light. Neutrinos, like protons, electrons and neutrons, carry an intrinsic angularmomentum, or spin, while photons have no spin at all. Matter is transparent to neutrinos,
which pass almost effortlessly through the Earth and through the Sun. Only a tiny fractionof them is stopped by the intervening matter. As I look up at the Sun for a second, a billion
neutrinos pass through my eyeball. Of course, they are not stopped at the retina as ordinary
photons are but continue unmolested through the back of my head. The curious part is that
if at night I look down at the ground, toward the place where the Sun would be (if the Earthwere not in the way), almost exactly the same number of solar neutrinos pass through myeyeball, pouring through an interposed Earth which is as transparent to neutrinos as a paneof clear glass is to visible light.
If our knowledge of the solar interior is as complete as we think, and if we also
understand the nuclear physics that makes neutrinos, then we should be able to calculate
with fair accuracy how many solar neutrinos we should receive in a given area - such as myeyeball - in a given unit of time, such as a second. Experimental confirmation of thecalculation is much more difficult. Since neutrinos pass directly through the Earth, wecannot catch a given one. But for a vast number of neutrinos, a small fraction will interact
with matter and in the appropriate circumstances might be detected. Neutrinos can on rareoccasion convert chlorine atoms into argon atoms, with the same total number of protons

and neutrons. To detect the predicted solar neutrino flux, you need an immense amount ofchlorine, so American physicists have poured a huge quantity of cleaning fluid into theHomestake Mine in Lead, South Dakota. The chlorine is microchemically swept for the
newly produced argon. The more argon found, the more neutrinos inferred. Theseexperiments imply that the Sun is dimmer in neutrinos than the calculations predict.
There is a real and unsolved mystery here. The low solar neutrino flux probably
does not put our view of stellar nucleosynthesis in jeopardy, but it surely means somethingimportant. Proposed explanations range from the hypothesis that neutrinos fall to pieces
during their passage between the Sun and the Earth to the idea that the nuclear fires in thesolar interior are temporarily banked, sunlight being generated in our time partly by slow
gravitational contraction. But neutrino astronomy is very new. For the moment we standamazed at having created a tool that can peer directly into the blazing heart of the Sun. As
the sensitivity of the neutrino telescope improves, it may become possible to probe nuclear
fusion in the deep interiors of the nearby stars.
But hydrogen fusion cannot continue forever: in the Sun or any other star, there is
only so much hydrogen fuel in its hot interior. The fate of a star, the end of its life cycle,
depends very much on its initial mass. If, after whatever matter it has lost to space, a starretains two or three times the mass of the Sun, it ends its life cycle in a startlingly differentmode than the Sun. But the Sun’s fate is spectacular enough. When the central hydrogen
has all reacted to form helium, five or six billion years from now, the zone of hydrogenfusion will slowly migrate outward, an expanding shell of thermonuclear reactions, until itreaches the place where the temperatures are less than about ten million degrees. Thenhydrogen fusion will shut itself off. Meanwhile the self-gravity of the Sun will force a
renewed contraction of its helium-rich core and a further increase in its interiortemperatures and pressures. The helium nuclei will be jammed together still more tightly,
so much so that they begin to stick together, the hooks of their short-range nuclear forces
becoming engaged despite the mutual electrical repulsion. The ash will become fuel, andthe Sun will be triggered into a second round of fusion reactions.
This process will generate the elements carbon and oxygen and provide additionalenergy for the Sun to continue shining for a limited time. A star is a phoenix, destined torise for a time from its own ashes.* Under the combined influence of hydrogen fusion in athin shell far from the solar interior and the high temperature helium fusion in the core, theSun will undergo a major change: its exterior will expand and cool. The Sun will become a
red giant star, its visible surface so far from its interior that the gravity at its surface grows
feeble, its atmosphere expanding into space in a kind of stellar gale. When the Sun, ruddy
and bloated, becomes a red giant, it will envelop and devour the planets Mercury and
Venus - and probably the Earth as well. The inner solar system will then reside within theSun.
* Stars more massive than the Sun achieve higher central temperatures and pressures in their lateevolutionary stages. They are able to rise more than once from their ashes, using carbon and oxygen as fuelfor synthesizing still heavier elements.

Billions of years from now, there will be a last perfect day on Earth. Thereafter the
Sun will slowly become red and distended, presiding over an Earth sweltering even at thepoles. The Arctic and Antarctic icecaps will melt, flooding the coasts of the world. The
high oceanic temperatures will release more water vapor into the air, increasing cloudiness,shielding the Earth from sunlight and delaying the end a little. But solar evolution is
inexorable. Eventually the oceans will boil, the atmosphere will evaporate away to space
and a catastrophe of the most immense proportions imaginable will overtake our planet.* In
the meantime, human beings will almost certainly have evolved into something quite
different. Perhaps our descendants will be able to control or moderate stellar evolution. Or
perhaps they will merely pick up and leave for Mars or Europa or Titan or, at last, as
Robert Goddard envisioned, seek out an uninhabited planet in some young and promisingplanetary system.
* The Aztecs foretold a time ‘when the Earth has become tired. . ., when the seed of Earth has
ended.’ On that day, they believed, the Sun will fall from the sky and the stars will be shaken from theheavens.

The Sun’s stellar ash can be reused for fuel only up to a point. Eventually the time
will come when the solar interior is all carbon and oxygen, when at the prevailingtemperatures and pressures no further nuclear reactions can occur. After the central helium
is almost all used up, the interior of the Sun will continue its postponed collapse, thetemperatures will rise again, triggering a last round of nuclear reactions and expanding thesolar atmosphere a little. In its death throes, the Sun will slowly pulsate, expanding andcontracting once every few millennia, eventually spewing its atmosphere into space in one
or more concentric shells of gas. The hot exposed solar interior will flood the shell withultraviolet light, inducing a lovely red and blue fluorescence extending beyond the orbit of
Pluto. Perhaps half the mass of the Sun will be lost in this way. The solar system will thenbe filled with an eerie radiance, the ghost of the Sun, outward bound.
When we look around us in our little corner of the Milky Way, we see many stars
surrounded by spherical shells of glowing gas, the planetary nebulae. (They have nothing to
do with planets, but some of them seemed reminiscent in inferior telescopes of the bluegreen discs of Uranus and Neptune.) They appear as rings, but only because, as with soap
bubbles, we see more of them at the periphery than at the center. Every planetary nebula isa token of a star in extremis. Near the central star there may be a retinue of dead worlds, theremnants of planets once full of life and now airless and ocean-free, bathed in a wraithlike
luminance. The remains of the Sun, the exposed solar core at first enveloped in its planetarynebula, will be a small hot star, cooling to space, collapsed to a density unheard of on
Earth, more than a ton per teaspoonful. Billions of years hence, the Sun will become adegenerate white dwarf, cooling like all those points of light we see at the centers ofplanetary nebulae from high surface temperatures to its ultimate state, a dark and deadblack dwarf.
Two stars of roughly the same mass will evolve roughly in parallel. But a more
massive star will spend its nuclear fuel faster, become a red giant sooner, and be first toenter the final white dwarf decline. There should therefore be, as there are, many cases ofbinary stars, one component a red giant, the other a white dwarf. Some such pairs are so
close together that they touch, and the glowing stellar atmosphere flows from the distended
red giant to the compact white dwarf, tending to fall on a particular province of the surfaceof the white dwarf. The hydrogen accumulates, compressed to higher and higher pressuresand temperatures by the intense gravity of the white dwarf, until the stolen atmosphere of
the red giant undergoes thermonuclear reactions, and the white dwarf briefly flares into
brilliance. Such a binary is called a nova and has quite a different origin from a supernova.

Novae occur only in binary systems and are powered by hydrogen fusion; supernovae occur
in single stars and are powered by silicon fusion.
Atoms synthesized in the interiors of stars are commonly returned to the interstellar
gas. Red giants find their outer atmospheres blowing away into space; planetary nebulae
are the final stages of Sunlike stars blowing their tops. Supernovae violently eject much of
their stellar mass into space. The atoms returned are, naturally, those most readily made in
the thermonuclear reactions in stellar interiors: Hydrogen fuses into helium, helium into
carbon, carbon into oxygen and thereafter, in massive stars, by the successive addition offurther helium nuclei, neon, magnesium, silicon, sulfur, and so on are built additions bystages, two protons and two neutrons per stage, all the way to iron. Direct fusion of siliconalso generates iron, a pair of silicon atoms, each with twenty-eight protons and neutrons,
joining, at a temperature of billions of degrees, to make an atom of iron with fifty-six
protons and neutrons.
These are all familiar chemical elements. We recognize their names. Such stellar
nuclear reactions do not readily generate erbium, hafnium, dysprosium, praseodymium or
yttrium, but rather the elements we know in everyday life, elements returned to theinterstellar gas, where they are swept up in a subsequent generation of cloud collapse andstar and planet formation. All the elements of the Earth except hydrogen and some helium
have been cooked by a kind of stellar alchemy billions of years ago in stars, some of which
are today inconspicuous white dwarfs on the other side of the Milky Way Galaxy. The
nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple
pies were made in the interiors of collapsing stars. We are made of starstuff.
Some of the rarer elements are generated in the supernova explosion itself. We haverelatively abundant gold and uranium on Earth only because many supernova explosions
had occurred just before the solar system formed. Other planetary systems may havesomewhat different amounts of our rare elements. Are there planets where the inhabitants
proudly display pendants of niobium and bracelets of protactinium, while gold is alaboratory curiosity? Would our lives be improved if gold and uranium were as obscure andunimportant on Earth as praseodymium?
The origin and evolution of life are connected in the most intimate way with theorigin and evolution of the stars. First: The very matter of which we are composed, theatoms that make life possible, were generated long ago and far away in giant red stars. The
relative abundance of the chemical elements found in the Cosmos matches the relative
abundance of atoms generated in stars so well as to leave little doubt that red giants andsupernovae are the ovens and crucibles in which matter has been forged. The Sun is asecond- or third-generation star. All the matter in it, all the matter you see around you, hasbeen through one or two previous cycles of stellar alchemy. Second: The existence of
certain varieties of heavy atoms on the Earth suggests that there was a nearby supernova
explosion shortly before the solar system was formed. But this is unlikely to be a mere
coincidence; more likely, the shock wave produced by the supernova compressedinterstellar gas and dust and triggered the condensation of the solar system. Third: When
the Sun turned on, its ultraviolet radiation poured into the atmosphere of the Earth; its
warmth generated lightning; and these energy sources sparked the complex organic
molecules that led to the origin of life. Fourth: Life on Earth runs almost exclusively onsunlight. Plants gather the photons and convert solar to chemical energy. Animals parasitize
the plants. Farming is simply the methodical harvesting of sunlight, using plants as
grudging intermediaries. We are, almost all of us, solar-powered. Finally, the hereditarychanges called mutations provide the raw material for evolution. Mutations, from which
nature selects its new inventory of life forms, are produced in part by cosmic rays - highenergy particles ejected almost at the speed of light in supernova explosions. The evolutionof life on Earth is driven in part by the spectacular deaths of distant, massive suns.
Imagine carrying a Geiger counter and a piece of uranium ore to some place deep
beneath the Earth - a gold mine, say, or a lava tube, a cave carved through the Earth by ariver of molten rock. The sensitive counter clicks when exposed to gamma rays or to suchhigh-energy charged particles as protons and helium nuclei. If we bring it close to theuranium ore, which is emitting helium nuclei in a spontaneous. nuclear decay, the countrate, the number of clicks per minute, increases dramatically. If we drop the uranium ore
into a heavy lead canister, the count rate declines substantially; the lead has absorbed theuranium radiation. But some clicks can still be heard. Of the remaining counts, a fraction
come from natural radioactivity in the walls of the cave. But there are more clicks than canbe accounted for by radioactivity. Some of them are caused by high-energy charged
particles penetrating the roof. We are listening to cosmic rays, produced in another age inthe depths of space. Cosmic rays, mainly electrons and protons, have bombarded the Earthfor the entire history of life on our planet. A star destroys itself thousands of light-years
away and produces cosmic rays that spiral through the Milky Way Galaxy for millions ofyears until, quite by accident, some of them strike the Earth, and our hereditary material.
Perhaps some key steps in the development of the genetic code, or the Cambrian explosion,or bipedal stature among our ancestors were initiated by cosmic rays.
On July 4, in the year 1054, Chinese astronomers recorded what they called a ‘guest
star’ in the constellation of Taurus, the Bull. A star never before seen became brighter thanany star in the sky. Halfway around the world, in the American Southwest, there was then ahigh culture, rich in astronomical tradition, that also witnessed this brilliant new star.*
From carbon 14 dating of the remains of a charcoal fire, we know that in the middle
eleventh century some Anasazi, the antecedents of the Hopi of today, were living under anoverhanging ledge in what is today New Mexico. One of them seems to have drawn on thecliff overhang, protected from the weather, a picture of the new star. Its position relative tothe crescent moon would have been just as was depicted. There is also a handprint, perhaps
the artist’s signature.
* Moslem observers noted it as well. But there is not a word about it in all the chronicles of Europe.

This remarkable star, 5,000 light-years distant, is now called the Crab Supernova,
because an astronomer centuries later was unaccountably reminded of a crab when looking
at the explosion remnant through his telescope. The Crab Nebula is the remains of a
massive star that blew itself up. The explosion was seen on Earth with the naked eye forthree months. Easily visible in broad daylight, you could read by it at night. On the average,a supernova occurs in a given galaxy about once every century. During the lifetime of atypical galaxy, about ten billion years, a hundred million stars will have exploded - a great
many, but still only about one star in a thousand. In the Milky Way, after the event of 1054,
there was a supernova observed in 1572, and described by Tycho Brahe, and another, justafter, in 1604, described by Johannes Kepler.* Unhappily, no supernova explosions havebeen observed in our Galaxy since the invention of the telescope, and astronomers havebeen chafing at the bit for some centuries.

* Kepler published in 1606 a book called De Stella Nova, ‘On the New Star,’ in which he wonders if
a supernova is the result of some random concatenation of atoms in the heavens. He presents what he says is‘. . . not my own opinion, but my wife’s: Yesterday, when weary with writing, I was called to supper, and asalad I had asked for was set before me. “It seems then,” I said, “if pewter dishes, leaves of lettuce, grains ofsalt, drops of water, vinegar, oil and slices of eggs had been flying about in the air for all eternity, it might atlast happen by chance that there would come a salad.” “Yes,” responded my lovely, “but not so nice as thisone of mine.” ’Supernovae are now routinely observed in other galaxies. Among my candidates forthe sentence that would most thoroughly astonish an astronomer of the early 1900’s is thefollowing, from a paper by David Helfand and Knox Long in the December 6, 1979, issueof the British journal Nature: ‘On 5 March, 1979, an extremely intense burst of hard x-rays
and gamma rays was recorded by the nine interplanetary spacecraft of the burst sensor
network, and localized by time-of-flight determinations to a position coincident with thesupernova remnant N49 in the Large Magellanic Cloud.’ (The Large Magellanic Cloud, socalled because the first inhabitant of the Northern Hemisphere to notice it was Magellan, isa small satellite galaxy of the Milky Way, 180,000 light-years distant. There is also, as you
might expect, a Small Magellanic Cloud.) However, in the same issue of Nature, E. P.
Mazets and colleagues of the Ioffe Institute, Leningrad - who observed this source with the
gamma-ray burst detector aboard the Venera 11 and 12 spacecraft on their way to land on
Venus - argue that what is being seen is a flaring pulsar only a few hundred light-years
away. But despite the close agreement in position Helfand and Long do not insist that the
gamma-ray outburst is associated with the supernova remnant. They charitably consider
many alternatives, including the surprising possibility that the source lies within the solar
system. Perhaps it is the exhaust of an alien starship on its long voyage home. But a rousingof the stellar fires in N49 is a simpler hypothesis: we are sure there are such things as
supernovae.
The fate of the inner solar system as the Sun becomes a red giant is grim enough.
But at least the planets will never be melted and frizzled by an erupting supernova. That isa fate reserved for planets near stars more massive than the Sun. Since such stars withhigher temperatures and pressures run rapidly through their store of nuclear fuel, their
lifetimes are much shorter than the Sun’s. A star tens of times more massive than the Sun
can stably convert hydrogen to helium for only a few million years before moving briefly
on to more exotic nuclear reactions. Thus there is almost certainly not enough time for theevolution of advanced forms of life on any accompanying planets; and it will be rare thatbeings elsewhere can ever know that their star will become a supernova: if they live longenough to understand supernovae, their star is unlikely to become one.
The essential preliminary to a supernova explosion is the generation by silicon
fusion of a massive iron core. Under enormous pressure, the free electrons in the stellarinterior are forceably melded with the protons of the iron nuclei, the equal and opposite
electrical charges canceling each other out; the inside of the star is turned into a single giant
atomic nucleus, occupying a much smaller volume than the precursor electrons and ironnuclei. The core implodes violently, the exterior rebounds and a supernova explosion
results. A supernova can be brighter than the combined radiance of all the other stars in thegalaxy within which it is embedded. All those recently hatched massive blue-whitesupergiant stars in Orion are destined in the next few million years to become supernovae, acontinuing cosmic fireworks in the constellation of the hunter.
The awesome supernova explosion ejects into space most of the matter of theprecursor star - a little residual hydrogen and helium and significant amounts of otheratoms, carbon and silicon, iron and uranium. Remaining is a core of hot neutrons, boundtogether by nuclear forces, a single, massive atomic nucleus with an atomic weight about
1056, a sun thirty kilometers across; a tiny, shrunken, dense, withered stellar fragment, a
rapidly rotating neutron star. As the core of a massive red giant collapses to form such aneutron star, it spins faster. The neutron star at the center of the Crab Nebula is an immense
atomic nucleus, about the size of Manhattan, spinning thirty times a second. Its powerfulmagnetic field, amplified during the collapse, traps charged particles rather as the muchtinier magnetic field of Jupiter does. Electrons in the rotating magnetic field emit beamed
radiation not only at radio frequencies but in visible light as well. If the Earth happens to lie
in the beam of this cosmic lighthouse, we see it flash once each rotation. This is the reason
it is called a pulsar. Blinking and ticking like a cosmic metronome, pulsars keep far better
time than the most accurate ordinary clock. Long-term timing of the radio pulse rate ofsome pulsars, for instance, one called PSR 0329+54, suggests that these objects may haveone or more small planetary companions. It is perhaps conceivable that a planet could
survive the evolution of a star into a pulsar; or a planet could be captured at a later time. I
wonder how the sky would look from the surface of such a planet.
Neutron star matter weighs -about the same as an ordinary mountain per
teaspoonful - so much that if you had a piece of it and let it go (you could hardly dootherwise), it might pass effortlessly through the Earth like a falling stone through air,carving a hole for itself completely through our planet and emerging out the other side perhaps in China. People there might be out for a stroll, minding their own business, whena tiny lump of neutron star plummets out of the ground, hovers for a moment, and thenreturns beneath the Earth, providing at least a diversion from the routine of the day. If apiece of neutron star matter were dropped from nearby space, with the Earth rotating
beneath it as it fell, it would plunge repeatedly through the rotating Earth, punching
hundreds of thousands of holes before friction with the interior of our planet stopped the
motion. Before it comes to rest at the center of the Earth, the inside of our planet mightlook briefly like a Swiss cheese until the subterranean flow of rock and metal healed thewounds. It is just as well that large lumps of neutron star matter are unknown on Earth. But
small lumps are everywhere. The awesome power of the neutron star is lurking in the
nucleus of every atom, hidden in every teacup and dormouse, every breath of air, every
apple pie. The neutron star teaches us respect for the commonplace.
A star like the Sun will end its days, as we have seen, as a red giant and then a whitedwarf. A collapsing star twice as massive as the Sun will become a supernova and then aneutron star. But a more massive star, left, after its supernova phase, with, say, five times
the Sun’s mass, has an even more remarkable fate reserved for it - its gravity will turn it
into a black hole. Suppose we had a magic gravity machine - a device with which we could
control the Earth’s gravity, perhaps by turning a dial. Initially the dial is set at 1 g* andeverything behaves as we have grown up to expect. The animals and plants on Earth andthe structures of our buildings are all evolved or designed for 1 g. If the gravity were muchless, there might be tall, spindly shapes that would not be tumbled or crushed by their own
weight. If the gravity were much more, plants and animals and architecture would have tobe short and squat and sturdy in order not to collapse. But even in a fairly strong gravity
field, light would travel in a straight line, as it does, of course, in everyday life.
* 1 g is the acceleration experienced by falling objects on the Earth, almost 10 meters per secondevery second. A falling rock will reach a speed of 10 meters per second after one second of fall, 20 meters per
second after two seconds, and so on until it strikes the ground or is slowed by friction with the air. On a world
where the gravitational acceleration was much greater, falling bodies would increase their speed bycorrespondingly greater amounts. On a world with 10 g acceleration, a rock would travel 10 x 10 m/sec oralmost 100 m/sec after the first second, 200 m/sec after the next second, and so on. A slight stumble could befatal. The acceleration due to gravity should always be written with a lowercase g, to distinguish it from the
Newtonian gravitational constant, G, which is a measure of the strength of gravity everywhere in the universe,
not merely on whatever world or sun we are discussing. (The Newtonian relationship of the two quantities is
F = mg = GMm/r2; g = GM/r2, where F is the gravitational force, M is the mass of the planet or star, m is themass of the falling object, and r is the distance from the falling object to the center of the planet or star.)

Consider a possibly typical group of Earth beings, Alice and her friends from Alicein Wonderland at the Mad Hatter’s tea party. As we lower the gravity, things weigh less.
Near 0 g the slightest motion sends our friends floating and tumbling up in the air. Spilled
tea - or any other liquid - forms throbbing spherical globs in the air: the surface tension of
the liquid overwhelms gravity. Balls of tea are everywhere. If now we dial 1 g again, we
make a rain of tea. When we increase the gravity a little - from 1 g to, say, 3 or 4 g’s everyone becomes immobilized: even moving a paw requires enormous effort. As akindness we remove our friends from the domain of the gravity machine before we dial
higher gravities still. The beam from a lantern travels in a perfectly straight line (as nearly
as we can see) at a few g’s, as it does at 0 g. At 1000 g’s, the beam is still straight, but trees
have become squashed and flattened; at 100,000 g’s, rocks are crushed by their own
weight. Eventually, nothing at all survives except, through a special dispensation, theCheshire cat. When the gravity approaches a billion g’s, something still more strange
happens. The beam of light, which has until now been heading straight up into the sky, isbeginning to bend. Under extremely strong gravitational accelerations, even light isaffected. If we increase the gravity still more, the light is pulled back to the ground near us.
Now the cosmic Cheshire cat has vanished; only its gravitational grin remains.
When the gravity is sufficiently high, nothing, not even light, can get out. Such aplace is called a black hole. Enigmatically indifferent to its surroundings, it is a kind of
cosmic Cheshire cat. When the density and gravity become sufficiently high, the black holewinks out and disappears from our universe. That is why it is called black: no light canescape from it. On the inside, because the light is trapped down there, things may beattractively well-lit. Even if a black hole is invisible from the outside, its gravitationalpresence can be palpable. If, on an interstellar voyage, you are not paying attention, you
can find yourself drawn into it irrevocably, your body stretched unpleasantly into a long,thin thread. But the matter accreting into a disk surrounding the black hole would be a sightworth remembering, in the unlikely case that you survived the trip.
Thermonuclear reactions in the solar interior support the outer layers of the Sun andpostpone for billions of years a catastrophic gravitational collapse. For white dwarfs, thepressure of the electrons, stripped from their nuclei, holds the star up. For neutron stars, thepressure of the neutrons staves off gravity. But for an elderly star left after supernova
explosions and other impetuosities with more than several times the Sun’s mass, there are
no forces known that can prevent collapse. The star shrinks incredibly, spins, reddens anddisappears. A star twenty times the mass of the Sun will shrink until it is the size of Greater
Los Angeles; the crushing gravity becomes 1010 g’s, and the star slips through a selfgenerated crack in the space-time continuum and vanishes from our universe.
Black holes were first thought of by the English astronomer John Michell in 1783.

But the idea seemed so bizarre that it was generally ignored until quite recently. Then, tothe astonishment of many, including many astronomers, evidence was actually found forthe existence of black holes in space. The Earth’s atmosphere is opaque to X-rays. To
determine whether astronomical objects emit such short wavelengths of light, an X-ray
telescope must be carried aloft. The first X-ray observatory was an admirably international
effort, orbited by the United States from an Italian launch platform in the Indian Ocean off
the coast of Kenya and named Uhuru, the Swahili word for ‘freedom’. In 1971, Uhuru
discovered a remarkably bright X-ray source in the constellation of Cygnus, the Swan,
flickering on and off a thousand times a second. The source, called Cygnus X-1, must
therefore be very small. Whatever the reason for the flicker, information on when to turn on
and off can cross Cyg X-1 no faster than the speed of light, 300,000 km/sec. Thus Cyg X-1
can be no larger than [300,000 km/sec] x [(1/ 1000) sec] = 300 kilometers across.
Something the size of an asteroid is a brilliant, blinking source of X-rays, visible overinterstellar distances. What could it possibly be? Cyg X-1 is in precisely the same place inthe sky as a hot blue supergiant star, which reveals itself in visible light to have a massive
close but unseen companion that gravitationally tugs it first in one direction and then inanother. The companion’s mass is about ten times that of the Sun. The supergiant is anunlikely source of X-rays, and it is tempting to identify the companion inferred in visible
light with the source detected in X-ray light. But an invisible object weighing ten times
more than the Sun and collapsed into a volume the size of an asteroid can only be a black
hole. The X-rays are plausibly generated by friction in the disk of gas and dust accreted
around Cyg X-1 from its supergiant companion. Other stars called V861 Scorpii, GX339-4,
SS433, and Circinus X-2 are also candidate black holes. Cassiopeia A is the remnant of asupernova whose light should have reached the Earth in the seventeenth century, when
there were a fair number of astronomers. Yet no one reported the explosion. Perhaps, as I.
S. Shklovskii has suggested, there is a black hole hiding there, which ate the exploding
stellar core and damped the fires of the supernova. Telescopes in space are the means forchecking these shards and fragments of data that may be the spoor, the trail, of thelegendary black hole.
A helpful way to understand black holes is to think about the curvature of space.
Consider a flat, flexible, lined two-dimensional surface, like a piece of graph paper made ofrubber. If we drop a small mass, the surface is deformed or puckered. A marble rolls aroundthe pucker in an orbit like that of a planet around the Sun. In this interpretation, which we
owe to Einstein, gravity is a distortion in the fabric of space. In our example, we see twodimensional space warped by mass into a third physical dimension. Imagine we live in athree-dimensional universe, locally distorted by matter into a fourth physical dimensionthat we cannot perceive directly. The greater the local mass, the more intense the local
gravity, and the more severe the pucker, distortion or warp of space. In this analogy, a blackhole is a kind of bottomless pit. What happens if you fall in? As seen from the outside, you
would take an infinite amount of time to fall in, because all your clocks - mechanical andbiological would be perceived as having stopped. But from your point of view, all yourclocks would be ticking away normally. If you could somehow survive the gravitational
tides and radiation flux, and (a likely assumption) if the black hole were rotating, it is justpossible that you might emerge in another part of space-time - somewhere else in space,somewhere else in time. Such worm holes in space, a little like those in an apple, have been
seriously suggested, although they have by no means been proved to exist. Might gravity
tunnels provide a kind of interstellar or intergalactic subway, permitting us to travel toinaccessible places much more rapidly than we could in the ordinary way? Can black holes
serve as time machines, carrying us to the remote past or the distant future? The fact thatsuch ideas are being discussed even semi-seriously shows how surreal the universe may be.
We are, in the most profound sense, children of the Cosmos. Think of the Sun’sheat on your upturned face on a cloudless summer’s day; think how dangerous it is to gazeat the Sun directly. From 150 million kilometers away, we recognize its power. Whatwould we feel on its seething self-luminous surface, or immersed in its heart of nuclear
fire? The Sun warms us and feeds us and permits us to see. It fecundated the Earth. It is
powerful beyond human experience. Birds greet the sunrise with an audible ecstasy. Evensome one-celled organisms know to swim to the light. Our ancestors worshiped the Sun,*and they were far from foolish. And yet the Sun is an ordinary, even a mediocre star. If wemust worship a power greater than ourselves, does it not make sense to revere the Sun andstars? Hidden within every astronomical investigation, sometimes so deeply buried that the
researcher himself is unaware of its presence, lies a kernel of awe.
* The early Sumerian pictograph for god was an asterisk, the symbol of the stars. The Aztec word for
god was Teotl, and its glyph was a representation of the Sun. The heavens were called the Teoatl, the godsea,
the cosmic ocean.

The Galaxy is an unexplored continent filled with exotic beings of stellar
dimensions. We have made a preliminary reconnaissance and have encountered some of the
inhabitants. A few of them resemble beings we know. Others are bizarre beyond our most
unconstrained fantasies. But we are at the very beginning of our exploration. Past voyagesof discovery suggest that many of the most interesting inhabitants of the galactic continentremain as yet unknown and unanticipated. Not far outside the Galaxy there are almost
certainly planets, orbiting stars in the Magellanic Clouds and in the globular clusters thatsurround the Milky Way. Such worlds would offer a breathtaking view of the Galaxy rising- an enormous spiral form comprising 400 billion stellar inhabitants, with collapsing gas
clouds, condensing planetary systems, luminous supergiants, stable middle-aged stars, red
giants, white dwarfs, planetary nebulae, novae, supernovae, neutron stars and black holes. It
would be clear from such a world, as it is beginning to be clear from ours, how our matter,our form and much of our character is determined by the deep connection between life andthe Cosmos.

CHAPTER X
The Edge of Forever
There is a thing confusedly formed,
Born before Heaven and Earth.
Silent and void
It stands alone and does not change,
Goes round and does not weary.
It is capable of being the mother of the world.
I know not its nameSo I style it ‘The Way.’
I give it the makeshift name of ‘The Great.’
Being great, it is further described as receding,
Receding, it is described as far away,
Being far away, it is described as turning back.
- Lao-tse, Tao Te Ching (China, about 600 B.C.)
There is a way on high, conspicuous in the clear heavens, called the Milky Way,brilliant with its own brightness. By it the gods go to the dwelling of the great
Thunderer and his royal abode . . . Here the famous and mighty inhabitants ofheaven have their homes. This is the region which I might make bold to call the
Palatine [Way] of the Great Sky.
- Ovid, Metamorphoses (Rome, first century)
Some foolish men declare that a Creator made the world. The doctrine that theworld was created is ill-advised, and should be rejected. If God created the world,
where was He before creation? . . .How could God have made the world without
any raw material? If you say He made this first, and then the world, you are facedwith an endless regression . . . Know that the world is uncreated, as time itself is,without beginning and end. And it is based on the principles . . .
- The Mahapurana (The Great Legend), Jinasena (India, ninth century)
Ten or twenty billion years ago, something happened - the Big Bang, the event that began
our universe. Why it happened is the greatest mystery we know. That it happened isreasonably clear. All the matter and energy now in the universe was concentrated at
extremely high density - a kind of cosmic egg, reminiscent of the creation myths of many
cultures - perhaps into a mathematical point with no dimensions at all. It was not that all thematter and energy were squeezed into a minor corner of the present universe; rather, theentire universe, matter and energy and the space they fill, occupied a very small volume.
There was not much room for events to happen in.
In that titanic cosmic explosion, the universe began an expansion which has neverceased. It is misleading to describe the expansion of the universe as a sort of distending
bubble viewed from the outside. By definition, nothing we can ever know about wasoutside. It is better to think of it from the inside, perhaps with grid lines imagined to adhere
to the moving fabric of space expanding uniformly in all directions. As space stretched, thematter and energy in the universe expanded with it and rapidly cooled. The radiation of thecosmic fireball, which, then as now, filled the universe, moved through the spectrum - fromgamma rays to X-rays to ultraviolet light; through the rainbow colors of the visible
spectrum; into the infrared and radio regions. The remnants of that fireball, the cosmic
background radiation, emanating from all parts of the sky can be detected by radio
telescopes today. In the early universe, space was brilliantly illuminated. As time passed,
the fabric of space continued to expand, the radiation cooled and, in ordinary visible light,
for the first time space became dark, as it is today.
The early universe was filled with radiation and a plenum of matter, originallyhydrogen and helium, formed from elementary particles in the dense primeval fireball.
There was very little to see, if there had been anyone around to do the seeing. Then little
pockets of gas, small nonuniformities, began to grow. Tendrils of vast gossamer gas clouds
formed, colonies of great lumbering, slowly spinning things, steadily brightening, each akind of beast eventually to contain a hundred billion shining points. The largest
recognizable structures in the universe had formed. We see them today. We ourselves
inhabit some lost corner of one. We call them galaxies.
About a billion years after the Big Bang, the distribution of matter in the universe
had become a little lumpy, perhaps because the Big Bang itself had not been perfectly
uniform. Matter was more densely compacted in these lumps than elsewhere. Their gravity
drew to them substantial quantities of nearby gas, growing clouds of hydrogen and heliumthat were destined to become clusters of galaxies. A very small initial nonuniformity
suffices to produce substantial condensations of matter later on.
As the gravitational collapse continued, the primordial galaxies spun increasingly
faster, because of the conservation of angular momentum. Some flattened, squashingthemselves along the axis of rotation where gravity is not balanced by centrifugal force.
These became the first spiral galaxies, great rotating pin-wheels of matter in open space.
Other protogalaxies with weaker gravity or less initial rotation flattened very little andbecame the first elliptical galaxies. There are similar galaxies, as if stamped from the samemold, all over the Cosmos because these simple laws of nature - gravity and theconservation of angular momentum - are the same all over the universe. The physics thatworks for falling bodies and pirouetting ice skaters down here in the microcosm of the
Earth makes galaxies up there in the macrocosm of the universe.
Within the nascent galaxies, much smaller clouds were also experiencing
gravitational collapse; interior temperatures became very high, thermonuclear reactions
were initiated, and the first stars turned on. The hot, massive young stars evolved rapidly,
profligates carelessly spending their capital of hydrogen fuel, soon ending their lives inbrilliant supernova explosions, returning thermonuclear ash - helium, carbon, oxygen andheavier elements - to the interstellar gas for subsequent generations of star formation.
Supernova explosions of massive early stars produced successive overlapping shock waves
in the adjacent gas, compressing the intergalactic medium and accelerating the generationof clusters of galaxies. Gravity is opportunistic, amplifying even small condensations ofmatter. Supernova shock waves may have contributed to accretions of matter at every scale.
The epic of cosmic evolution had begun, a hierarchy in the condensation of matter from thegas of the Big Bang - clusters of galaxies, galaxies, stars, planets, and, eventually, life andan intelligence able to understand a little of the elegant process responsible for its origin.
Clusters of galaxies fill the universe today. Some are insignificant, paltry collectionsof a few dozen galaxies. The affectionately titled ‘Local Group’ contains only two largegalaxies of any size, both spirals: the Milky Way and M31. Other clusters run to immense
hordes of thousands of galaxies in mutual gravitational embrace. There is some hint that theVirgo cluster contains tens of thousands of galaxies.
On the largest scale, we inhabit a universe of galaxies, perhaps a hundred billion
exquisite examples of cosmic architecture and decay, with order and disorder equallyevident: normal spirals, turned at various angles to our earthly line of sight (face-on we see
the spiral arms, edge-on, the central lanes of gas and dust in which the arms are formed);barred spirals with a river of gas and dust and stars running through the center, connecting
the spiral arms on opposite sides; stately giant elliptical galaxies containing more than atrillion stars which have grown so large because they have swallowed and merged withother galaxies; a plethora of dwarf ellipticals, the galactic midges, each containing some
paltry millions of suns; an immense variety of mysterious irregulars, indications that in theworld of galaxies there are places where something has gone ominously wrong; andgalaxies orbiting each other so closely that their edges are bent by the gravity of theircompanions and in some cases streamers of gas and stars are drawn out gravitationally, abridge between the galaxies.
Some clusters have their galaxies arranged in an unambiguously sphericalgeometry; they are composed chiefly of ellipticals, often dominated by one giant elliptical,
the presumptive galactic cannibal. Other clusters with a far more disordered geometry have,comparatively, many more spirals and irregulars. Galactic collisions distort the shape of an
originally spherical cluster and may also contribute to the genesis of spirals and irregulars
from ellipticals. The form and abundance of the galaxies have a story to tell us of ancient
events on the largest possible scale, a story we are just beginning to read.
The development of high-speed computers make possible numerical experiments on
the collective motion of thousands or tens of thousands of points, each representing a star,each under the gravitational influence of all the other points. In some cases, spiral arms
form all by themselves in a galaxy that has already flattened to a disk. Occasionally a spiralarm may be produced by the close gravitational encounter of two galaxies, each of course
composed of billions of stars. The gas and dust diffusely spread through such galaxies will
collide and become warmed. But when two galaxies collide, the stars pass effortlessly by
one another, like bullets through a swarm of bees, because a galaxy is made mostly ofnothing and the spaces between the stars are vast. Nevertheless, the configuration of thegalaxies can be distorted severely. A direct impact on one galaxy by another can send theconstituent stars pouring and careening through intergalactic space, a galaxy wasted. Whena small galaxy runs into a larger one face-on it can produce one of the loveliest of the rareirregulars, a ring galaxy thousands of light-years across, set against the velvet ofintergalactic space. It is a splash in the galactic pond, a temporary configuration of
disrupted stars, a galaxy with a central piece torn out.
The unstructured blobs of irregular galaxies, the arms of spiral galaxies and thetorus of ring galaxies exist for only a few frames in the cosmic motion picture, thendissipate, often to be reformed again. Our sense of galaxies as ponderous rigid bodies is
mistaken. They are fluid structures with 100 billion stellar components. Just as a humanbeing, a collection of 100 trillion cells, is typically in a steady state between synthesis anddecay and is more than the sum of its parts, so also is a galaxy.
The suicide rate among galaxies is high. Some nearby examples, tens or hundreds ofmillions of light-years away, are powerful sources of X-rays, infrared radiation and radio
waves, have extremely luminous cores and fluctuate in brightness on time scales of weeks.

Some display jets of radiation, thousand-light-year-long plumes, and disks of dust in
substantial disarray. These galaxies are blowing themselves up. Black holes ranging frommillions to billions of times more massive than the Sun are suspected in the cores of giant
elliptical galaxies such as NGC 6251 and M87. There is something very massive, very
dense, and very small ticking and purring inside M87 - from a region smaller than the solar
system. A black hole is implicated. Billions of light-years away are still more tumultuous
objects, the quasars, which may be the colossal explosions of young galaxies, the mightiest
events in the history of the universe since the Big Bang itself.
The word ‘quasar’ is an acronym for ‘quasi-stellar radio source.’ After it became
clear that not all of them were powerful radio sources, they were called QSO’s (‘quasistellar objects’). Because they are starlike in appearance, they were naturally thought to bestars within our own galaxy. But spectroscopic observations of their red shift (see below)
show them likely to be immense distances away. They seem to partake vigorously in the
expansion of the universe, some receding from us at more than 90 percent the speed oflight. If they are very far, they must be intrinsically extremely bright to be visible over such
distances; some are as bright as a thousand supernovae exploding at once. Just as for Cyg
X-1, their rapid fluctuations show their enormous brightness to be confined to a very small
volume, in this case less than the size of the solar system. Some remarkable process must
be responsible for the vast outpouring of energy in a quasar. Among the proposed
explanations are: (1) quasars are monster versions of pulsars, with a rapidly rotating,supermassive core connected to a strong magnetic field; (2) quasars are due to multiple
collisions of millions of stars densely packed into the galactic core, tearing away the outerlayers and exposing to full view the billion-degree temperatures of the interiors of massive
stars; (3), a related idea, quasars are galaxies in which the stars are so densely packed that asupernova explosion in one will rip away the outer layers of another and make it asupernova, producing a stellar chain reaction; (4) quasars are powered by the violent mutual
annihilation of matter and antimatter, somehow preserved in the quasar until now; (5) aquasar is the energy released when gas and dust and stars fall into an immense black hole inthe core of such a galaxy, perhaps itself the product of ages of collision and coalescence ofsmaller black holes; and (6) quasars are ‘white holes,’ the other side of black holes, afunneling and eventual emergence into view of matter pouring into a multitude of black
holes in other parts of the universe, or even in other universes.
In considering the quasars, we confront profound mysteries. Whatever the cause ofa quasar explosion, one thing seems clear: such a violent event must produce untold havoc.
In every quasar explosion millions of worlds - some with life and the intelligence tounderstand what is happening - may be utterly destroyed. The study of the galaxies reveals
a universal order and beauty. It also shows us chaotic violence on a scale hitherto
undreamed of. That we live in a universe which permits life is remarkable. That we live in
one which destroys galaxies and stars and worlds is also remarkable. The universe seems
neither benign nor hostile, merely indifferent to the concerns of such puny creatures as we.
Even a galaxy so seemingly well-mannered as the Milky Way has its stirrings andits dances. Radio observations show two enormous clouds of hydrogen gas, enough tomake millions of suns, plummeting out from the galactic core, as if a mild explosion
happened there every now and then. A high-energy astronomical observatory in Earth orbit
has found the galactic core to be a strong source of a particular gamma ray spectral line,
consistent with the idea that a massive black hole is hidden there. Galaxies like the Milky
Way may represent the staid middle age in a continuous evolutionary sequence, whichencompasses, in their violent adolescence, quasars, and exploding galaxies: because thequasars are so distant, we see them in their youth, as they were billions of years ago.
The stars of the Milky Way move with systematic grace. Globular clusters plunge
through the galactic plane and out the other side, where they slow, reverse and hurtle backagain. If we could follow the motion of individual stars bobbing about the galactic plane,
they would resemble a froth of popcorn. We have never seen a galaxy change its form
significantly only because it takes so long to move. The Milky Way rotates once every
quarter billion years. If we were to speed the rotation, we would see that the Galaxy is a
dynamic, almost organic entity, in some ways resembling a multi-cellular organism. Any
astronomical photograph of a galaxy is merely a snapshot of one stage in its ponderous
motion and evolution.* The inner region of a galaxy rotates as a solid body. But, beyond
that, like the planets around the Sun following Kepler’s third law, the outer provinces rotate
progressively more slowly. The arms have a tendency to wind up around the core in anever-tightening spiral, and gas and dust accumulate in spiral patterns of greater density,
which are in turn the locales for the formation of young, hot, bright stars, the stars thatoutline the spiral arms. These stars shine for ten million years or so, a period corresponding
to only 5 percent of a galactic rotation. But as the stars that outline a spiral arm burn out,
new stars and their associated nebulae are formed just behind them, and the spiral patternpersists. The stars that outline the arms do not survive even a single galactic rotation; only
the spiral pattern remains.
* This is not quite true. The near side of a galaxy is tens of thousands of light-years closer to us than
the far side; thus we see the front as it was tens of thousands of years before the back. But typical events ingalactic dynamics occupy tens of millions of years, so the error in thinking of an image of a galaxy as frozen
in one moment of time is small.

The speed of any given star around the center of the Galaxy is generally not thesame as that of the spiral pattern. The Sun has been in and out of spiral arms often in thetwenty times it has gone around the Milky Way at 200 kilometers per second (roughly halfa million miles per hour). On the average, the Sun and the planets spend forty million years
in a spiral arm, eighty million outside, another forty million in, and so on. Spiral arms
outline the region where the latest crop of newly hatched stars is being formed, but notnecessarily where such middle-aged stars as the Sun happen to be. In this epoch, we live
between spiral arms.
The periodic passage of the solar system through spiral arms may conceivably have
had important consequences for us. About ten million years ago, the Sun emerged from theGould Belt complex of the Orion Spiral Arm, which is now a little less than a thousand
light-years away. (Interior to the Orion arm is the Sagittarius arm; beyond the Orion arm is
the Perseus arm.) When the Sun passes through a spiral arm it is more likely than it is atpresent to enter into gaseous nebulae and interstellar dust clouds and to encounter objectsof substellar mass. It has been suggested that the major ice ages on our planet, which recur
every hundred million years or so, may be due to the interposition of interstellar matter
between the Sun and the Earth. W. Napier and S. Clube have proposed that a number of themoons, asteroids, comets and circumplanetary rings in the solar system once freely
wandered in interstellar space until they were captured as the Sun plunged through theOrion spiral arm. This is an intriguing idea, although perhaps not very likely. But it istestable. All we need do is procure a sample of, say, Phobos or a comet and examine its

magnesium isotopes. The relative abundance of magnesium isotopes (all sharing the same
number of protons, but having differing numbers of neutrons) depends on the precise
sequence of stellar nucleosynthetic events, including the timing of nearby supernova
explosions, that produced any particular sample of magnesium. In a different corner of the
Galaxy, a different sequence of events should have occurred and a different ratio of
magnesium isotopes should prevail.
The discovery of the Big Bang and the recession of the galaxies came from acommonplace of nature called the Doppler effect. We are used to it in the physics of sound.
An automobile driver speeding by us blows his horn. Inside the car, the driver hears a
steady blare at a fixed pitch. But outside the car, we hear a characteristic change in pitch.
To us, the sound of the horn elides from high frequencies to low. A racing car traveling at
200 kilometers per hour (120 miles her hour) is going almost one-fifth the speed of sound.
Sound is a succession of waves in air, a crest and a trough, a crest and a trough. The closer
together the waves are, the higher the frequency or pitch; the farther apart the waves are,the lower the pitch. If the car is racing away from us, it stretches out the sound waves,moving them, from our point of view, to a lower pitch and producing the characteristic
sound with which we are all familiar. If the car were racing toward us, the sound waves
would be squashed together, the frequency would be increased, and we would hear a highpitched wail. If we knew what the ordinary pitch of the horn was when the car was at rest,
we could deduce its speed blindfolded, from the change in pitch.
Light is also a wave. Unlike sound, it travels perfectly well through a vacuum. The
Doppler effect works here as well. If instead of sound the automobile were for some reason
emitting, front and back, a beam of pure yellow light, the frequency of the light would
increase slightly as the car approached and decrease slightly as the car receded. At ordinary
speeds the effect would be imperceptible. If, however, the car were somehow traveling at agood fraction of the speed of light, we would be able to observe the color of the lightchanging toward higher frequency, that is, toward blue, as the car approached us; andtoward lower frequencies, that is, toward red, as the car receded from us. An object
approaching us at very high velocities is perceived to have the color of its spectral lines
blue-shifted. An object receding from us at very high velocities has its spectral lines redshifted.* This red shift, observed in the spectral lines of distant galaxies and interpreted as a
Doppler effect, is the key to cosmology.
* The object itself might be any color, even blue. The red shift means only that each spectral line
appears at longer wavelengths than when the object is at rest; the amount of the red shift is proportional bothto the velocity and to the wavelength of the spectral line when the object is at rest.

During the early years of this century, the world’s largest telescope, destined to
discover the red shift of remote galaxies, was being built on Mount Wilson, overlooking
what were then the clear skies of Los Angeles. Large pieces of the telescope had to behauled to the top of the mountain, a job for mule teams. A young mule skinner named
Milton Humason helped to transport mechanical and optical equipment, scientists,
engineers and dignitaries up the mountain. Humason would lead the column of mules onhorseback, his white terrier standing just behind the saddle, its front paws on Humason’s
shoulders. He was a tobacco-chewing roustabout, a superb gambler and pool player andwhat was then called a ladies’ man. In his formal education, he had never gone beyond theeighth grade. But he was bright and curious and naturally inquisitive about the equipment

he had laboriously carted to the heights. Humason was keeping company with the daughterof one of the observatory engineers, a man who harbored reservations about his daughter
seeing a young man who had no higher ambition than to be a mule skinner. So Humason
took odd jobs at the observatory - electrician’s assistant, janitor, swabbing the floors of thetelescope he had helped to build. One evening, so the story goes, the night telescope
assistant fell ill and Humason was asked if he might fill in. He displayed such skill and carewith the instruments that he soon became a permanent telescope operator and observing
aide.
After World War I, there came to Mount Wilson the soon-to-be famous Edwin
Hubble - brilliant, polished, gregarious outside the astronomical community, with an
English accent acquired during a single year as Rhodes scholar at Oxford. It was Hubble
who provided the final demonstration that the spiral nebulae were in fact ‘island universes,’
distant aggregations of enormous numbers of stars, like our own Milky Way Galaxy; he
had figured out the stellar standard candle required to measure the distances to the galaxies.
Hubble and Humason hit it off splendidly, a perhaps unlikely pair who worked together at
the telescope harmoniously. Following a lead by the astronomer V. M. Slipher at LowellObservatory, they began measuring the spectra of distant galaxies. It soon became clear that
Humason was better able to obtain high-quality spectra of distant galaxies than any
professional astronomer in the world. He became a full staff member of the Mount Wilson
Observatory, learned many of the scientific underpinnings of his work and died rich in therespect of the astronomical community.
The light from a galaxy is the sum of the light emitted by the billions of stars within
it. As the light leaves these stars, certain frequencies or colors are absorbed by the atoms inthe stars’ outermost layers. The resulting lines permit us to tell that stars millions of lightyears away contain the same chemical elements as our Sun and the nearby stars. Humasonand Hubble found, to their amazement, that the spectra of all the distant galaxies are redshifted and, still more startling, that the more distant the galaxy was, the more red-shifted
were its spectral lines.
The most obvious explanation of the red shift was in terms of the Doppler effect:
the galaxies were receding from us; the more distant the galaxy the greater its speed ofrecession. But why should the galaxies be fleeing us? Could there be something special
about our location in the universe, as if the Milky Way had performed some inadvertent but
offensive act in the social life of galaxies? It seemed much more likely that the universe
itself was expanding, carrying the galaxies with it. Humason and Hubble, it graduallybecame clear, had discovered the Big Bang - if not the origin of the universe then at least its
most recent incarnation.
Almost all of modern cosmology - and especially the idea of an expanding universe
and a Big Bang - is based on the idea that the red shift of distant galaxies is a Doppler
effect and arises from their speed of recession. But there are other kinds of red shifts innature. There is, for example, the gravitational red shift, in which the light leaving an
intense gravitational field has to do so much work to escape that it loses energy during thejourney, the process perceived by a distant observer as a shift of the escaping light to longer
wavelengths and redder colors. Since we think there may be massive black holes at thecenters of some galaxies, this is a conceivable explanation of their red shifts. However, theparticular spectral lines observed are often characteristic of very thin, diffuse gas, and not
the astonishingly high density that must prevail near black holes. Or the red shift might be a
Doppler effect due not to the general expansion of the universe but rather to a more modest

and local galactic explosion. But then we should expect as many explosion fragments
traveling toward us as away from us, as many blue shifts as red shifts. What we actuallysee, however, is almost exclusively red shifts no matter what distant objects beyond the
Local Group we point our telescopes to.
There is nevertheless a nagging suspicion among some astronomers that all may not
be right with the deduction, from the red shifts of galaxies via the Doppler effect, that theuniverse is expanding. The astronomer Halton Arp has found enigmatic and disturbing
cases where a galaxy and a quasar, or a pair of galaxies, that are in apparent physicalassociation have very different red shifts. Occasionally there seems to be a bridge of gas
and dust and stars connecting them. If the red shift is due to the expansion of the universe,very different red shifts imply very different distances. But two galaxies that are physicallyconnected can hardly also be greatly separated from each other - in some cases by a billion
light-years. Skeptics say that the association is purely statistical: that, for example, a nearby
bright galaxy and a much more distant quasar, each having very different red shifts andvery different speeds of recession, are merely accidentally aligned along the line of sight;that they have no real physical association. Such statistical alignments must happen bychance every now and then. The debate centers on whether the number of coincidences ismore than would be expected by chance. Arp points to other cases in which a galaxy with asmall red shift is flanked by two quasars of large and almost identical red shift. He believes
the quasars are not at cosmological distances but instead are being ejected, left and right, bythe ‘foreground’ galaxy; and that the red shifts are the result of some as-yet-unfathomedmechanism. Skeptics argue coincidental alignment and the conventional Hubble-Humason
interpretation of the red shift. If Arp is right, the exotic mechanisms proposed to explain theenergy source of distant quasars - supernova chain reactions, supermassive black holes andthe like - would prove unnecessary. Quasars need not then be very distant. But some otherexotic mechanism will be required to explain the red shift. In either case, something very
strange is going on in the depths of space.
The apparent recession of the galaxies, with the red shift interpreted through the
Doppler effect, is not the only evidence for the Big Bang. Independent and quite persuasive
evidence derives from the cosmic black body background radiation, the faint static of radio
waves coming quite uniformly from all directions in the Cosmos at just the intensity
expected in our epoch from the now substantially cooled radiation of the Big Bang. But
here also there is something puzzling. Observations with a sensitive radio antenna carriednear the top of the Earth’s atmosphere in a U-2 aircraft have shown that the backgroundradiation is, to first approximation, just as intense in all directions - as if the fireball of the
Big Bang expanded quite uniformly, an origin of the universe with a very precise
symmetry. But the background radiation, when examined to finer precision, proves to be
imperfectly symmetrical. There is a small systematic effect that could be understood if theentire Milky Way Galaxy (and presumably other members of the Local Group) werestreaking toward the Virgo cluster of galaxies at more than a million miles an hour (600kilometers per second). At such a rate, we will reach it in ten billion years, andextragalactic astronomy will then be a great deal easier. The Virgo cluster is already therichest collection of galaxies known, replete with spirals and ellipticals and irregulars, ajewel box in the sky. But why should we be rushing toward it? George Smoot and hiscolleagues, who made these high-altitude observations, suggest that the Milky Way is being
gravitationally dragged toward the center of the Virgo cluster; that the cluster has many
more galaxies than have been detected heretofore; and, most startling, that the cluster is ofimmense proportions, stretching across one or two billion light-years of space.
The observable universe itself is only a few tens of billions of light-years acrossand, if there is a vast supercluster in the Virgo group, perhaps there are other suchsuperclusters at much greater distances, which are correspondingly more difficult to detect.
In the lifetime of the universe there has apparently not been enough time for an initial
gravitational nonuniformity to collect the amount of mass that seems to reside in the Virgosupercluster. Thus Smoot is tempted to conclude that the Big Bang was much less uniform
than his other observations suggest, that the original distribution of matter in the universe
was very lumpy. (Some little lumpiness is to be expected, and indeed even needed tounderstand the condensation of galaxies; but a lumpiness on this scale is a surprise.)
Perhaps the paradox can be resolved by imagining two or more nearly simultaneous Big
Bangs.
If the general picture of an expanding universe and a Big Bang is correct, we mustthen confront still more difficult questions. What were conditions like at the time of the Big
Bang? What happened before that? Was there a tiny universe, devoid of all matter, and thenthe matter suddenly created from nothing? How does that happen? In many cultures it iscustomary to answer that God created the universe out of nothing. But this is mere
temporizing. If we wish courageously to pursue the question, we must, of course ask next
where God comes from. And if we decide this to be unanswerable, why not save a step anddecide that the origin of the universe is an unanswerable question? Or, if we say that God
has always existed, why not save a step and conclude that the universe has always existed?
Every culture has a myth of the world before creation, and of the creation of theworld, often by the mating of the gods or the hatching of a cosmic egg. Commonly, theuniverse is naively imagined to follow human or animal precedent. Here, for example, arefive small extracts from such myths, at different levels of sophistication, from the PacificBasin:In the very beginning everything was resting in perpetual darkness: night oppressed
everything like an impenetrable thicket.
- The Great Father myth of the Aranda people of Central Australia
All was in suspense, all calm, all in silence; all motionless and still; and the expanseof the sky was empty.
- The Popol Vuh of the Quiché Maya
Na Arean sat alone in space as a cloud that floats in nothingness. He slept not, forthere was no sleep; he hungered not, for as yet there was no hunger. So he remained
for a great while, until a thought came to his mind. He said to himself, ‘I will makea thing.’- A myth from Maiana, Gilbert Islands
First there was the great cosmic egg. Inside the egg was chaos, and floating in chaos
was Pan Ku, the Undeveloped, the divine Embryo. And Pan Ku burst out of the egg,
four times larger than any man today, with a hammer and chisel in his hand withwhich he fashioned the world.
- The P’an Ku Myths, China (around third century)Before heaven and earth had taken form all was vague and amorphous . . . That
which was clear and light drifted up to become heaven, while that which was heavy
and turbid solidified to become earth. It was very easy for the pure, fine material tocome together, but extremely difficult for the heavy, turbid material to solidify.
Therefore heaven was completed first and earth assumed shape after. When heavenand earth were joined in emptiness and all was unwrought simplicity, then without
having been created things came into being. This was the Great Oneness. All things
issued from this Oneness but all became different . . .
- Huai-nan Tzu, China (around first century B.C.)
These myths are tributes to human audacity. The chief difference between them and ourmodern scientific myth of the Big Bang is that science is self-questioning, and that we canperform experiments and observations to test our ideas. But those other creation stories areworthy of our deep respect.
Every human culture rejoices in the fact that there are cycles in nature. But how, it
was thought, could such cycles come about unless the gods willed them? And if there arecycles in the years of humans, might there not be cycles in the aeons of the gods? The
Hindu religion is the only one of the world’s great faiths dedicated to the idea that the
Cosmos itself undergoes an immense, indeed an infinite, number of deaths and rebirths. It
is the only religion in which the time scales correspond, no doubt by accident, to those ofmodern scientific cosmology. Its cycles run from our ordinary day and night to a day andnight of Brahma, 8.64 billion years long, longer than the age of the Earth or the Sun andabout half the time since the Big Bang. And there are much longer time scales still.
There is the deep and appealing notion that the universe is but the dream of the god
who, after a hundred Brahma years, dissolves himself into a dreamless sleep. The universe
dissolves with him - until, after another Brahma century, he stirs, recomposes himself andbegins again to dream the great cosmic dream. Meanwhile, elsewhere, there are an infinite
number of other universes, each with its own god dreaming the cosmic dream. These greatideas are tempered by another, perhaps still greater. It is said that men may not be the
dreams of the gods, but rather that the gods are the dreams of men.
In India there are many gods, and each god has many manifestations. The Cholabronzes, cast in the eleventh century, include several different incarnations of the god
Shiva. The most elegant and sublime of these is a representation of the creation of theuniverse at the beginning of each cosmic cycle, a motif known as the cosmic dance of
Shiva. The god, called in this manifestation Nataraja, the Dance King, has four hands. In
the upper right hand is a drum whose sound is the sound of creation. In the upper left handis a tongue of flame, a reminder that the universe, now newly created, will billions of years
from now be utterly destroyed.
These profound and lovely images are, I like to imagine, a kind of premonition ofmodern astronomical ideas.* Very likely, the universe has been expanding since the Big
Bang, but it is by no means clear that it will continue to expand forever. The expansion may
gradually slow, stop and reverse itself. If there is less than a certain critical amount ofmatter in the universe, the gravitation of the receding galaxies will be insufficient to stopthe expansion, and the universe will run away forever. But if there is more matter than we
can see - hidden away in black holes, say, or in hot but invisible gas between the galaxies -then the universe will hold together gravitationally and partake of a very Indian successionof cycles, expansion followed by contraction, universe upon universe, Cosmos without end.
If we live in such an oscillating universe, then the Big Bang is not the creation of the
Cosmos but merely the end of the previous cycle, the destruction of the last incarnation of
the Cosmos.
* The dates on Mayan inscriptions also range deep into the past and occasionally far into the future.
One inscription refers to a time more than a million years ago and another perhaps refers to events of 400
million years ago, although this is in some dispute among Mayan scholars. The events memorialized may be
mythical, but the time scales are prodigious. A millennium before Europeans were willing to divest
themselves of the Biblical idea that the world was a few thousand years old, the Mayans were thinking of
millions, and the Indians of billions.

Neither of these modern cosmologies may be altogether to our liking. In one, theuniverse is created, somehow, ten or twenty billion years ago and expands forever, thegalaxies mutually receding until the last one disappears over our cosmic horizon. Then thegalactic astronomers are out of business, the stars cool and die, matter itself decays and theuniverse becomes a thin cold haze of elementary particles. In the other, the oscillating
universe, the Cosmos has no beginning and no end, and we are in the midst of an infinite
cycle of cosmic deaths and rebirths with no information trickling through the cusps of theoscillation. Nothing of the galaxies, stars, planets, life forms or civilizations evolved in theprevious incarnation of the universe oozes into the cusp, flutters past the Big Bang, to be
known in our present universe. The fate of the universe in either cosmology may seem alittle depressing, but we may take solace in the time scales involved. These events will
occupy tens of billions of years, or more. Human beings and our descendants, whoever theymight be, can accomplish a great deal in tens of billions of years, before the Cosmos dies.
If the universe truly oscillates, still stranger questions arise. Some scientists thinkthat when expansion is followed by contraction, when the spectra of distant galaxies are all
blue-shifted, causality will be inverted and effects will precede causes. First the ripples
spread from a point on the water’s surface, then I throw a stone into the pond. First thetorch bursts into flame and then I light it. We cannot pretend to understand what suchcausality inversion means. Will people at such a time be born in the grave and die in thewomb? Will time flow backwards? Do these questions have any meaning?
Scientists wonder about what happens in an oscillating universe at the cusps, at thetransition from contraction to expansion. Some think that the laws of nature are thenrandomly reshuffled, that the kind of physics and chemistry that orders this universe
represent only one of an infinite range of possible natural laws. It is easy to see that only avery restricted range of laws of nature are consistent with galaxies and stars, planets, life
and intelligence. If the laws of nature are unpredictably reassorted at the cusps, then it is
only by the most extraordinary coincidence that the cosmic slot machine has this time comeup with a universe consistent with us.** The laws of nature cannot be randomly reshuffled at the cusps. If the universe has already gone
through many oscillations, many possible laws of gravity would have been so weak that, for any given initial
expansion, the universe would not have held together. Once the universe stumbles upon such a gravitationallaw, it flies apart and has no further opportunity to experience another oscillation and another cusp andanother set of laws of nature. Thus we can deduce from the fact that the universe exists either a finite age, or asevere restriction on the kinds of laws of nature permitted in each oscillation. If the laws of physics are notrandomly reshuffled at the cusps, there must be a regularity, a set of rules, that determines which laws arepermissible and which are not. Such a set of rules would comprise a new physics standing over the existing
physics. Our language is impoverished; there seems to be no suitable name for such a new physics. Both‘paraphysics’ and ‘metaphysics’ have been preempted by other rather different and, quite possibly, whollyirrelevant activities. Perhaps ‘transphysics’ would do.

Do we live in a universe that expands forever or in one in which there is an infinite
set of cycles? There are ways to find out: by making an accurate census of the total amountof matter in the universe, or by seeing to the edge of the Cosmos.
Radio telescopes can detect very faint, very distant objects. As we look deep into
space we also look far back into time. The nearest quasar is perhaps half a billion lightyears away. The farthest may be ten or twelve or more billions. But if we see an object
twelve billion light-years away, we are seeing it as it was twelve billion years ago in time.
By looking far out into space we are also looking far back into time, back toward thehorizon of the universe, back toward the epoch of the Big Bang.
The Very Large Array (VLA) is a collection of twenty-seven separate radio
telescopes in a remote region of New Mexico. It is a phased array, the individual telescopes
electronically connected, as if it were a single telescope of the same size as its remotest
elements, as if it were a radio telescope tens of kilometers across. The VLA is able toresolve or discriminate fine detail in the radio regions of the spectrum comparable to what
the largest ground-based telescopes can do in the optical region of the spectrum.
Sometimes such radio telescopes are connected with telescopes on the other side of
the Earth, forming a baseline comparable to the Earth’s diameter - in a certain sense, atelescope as large as the planet. In the future we may have telescopes in the Earth’s orbit,
around toward the other side of the Sun, in effect a radio telescope as large as the innersolar system. Such telescopes may reveal the internal structure and nature of quasars.
Perhaps a quasar standard candle will be found, and the distances to the quasars determined
independent of their red shifts. By understanding the structure and the red shift of the most
distant quasars it may be possible to see whether the expansion of the universe was fasterbillions of years ago, whether the expansion is slowing down, whether the universe will one
day collapse.
Modern radio telescopes are exquisitely sensitive; a distant quasar is so faint that its
detected radiation amounts perhaps to a quadrillionth of a watt. The total amount of energy
from outside the solar system ever received by all the radio telescopes on the planet Earth isless than the energy of a single snowflake striking the ground. In detecting the cosmic
background radiation, in counting quasars, in searching for intelligent signals from space,radio astronomers are dealing with amounts of energy that are barely there at all.
Some matter, particularly the matter in the stars, glows in visible light and is easy tosee. Other matter, gas and dust in the outskirts of galaxies, for example, is not so readily
detected. It does not give off visible light, although it seems to give off radio waves. This is
one reason that the unlocking of the cosmological mysteries requires us to use exotic
instruments and frequencies different from the visible light to which our eyes are sensitive.
Observatories in Earth orbit have found an intense X-ray glow between the galaxies. It was
first thought to be hot intergalactic hydrogen, an immense amount of it never before seen,
perhaps enough to close the Cosmos and to guarantee that we are trapped in an oscillating
universe. But more recent observations by Ricardo Giacconi may have resolved the X-ray
glow into individual points, perhaps an immense horde of distant quasars. They contributepreviously unknown mass to the universe as well. When the cosmic inventory is completed,and the mass of all the galaxies, quasars, black holes, intergalactic hydrogen, gravitational
waves and still more exotic denizens of space is summed up, we will know what kind ofuniverse we inhabit.
In discussing the large-scale structure of the Cosmos, astronomers are fond ofsaying that space is curved, or that there is no center to the Cosmos, or that the universe isfinite but unbounded. Whatever are they talking about? Let us imagine we inhabit a strange
country where everyone is perfectly flat. Following Edwin Abbott, a Shakespearean scholar
who lived in Victorian England, we call it Flatland. Some of us are squares; some aretriangles; some have more complex shapes. We scurry about, in and out of our flatbuildings, occupied with our flat businesses and dalliances. Everyone in Flatland has widthand length, but no height whatever. We know about left-right and forward-back, but haveno hint, not a trace of comprehension, about up-down - except for flat mathematicians.
They say, ‘Listen, it’s really very easy. Imagine left-right. Imagine forward-back. Okay, so
far? Now imagine another dimension, at right angles to the other two.’ And we say, ‘What
are you talking about? “At right angles to the other two!” There are only two dimensions.
Point to that third dimension. Where is it?’ So the mathematicians, disheartened, amble off.
Nobody listens to mathematicians.
Every square creature in Flatland sees another square as merely a short line
segment, the side of the square nearest to him. He can see the other side of the square only
by taking a short walk. But the inside of a square is forever mysterious, unless some terrible
accident or autopsy breaches the sides and exposes the interior parts.
One day a three-dimensional creature - shaped like an apple, say - comes upon
Flatland, hovering above it. Observing a particularly attractive and congenial-looking
square entering its flat house, the apple decides, in a gesture of interdimensional amity, tosay hello. ‘How are you?’ asks the visitor from the third dimension. ‘I am a visitor from thethird dimension.’ The wretched square looks about his closed house and sees no one. Whatis worse, to him it appears that the greeting, entering from above, is emanating from his
own flat body, a voice from within. A little insanity, he perhaps reminds himself gamely,
runs in the family.
Exasperated at being judged a psychological aberration, the apple descends into
Flatland. Now a three-dimensional creature can exist, in Flatland, only partially; only across section can be seen, only the points of contact with the plane surface of Flatland. An
apple slithering through Flatland would appear first as a point and then as progressively
larger, roughly circular slices. The square sees a point appearing in a closed room in his
two-dimensional world and slowly growing into a near circle. A creature of strange andchanging shape has appeared from nowhere.
Rebuffed, unhappy at the obtuseness of the very flat, the apple bumps the squareand sends him aloft, fluttering and spinning into that mysterious third dimension. At firstthe square can make no sense of what is happening; it is utterly outside his experience. But
eventually he realizes that he is viewing Flatland from a peculiar vantage point: ‘above’. He
can see into closed rooms. He can see into his flat fellows. He is viewing his universe froma unique and devastating perspective. Traveling through another dimension provides, as anincidental benefit, a kind of X-ray vision. Eventually, like a falling leaf, our square slowly
descends to the surface. From the point of view of his fellow Flatlanders, he has
unaccountably disappeared from a closed room and then distressingly materialized fromnowhere. ‘For heaven’s sake,’ they say, ‘what’s happened to you?’ ‘I think,’ he finds
himself replying, ‘I was “up.” ’ They pat him on his sides and comfort him. Delusions

always ran in his family.
In such interdimensional contemplations, we need not be restricted to twodimensions. We can, following Abbott, imagine a world of one dimension, where everyone
is a line segment, or even the magical world of zero-dimensional beasts, the points. But
perhaps more interesting is the question of higher dimensions. Could there be a fourth
physical dimension?** If a fourth-dimensional creature existed it could, in our three-dimensional universe, appear anddematerialize at will, change shape remarkably, pluck us out of locked rooms and make us appear fromnowhere. It could also turn us inside out. There are several ways in which we can be turned inside out: theleast pleasant would result in our viscera and internal organs being on the outside and the entire Cosmos glowing intergalactic gas, galaxies, planets, everything - on the inside. I am not sure I like the idea.

We can imagine generating a cube in the following way: Take a line segment of acertain length and move it an equal length at right angles to itself. That makes a square.
Move the square an equal length at right angles to itself, and we have a cube. We
understand this cube to cast a shadow, which we usually draw as two squares with their
vertices connected. If we examine the shadow of a cube in two dimensions, we notice that
not all the lines appear equal, and not all the angles are right angles. The three-dimensional
object has not been perfectly represented in its transfiguration into two dimensions. This is
the cost of losing a dimension in the geometrical projection. Now let us take our threedimensional cube and carry it, at right angles to itself, through a fourth physical dimension:
not left-right, not forward-back, not up-down, but simultaneously at right angles to all those
directions. I cannot show you what direction that is, but I can imagine it to exist. In such acase, we would have generated a four-dimensional hypercube, also called a tesseract. I
cannot show you a tesseract, because we are trapped in three dimensions. But what I canshow you is the shadow in three dimensions of a tesseract. It resembles two nested cubes,
all the vertices connected by lines. But for a real tesseract, in four dimensions, all the lines
would be of equal length and all the angles would be right angles.
Imagine a universe just like Flatland, except that unbeknownst to the inhabitants,
their two-dimensional universe is curved through a third physical dimension. When the
Flatlanders take short excursions, their universe looks flat enough. But if one of them takes
a long enough walk along what seems to be a perfectly straight line, he uncovers a great
mystery: although he has not reached a barrier and has never turned around, he has
somehow come back to the place from which he started. His two-dimensional universe
must have been warped, bent or curved through a mysterious third dimension. He cannot
imagine that third dimension, but he can deduce it. Increase all dimensions in this story by
one, and you have a situation that may apply to us.
Where is the center of the Cosmos? Is there an edge to the universe? What lies
beyond that? In a two-dimensional universe, curved through a third dimension, there is no
center - at least not on the surface of the sphere. The center of such a universe is not in that
universe; it lies, inaccessible, in the third dimension, inside the sphere. While there is only
so much area on the surface of the sphere, there is no edge to this universe - it is finite but
unbounded. And the question of what lies beyond is meaningless. Flat creatures cannot, on
their own, escape their two dimensions.
Increase all dimensions by one, and you have the situation that may apply to us: theuniverse as a four-dimensional hypersphere with no center and no edge, and nothing

beyond. Why do all the galaxies seem to be running away from us? The hypersphere isexpanding from a point, like a four-dimensional balloon being inflated, creating in every
instant more space in the universe. Sometime after the expansion begins, galaxies condense
and are carried outward on the surface of the hypersphere. There are astronomers in eachgalaxy, and the light they see is also trapped on the curved surface of the hypersphere. As
the sphere expands, an astronomer in any galaxy will think all the other galaxies are
running away from him. There are no privileged reference frames.* The farther away thegalaxy, the faster its recession. The galaxies are embedded in, attached to space, and thefabric of space is expanding. And to the question, Where in the present universe did the Big
Bang occur? the answer is clearly, everywhere.
* The view that the universe looks by and large the same no matter from where we happen to view it
was first proposed, so far as we know, by Giordano Bruno.

If there is insufficient matter to prevent the universe from expanding forever, it
must have an open shape, curved like a saddle with a surface extending to infinity in our
three-dimensional analogy. If there is enough matter, then it has a closed shape, curved likea sphere in our three-dimensional analogy. If the universe is closed, light is trapped within
it. In the 1920’s, in a direction opposite to M31, observers found a distant pair of spiralgalaxies. Was it possible, they wondered, that they were seeing the Milky Way and M31
from the other direction - like seeing the back of your head with light that has
circumnavigated the universe? We now know that the universe is much larger than they
imagined in the 1920’s. It would take more than the age of the universe for light tocircumnavigate it. And the galaxies are younger than the universe. But if the Cosmos is
closed and light cannot escape from it, then it may be perfectly correct to describe theuniverse as a black hole. If you wish to know what it is like inside a black hole, look aroundyou.
We have previously mentioned the possibility of wormholes to get from one placein the universe to another without covering the intervening distance - through a black hole.
We can imagine these wormholes as tubes running through a fourth physical dimension.
We do not know that such wormholes exist. But if they do, must they always hook up withanother place in our universe? Or is it just possible that wormholes connect with otheruniverses, places that would otherwise be forever inaccessible to us? For all we know, there
may be many other universes. Perhaps they are, in some sense, nested within one another.
There is an idea - strange, haunting, evocative - one of the most exquisite
conjectures in science or religion. It is entirely undemonstrated; it may never be proved.
But it stirs the blood. There is, we are told, an infinite hierarchy of universes, so that anelementary particle, such as an electron, in our universe would, if penetrated, reveal itself tobe an entire closed universe. Within it, organized into the local equivalent of galaxies andsmaller structures, are an immense number of other, much tinier elementary particles,
which are themselves universe at the next level, and so on forever - an infinite downward
regression, universes within universes, endlessly. And upward as well. Our familiar
universe of galaxies and stars, planets and people, would be a single elementary particle in
the next universe up, the first step of another infinite regress.
This is the only religious idea I know that surpasses the endless number of infinitely
old cycling universes in Hindu cosmology. What would those other universes be like?
Would they be built on different laws of physics? Would they have stars and galaxies andworlds, or something quite different? Might they be compatible with some unimaginably
different form of life? To enter them, we would somehow have to penetrate a fourth
physical dimension - not an easy undertaking, surely but perhaps a black hole would
provide a way. There may be small black holes in the solar neighborhood. Poised at theedge of forever, we would jump off.

CHAPTER XI
The Persistence of Memory
Now that the destinies of Heaven and Earth have been fixed;
Trench and canal have been given their proper course;
The banks of the Tigris and the Euphrates have been established;
What else shall we do?
What else shall we create?
Oh Anunaki, you great gods of the sky, what else shall we do?
- The Assyrian account of the creation of Man, 800 B.C.
When he, whoever of the gods it was, had thus arranged in order and resolved thatchaotic mass, and reduced it, thus resolved, to cosmic parts, he first moulded theEarth into the form of a mighty ball so that it might be of like form on every side . .
. And, that no region might be without its own forms of animate life, the stars anddivine forms occupied the floor of heaven, the sea fell to the shining fishes for theirhome, Earth received the beasts, and the mobile air the birds . . . Then Man wasborn: . . . though all other animals are prone, and fix their gaze upon the earth, hegave to Man an uplifted face and bade him stand erect and turn his eyes to heaven.
- Ovid, Metamorphoses, first century
In the great cosmic dark there are countless stars and planets both younger and older thanour solar system. Although we cannot yet be certain, the same processes that led on Earth
to the evolution of life and intelligence should have been operating throughout the Cosmos.
There may be a million worlds in the Milky Way Galaxy alone that at this moment are
inhabited by beings who are very different from us, and far more advanced. Knowing agreat deal is not the same as being smart; intelligence is not information alone but also
judgment, the manner in which information is coordinated and used. Still, the amount of
information to which we have access is one index of our intelligence. The measuring rod,the unit of information, is something called a bit (for binary digit). It is an answer - either
yes or no - to an unambiguous question. To specify whether a lamp is on or off requires asingle bit of information. To designate one letter out of the twenty-six in the Latin alphabet
takes five bits (25 = 2 x 2 x 2 x 2 x 2 = 32, which is more than 26). The verbal information
content of this book is a little less than ten million bits, 107. The total number of bits thatcharacterizes an hour-long television program is about 1012. The information in the words
and pictures of different books in all the libraries on the Earth is something like 1016 or 1017
bits.* Of course much of it is redundant. Such a number calibrates crudely what humans
know. But elsewhere, on older worlds, where life has evolved billions of years earlier thanon Earth, perhaps they know 1020 bits or 1030 - not just more information but significantly
different information.
* Thus all of the books in the world contain no more information than is broadcast as video in asingle large American city in a single year. Not all bits have equal value.

Of those million worlds inhabited by advanced intelligencies, consider a rare planet,the only one in its system with a surface ocean of liquid water. In this rich aquatic
environment, many relatively intelligent creatures live - some with eight appendages forgrasping; others that communicate among themselves by changing an intricate pattern ofbright and dark mottling on their bodies; even clever little creatures from the land who
make brief forays into the ocean in vessels of wood or metal. But we seek the dominant
intelligences, the grandest creatures on the planet, the sentient and graceful masters of the
deep ocean, the great whales.
They are the largest animals* ever to evolve on the planet Earth, larger by far thanthe dinosaurs. An adult blue whale can be thirty meters long and weigh 150 tons. Many,
especially the baleen whales, are placid browsers, straining through vast volumes of ocean
for the small animals on which they graze; others eat fish and krill. The whales are recent
arrivals in the ocean. Only seventy million years ago their ancestors were carnivorous
mammals who migrated in slow steps from the land into the ocean. Among the whales,
mothers suckle and care tenderly for their offspring. There is a long childhood in which theadults teach the young. Play is a typical pastime. These are all mammalian characteristics,
all important for the development of intelligent beings.
* Some sequoia trees are both larger and more massive than any whale.

The sea is murky. Sight and smell, which work well for mammals on the land, are
not of much use in the depths of the ocean. Those ancestors of the whales who relied onthese senses to locate a mate or a baby or a predator did not leave many offspring. So
another method was perfected by evolution; it works superbly well and is central to any
understanding of the whales: the sense of sound. Some whale sounds are called songs, but
we are still ignorant of their true nature and meaning. They range over a broad band of
frequencies, down to well below the lowest sound the human ear can detect. A typicalwhale song lasts for perhaps fifteen minutes; the longest, about an hour. Often it isrepeated, identically, beat for beat, measure for measure, note for note. Occasionally agroup of whales will leave their winter waters in the midst of a song and six months later
return to continue at precisely the right note, as if there had been no interruption. Whales
are very good at remembering. More often, on their return, the vocalizations have changed.
New songs appear on the cetacean hit parade.
Very often the members of the group will sing the same song together. By somemutual consensus, some collaborative songwriting, the piece changes month by month,slowly and predictably. These vocalizations are complex. If the songs of the humpback
whale are enunciated as a tonal language, the total information content, the number of bitsof information in such songs, is some 106 bits, about the same as the information content of
the Iliad or the Odyssey. We do not know what whales or their cousins the dolphins have totalk or sing about. They have no manipulative organs, they make no engineering constructs,
but they are social creatures. They hunt, swim, fish, browse, frolic, mate, play, run frompredators. There may be a great deal to talk about.
The primary danger to the whales is a newcomer, an upstart animal, only recently,
through technology, become competent in the oceans, a creature that calls itself human. For
99.99 percent of the history of the whales, there were no humans in or on the deep oceans.
During this period the whales evolved their extraordinary audio communication system.
The finbacks, for example, emit extremely loud sounds at a frequency of twenty Hertz,
down near the lowest octave on the piano keyboard.. (A Hertz is a unit of sound frequency

that represents one sound wave, one crest and one trough, entering your ear every second.)
Such low-frequency sounds are scarcely absorbed in the ocean. The American biologist
Roger Payne has calculated that using the deep ocean sound channel, two whales could
communicate with each other at twenty Hertz essentially anywhere in the world. One might
be off the Ross Ice Shelf in Antarctica and communicate with another in the Aleutians. For
most of their history, the whales may have established a global communications network.
Perhaps when separated by 15,000 kilometers, their vocalizations are love songs, casthopefully into the vastness of the deep.
For tens of millions of years these enormous, intelligent, communicative creatures
evolved with essentially no natural enemies. Then the development of the steamship in thenineteenth century introduced an ominous source of noise pollution. As commercial andmilitary vessels became more abundant, the noise background in the oceans, especially at afrequency of twenty Hertz, became noticeable. Whales communicating across the oceansmust have experienced increasingly greater difficulties. The distance over which they could
communicate must have decreased steadily. Two hundred years ago, a typical distanceacross which finbacks could communicate was perhaps 10,000 kilometers. Today, thecorresponding number is perhaps a few hundred kilometers. Do whales know each other'snames? Can they recognize each other as individuals by sounds alone? We have cut thewhales off from themselves. Creatures that communicated for tens of millions of years have
now effectively been silenced.** There is a curious counterpoint to this story. The preferred radio channel for interstellar
communication with other technical civilizations is near a frequency of 1.42 billion Hertz, marked by a radio
spectral line of hydrogen, the most abundant atom in the Universe. We are just beginning to listen here forsignals of intelligent origin. But the frequency band is being increasingly encroached upon by civilian andmilitary communications traffic on Earth, and not only by the major powers. We are jamming the interstellar
channel. Uncontrolled growth of terrestrial radio technology may prevent us from ready communication withintelligent beings on distant worlds. Their songs may go unanswered because we have not the will to control
our radio-frequency pollution and listen.

And we have done worse than that, because there persists to this day a traffic in thedead bodies of whales. There are humans who hunt and slaughter whales and market the
products for lipstick or industrial lubricant. Many nations understand that the systematic
murder of such intelligent creatures is monstrous, but the traffic continues, promoted
chiefly by Japan, Norway and the Soviet Union. We humans, as a species, are interested incommunication with extraterrestrial intelligence. Would not a good beginning be improved
communication with terrestrial intelligence, with other human beings of different culturesand languages, with the great apes, with the dolphins, but particularly with those intelligent
masters of the deep, the, great whales?
For a whale to live there are many things it must know how to do. This knowledge
is stored in its genes and in its brain. The genetic information includes how to convert
plankton into blubber; or how to hold your breath on a dive one kilometer below thesurface. The information in the brain, the learned information, includes such things as who
your mother is, or the meaning of the song you are hearing just now. The whale, like all theother animals on the Earth, has a gene library and a brain library.
The genetic material of the whale, like the genetic material of human beings, ismade of nucleic acids, those extraordinary molecules capable of reproducing themselvesfrom the chemical building blocks that surround them, and of turning hereditaryinformation into action. For example, one whale enzyme, identical to one you have in every
cell of your body, is called hexokinase, the first of more than two dozen enzyme-mediatedsteps required to convert a molecule of sugar obtained from the plankton in the whale's dietinto a little energy - perhaps a contribution to a single low-frequency note in the music of
the whale.
The information stored in the DNA double helix of a whale or a human or any otherbeast or vegetable on Earth is written in a language of four letters - the four different kindsof nucleotides, the molecular components that make up DNA. How many bits of
information are contained in the hereditary material of various life forms? How many
yes/no answers to the various biological questions are written in the language of life? Avirus needs about 10,000 bits - roughly equivalent to the amount of information on thispage. But the viral information is simple, exceedingly compact, extraordinarily efficient.
Reading it requires very close attention. These are the instructions it needs to infect someother organism and to reproduce itself - the only things that viruses are any good at. A
bacterium uses roughly a million bits of information - which is about 100 printed pages.
Bacteria have a lot more to do than viruses. Unlike the viruses, they are not thoroughgoingparasites. Bacteria have to make a living. And a free-swimming one-celled amoeba is much
more sophisticated; with about four hundred million bits in its DNA, it would require someeighty 500-page volumes to make another amoeba.
A whale or a human being needs something like five billion bits. The 5 x 109 bits of
information in our encyclopaedia of life - in the nucleus of each of our cells - if written out
in, say, English, would fill a thousand volumes. Every one of your hundred trillion cells
contains a complete library of instructions on how to make every part of you. Every cell inyour body arises by successive cell divisions from a single cell, a fertilized egg generated
by your parents. Every time that cell divided, in the many embryological steps that went
into making you, the original set of genetic instructions was duplicated with great fidelity.
So your liver cells have some unemployed knowledge about how to make your bone cells,and vice versa. The genetic library contains everything your body knows how to do on its
own. The ancient information is written in exhaustive, careful, redundant detail - how tolaugh, how to sneeze, how to walk, how to recognize patterns, how to reproduce, how todigest an apple.
Eating an apple is an immensely complicated process. In fact, if I had to synthesize
my own enzymes, if I consciously had to remember and direct all the chemical steps
required to get energy out of food, I would probably starve. But even bacteria do anaerobic
glycolysis, which is why apples rot: lunchtime for the microbes. They and we and all
creatures in between possess many similar genetic instructions. Our separate gene libraries
have many pages in common, another reminder of our common evolutionary heritage. Our
technology can duplicate only a tiny fraction of the intricate biochemistry that our bodies
effortlessly perform: we have only just begun to study these processes. Evolution, however,has had billions of years of practice. DNA knows.
But suppose what you had to do was so complicated that even several billion bits
was insufficient. Suppose the environment was changing so fast that the precoded genetic
encyclopaedia, which served perfectly well before, was no longer entirely adequate. Theneven a gene library of 1,000 volumes would not be enough. That is why we have brains.
Like all our organs, the brain has evolved, increasing in complexity and information
content, over millions of years. Its structure reflects all the stages through which it has
passed. The brain evolved from the inside out. Deep inside is the oldest part, the brainstem,

which conducts the basic biological functions, including the rhythms of life - heartbeat andrespiration. According to a provocative insight by Paul MacLean, the higher functions of
the brain evolved in three successive stages. Capping the brainstem is the R-complex, theseat of aggression, ritual, territoriality and social hierarchy, which evolved hundreds ofmillions of years ago in our reptilian ancestors. Deep inside the skull of every one of us
there is something like the brain of a crocodile. Surrounding the R-complex is the limbic
system or mammalian brain, which evolved tens of millions of years ago in ancestors who
were mammals but not yet primates. It is a major source of our moods and emotions, of ourconcern and care for the young.
And finally, on the outside, living in uneasy truce with the more primitive brains
beneath, is the cerebral cortex, which evolved millions of years ago in our primate
ancestors. The cerebral cortex, where matter is transformed into consciousness, is the pointof embarkation for all our cosmic voyages. Comprising more than two-thirds of the brain
mass, it is the realm of both intuition and critical analysis. It is here that we have ideas andinspirations, here that we read and write, here that we do mathematics and compose music.
The cortex regulates our conscious lives. It is the distinction of our species, the seat of ourhumanity. Civilization is a product of the cerebral cortex.
The language of the brain is not the DNA language of the genes. Rather, what weknow is encoded in cells called neurons - microscopic electrochemical switching elements,
typically a few hundredths of a millimeter across. Each of us has perhaps a hundred billion
neurons, comparable to the number of stars in the Milky Way Galaxy. Many neurons havethousands of connections with their neighbors. There are something like a hundred trillion,
1014, such connections in the human cerebral cortex.
Charles Sherrington imagined the activities in the cerebral cortex upon awakening:[The cortex] becomes now a sparkling field of rhythmic flashing points with trainsof traveling sparks hurrying hither and thither. The brain is waking and with it themind is returning. It is as if the Milky Way entered upon some cosmic dance.
Swiftly the [cortex] becomes an enchanted loom where millions of flashing shuttles
weave a dissolving pattern, always a meaningful pattern though never an abiding
one; a shifting harmony of sub-patterns. Now as the waking body rouses, subpatterns of this great harmony of activity stretch down into the unlit tracks of the[lower brain]. Strings of flashing and traveling sparks engage the links of it. Thismeans that the body is up and rises to meet its waking day.
Even in sleep, the brain is pulsing, throbbing and flashing with the complex business ofhuman life - dreaming, remembering, figuring things out. Our thoughts, visions andfantasies have a physical reality. A thought is made of hundreds of electrochemical
impulses. If we were shrunk to the level of the neurons, we might witness elaborate,
intricate, evanescent patterns. One might be the spark of a memory of the smell of lilacs ona country road in childhood. Another might be part of an anxious all-points bulletin:‘Where did I leave the keys?’
There are many valleys in the mountains of the mind, convolutions that greatly
increase the surface area available in the cerebral cortex for information storage in a skullof limited size. The neurochemistry of the brain is astonishingly busy, the circuitry of amachine more wonderful than any devised by humans. But there is no evidence that its
functioning is due to anything more than the 1014 neural connections that build an elegant

architecture of consciousness. The world of thought is divided roughly into twohemispheres. The right hemisphere of the cerebral cortex is mainly responsible for patternrecognition, intuition, sensitivity, creative insights. The left hemisphere presides overrational, analytical and critical thinking. These are the dual strengths, the essential
opposites, that characterize human thinking. Together, they provide the means both forgenerating ideas and for testing their validity. A continuous dialogue is going on between
the two hemispheres, channeled through an immense bundle of nerves, the corpus
callosum, the bridge between creativity and analysis, both of which are necessary tounderstand the world.
The information content of the human brain expressed in bits is probably
comparable to the total number of connections among the neurons - about a hundred
trillion, 1014, bits. If written out in English, say, that information would fill some twenty
million volumes, as many as in the world's largest libraries. The equivalent of twenty
million books is inside the heads of every one us. The brain is a very big place in a verysmall space. Most of the books in the brain are in the cerebral cortex. Down in thebasement are the functions our remote ancestors mainly depended on - aggression, childrearing, fear, sex, the willingness to follow leaders blindly. Of the higher brain functions,some - reading, writing, speaking - seem to be localized in particular places in the cerebral
cortex. Memories, on the other hand, are stored redundantly in many locales. If such a thingas telepathy existed, one of its glories would be the opportunity for each of us to read thebooks in the cerebral cortices of our loved ones. But there is no compelling evidence fortelepathy, and the communication of such information remains the task of artists andwriters.
The brain does much more than recollect. It compares, synthesizes, analyzes,
generates abstractions. We must figure out much more than our genes can know. That is
why the brain library is some ten thousand times larger than the gene library. Our passion
for learning, evident in the behavior of every toddler, is the tool for our survival. Emotions
and ritualized behavior patterns are built deeply into us. They are part of our humanity. But
they are not characteristically human. Many other animals have feelings. Whatdistinguishes our species is thought. The cerebral cortex is a liberation. We need no longer
be trapped in the genetically inherited behavior patterns of lizards and baboons. We are,each of us, largely responsible for what gets put into our brains, for what, as adults, wewind up caring for and knowing about. No longer at the mercy of the reptile brain, we canchange ourselves.
Most of the world's great cities have grown haphazardly, little by little, in responseto the needs of the moment; very rarely is a city planned for the remote future. The
evolution of a city is like the evolution of the brain: it develops from a small center andslowly grows and changes, leaving many old parts still functioning. There is no way forevolution to rip out the ancient interior of the brain because of its imperfections and replace
it with something of more modern manufacture. The brain must function during therenovation. That is why the brainstem is surrounded by the R-complex, then the limbic
system and finally the cerebral cortex. The old parts are in charge of too many fundamental
functions for them to be replaced altogether. So they wheeze along, out-of-date andsometimes counterproductive, but a necessary consequence of our evolution.
In New York City, the arrangement of many of the major streets dates to theseventeenth century, the stock exchange to the eighteenth century, the waterworks to thenineteenth, the electrical power system to the twentieth. The arrangement might be more

efficient if all civic systems were constructed in parallel and replaced periodically (which iswhy disastrous fires - the great conflagrations of London and Chicago, for example - aresometimes an aid in city planning). But the slow accretion of new functions permits the city
to work more or less continuously through the centuries. In the seventeenth century you
traveled between Brooklyn and Manhattan across the East River by ferry. In the nineteenth
century, the technology became available to construct a suspension bridge across the river.
It was built precisely at the site of the ferry terminal, both because the city owned the landand because major thoroughfares were already converging on the pre-existing ferry service.
Later when it was possible to construct a tunnel under the river, it too was built in the sameplace for the same reasons, and also because small abandoned precursors of tunnels, called
caissons, had already been emplaced during the construction of the bridge. This use andrestructuring of previous systems for new purposes is very much like the pattern ofbiological evolution.
When our genes could not store all the information necessary for survival, we
slowly invented brains. But then the time came, perhaps ten thousand years ago, when we
needed to know more than could conveniently be contained in brains. So we learned tostockpile enormous quantities of information outside our bodies. We are the only specieson the planet, so far as we know, to have invented a communal memory stored neither inour genes nor in our brains. The warehouse of that memory is called the library.
A book is made from a tree. It is an assemblage of flat, flexible parts (still called‘leaves’) imprinted with dark pigmented squiggles. One glance at it and you hear the voiceof another person - perhaps someone dead for thousands of years. Across the millennia, theauthor is speaking, clearly and silently, inside your head, directly to you. Writing is perhaps
the greatest of human inventions, binding together people, citizens of distant epochs, who
never knew one another. Books break the shackles of time, proof that humans can work
magic.
Some of the earliest authors wrote on clay. Cuneiform writing, the remote ancestorof the Western alphabet, was invented in the Near East about 5,000 years ago. Its purpose
was to keep records: the purchase of grain, the sale of land, the triumphs of the king, thestatutes of the priests, the positions of the stars, the prayers to the gods. For thousands ofyears, writing was chiseled into clay and stone, scratched onto wax or bark or leather;painted on bamboo or papyrus or silk - but always one copy at a time and, except for the
inscriptions on monuments, always for a tiny readership. Then in China between the secondand sixth centuries, paper, ink and printing with carved wooden blocks were all invented,
permitting many copies of a work to be made and distributed. It took a thousand years forthe idea to catch on in remote and backward Europe. Then, suddenly, books were being
printed all over the world. Just before the invention of movable type, around 1450, there
were no more than a few tens of thousands of books in all of Europe, all handwritten; about
as many as in China in 100 B.C., and a tenth as many as in the Great Library of Alexandria.
Fifty years later, around 1500, there were ten million printed books. Learning had become
available to anyone who could read. Magic was everywhere.
More recently, books, especially paperbacks, have been printed in massive andinexpensive editions. For the price of a modest meal you can ponder the decline and fall of
the Roman Empire, the origin of species, the interpretation of dreams, the nature of things.
Books are like seeds. They can lie dormant for centuries and then flower in the most
unpromising soil.
The great libraries of the world contain millions of volumes, the equivalent of about1014 bits of information in words, and perhaps 1015 bits in pictures. This is ten thousand
times more information than in our genes, and about ten times more than in our brains. If I
finish a book a week, I will read only a few thousand books in my lifetime, about a tenth ofa percent of the contents of the greatest libraries of our time. The trick is to know whichbooks to read. The information in books is not preprogrammed at birth but constantly
changed, amended by events, adapted to the world. It is now twenty-three centuries since
the founding of the Alexandrian Library. If there were no books, no written records, think
how prodigious a time twenty-three centuries would be. With four generations per century,
twenty-three centuries occupies almost a hundred generations of human beings. If
information could be passed on merely by word of mouth, how little we should know of ourpast, how slow would be our progress! Everything would depend on what ancient findings
we had accidentally been told about, and how accurate the account was. Past information
might be revered, but in successive retellings it would become progressively more muddled
and eventually lost. Books permit us to voyage through time, to tap the wisdom of ourancestors. The library connects us with the insights and knowledge, painfully extracted
from Nature, of the greatest minds that ever were, with the best teachers, drawn from theentire planet and from all of our history, to instruct us without tiring, and to inspire us tomake our own contribution to the collective knowledge of the human species. Public
libraries depend on voluntary contributions. I think the health of our civilization, the depthof our awareness about the underpinnings of our culture and our concern for the future canall be tested by how well we support our libraries.
Were the Earth to be started over again with all its physical features identical, it isextremely unlikely that anything closely resembling a human being would ever again
emerge. There is a powerful random character to the evolutionary process. A cosmic ray
striking a different gene, producing a different mutation, can have small consequences early
but profound consequences late. Happenstance may play a powerful role in biology, as it
does in history. The farther back the critical events occur, the more powerfully can they
influence the present.
For example, consider our hands. We have five fingers, including one opposable
thumb. They serve us quite well. But I think we would be served equally well with six
fingers including a thumb, or four fingers including a thumb, or maybe five fingers and two
thumbs. There is nothing intrinsically best about our particular configuration of fingers,which we ordinarily think of as so natural and inevitable. We have five fingers because wehave descended from a Devonian fish that had five phalanges or bones in its fins. Had wedescended from a fish with four or six phalanges, we would have four or six fingers on eachhand and would think them perfectly natural. We use base ten arithmetic only because wehave ten fingers on our hands.* Had the arrangement been otherwise, we would use baseeight or base twelve arithmetic and relegate base ten to the New Math. The same point
applies, I believe, to many more essential aspects of our being - our hereditary material, ourinternal biochemistry, our form, stature, organ systems, loves and hates, passions anddespairs, tenderness and aggression, even our analytical processes - all of these are, at leastin part, the result of apparently minor accidents in our immensely long evolutionary history.
Perhaps if one less dragonfly had drowned in the Carboniferous swamps, the intelligent
organisms on our planet today would have feathers and teach their young in rookeries. The
pattern of evolutionary causality is a web of astonishing complexity; the incompleteness of
our understanding humbles us.

* The arithmetic based on the number 5 or 10 seems so obvious that the ancient Greek equivalent of‘to count’ literally means ‘to five.’

Just sixty-five million years ago our ancestors were the most unprepossessing of
mammals - creatures with the size and intelligence of moles or tree shrews. It would havetake a very audacious biologist to guess that such animals would eventually produce theline that dominates the Earth today. The Earth then was full of awesome, nightmarish
lizards - the dinosaurs, immensely successful creatures, which filled virtually every
ecological niche. There were swimming reptiles, flying reptiles, and reptiles - some as tallas a six-story building - thundering across the face of the Earth. Some of them had rather
large brains, an upright posture and two little front legs very much like hands, which theyused to catch small, speedy mammals - probably including our distant ancestors - fordinner. If such dinosaurs had survived, perhaps the dominant intelligent species on ourplanet today would be four meters tall with green skin and sharp teeth, and the human form
would be considered a lurid fantasy of saurian science fiction. But the dinosaurs did notsurvive. In one catastrophic event all of them and many, perhaps most, of the other specieson the Earth, were destroyed.* But not the tree shrews. Not the mammals. They survived.
* A recent analysis suggests that 96 per cent of all the species in the oceans may have died at thistime. With such an enormous extinction rate, the organisms of today can have evolved from only a small andunrepresentative sampling of the organisms that lived in late Mesozoic times.

No one knows what wiped out the dinosaurs. One evocative idea is that it was a
cosmic catastrophe, the explosion of a nearby star - a supernova like the one that produced
the Crab Nebula. If there were by chance a supernova within ten or twenty light-years of
the solar system some sixty-five million years ago, it would have sprayed an intense flux ofcosmic rays into space, and some of these, entering the Earth's envelope of air, would haveburned the atmospheric nitrogen. The oxides of nitrogen thus generated would haveremoved the protective layer of ozone from the atmosphere, increasing the flux of solar
ultraviolet radiation at the surface and frying and mutating the many organisms imperfectly
protected against intense ultraviolet light. Some of those organisms may have been staplesof the dinosaur diet.
The disaster, whatever it was, that cleared the dinosaurs from the world stage
removed the pressure on the mammals. Our ancestors no longer had to live in the shadow
of voracious reptiles. We diversified exuberantly and flourished. Twenty million years ago,
our immediate ancestors probably still lived in the trees, later descending because theforests receded during a major ice age and were replaced by grassy savannahs. It is notmuch good to be supremely adapted to life in the trees if there are very few trees. Manyarboreal primates must have vanished with the forests. A few eked out a precarious
existence on the ground and survived. And one of those lines evolved to become us. No one
knows the cause of that climatic change. It may have been a small variation in the intrinsic
luminosity of the Sun or in the orbit of the Earth; or massive volcanic eruptions injecting
fine dust into the stratosphere, reflecting more sunlight back into space and cooling the
Earth. It may have been due to changes in the general circulation of the oceans. Or perhaps
the passage of the Sun through a galactic dust cloud. Whatever the cause, we see again howtied our existence is to random astronomical and geological events.

After we came down from the trees, we evolved an upright posture; our hands were
free; we possessed excellent binocular vision - we had acquired many of the preconditions
for making tools. There was now a real advantage in possessing a large brain and incommunicating complex thoughts. Other things being equal, it is better to be smart than to
be stupid. Intelligent beings can solve problems better, live longer and leave more
offspring; until the invention of nuclear weapons, intelligence powerfully aided survival. In
our history it was some horde of furry little mammals who hid from the dinosaurs,colonized the treetops and later scampered down to domesticate fire, invent writing,construct observatories and launch space vehicles. If things had been a little different, it
might have been some other creature whose intelligence and manipulative ability would
have led to comparable accomplishments. Perhaps the smart bipedal dinosaurs, or the
raccoons, or the otters, or the squid. It would be nice to know how different otherintelligences can be; so we study the whales and the great apes. To learn a little about what
other kinds of civilizations are possible, we can study history and cultural anthropology.
But we are all of us - us whales, us apes, us people - too closely related. As long as ourinquiries are limited to one or two evolutionary lines on a single planet, we will remain
forever ignorant of the possible range and brilliance of other intelligences and othercivilizations.
On another planet, with a different sequence of random processes to makehereditary diversity and a different environment to select particular combinations of genes,
the chances of finding beings who are physically very similar to us is, I believe, near zero.
The chances of finding another form of intelligence is not. Their brains may well have
evolved from the inside out. They may have switching elements analogous to our neurons.
But the neurons may be very different; perhaps superconductors that work at very low
temperatures rather than organic devices that work at room temperature, in which case their
speed of thought will be 107 times faster than ours. Or perhaps the equivalent of neurons
elsewhere would not be in direct physical contact but in radio communication so that asingle intelligent being could be distributed among many different organisms, or even manydifferent planets, each with a part of the intelligence of the whole, each contributing byradio to an intelligence much greater than itself.* There may be planets where theintelligent beings have about 1014 neural connections, as we do. But there may be places
where the number is 1024 or 1034. I wonder what they would know. Because we inhabit thesame universe as they, we and they must share some substantial information in common. If
we could make contact, there is much in their brains that would be of great interest to ours.
But the opposite is also true. I think extraterrestrial intelligence - even beings substantiallyfurther evolved than we - will be interested in us, in what we know, how we think, what ourbrains are like, the course of our evolution, the prospects for our future.
* In some sense such a radio integration of separate individuals is already beginning to happen on theplanet Earth.

If there are intelligent beings on the planets of fairly nearby stars, could they know
about us? Might they somehow have an inkling of the long evolutionary progression fromgenes to brains to libraries that has occurred on the obscure planet Earth? If theextraterrestrials stay at home, there are at least two ways in which they might find out about
us. One way would be to listen with large radio telescopes. For billions of years they would
have heard only weak and intermittent radio static caused by lightning and the trappedelectrons and protons whistling within the Earth's magnetic field. Then, less than a century
ago, the radio waves leaving the Earth would become stronger, louder, less like noise andmore like signals. The inhabitants of Earth had finally stumbled upon radio communication.
Today there is a vast international radio, television and radar communications traffic. At
some radio frequencies the Earth has become by far the brightest object, the most powerfulradio source, in the solar system - brighter than Jupiter, brighter than the Sun. An
extraterrestrial civilization monitoring the radio emission from Earth and receiving suchsignals could not fail to conclude that something interesting had been happening here
lately.
As the Earth rotates, our more powerful radio transmitters slowly sweep the sky. A
radio astronomer on a planet of another star would be able to calculate the length of the day
on Earth from the times of appearance and disappearance of our signals. Some of our mostpowerful sources are radar transmitters; a few are used for radar astronomy, to probe withradio fingers the surfaces of the nearby planets. The size of the radar beam projectedagainst the sky is much larger than the size of the planets, and much of the signal wafts on,out of the solar system into the depths of interstellar space to any sensitive receivers that
may be listening. Most radar transmissions are for military purposes; they scan the skies inconstant fear of a massive launch of missiles with nuclear warheads, an augury fifteen
minutes early of the end of human civilization. The information content of these pulses isnegligible: a succession of simple numerical patterns coded into beeps.
Overall, the most pervasive and noticeable source of radio transmissions from the
Earth is our television programming. Because the Earth is turning, some television stations
will appear at one horizon of the Earth while others disappear over the other. There will bea confused jumble of programs. Even these might be sorted out and pieced together by anadvanced civilization on a planet of a nearby star. The most frequently repeated messages
will be station call signals and appeals to purchase detergents, deodorants, headache tablets,
and automobile and petroleum products. The most noticeable messages will be thosebroadcast simultaneously by many transmitters in many time zones - for example, speechesin times of international crisis by the President of the United States or the Premier of theSoviet Union. The mindless contents of commercial television and the integuments ofinternational crisis and internecine warfare within the human family are the principal
messages about life on Earth that we choose to broadcast to the Cosmos. What must they
think of us?
There is no calling those television programs back. There is no way of sending afaster message to overtake them and revise the previous transmission. Nothing can travel
faster than light. Large-scale television transmission on the planet Earth began only in thelate 1940’s. Thus, there is a spherical wave front centered on the Earth expanding at the
speed of light and containing Howdy Doody, the ‘Checkers’ speech of then Vice-President
Richard M. Nixon and the televised inquisitions by Senator Joseph McCarthy. Because
these transmissions were broadcast a few decades ago, they are only a few tens of lightyears away from the Earth. If the nearest civilization is farther away than that, then we cancontinue to breathe easy for a while. In any case, we can hope that they will find these
programs incomprehensible.
The two Voyager spacecraft are bound for the stars. Affixed to each is a gold-plated
copper phonograph record with a cartridge and stylus and, on the aluminum record jacket,
instructions for use. We sent something about our genes, something about our brains, andsomething about our libraries to other beings who might sail the sea of interstellar space.

But we did not want to send primarily scientific information. Any civilization able tointercept Voyager in the depths of interstellar space, its transmitters long dead, would know
far more science than we do. Instead, we wanted to tell those other beings something about
what seems unique about ourselves. The interests of the cerebral cortex and limbic system
are well represented; the R-complex less so. Although the recipients may not know any
languages of the Earth, we included greetings in sixty human tongues, as well as the hellosof the humpback whales. We sent photographs of humans from all over the world caring
for one another, learning, fabricating tools and art and responding to challenges. There is anhour and a half of exquisite music from many cultures, some of it expressing our sense ofcosmic loneliness, our wish to end our isolation, our longing to make contact with otherbeings in the Cosmos. And we have sent recordings of the sounds that would have been
heard on our planet from the earliest days before the origin of life to the evolution of thehuman species and our most recent burgeoning technology. It is, as much as the sounds ofany baleen whale, a love song cast upon the vastness of the deep. Many, perhaps most, of
our messages will be indecipherable. But we have sent them because it is important to try.
In this spirit we included on the Voyager spacecraft the thoughts and feelings of one
person, the electrical activity of her brain, heart, eyes and muscles, which were recorded foran hour, transcribed into sound, compressed in time and incorporated into the record. In one
sense we have launched into the Cosmos a direct transcription of the thoughts and feelingsof a single human being in the month of June in the year 1977 on the planet Earth. Perhaps
the recipients will make nothing of it, or think it is a recording of a pulsar, which in somesuperficial sense it resembles. Or perhaps a civilization unimaginably more advanced thanours will be able to decipher such recorded thoughts and feelings and appreciate our efforts
to share ourselves with them.
The information in our genes is very old - most of it more than millions of years
old, some of it billions of years old. In contrast, the information in our books is at mostthousands of years old, and that in our brains is only decades old. The long-lived
information is not the characteristically human information. Because of erosion on the
Earth, our monuments and artifacts will not, in the natural course of things, survive to the
distant future. But the Voyager record is on its way out of the solar system. The erosion ininterstellar space - chiefly cosmic rays and impacting dust grains - is so slow that the
information on the record will last a billion years. Genes and brains and books encode
information differently and persist through time at different rates. But the persistence of thememory of the human species will be far longer in the impressed metal grooves on the
Voyager interstellar record.
The Voyager message is traveling with agonizing slowness. The fastest object everlaunched by the human species, it will still take tens of thousands of years to go the
distance to the nearest star. Any television program will traverse in hours the distance that
Voyager has covered in years. A television transmission that has just finished being aired
will, in only a few hours, overtake the Voyager spacecraft in the region of Saturn andbeyond and speed outward to the stars. If it is headed that way, the signal will reach Alpha
Centauri in a little more than four years. If, some decades or centuries hence, anyone out
there in space hears our television broadcasts, I hope they will think well of us, a product offifteen billion years of cosmic evolution, the local transmogrification of matter into
consciousness. Our intelligence has recently provided us with awesome powers. It is not yet
clear that we have the wisdom to avoid our own self-destruction. But many of us are trying
very hard. We hope that very soon in the perspective of cosmic time we will have unified

our planet peacefully into an organization cherishing the life of every living creature on it
and will be ready to take that next great step, to become part of a galactic society ofcommunicating civilizations.

CHAPTER XII
Encyclopaedia Galactica‘What are you? From where did you come? I have never seen anything like you.’
The Creator Raven looked at Man and was . . . surprised to find that this strange
new being was so much like himself.
- An Eskimo creation myth
Heaven is founded,
Earth is founded,
Who now shall be alive, oh gods?
- The Aztec chronicle, The History of the Kingdoms
I know some will say, we are a little too bold in these Assertions of the Planets, andthat we mounted hither by many Probabilities, one of which, if it chanced to befalse, and contrary to our Supposition, would, like a bad Foundation, ruin the whole
Building, and make it fall to the ground. But . . . supposing the Earth, as we did, oneof the Planets of equal dignity and honor with the rest, who would venture to say,that nowhere else were to be found any that enjoy’d the glorious sight of Nature’s
Opera? Or if there were any Fellow-Spectators, yet we were the only ones that had
dived deep to the secrets and knowledge of it?
- Christiaan Huygens in New Conjectures Concerning the Planetary Worlds, Their
Inhabitants and Productions, c. 1690
The author of Nature . . . has made it impossible for us to have any communicationfrom this earth with the other great bodies of the universe, in our present state; andit is highly possible that he has likewise cut off all communication betwixt the otherplanets, and betwixt the different systems . . . We observe, in all of them, enough toraise our curiosity, but not to satisfy it . . . It does not appear to be suitable to thewisdom that shines throughout all nature, to suppose that we should see so far, andhave our curiosity so much raised . . . only to be disappointed at the end . . . This,
therefore, naturally leads us to consider our present state as only the dawn or
beginning of our existence, and as a state of preparation or probation for farther
advancement . . .
- Colin Maclaurin, 1748
There cannot be a language more universal and more simple, more free from errors
and obscurities . . . more worthy to express the invariable relations of natural things[than mathematics]. It interprets [all phenomena] by the same language, as if toattest the unity and simplicity of the plan of the universe, and to make still moreevident that unchangeable order which presides over all natural causes.
- Joseph Fourier, Analytic Theory of Heat, 1822
We have launched four ships to the stars, Pioneers 10 and 11 and Voyagers 1 and 2. They
are backward and primitive craft, moving, compared to the immense interstellar distances,with the slowness of a race in a dream. But in the future we will do better. Our ships willtravel faster. There will be designated interstellar objectives, and sooner or later our
spacecraft will have human crews. In the Milky Way Galaxy there must be many planets
millions of years older than Earth, and some that are billions of years older. Should we nothave been visited? In all the billions of years since the origin of our planet, has there notbeen even once a strange craft from a distant civilization surveying our world from above,and slowly settling down to the surface to be observed by iridescent dragonflies, incurious
reptiles, screeching primates or wondering humans? The idea is natural enough. It has
occurred to everyone who has contemplated, even casually, the question of intelligent lifein the universe. But has it happened in fact? The critical issue is the quality of the purported
evidence, rigorously and skeptically scrutinized - not what sounds plausible, not theunsubstantiated testimony of one or two self-professed eyewitnesses. By this standard there
are no compelling cases of extraterrestrial visitation, despite all the claims about UFOs andancient astronauts which sometimes make it seem that our planet is awash in uninvited
guests. I wish it were otherwise. There is something irresistible about the discovery of evena token, perhaps a complex inscription, but, best by far, a key to the understanding of analien and exotic civilization. It is an appeal we humans have felt before.
In 1801 a physicist named Joseph Fourier* was the prefect of a departement ofFrance called Isère. While inspecting the schools in his province, Fourier discovered aneleven-year-old boy whose remarkable intellect and flair for oriental languages had alreadyearned him the admiring attention of scholars. Fourier invited him home for a chat. The boy
was fascinated by Fourier’s collection of Egyptian artifacts, collected during the
Napoleonic expedition where he had been responsible for cataloging the astronomicalmonuments of that ancient civilization. The hieroglyphic inscriptions roused the boy’s
sense of wonder. ‘But what do they mean?’ he asked. ‘Nobody knows,’ was the reply. The
boy’s name was Jean François Champollion. Fired by the mystery of language no one could
read, he became a superb linguist and passionately immersed himself in ancient Egyptian
writing. France at that time was flooded with Egyptian artifacts, stolen by Napoleon andlater made available to Western scholars. The description of the expedition was published,
and devoured by the young Champollion. As an adult, Champollion succeeded; fulfilling
his childhood ambition, he provided a brilliant decipherment of the ancient Egyptian
hieroglyphics. But it was not until 1828, twenty-seven years after his meeting with Fourier,that Champollion first set foot in Egypt, the land of his dreams, and sailed upstream from
Cairo, following the course of the Nile, paying homage to the culture he had worked so
hard to understand. It was an expedition in time, a visit to an alien civilization:
The evening of the 16th we finally arrived at Dendera. There was magnificent
moonlight and we were only an hour away from the Temples: Could we resist thetemptation? I ask the coldest of you mortals! To dine and leave immediately were
the orders of the moment: alone and without guides, but armed to the teeth wecrossed the fields . . . the Temple appeared to us at last . . . One could well measure
it but to give an idea of it would be impossible. It is the union of grace and majesty
in the highest degree. We stayed there two hours in ecstasy, running through thehuge rooms . . . and trying to read the exterior inscriptions in the moonlight. We did
not return to the boat until three in the morning, only to return to the Temple atseven . . . What had been magnificent in the moonlight was still so when the
sunlight revealed to us all the details . . . We in Europe are only dwarfs and nonation, ancient or modern, has conceived the art of architecture on such a sublime,
great, and imposing style, as the ancient Egyptians. They ordered everything to bedone for people who are a hundred feet high.
* Fourier is now famous for his study of the propagation of heat in solids, used today to understandthe surface properties of the planets, and for his investigation of waves and other periodic motion - a branchof mathematics known as Fourier analysis.

On the walls and columns of Karnak, at Dendera, everywhere in Egypt, Champollion
delighted to find that he could read the inscriptions almost effortlessly. Many before him
had tried and failed to decipher the lovely hieroglyphics, a word that means ‘sacredcarvings.’ Some scholars had believed them to be a kind of picture code, rich in murky
metaphor, mostly about eyeballs and wavy lines, beetles, bumblebees and birds - especiallybirds. Confusion was rampant. There were those who deduced that the Egyptians werecolonists from ancient China. There were those who concluded the opposite. Enormous
folio volumes of spurious translations were published. One interpreter glanced at the
Rosetta stone, whose hieroglyphic inscription was then still undeciphered, and instantly
announced its meaning. He said that the quick decipherment enabled him ‘to avoid thesystematic errors which invariably arise from prolonged reflection.’ You get better results,
he argued, by not thinking too much. As with the search for extraterrestrial life today, theunbridled speculation of amateurs had frightened many professionals out of the field.
Champollion resisted the idea of hieroglyphs as pictorial metaphors. Instead, withthe aid of a brilliant insight by the English physicist Thomas Young, he proceeded
something like this: The Rosetta stone had been uncovered in 1799 by a French soldier
working on the fortifications of the Nile Delta town of Rashid, which the Europeans,largely ignorant of Arabic, called Rosetta. It was a slab from an ancient temple, displaying
what seemed clearly to be the same message in three different writings: in hieroglyphics at
top, in a kind of cursive hieroglyphic called demotic in the middle, and, the key to theenterprise, in Greek at the bottom. Champollion, who was fluent in ancient Greek, read thatthe stone had been inscribed to commemorate the coronation of Ptolemy V Epiphanes, inthe spring of the year 196 B.C. On this occasion the king released political prisoners,
remitted taxes, endowed temples, forgave rebels, increased military preparedness and, inshort, did all the things that modern rulers do when they wish to stay in office.
The Greek text mentions Ptolemy many times. In roughly the same positions in thehieroglyphic text is a set of symbols surrounded by an oval or cartouche. This,Champollion reasoned, very probably also denotes Ptolemy. If so, the writing could not be
fundamentally pictographic or metaphorical; rather, most of the symbols must stand forletters or syllables. Champollion also had the presence of mind to count up the number of
Greek words and the number of individual hieroglyphs in what were presumably equivalenttexts. There were many fewer of the former, again suggesting that the hieroglyphs weremainly letters and syllables. But which hieroglyphs correspond to which letters?
Fortunately, Champollion had available to him an obelisk, which had been excavated at
Philae, that included the hieroglyphic equivalent of the Greek name Cleopatra. Ptolemy
begins with P; the first symbol in the cartouche is a square. Cleopatra has for its fifth lettera P, and in the Cleopatra cartouche in the fifth position is the same square. P it is. The
fourth letter in Ptolemy is an L. It is represented by the lion. The second letter of Cleopatra
is an L and, in hieroglyphics, here is a lion again. The eagle is an A, appearing twice inCleopatra, as it should. A clear pattern is emerging. Egyptian hieroglyphics are, insignificant part, a simple substitution cipher. But not every hieroglyph is a letter or syllable.
Some are pictographs. The end of the Ptolemy cartouche means ‘Ever-living, beloved of
the god Ptah.’ The semicircle and egg at the end of Cleopatra are a conventional ideogramfor ‘daughter of Isis.’ This mix of letters and pictographs caused some grief for earlierinterpreters.
In retrospect it sounds almost easy. But it had taken many centuries to figure out,
and there was a great deal more to do, especially in the decipherment of the hieroglyphs ofmuch earlier times. The cartouches were the key within the key, almost as if the pharaohsof Egypt had circled their own names to make the going easier for the Egyptologists two
thousand years in the future. Champollion walked the Great Hypostyle Hall at Karnak andcasually read the inscriptions, which had mystified everyone else, answering the question
he had posed as a child to Fourier. What a joy it must have been to open this one-way
communication channel with another civilization, to permit a culture that had been mute formillennia to speak of its history, magic, medicine, religion, politics and philosophy.
Today we are again seeking messages from an ancient and exotic civilization, thistime hidden from us not only in time but also in space. If we should receive a radio
message from an extraterrestrial civilization, how could it possibly be understood?
Extraterrestrial intelligence will be elegant, complex, internally consistent and utterly alien.
Extraterrestrials would, of course, wish to make a message sent to us as comprehensible aspossible. But how could they? Is there in any sense an interstellar Rosetta stone? Webelieve there is. We believe there is a common language that all technical civilizations, nomatter how different, must have. That common language is science and mathematics. The
laws of Nature are the same everywhere. The patterns in the spectra of distant stars andgalaxies are the same as those for the Sun or for appropriate laboratory experiments: notonly do the same chemical elements exist everywhere in the universe, but also the samelaws of quantum mechanics that govern the absorption and emission of radiation by atoms
apply everywhere as well. Distant galaxies revolving about one another follow the samelaws of gravitational physics as govern the motion of an apple falling to Earth, or Voyager
on its way to the stars. The patterns of Nature are everywhere the same. An interstellar
message, intended to be understood by an emerging civilization; should be easy to decode.
We do not expect an advanced technical civilization on any other planet in our solar
system. If one were only a little behind us - 10,000 years, say - it would have no advanced
technology at all. If it were only a little ahead of us - we who are already exploring the solar
system - its representatives should by now be here. To communicate with othercivilizations, we require a method adequate not merely for interplanetary distances but forinterstellar distances. Ideally, the method should be inexpensive, so that a huge amount of
information could be sent and received at very little cost; fast, so an interstellar dialogue isrendered possible; and obvious, so any technological civilization, no matter what its
evolutionary path, will discover it early. Surprisingly, there is such a method. It is called
radio astronomy.
The largest semi-steerable radio/radar observatory on the planet Earth is the Arecibo
facility, which Cornell University operates for the National Science Foundation. In the
remote hinterland of the island of Puerto Rico, it is 305 meters (a thousand feet) across, its
reflecting surface a section of a sphere laid down in a pre-existing bowl-shaped valley. It
receives radio waves from the depths of space, focusing them onto the feed arm antennahigh above the dish, which is in turn electronically connected to the control room, wherethe signal is analyzed. Alternatively, when the telescope is used as a radar transmitter, thefeed arm can broadcast a signal into the dish, which reflects it into space. The Arecibo
Observatory has been used both to search for intelligent signals from civilizations in space
and, just once, to broadcast a message - to M13, a distant globular cluster of stars, so that
our technical capability to engage in both sides of an interstellar dialogue would be clear, atleast to us.
In a period of a few weeks, the Arecibo Observatory could transmit to a comparable
observatory on a planet of a nearby star all of the Encyclopaedia Britannica. Radio waves
travel at the speed of light, 10,000 times faster than a message attached to our fastest
interstellar spaceship. Radio telescopes generate, in narrow frequency ranges, signals so
intense they can be detected over immense interstellar distances. The Arecibo Observatory
could communicate with an identical radio telescope on a planet 15,000 light-years away,
halfway to the center of the Milky Way Galaxy, if we knew precisely where to point it. And
radio astronomy is a natural technology. Virtually any planetary atmosphere, no matter
what its composition, should be partially transparent to radio waves. Radio messages are
not much absorbed or scattered by the gas between the stars, just as a San Francisco radio
station can be heard easily in Los Angeles even when smog there has reduced the visibility
at optical wavelengths to a few kilometers. There are many natural cosmic radio sourceshaving nothing to do with intelligent life - pulsars and quasars, the radiation belts of planetsand the outer atmospheres of stars; from almost any planet there are bright radio sources todiscover early in the local development of radio astronomy. Moreover, radio represents alarge fraction of the electromagnetic spectrum. Any technology able to detect radiation of
any wavelength would fairly soon stumble on the radio part of the spectrum.
There may be other effective methods of communication that have substantial merit:interstellar spacecraft; optical or infrared lasers; pulsed neutrinos; modulated gravity
waves; or some other kind of transmission that we will not discover for a thousand years.
Advanced civilizations may have graduated far beyond radio for their own
communications. But radio is powerful, cheap, fast and simple. They will know that abackward civilization like ours,, wishing to receive messages from the skies, is likely toturn first to radio technology. Perhaps they will have to wheel the radio telescopes out of
the Museum of Ancient Technology. If we were to receive a radio message we would knowthat there would be at the very least one thing we could talk about: radio astronomy.
But is there anyone out there to talk to? With a third or half a trillion stars in our
Milky Way Galaxy alone, could ours be the only one accompanied by an inhabited planet?
How much more likely it is that technical civilizations are a cosmic commonplace, that the
Galaxy is pulsing and humming with advanced societies, and, therefore, that the nearest
such culture is not so very far away - perhaps transmitting from antennas established on aplanet of a naked-eye star just next door. Perhaps when we look up at the sky at night, near
one of those faint pinpoints of light is a world on which someone quite different from us isthen glancing idly at a star we call the Sun and entertaining, for just a moment, an
outrageous speculation.
It is very hard to be sure. There may be severe impediments to the evolution of atechnical civilization. Planets may be rarer than we think. Perhaps the origin of life is not soeasy as our laboratory experiments suggest. Perhaps the evolution of advanced life forms is
improbable. Or it may be that complex life forms evolve readily, but intelligence andtechnical societies require an unlikely set of coincidences - just as the evolution of thehuman species depended on the demise of the dinosaurs and the ice-age recession of theforests in whose trees our ancestors screeched and dimly wondered. Or perhaps
civilizations arise repeatedly, inexorably, on innumerable planets in the Milky Way, but are
generally unstable; so all but a tiny fraction are unable to survive their technology andsuccumb to greed and ignorance, pollution and nuclear war.
It is possible to explore this great issue further and make a crude estimate of N, thenumber of advanced technical civilizations in the Galaxy. We define an advanced
civilization as one capable of radio astronomy. This is, of course, a parochial if essential
definition. There may be countless worlds on which the inhabitants are accomplishedlinguists or superb poets but indifferent radio astronomers. We will not hear from them. N
can be written as the product or multiplication of a number of factors, each a kind of filter,
every one of which must be sizable for there to be large number of civilizations:N*, the number of stars in the Milky Way Galaxy;fp, the fraction of stars that have planetary systems;
ne, the number of planets in a given system that are ecologically suitable for life;
fl, the fraction of otherwise suitable planets on which life actually arises;fi, the fraction of inhabited planets on which an intelligent form of life evolves;fc, the fraction of planets inhabited by intelligent beings on which a communicativetechnical civilization develops; andfL, the fraction of a planetary lifetime graced by a technical civilization.
Written out, the equation reads N = N*fpneflfifcfL. All the f’s are fractions, having values
between 0 and 1; they will pare down the large value of N*.
To derive N we must estimate each of these quantities. We know a fair amount about theearly factors in the equation, the numbers of stars and planetary systems. We know very
little about the later factors, concerning the evolution of intelligence or the lifetime oftechnical societies. In these cases our estimates will be little better than guesses. I invite
you, if you disagree with my estimates below, to make your own choices and see what
implications your alternative suggestions have for the number of advanced civilizations in
the Galaxy. One of the great virtues of this equation, due originally to Frank Drake ofCornell, is that it involves subjects ranging from stellar and planetary astronomy to organic
chemistry, evolutionary biology, history, politics and abnormal psychology. Much of the
Cosmos is in the span of the Drake equation.
We know N*, the number of stars in the Milky Way Galaxy, fairly well, by carefulcounts of stars in small but representative regions of the sky. It is a few hundred billion;some recent estimates place it at 4x 1011. Very few of these stars are of the massive shortlived variety that squander their reserves of thermonuclear fuel. The great majority have
lifetimes of billions or more years in which they are shining stably, providing a suitableenergy source for the origin and evolution of life on nearby planets.
There is evidence that planets are a frequent accompaniment of star formation: inthe satellite systems of Jupiter, Saturn and Uranus, which are like miniature solar systems;in theories of the origin of the planets; in studies of double stars; in observations ofaccretion disks around stars; and in some preliminary investigations of gravitational
perturbations of nearby stars. Many, perhaps even most, stars may have planets. We takethe fraction of stars that have planets, fp, as roughly equal to 1/3 . Then the total number ofplanetary systems in the Galaxy would be N*fp 1.3 x 1011 (the symbol means‘approximately equal to’). If each system were to have about ten planets, as ours does, thetotal number of worlds in the Galaxy would be more than a trillion, a vast arena for thecosmic drama.
In our own solar system there are several bodies that may be suitable for life ofsome sort: the Earth certainly, and perhaps Mars, Titan and Jupiter. Once life originates, it
tends to be very adaptable and tenacious. There must be many different environments
suitable for life in a given planetary system. But conservatively we choose ne = 2. Then thenumber of planets in the Galaxy suitable for life becomes N*fpne 3 x 1011.
Experiments show that under the most common cosmic conditions the molecularbasis of life is readily made, the building blocks of molecules able to make copies ofthemselves. We are now on less certain ground; there may, for example, be impediments inthe evolution of the genetic code, although I think this unlikely over billions of years ofprimeval chemistry. We choose fl 1/3, implying a total number of planets in the Milky
Way on which life has arisen at least once as N*fpnefl 1 x 1011, a hundred billion inhabitedworlds. That in itself is a remarkable conclusion. But we are not yet finished.
The choices of fi and fc are more difficult. On the one hand, many individuallyunlikely steps had to occur in biological evolution and human history for our present
intelligence and technology to develop. On the other hand, there must be many quite
different pathways to an advanced civilization of specified capabilities. Considering theapparent difficulty in the evolution of large organisms represented by the Cambrian
explosion, let us choose fi x fc = 1/100, meaning that only 1 percent of planets on which life
arises eventually produce a technical civilization. This estimate represents some middle
ground among the varying scientific opinions. Some think that the equivalent of the stepfrom the emergence of trilobites to the domestication of fire goes like a shot in all planetarysystems; others think that, even given ten or fifteen billion years, the evolution of technicalcivilizations is unlikely. This is not a subject on which we can do much experimentation aslong as our investigations are limited to a single planet. Multiplying these factors together,
we find N*fpneflfifc 1 x 109, a billion planets on which technical civilizations have arisen
at least once. But that is very different from saying that there are a billion planets on which
technical civilizations now exist. For this, we must also estimate fL.
What percentage of the lifetime of a planet is marked by a technical civilization?
The Earth has harbored a technical civilization characterized by radio astronomy for only a
few decades out of a lifetime of a few billion years. So far, then, for our planet fL, is lessthan 1/108, a millionth of a percent. And it is hardly out of the question that we might
destroy ourselves tomorrow. Suppose this were to be a typical case, and the destruction socomplete that no other technical civilization - of the human or any other species - were able
to emerge in the five or so billion years remaining before the Sun dies. Then N =N*fpneflfifcfL 10, and at any given time there would be only a tiny smattering, a handful, apitiful few technical civilizations in the Galaxy, the steady state number maintained as
emerging societies replace those recently self-immolated. The number N might even be as
small as 1. If civilizations tend to destroy themselves soon after reaching a technologicalphase, there might be no one for us to talk with but ourselves. And that we do but poorly.
Civilizations would take billions of years of tortuous evolution to arise, and then snuff
themselves out in an instant of unforgivable neglect.
But consider the alternative, the prospect that at least some civilizations learn to livewith high technology; that the contradictions posed by the vagaries of past brain evolution
are consciously resolved and do not lead to self-destruction; or that, even if major
disturbances do occur, they are reversed in the subsequent billions of years of biologicalevolution. Such societies might live to a prosperous old age, their lifetimes measured
perhaps on geological or stellar evolutionary time scales. If 1 percent of civilizations cansurvive technological adolescence, take the proper fork at this critical historical branch
point and achieve maturity, then fL 1/100, N 107, and the number of extant civilizationsin the Galaxy is in the millions. Thus, for all our concern about the possible unreliability of
our estimates of the early factors in the Drake equation, which involved astronomy, organic
chemistry and evolutionary biology, the principal uncertainty comes down to economics
and politics and what, on Earth, we call human nature. It seems fairly clear that if selfdestruction is not the overwhelmingly preponderant fate of galactic civilizations, then thesky is softly humming with messages from the stars.
These estimates are stirring. They suggest that the receipt of a message from spaceis, even before we decode it, a profoundly hopeful sign. It means that someone has learned
to live with high technology; that it is possible to survive technological adolescence. This
alone, quite apart from the contents of the message, provides a powerful justification for the
search for other civilizations.
If there are millions of civilizations distributed more or less randomly through the
Galaxy, the distance to the nearest is about two hundred light-years. Even at the speed oflight it would take two centuries for a radio message to get from there to here. If we had
initiated the dialogue, it would be as if the question had been asked by Johannes Kepler andthe answer received by us. Especially because we, new to radio astronomy, must becomparatively backward, and the transmitting civilization advanced, it makes more sense
for us to listen than to send. For a more advanced civilization, the positions are, of course,
reversed.
We are at the earliest stages of our radio search for other civilizations in space. In an
optical photograph of a dense star field, there are hundreds of thousands of stars. By ourmore optimistic estimates, one of them is the site of an advanced civilization. But whichone? Toward which stars should we point our radio telescopes? Of the millions of stars that
may mark the location of advanced civilizations, we have so far examined by radio no morethan thousands. We have made about one-tenth of one percent of the required effort. But a
serious, rigorous, systematic search will come soon. The preparatory steps are now
underway, both in the United States and in the Soviet Union. It is comparatively
inexpensive: the cost of a single naval vessel of intermediate size - a modern destroyer, say- would pay for a decade-long program in the search for extraterrestrial intelligence.
Benevolent encounters have not been the rule in human history, where transculturalcontacts have been direct and physical, quite different from the receipt of a radio signal, acontact as light as a kiss. Still, it is instructive to examine one or two cases from our past, if
only to calibrate our expectations: Between the times of the American and the French
Revolutions, Louis XVI of France outfitted an expedition to the Pacific Ocean, a voyage
with scientific, geographic, economic and nationalistic objectives. The commander was theCount of La Pérouse, a noted explorer who had fought for the United States in its War of
Independence. In July 1786, almost a year after setting sail, he reached the coast of Alaska,
a place now called Lituya Bay. He was delighted with the harbor and wrote: ‘Not a port in
the universe could afford more conveniences.’ In this exemplary location, La Pérouseperceived some savages, who made signs of friendship, by displaying and waving
white mantles, and different skins. Several of the canoes of these Indians werefishing in the Bay . . . [We were] continually surrounded by the canoes of thesavages, who offered us fish, skins of otters and other animals, and different little
articles of their dress in exchange for our iron. To our great surprise, they appeared
well accustomed to traffic, and bargained with us with as much skill as any
tradesman of Europe.
The Native Americans drove increasingly harder bargains. To La Pérouse’s annoyance,
they also resorted to pilferage, largely of iron objects, but once of the uniforms of Frenchnaval officers hidden under their pillows as they were sleeping one night surrounded byarmed guards - a feat worthy of Harry Houdini. La Pérouse followed his royal orders to
behave peaceably but complained that the natives ‘believed our forbearance inexhaustible.’
He was disdainful of their society. But no serious damage was done by either culture to theother. After reprovisioning his two ships La Pérouse sailed out of Lituya Bay, never toreturn. The expedition was lost in the South Pacific in 1788; La Pérouse and all but one of
the members of his crew perished.** When La Pérouse was mustering the ship’s company in France, there were many bright and eager
young men who applied but were turned down. One of them was a Corsican artillery officer named Napoleon
Bonaparte. It was an interesting branch point in the history of the world. If La Pérouse had accepted
Bonaparte, the Rosetta stone might never have been found, Champollion might never have decrypted
Egyptian hieroglyphics, and in many more important respects our recent history might have been changed
significantly.

Exactly a century later Cowee, a chief of the Tlingit, related to the Canadiananthropologist G. T. Emmons a story of the first meeting of his ancestors with the whiteman, a narrative handed down by word of mouth only. The Tlingit possessed no written
records, nor had Cowee ever heard of La Pérouse. This is a paraphrase of Cowee’s story:
Late one spring a large party of Tlingit ventured North to Yakutat to trade forcopper. Iron was even more precious, but it was unobtainable. In entering Lituya
Bay four canoes were swallowed by the waves. As the survivors made camp andmourned for their lost companions two strange objects entered the Bay. No one
knew what they were. They seemed to be great black birds with immense whitewings. The Tlingit believed the world had been created by a great bird which often
assumed the form of a raven, a bird which had freed the Sun, the Moon, and thestars from boxes in which they had been imprisoned. To look upon the Raven was
to be turned to stone. In their fright, the Tlingit fled into the forest and hid. But after
a while, finding that no harm had come to them, a few more enterprising souls creptout and rolled leaves of the skunk cabbage into crude telescopes, believing that thiswould prevent being turned to stone. Through the skunk cabbage, it seemed that thegreat birds were folding their wings and that flocks of small black messengers arosefrom their bodies and crawled upon their feathers.
Now one nearly blind old warrior gathered the people together andannounced that his life was far behind him; for the common good he would
determine whether the Raven would turn his children into stone. Putting on his robeof sea otter fur, he entered his canoe and was paddled seaward to the Raven. He
climbed upon it and heard strange voices. With his impaired vision he could barely
make out the many black forms moving before him. Perhaps they were crows.
When he returned safely to his people they crowded about him, surprised to see him
alive. They touched him and smelled him to see if it was really he. After muchthought the old man convinced himself that it was not the god-raven that he had
visited, but rather a giant Canoe made by men. The black figures were not crows but
people of a different sort. He convinced the Tlingit, who then visited the ships andexchanged their furs for many strange articles, chiefly iron.
The Tlingit had preserved in oral tradition an entirely recognizable and accurate account of
their first, almost fully peaceable encounter with an alien culture.** The account of Cowee, the Tlingit chief, shows that even in a preliterate culture a recognizableaccount of contact with an advanced civilization can be preserved for generations. If the Earth had been
visited hundreds or thousands of years ago by an advanced extraterrestrial civilization, even if the contacted
culture was preliterate, we might well expect to have some recognizable form of the encounter preserved. But
there is not a single case in which a legend reliably dated from earlier pretechnological times can beunderstood only in terms of contact with an extraterrestrial civilization.

If someday we make contact with a more advanced extraterrestrial civilization, will
the encounter be largely peaceable, even if lacking a certain rapport, like that of the French
among the Tlingit, or will it follow some more ghastly prototype, where the society that
was a little more advanced utterly destroyed the society that was technically more
backward? In the early sixteenth century a high civilization flourished in central Mexico.
The Aztecs had monumental architecture, elaborate record-keeping, exquisite art and an
astronomical calendar superior to that of any in Europe. Upon viewing the Aztec artifacts
returned by the first Mexican treasure ships, the artist Albrecht Dürer wrote in August
1520: ‘I have never seen anything heretofore that has so rejoiced my heart. I have seen . . .
a sun entirely of gold a whole fathom broad [in fact, the Aztec astronomical calendar];likewise a moon entirely of silver, equally large . . . also two chambers full of all sorts of
weapons, armor, and other wonderous arms, all of which is fairer to see than marvels.’Intellectuals were stunned at the Aztec books, ‘which,’ one of them said, ‘almost resemble
those of the Egyptians.’ Hernán Cortés described their capital Tenochtitlán as ‘one of themost beautiful cities in the world . . . The people’s activities and behavior are on almost as
high a level as in Spain, and as well-organized and orderly. Considering that these people
are barbarous, lacking knowledge of God and communication with other civilized nations,
it is remarkable to see all that they have.’ Two years after writing these words, Cortésutterly destroyed Tenochtitlán along with the rest of the Aztec civilization. Here is an Aztec
account:
Moctezuma [the Aztec Emperor] was shocked, terrified by what he heard. He wasmuch puzzled by their food, but what made him almost faint away was the telling of
how the great Lombard gun, at the Spaniards’ command, expelled the shot whichthundered as it went off. The noise weakened one, dizzied one. Something like astone came out of it in a shower of fire and sparks. The smoke was foul; it had asickening, fetid smell. And the shot, which struck a mountain, knocked it to bits -dissolved it. It reduced a tree to sawdust - the tree disappeared as if they had blown
it away . . . When Moctezuma was told all this, he was terror-struck. He felt faint.
His heart failed him.
Reports continued to arrive: ‘We, are not as strong as they,’ Moctezuma was told.
‘We are nothing compared to them: The Spaniards began to be called ‘the Gods come fromthe Heavens.’ Nevertheless, the Aztecs had no illusions about the Spaniards, whom theydescribed in these words:
They seized upon the gold as if they were monkeys, their faces gleaming. For
clearly their thirst for gold was insatiable; they starved for it; they lusted for it; theywanted to stuff themselves with it as if they were pigs. So they went about
fingering, taking up the streamers of gold, moving them back and forth, grabbing
them to themselves, babbling, talking gibberish among themselves.
But their insight into the Spanish character did not help them defend themselves. In 1517 agreat comet had been seen in Mexico. Moctezuma, captured by the legend of the return of
the Aztec god Quetzalcoatl as a white-skinned man arriving across the Eastern sea,
promptly executed his astrologers. They had not predicted the comet, and they had not
explained it. Certain of forthcoming disaster, Moctezuma became distant and gloomy.
Aided by the superstition of the Aztecs and their own superior technology, an armed partyof 400 Europeans and their native allies in the year 1521 entirely vanquished and utterly
destroyed a high civilization of a million people. The Aztecs had never seen a horse; there
were none in the New World. They had not applied iron metallurgy to warfare. They had
not invented firearms. Yet the technological gap between them and the Spaniards was notvery great, perhaps a few centuries.
We must be the most backward technical society in the Galaxy. Any society still
more backward would not have radio astronomy at all. If the doleful experience of cultural
conflict on Earth were the galactic standard, it seems we would already have been
destroyed, perhaps with some passing admiration expressed for Shakespeare, Bach andVermeer. But this has not happened. Perhaps alien intentions are uncompromisingly
benign, more like La Pérouse than Cortés. Or might it be, despite all the pretensions about
UFOs and ancient astronauts, that our civilization has not yet been discovered?
On the one hand, we have argued that if even a small fraction of technicalcivilizations learn to live with themselves and with weapons of mass destruction, there
should now be an enormous number of advanced civilizations in the Galaxy. We alreadyhave slow interstellar flight, and think fast interstellar flight a possible goal for the humanspecies. On the other hand, we maintain that there is no credible evidence for the Earthbeing visited, now or ever. Is this not a contradiction? If the nearest civilization is, say, 200
light-years away, it takes only 200 years to get from there to here at close to the speed oflight. Even at 1 percent or a tenth of a percent of the speed of light, beings from nearby
civilizations could have come during the tenure of humanity on Earth. Why are they not
here? There are many possible answers. Although it runs contrary to the heritage ofAristarchus and Copernicus, perhaps we are the first. Some technical civilization must be
the first to emerge in the history of the Galaxy. Perhaps we are mistaken in our belief that atleast occasional civilizations avoid self-destruction. Perhaps there is some unforeseenproblem to interstellar spaceflight - although, at speeds much less than the velocity of light
it is difficult to see what such an impediment might be. Or perhaps they are here, but in
hiding because of some Lex Galactica, some ethic of noninterference with emerging
civilizations. We can imagine them, curious and dispassionate, observing us, as we would
watch a bacterial culture in a dish of agar, to determine whether, this year again, wemanage to avoid self-destruction.
But there is another explanation that is consistent with everything we know. If agreat many years ago an advanced interstellar spacefaring civilization emerged 200 lightyears away, it would have no reason to think there was something special about the Earthunless it had been here already. No artifact of human technology, not even our radiotransmissions, has had time, even traveling at the speed of light, to go 200 light-years. Fromtheir point of view, all nearby star systems are more or less equally attractive for
exploration or colonization.** There may be many motivations to go to the stars. If our Sun or a nearby star were about to gosupernova, a major program of interstellar spaceflight might suddenly become attractive. If we were very
advanced, the discovery that the galactic core was imminently to explode might even generate serious interestin transgalactic or intergalactic spaceflight. Such cosmic violence occurs sufficiently often that nomadic
spacefaring civilizations may not be uncommon. Even so, their arrival here remains unlikely.

An emerging technical civilization, after exploring its home planetary system anddeveloping interstellar spaceflight, would slowly and tentatively begin exploring the nearbystars. Some stars would have no suitable planets - perhaps they would all be giant gas
worlds, or tiny asteroids. Others would carry an entourage of suitable planets, but somewould be already inhabited, or the atmosphere would be poisonous or the climate
uncomfortable. In many cases the colonists might have to change - or as we would
parochially say, terraform - a world to make it adequately clement. The reengineering of aplanet will take time. Occasionally, an already suitable world would be found andcolonized. The utilization of planetary resources so that new interstellar spacecraft could beconstructed locally would be a slow process. Eventually a second-generation mission of
exploration and colonization would take off toward stars where no one had yet been. And
in this way a civilization might slowly wend its way like a vine among the worlds.
It is possible that at some later time with third and higher orders of colonies
developing new worlds, another independent expanding civilization would be discovered.
Very likely mutual contact would already have been made by radio or other remote means.
The new arrivals might be a different sort of colonial society. Conceivably two expanding
civilizations with different planetary requirements would ignore each other, their filigree
patterns of expansion intertwining, but not conflicting. They might cooperate in the
exploration of a province of the Galaxy. Even nearby civilizations could spend millions ofyears in such separate or joint colonial ventures without ever stumbling upon our obscuresolar system.
No civilization can possibly survive to an interstellar spacefaring phase unless it
limits its numbers. Any society with a marked population explosion will be forced to
devote all its energies and technological skills to feeding and caring for the population on
its home planet. This is a very powerful conclusion and is in no way based on theidiosyncrasies of a particular civilization. On any planet, no matter what its biology orsocial system, an exponential increase in population will swallow every resource.

Conversely, any civilization that engages in serious interstellar exploration and colonizationmust have exercised zero population growth or something very close to it for many
generations. But a civilization with a low population growth rate will take a long time tocolonize many worlds, even if the strictures on rapid population growth are eased after
reaching some lush Eden.
My colleague William Newman and I have calculated that if a million years ago aspacefaring civilization with a low population growth rate emerged two hundred light-years
away and spread outward, colonizing suitable worlds along the way, their survey starships
would be entering our solar system only about now. But a million years is a very long
period of time. If the nearest civilization is younger than this, they would not have reached
us yet. A sphere two hundred light-years in radius contains 200,000 suns and perhaps acomparable number of worlds suitable for colonization. It is only after 200,000 otherworlds have been colonized that, in the usual course of things, our solar system would be
accidentally discovered to harbor an indigenous civilization.
What does it mean for a civilization to be a million years old? We have had radio
telescopes and spaceships for a few decades; our technical civilization is a few hundredyears old, scientific ideas of a modern cast a few thousand, civilization in general a few tensof thousands of years; human beings evolved on this planet only a few million years ago.
At anything like our present rate of technical progress, an advanced civilization millions ofyears old is as much beyond us as we are beyond a bush baby or a macaque. Would weeven recognize its presence? Would a society a million years in advance of us be interested
in colonization or interstellar spaceflight? People have a finite lifespan for a reason.
Enormous progress in the biological and medical sciences might uncover that reason andlead to suitable remedies. Could it be that we are so interested in spaceflight because it is a
way of perpetuating ourselves beyond our own lifetimes? Might a civilization composed ofessentially immortal beings consider interstellar exploration fundamentally childish? It may
be that we have not been visited because the stars are strewn abundantly in the expanse of
space, so that before a nearby civilization arrives, it has altered its exploratory motivations
or evolved into forms indetectable to us.
A standard motif in science fiction and UFO literature assumes extraterrestrials
roughly as capable as we. Perhaps they have a different sort of spaceship or ray gun, but inbattle - and science fiction loves to portray battles between civilizations - they and we arerather evenly matched. In fact, there is almost no chance that two galactic civilizations willinteract at the same level. In any confrontation, one will always utterly dominate the other.
A million years is a great many. If an advanced civilization were to arrive in our solar
system, there would be nothing whatever we could do about it. Their science andtechnology would be far beyond ours. It is pointless to worry about the possible malevolent
intentions of an advanced civilization with whom we might make contact. It is more likelythat the mere fact they have survived so long means they have learned to live withthemselves and others. Perhaps our fears about extraterrestrial contact are merely aprojection of our own backwardness, an expression of our guilty conscience about our pasthistory: the ravages that have been visited on civilizations only slightly more backward
than we. We remember Columbus and the Arawaks, Cortés and the Aztecs, even the fate of
the Tlingit in the generations after La Pérouse. We remember and we worry. But if aninterstellar armada appears in our skies, I predict we will be very accommodating.
A very different kind of contact is much more likely - the case we have alreadydiscussed in which we receive a rich, complex message, probably by radio, from anothercivilization in space, but do not make, at least for a while, physical contact with them. In
this case there is no way for the transmitting civilization to know whether we have received
the message. If we find the contents offensive or frightening, we are not obliged to reply.
But if the message contains valuable information, the consequences for our own civilization
will be stunning - insights on alien science and technology, art, music, politics, ethics,
philosophy and religion, and most of all, a profound deprovincialization of the humancondition. We will know what else is possible.
Because we will share scientific and mathematical insights with any othercivilization, I believe that understanding the interstellar message will be the easiest part ofthe problem. Convincing the U.S. Congress and the Council of Ministers of the U.S.S.R. tofund a search for extraterrestrial intelligence is the hard part.* In fact, it may be thatcivilizations can be divided into two great categories: one in which the scientists are unable
to convince nonscientists to authorize a search for extraplanetary intelligence, in whichenergies are directed exclusively inward, in which conventional perceptions remainunchallenged and society falters and retreats from the stars; and another category in which
the grand vision of contact with other civilizations is shared widely, and a major search isundertaken.
* Or other national organs. Consider this pronouncement from a British Defence Department
spokesman as reported in the London Observer for February 26, 1978: ‘Any messages transmitted from outerspace are the responsibility of the BBC and the Post Office. It is their responsibility to track down illegal
broadcasts.’

This is one of the few human endeavors where even a failure is a success. If wewere to carry out a rigorous search for extraterrestrial radio signals encompassing millionsof stars and heard nothing, we would conclude that galactic civilizations were at best
extremely rare, a calibration of our place in the universe. It would speak eloquently of how
rare are the living things of our planet, and would underscore, as nothing else in humanhistory has, the individual worth of every human being. If we were to succeed, the historyof our species and our planet would be changed forever.
It would be easy for extraterrestrials to make an unambiguously artificial interstellar
message. For example, the first ten prime numbers - numbers divisible only by themselves
and by one - are 1, 2, 3, 5, 7, 11, 13, 17, 19, 23. It is extremely unlikely that any natural
physical process could transmit radio messages containing prime numbers only. If we
received such a message we would deduce a civilization out there that was at least fond ofprime numbers. But the most likely case is that interstellar communication will be a kind of
palimpsest, like the palimpsests of ancient writers short of papyrus or stone who
superimposed their messages on top of preexisting messages. Perhaps at an adjacentfrequency or a faster timing, there would be another message, which would turn out to be aprimer, an introduction to the language of interstellar discourse. The primer would be
repeated again and again because the transmitting civilization would have no way to know
when we turned in on the message. And then, deeper in the palimpsest, underneath theannouncement signal and the primer, would be the real message. Radio technology permits
that message to be inconceivably rich. Perhaps when we tuned in, we would find ourselvesin the midst of Volume 3,267 of the Encyclopaedia Galactica.
We would discover the nature of other civilizations. There would be many of them,each composed of organisms astonishingly different from anything on this planet. Theywould view the universe somewhat differently. They would have different arts and social
functions. They would be interested in things we never thought of. By comparing ourknowledge with theirs, we would grow immeasurably. And with our newly acquired
information sorted into a computer memory, we would be able to see which sort ofcivilization lived where in the Galaxy. Imagine a huge galactic computer, a repository,
more or less up-to-date, of information on the nature and activities of all the civilizations in
the Milky Way Galaxy, a great library of life in the Cosmos. Perhaps among the contents ofthe Encyclopaedia Galactica will be a set of summaries of such civilizations, theinformation enigmatic, tantalizing, evocative - even after we succeed in translating it.
Eventually, taking as much time as we wished, we would decide to reply. We would
transmit some information about ourselves - just the basics at first - as the start of a longinterstellar dialogue which we would begin but which, because of the vast distances ofinterstellar space and the finite velocity of light, would be continued by our remote
descendants. And someday, on a planet of some far distant star, a being very different fromany of us would request a printout from the latest edition of the Encyclopaedia Galactica
and acquire a little information about the newest society to join the community of galactic
civilizations.

CHAPTER XIII
Who Speaks for Earth?
To what purpose should I trouble myself in searching out the secrets of the stars,
having death or slavery continually before my eyes?
- A question put to Pythagoras by Anaximenes (c. 600 B.C.), according toMontaigne
How vast those Orbs must be, and how inconsiderable this Earth, the Theatre uponwhich all our mighty Designs, all our Navigations, and all our Wars are transacted,
is when compared to them. A very fit consideration, and matter of Reflection, forthose Kings and Princes who sacrifice the Lives of so many People, only to flatter
their Ambition in being Masters of some pitiful corner of this small Spot.
- Christiaan Huygens, New Conjectures Concerning the Planetary Worlds, Their
Inhabitants and Productions, c. 1690
‘To the entire world,’ added our Father the Sun, ‘I give my light and my radiance; I
give men warmth when they are cold; I cause their fields to fructify and their cattle
to multiply; each day that passes I go around the world to secure a better knowledgeof men’s needs and to satisfy those needs. Follow my example.’- An Inca myth recorded in ‘The Royal Commentaries’ of Garcilaso de la Vega,1556
We look back through countless millions of years and see the great will to live
struggling out of the intertidal slime, struggling from shape to shape and frompower to power, crawling and then walking confidently upon the land, struggling
generation after generation to master the air, creeping down into the darkness of thedeep; we see it turn upon itself in rage and hunger and reshape itself anew, wewatch it draw nearer and more akin to us, expanding, elaborating itself, pursuing its
relentless inconceivable purpose, until at last it reaches us and its being beatsthrough our brains and arteries . . . It is possible to believe that all the past is but the
beginning of a beginning, and that all that is and has been is but the twilight of thedawn. It is possible to believe that all that the human mind has ever accomplished isbut the dream before the awakening . . . Out of our . . . lineage, minds will spring,that will reach back to us in our littleness to know us better than we know ourselves.
A day will come, one day in the unending succession of days, when beings, beings
who are now latent in our thoughts and hidden in our loins, shall stand upon thisearth as one stands upon a footstool, and shall laugh and reach out their hands
amidst the stars.
- H. G. Wells, ‘The Discovery of the Future,’ Nature 65, 326 (1902)
The Cosmos was discovered only yesterday. For a million years it was clear to everyone
that there were no other places than the Earth. Then in the last tenth of a percent of the
lifetime of our species, in the instant between Aristarchus and ourselves, we reluctantly
noticed that we were not the center and purpose of the Universe, but rather lived on a tiny

and fragile world lost in immensity and eternity, drifting in a great cosmic ocean dottedhere and there with a hundred billion galaxies and a billion trillion stars. We have bravely
tested the waters and have found the ocean to our liking, resonant with our nature.
Something in us recognizes the Cosmos as home. We are made of stellar ash. Our origin
and evolution have been tied to distant cosmic events. The exploration of the Cosmos is avoyage of self-discovery.
As the ancient mythmakers knew, we are the children equally of the sky and theEarth. In our tenure on this planet we have accumulated dangerous evolutionary baggage,hereditary propensities for aggression and ritual, submission to leaders and hostility tooutsiders, which place our survival in some question. But we have also acquiredcompassion for others, love for our children and our children’s children, a desire to learn
from history, and a great soaring passionate intelligence - the clear tools for our continued
survival and prosperity. Which aspects of our nature will prevail is uncertain, particularly
when our vision and understanding and prospects are bound exclusively to the Earth - or,worse, to one small part of it. But up there in the immensity of the Cosmos, an inescapable
perspective awaits us. There are not yet any obvious signs of extraterrestrial intelligenceand this makes us wonder whether civilizations like ours always rush implacably, headlong,
toward self-destruction. National boundaries are not evident when we view the Earth fromspace. Fanatical ethnic or religious or national chauvinisms are a little difficult to maintainwhen we see our planet as a fragile blue crescent fading to become an inconspicuous pointof light against the bastion and citadel of the stars. Travel is broadening.
There are worlds on which life has never arisen. There are worlds that have beencharred and ruined by cosmic catastrophes. We are fortunate: we are alive; we are
powerful; the welfare of our civilization and our species is in our hands. If we do not speakfor Earth, who will? If we are not committed to our own survival, who will be?
The human species is now undertaking a great venture that if successful will be asimportant as the colonization of the land or the descent from the trees. We are haltingly,
tentatively breaking the shackles of Earth - metaphorically, in confronting and taming theadmonitions of those more primitive brains within us; physically, in voyaging to the planetsand listening for the messages from the stars. These two enterprises are linked indissolubly.
Each, I believe, is a necessary condition for the other. But our energies are directed far
more toward war. Hypnotized by mutual mistrust, almost never concerned for the species
or the planet, the nations prepare for death. And because what we are doing is so horrifying,
we tend not to think of it much. But what we do not consider we are unlikely to put right.
Every thinking person fears nuclear war, and every technological state plans for it.
Everyone knows it is madness, and every nation has an excuse. There is a dreary chain of
causality: The Germans were working on the bomb at the beginning of World War II; so
the Americans had to make one first: If the Americans had one, the Soviets had to haveone, and then the British, the French, the Chinese, the Indians, the Pakistanis. . . By the endof the twentieth century many nations had collected nuclear weapons. They were easy todevise. Fissionable material could be stolen from nuclear reactors. Nuclear weapons
became almost a home handicraft industry.
The conventional bombs of World War II were called blockbusters. Filled withtwenty tons of TNT, they could destroy a city block. All the bombs dropped on all the cities
in World War II amounted to some two million tons, two megatons, of TNT - Coventry andRotterdam, Dresden and Tokyo, all the death that rained from the skies between 1939 and
1945: a hundred thousand blockbusters, two megatons. By the late twentieth century, twomegatons was the energy released in the explosion of a single more or less humdrum
thermonuclear bomb: one bomb with the destructive force of the Second World War. But
there are tens of thousands of nuclear weapons. By the ninth decade of the twentiethcentury the strategic missile and bomber forces of the Soviet Union and the United States
were aiming warheads at over 15,000 designated targets. No place on the planet was safe.
The energy contained in these weapons, genies of death patiently awaiting the rubbing of
the lamps, was far more than 10,000 megatons - but with the destruction concentrated
efficiently, not over six years but over a few hours, a blockbuster for every family on theplanet, a World War II every second for the length of a lazy afternoon.
The immediate causes of death from nuclear attack are the blast wave, which canflatten heavily reinforced buildings many kilometers away, the firestorm, the gamma rays
and the neutrons, which effectively fry the insides of passersby. A school girl who survived
the American nuclear attack on Hiroshima, the event that ended the Second World War,wrote this first-hand account:
Through a darkness like the bottom of hell, I could hear the voices of the otherstudents calling for their mothers. And at the base of the bridge, inside a big cisternthat had been dug out there, was a mother weeping, holding above her head a naked
baby that was burned red all over its body. And another mother was crying andsobbing as she gave her burned breast to her baby. In the cistern the students stoodwith only their heads above the water, and their two hands, which they clasped asthey imploringly cried and screamed, calling for their parents. But every single
person who passed was wounded, all of them, and there was no one, there was noone to turn to for help. And the singed hair on the heads of the people was frizzled
and whitish and covered with dust. They did not appear to be human, not creaturesof this world.
The Hiroshima explosion, unlike the subsequent Nagasaki explosion, was an air burst highabove the surface, so the fallout was insignificant. But on March 1, 1954, a thermonuclear
weapons test at Bikini in the Marshall Islands detonated at higher yield than expected. A
great radioactive cloud was deposited on the tiny atoll of Rongalap, 150 kilometers away,
where the inhabitants likened the explosion to the Sun rising in the West. A few hours later,
radioactive ash fell on Rongalap like snow. The average dose received was only about 175
rads, a little less than half the dose needed to kill an average person. Being far from the
explosion, not many people died. Of course, the radioactive strontium they ate wasconcentrated in their bones, and the radioactive iodine was concentrated in their thyroids.
Two-thirds of the children and one-third of the adults later developed thyroid abnormalities,
growth retardation or malignant tumors. In compensation, the Marshall Islanders received
expert medical care.
The yield of the Hiroshima bomb was only thirteen kilotons, the equivalent ofthirteen thousand tons of TNT. The Bikini test yield was fifteen megatons. In a full nuclear
exchange, in the paroxysm of thermonuclear war, the equivalent of a million Hiroshimabombs would be dropped all over the world. At the Hiroshima death rate of some hundred
thousand people killed per equivalent thirteen-kiloton weapon, this would be enough to killa hundred billion people. But there were less than five billion people on the planet in thelate twentieth century. Of course, in such an exchange, not everyone would be killed by theblast and the firestorm, the radiation and the fallout - although fallout does last for a longish

time: 90 percent of the strontium 90 will decay in 96 years; 90 percent of the cesium 137,
in 100 years; 90 percent of the iodine 131 in only a month.
The survivors would witness more subtle consequences of the war. A full nuclear
exchange would burn the nitrogen in the upper air, converting it to oxides of nitrogen,which would in turn destroy a significant amount of the ozone in the high atmosphere,
admitting an intense dose of solar ultraviolet radiation.* The increased ultraviolet flux
would last for years. It would produce skin cancer preferentially in light-skinned people.
Much more important, it would affect the ecology of our planet in an unknown way.
Ultraviolet light destroys crops. Many microorganisms would be killed; we do not know
which ones or how many, or what the consequences might be. The organisms killed might,
for all we know, be at the base of a vast ecological pyramid at the top of which totter we.
* The process is similar to, but much more dangerous than, the destruction of the ozone layer by thefluorocarbon propellants in aerosol spray cans, which have accordingly been banned by a number of nations;and to that invoked in the explanation of the extinction of the dinosaurs by a supernova explosion a few dozen
light-years away.

The dust put into the air in a full nuclear exchange would reflect sunlight and cool
the Earth a little. Even a little cooling can have disastrous agricultural consequences. Birds
are more easily killed by radiation than insects. Plagues of insects and consequent further
agricultural disorders are a likely consequence of nuclear war. There is also another kind ofplague to worry about: the plague bacillus is endemic all over the Earth. In the latetwentieth century humans did not much die of plague - not because it was absent, but
because resistance was high. However, the radiation produced in a nuclear war, among its
many other effects, debilitates the body’s immunological system, causing a deterioration of
our ability to resist disease. In the longer term, there are mutations, new varieties of
microbes and insects, that might cause still further problems for any human survivors of a
nuclear holocaust; and perhaps after a while, when there has been enough time for therecessive mutations to recombine and be expressed, new and horrifying varieties ofhumans. Most of these mutations, when expressed, would be lethal. A few would not. And
then there would be other agonies: the loss of loved ones; the legions of the burned, theblind and the mutilated; disease, plague, long-lived radioactive poisons in the air and water;
the threat of tumors and stillbirths and malformed children; the absence of medical care; thehopeless sense of a civilization destroyed for nothing; the knowledge that we could have
prevented it and did not.
L. F. Richardson was a British meteorologist interested in war. He wished tounderstand its causes. There are intellectual parallels between war and weather. Both arecomplex. Both exhibit regularities, implying that they are not implacable forces but natural
systems that can be understood and controlled. To understand the global weather you mustfirst collect a great body of meteorological data; you must discover how the weather
actually behaves. Our approach must be the same, Richardson decided, if we are tounderstand warfare. So, for the years between 1820 and 1945, he collected data on thehundreds of wars that had then been fought on our poor planet.
Richardson’s results were published posthumously in a book called The Statistics ofDeadly Quarrels. Because he was interested in how long you had to wait for a war thatwould claim a specified number of victims, he defined an index, M, the magnitude of awar, a measure of the number of immediate deaths it causes. A war of magnitude M = 3

might be merely a skirmish, killing only a thousand people (103). M = 5 or M = 6 denote
more serious wars, where a hundred thousand (105) or a million (106) people are killed.
World Wars I and II had larger magnitudes. He found that the more people killed in a war,
the less likely it was to occur, and the longer before you would witness it, just as violent
storms occur less frequently than cloudbursts. From his data we can construct a graphwhich shows how long on the average during the past century and a half you would have to
wait to witness the war of magnitude M.
Richardson proposed that if you continue the curve to very small values of M, all
the way to M = 0, it roughly predicts the worldwide incidence of murder; somewhere in theworld someone is murdered every five minutes. Individual killings and wars on the largest
scale are, he said, two ends of a continuum, an unbroken curve. It follows, not only in atrivial sense but also I believe in a very deep psychological sense, that war is murder writlarge. When our well-being is threatened, when our illusions about ourselves arechallenged, we tend - some of us at least - to fly into murderous rages. And when the sameprovocations are applied to nation states, they, too, sometimes fly into murderous rages,
egged on often enough by those seeking personal power or profit. But as the technology ofmurder improves and the penalties of war increase, a great many people must be made to
fly into murderous rages simultaneously for a major war to be mustered. Because theorgans of mass communication are often in the hands of the state, this can commonly bearranged. (Nuclear war is the exception. It can be triggered by a very small number ofpeople.)We see here a conflict between our passions and what is sometimes called our better
natures; between the deep, ancient reptilian part of the brain, the R-complex, in charge of
murderous rages, and the more recently evolved mammalian and human parts of the brain,
the limbic system and the cerebral cortex. When humans lived in small groups, when ourweapons were comparatively paltry, even an enraged warrior could kill only a few. As ourtechnology improved, the means of war also improved. In the same brief interval, we also
have improved. We have tempered our anger, frustration and despair with reason. We haveameliorated on a planetary scale injustices that only recently were global and endemic. But
our weapons can now kill billions. Have we improved fast enough? Are we teaching reason
as effectively as we can? Have we courageously studied the causes of war?
What is often called the strategy of nuclear deterrence is remarkable for its relianceon the behavior of our nonhuman ancestors. Henry Kissinger, a contemporary politician,wrote: ‘Deterrence depends, above all, on psychological criteria. For purposes ofdeterrence, a bluff taken seriously is more useful than a serious threat interpreted as a
bluff.’ Truly effective nuclear bluffing, however, includes occasional postures ofirrationality, a distancing from the horrors of nuclear war. Then the potential enemy istempted to submit on points of dispute rather than unleash a global confrontation, which theaura of irrationality has made plausible. The chief danger of adopting a credible pose ofirrationality is that to succeed in the pretense you have to be very good. After a while, you
get used to it. It becomes pretense no longer.
The global balance of terror, pioneered by the United States and the Soviet Union,
holds hostage the citizens of the Earth. Each side draws limits on the permissible behaviorof the other. The potential enemy is assured that if the limit is transgressed, nuclear war
will follow. However, the definition of the limit changes from time to time. Each side must
be quite confident that the other understands the new limits. Each side is tempted toincrease its military advantage, but not in so striking a way as seriously to alarm the other.

Each side continually explores the limits of the other’s tolerance, as in flights of nuclear
bombers over the Arctic wastes; the Cuban missile crisis; the testing of anti-satellite
weapons; the Vietnam and Afghanistan wars - a few entries from a long and dolorous list.
The global balance of terror is a very delicate balance. It depends on things not goingwrong, on mistakes not being made, on the reptilian passions not being seriously aroused.
And so we return to Richardson. In his diagram a solid line is the waiting time for a
war of magnitude M - that is, the average time we would have to wait to witness a war thatkills 10M people (where M represents the number of zeroes after the one in our usual
exponential arithmetic). Also shown, as a vertical bar at the right of the diagram, is theworld population in recent years, which reached one billion people (M = 9) around 1835
and is now about 4.5 billion people (M = 9.7). When the Richardson curve crosses thevertical bar we have specified the waiting time to Doomsday: how many years until the
population of the Earth is destroyed in some great war. With Richardson’s curve and thesimplest extrapolation for the future growth of the human population, the two curves do notintersect until the thirtieth century or so, and Doomsday is deferred.
But World War II was of magnitude 7.7: some fifty million military personnel andnoncombatants were killed. The technology of death advanced ominously. Nuclear
weapons were used for the first time. There is little indication that the motivations andpropensities for warfare have diminished since, and both conventional and nuclear
weaponry has become far more deadly. Thus, the top of the Richardson curve is shifting
downward by an unknown amount. If its new position is somewhere in the shaded region of
the figure, we may have only another few decades until Doomsday. A more detailed
comparison of the incidence of wars before and after 1945 might help to clarify thisquestion. It is of more than passing concern.
This is merely another way of saying what we have known for decades: thedevelopment of nuclear weapons and their delivery systems will, sooner or later, lead to
global disaster. Many of the American and European émigré scientists who developed thefirst nuclear weapons were profoundly distressed about the demon they had let loose on theworld. They pleaded for the global abolition of nuclear weapons. But their pleas went
unheeded; the prospect of a national strategic advantage galvanized both the U.S.S.R. andthe United States, and the nuclear arms race began.
In the same period, there was a burgeoning international trade in the devastating
non-nuclear weapons coyly called ‘conventional’. In the past twenty-five years, in dollars
corrected for inflation, the annual international arms trade has gone from $300 million tomuch more than $20 billion. In the years between 1950 and 1968, for which good statistics
seem to be available, there were, on the average, worldwide several accidents involving
nuclear weapons per year, although perhaps no more than one or two accidental nuclear
explosions. The weapons establishments in the Soviet Union, the United States and othernations are large and powerful. In the United States they include major corporations famous
for their homey domestic manufactures. According to one estimate, the corporate profits inmilitary weapons procurement are 30 to 50 percent higher than in an equally technologicalbut competitive civilian market. Cost overruns in military weapons systems are permitted
on a scale that would be considered unacceptable in the civilian sphere. In the Soviet Union
the resources, quality, attention and care given to military production is in striking contrast
to the little left for consumer goods. According to some estimates, almost half the scientists
and high technologists on Earth are employed full- or part-time on military matters. Thoseengaged in the development and manufacture of weapons of mass destruction are givensalaries, perquisites of power and, where possible, public honors at the highest levels
available in their respective societies. The secrecy of weapons development, carried toespecially extravagant lengths in the Soviet Union, implies that individuals so employed
need almost never accept responsibility for their actions. They are protected andanonymous. Military secrecy makes the military the most difficult sector of any society forthe citizens to monitor. If we do not know what they do, it is very hard for us to stop them.
And with the rewards so substantial, with the hostile military establishments beholden toeach other in some ghastly mutual embrace, the world discovers itself drifting toward theultimate undoing of the human enterprise.
Every major power has some widely publicized justification for its procurement andstockpiling of weapons of mass destruction, often including a reptilian reminder of thepresumed character and cultural defects of potential enemies (as opposed to us stout
fellows), or of the intentions of others, but never ourselves, to conquer the world. Everynation seems to have its set of forbidden possibilities, which its citizenry and adherents
must not at any cost be permitted to think seriously about. In the Soviet Union these include
capitalism, God, and the surrender of national sovereignty; in the United States, socialism,
atheism, and the surrender of national sovereignty. It is the same all over the world.
How would we explain the global arms race to a dispassionate extraterrestrial
observer? How would we justify the most recent destabilizing developments of killersatellites, particle beam weapons, lasers, neutron bombs, cruise missiles, and the proposed
conversion of areas the size of modest countries to the enterprise of hiding eachintercontinental ballistic missile among hundreds of decoys? Would we argue that ten
thousand targeted nuclear warheads are likely to enhance the prospects for our survival?
What account would we give of our stewardship of the planet Earth? We have heard therationales offered by the nuclear superpowers. We know who speaks for the nations. But
who speaks for the human species? Who speaks for Earth?
About two-thirds of the mass of the human brain is in the cerebral cortex, devoted
to intuition and reason. Humans have evolved gregariously. We delight in each other’scompany; we care for one another. We cooperate. Altruism is built into us. We havebrilliantly deciphered some of the patterns of Nature. We have sufficient motivation towork together and the ability to figure out how to do it. If we are willing to contemplate
nuclear war and the wholesale destruction of our emerging global society, should we notalso be willing to contemplate a wholesale restructuring of our societies? From an
extraterrestrial perspective, our global civilization is clearly on the edge of failure in themost important task it faces: to preserve the lives and well-being of the citizens of theplanet. Should we not then be willing to explore vigorously, in every nation, major changes
in the traditional ways of doing things, a fundamental redesign of economic, political,
social and religious institutions?
Faced with so disquieting an alternative, we are always tempted to minimize theseriousness of the problem, to argue that those who worry about doomsdays are alarmists;
to hold that fundamental changes in our institutions are impractical or contrary to ‘humannature’, as if nuclear war were practical, or as if there were only one human nature. Fullscale nuclear war has never happened. Somehow this it taken to imply that it never will.
But we can experience it only once. By then it will be too late to reformulate the statistics.
The United States is one of the few governments that actually supports an agency
devoted to reversing the arms race. But the comparative budgets of the Department of
Defense (153 billion dollars per year in 1980) and of the Arms Control and Disarmament

Agency (0.018 billion dollars per year) remind us of the relative importance we haveassigned to the two activities. Would not a rational society spend more on understanding
and preventing, than on preparing for, the next war? It is possible to study the causes ofwar. At present our understanding is meager - probably because disarmament budgets have,since the time of Sargon of Akkad, been somewhere between ineffective and nonexistent.
Microbiologists and physicians study diseases mainly to cure people. Rarely are theyrooting for the pathogen. Let us study war as if it were, as Einstein aptly called it, an illnessof childhood. We have reached the point where proliferation of nuclear arms and resistanceto nuclear disarmament threaten every person on the planet. There are no more special
interests or special cases. Our survival depends on committing our intelligence andresources on a massive scale to take charge of our own destiny, to guarantee that
Richardson’s curve does not veer to the right.
We, the nuclear hostages - all the peoples of the Earth - must educate ourselves
about conventional and nuclear warfare. Then we must educate our governments. We must
learn the science and technology that provide the only conceivable tools for our survival.
We must be willing to challenge courageously the conventional social, political, economic
and religious wisdom. We must make every effort to understand that our fellow humans, allover the world, are human. Of course, such steps are difficult. But as Einstein many times
replied when his suggestions were rejected as impractical or as inconsistent with ‘humannature’: What is the alternative?
Mammals characteristically nuzzle, fondle, hug, caress, pet, groom and love their young,behavior essentially unknown among the reptiles. If it is really true that the R-complex andlimbic systems live in an uneasy truce within our skulls and still partake of their ancient
predilections, we might expect affectionate parental indulgence to encourage ourmammalian natures, and the absence of physical affection to prod reptilian behavior. Thereis some evidence that this is the case. In laboratory experiments, Harry and Margaret
Harlow found that monkeys raised in cages and physically isolated - even though they
could see, hear and smell their simian fellows - developed a range of morose, withdrawn,
self-destructive and otherwise abnormal characteristics. In humans the same is observed forchildren raised without physical affection - usually in institutions - where they are clearly ingreat pain.
The neuropsychologist James W. Prescott has performed a startling cross-cultural
statistical analysis of 400 preindustrial societies and found that cultures that lavish physicalaffection on infants tend to be disinclined to violence. Even societies without notablefondling of infants develop nonviolent adults, provided sexual activity in adolescents is not
repressed. Prescott believes that cultures with a predisposition for violence are composed ofindividuals who have been deprived - during at least one of two critical stages in life,
infancy and adolescence - of the pleasures of the body. Where physical affection is
encouraged, theft, organized religion and invidious displays of wealth are inconspicuous;
where infants are physically punished, there tends to be slavery, frequent killing, torturingand mutilation of enemies, a devotion to the inferiority of women, and a belief in one ormore supernatural beings who intervene in daily life.
We do not understand human behavior well enough to be sure of the mechanisms
underlying these relationships, although we can conjecture. But the correlations are
significant. Prescott writes: ‘The percent likelihood of a society becoming physicallyviolent if it is physically affectionate toward its infants and tolerant of premarital sexual

behavior is 2 percent. The probability of this relationship occurring by chance is 125,000 toone. I am not aware of any other developmental variable that has such a high degree ofpredictive validity.’ Infants hunger for physical affection; adolescents are strongly driven tosexual activity. If youngsters had their way, societies might develop in which adults havelittle tolerance for aggression, territoriality, ritual and social hierarchy (although in thecourse of growing up the children might well experience these reptilian behaviors). If
Prescott is right, in an age of nuclear weapons and effective contraceptives, child abuse andsevere sexual repression are crimes against humanity. More work on this provocative thesis
is clearly needed. Meanwhile, we can each make a personal and noncontroversial
contribution to the future of the world by hugging our infants tenderly.
If the inclinations toward slavery and racism, misogyny and violence are connected- as individual character and human history, as well as cross-cultural studies, suggest - thenthere is room for some optimism. We are surrounded by recent fundamental changes insociety. In the last two centuries, abject slavery, with us for thousands of years or more, hasbeen almost eliminated in a stirring planet-wide revolution. Women, patronized formillennia, traditionally denied real political and economic power, are gradually becoming,
even in the most backward societies, equal partners with men. For the first time in modern
history, major wars of aggression were stopped partly because of the revulsion felt by the
citizens of the aggressor nations. The old exhortations to nationalist fervor and jingoist
pride have begun to lose their appeal. Perhaps because of rising standards of living,children are being treated better worldwide. In only a few decades, sweeping globalchanges have begun to move in precisely the directions needed for human survival. A new
consciousness is developing which recognizes that we are one species.
‘Superstition [is] cowardice in the face of the Divine,’ wrote Theophrastus, who lived
during the founding of the Library of Alexandria. We inhabit a universe where atoms are
made in the centers of stars; where each second a thousand suns are born; where life issparked by sunlight and lightning in the airs and waters of youthful planets; where the raw
material for biological evolution is sometimes made by the explosion of a star halfway
across the Milky Way; where a thing as beautiful as a galaxy is formed a hundred billion
times - a Cosmos of quasars and quarks, snowflakes and fireflies, where there may be black
holes and other universes and extraterrestrial civilizations whose radio messages are at thismoment reaching the Earth. How pallid by comparison are the pretensions of superstitionand pseudoscience; how important it is for us to pursue and understand science, thatcharacteristically human endeavor.
Every aspect of Nature reveals a deep mystery and touches our sense of wonder andawe. Theophrastus was right. Those afraid of the universe as it really is, those who pretend
to nonexistent knowledge and envision a Cosmos centered on human beings will prefer the
fleeting comforts of superstition. They avoid rather than confront the world. But those withthe courage to explore the weave and structure of the Cosmos, even where it differs
profoundly from their wishes and prejudices, will penetrate its deepest mysteries.
There is no other species on Earth that does science. It is, so far, entirely a humaninvention, evolved by natural selection in the cerebral cortex for one simple reason: itworks. It is not perfect. It can be misused. It is only a tool. But it is by far the best tool wehave, self-correcting, ongoing, applicable to everything. It has two rules. First: there are nosacred truths; all assumptions must be critically examined; arguments from authority areworthless. Second: whatever is inconsistent with the facts must be discarded or revised. We

must understand the Cosmos as it is and not confuse how it is with how we wish it to be.
The obvious is sometimes false; the unexpected is sometimes true. Humans everywhere
share the same goals when the context is large enough. And the study of the Cosmos
provides the largest possible context. Present global culture is a kind of arrogant newcomer.
It arrives on the planetary stage following four and a half billion years of other acts, andafter looking about for a few thousand years declares itself in possession of eternal truths.
But in a world that is changing as fast as ours, this is a prescription for disaster. No nation,
no religion, no economic system, no body of knowledge, is likely to have all the answers
for our survival. There must be many social systems that would work far better than any
now in existence. In the scientific tradition, our task is to find them.
Only once before in our history was there the promise of a brilliant scientific civilization.
Beneficiary of the Ionian Awakening, it had its citadel at the Library of Alexandria, where
2,000 years ago the best minds of antiquity established the foundations for the systematic
study of mathematics, physics, biology, astronomy, literature, geography and medicine: We
build on those foundations still. The Library was constructed and supported by the
Ptolemys, the Greek kings who inherited the Egyptian portion of the empire of Alexander
the Great. From the time of its creation in the third century B.C. until its destruction sevencenturies later, it was the brain and heart of the ancient world.
Alexandria was the publishing capital of the planet. Of course, there were no
printing presses then. Books were expensive; every one of them was copied by hand. The
Library was the repository of the most accurate copies in the world. The art of criticalediting was invented there. The Old Testament comes down to us mainly from the Greek
translations made in the Alexandrian Library. The Ptolemys devoted much of theirenormous wealth to the acquisition of every Greek book, as well as works from Africa,
Persia, India, Israel and other parts of the world. Ptolemy III Euergetes wished to borrowfrom Athens the original manuscripts or official state copies of the great ancient tragedies
of Sophocles, Aeschylus and Euripides. To the Athenians, these were a kind of cultural
patrimony - something like the original handwritten copies and first folios of Shakespeare
might be in England. They were reluctant to let the manuscripts out of their hands even fora moment. Only after Ptolemy guaranteed their return with an enormous cash deposit did
they agree to lend the plays. But Ptolemy valued those scrolls more than gold or silver. He
forfeited the deposit gladly and enshrined, as well he might, the originals in the Library.
The outraged Athenians had to content themselves with the copies that Ptolemy, only alittle shamefacedly, presented to them. Rarely has a state so avidly supported the pursuit ofknowledge.
The Ptolemys did not merely collect established knowledge; they encouraged andfinanced scientific research and so generated new knowledge. The results were amazing:
Eratosthenes accurately calculated the size of the Earth, mapped it, and argued that India
could be reached by sailing westward from Spain. Hipparchus anticipated that stars comeinto being, slowly move during the course of centuries, and eventually perish; it was he
who first catalogued the positions and magnitudes of the stars to detect such changes.
Euclid produced a textbook on geometry from which humans learned for twenty-threecenturies, a work that was to help awaken the scientific interest of Kepler, Newton andEinstein. Galen wrote basic works on healing and anatomy which dominated medicine until
the Renaissance. There were, as we have noted, many others.
Alexandria was the greatest city the Western world had ever seen. People of allnations came there to live, to trade, to learn. On any given day, its harbors were thronged
with merchants, scholars and tourists. This was a city where Greeks, Egyptians, Arabs,
Syrians, Hebrews, Persians, Nubians, Phoenicians, Italians, Gauls and Iberians exchanged
merchandise and ideas. It is probably here that the word cosmopolitan realized its true
meaning - citizen, not just of a nation, but of the Cosmos.* To be a citizen of the Cosmos . .
.
* The word cosmopolitan was first invented by Diogenes, the rationalist philosopher and critic of
Plato.

Here clearly were the seeds of the modern world. What prevented them from taking
root and flourishing? Why instead did the West slumber through a thousand years ofdarkness until Columbus and Copernicus and their contemporaries rediscovered the work
done in Alexandria? I cannot give you a simple answer. But I do know this: there is norecord, in the entire history of the Library, that any of its illustrious scientists and scholars
ever seriously challenged the political, economic and religious assumptions of their society.
The permanence of the stars was questioned; the justice of slavery was not. Science andlearning in general were the preserve of a privileged few. The vast population of the city
had not the vaguest notion of the great discoveries taking place within the Library. New
findings were not explained or popularized. The research benefited them little. Discoveries
in mechanics and steam technology were applied mainly to the perfection of weapons, theencouragement of superstition, the amusement of kings. The scientists never grasped thepotential of machines to free people.* The great intellectual achievements of antiquity had
few immediate practical applications. Science never captured the imagination of themultitude. There was no counterbalance to stagnation, to pessimism, to the most abject
surrenders to mysticism. When, at long last, the mob came to burn the Library down, there
was nobody to stop them.
* With the single exception of Archimedes, who during his stay at the Alexandrian Library invented
the water screw, which is used in Egypt to this day for the irrigation of cultivated fields. But even heconsidered such mechanical contrivances far beneath the dignity of science.

The last scientist who worked in the Library was a mathematician, astronomer,
physicist and the head of the Neoplatonic school of philosophy - an extraordinary range ofaccomplishments for any individual in any age. Her name was Hypatia. She was born inAlexandria in 370. At a time when women had few options and were treated as property,
Hypatia moved freely and unselfconsciously through traditional male domains. By all
accounts she was a great beauty. She had many suitors but rejected all offers of marriage.
The Alexandria of Hypatia’s time - by then long under Roman rule - was a city under grave
strain. Slavery had sapped classical civilization of its vitality. The growing Christian
Church was consolidating its power and attempting to eradicate pagan influence andculture. Hypatia stood at the epicenter of these mighty social forces. Cyril, the Archbishopof Alexandria, despised her because of her close friendship with the Roman governor, andbecause she was a symbol of learning and science, which were largely identified by theearly Church with paganism. In great personal danger, she continued to teach and publish,
until, in the year 415, on her way to work she was set upon by a fanatical mob of Cyril’s
parishioners. They dragged her from her chariot, tore off her clothes, and, armed with

abalone shells, flayed her flesh from her bones. Her remains were burned, her works
obliterated, her name forgotten. Cyril was made a saint.
The glory of the Alexandrian Library is a dim memory. Its last remnants weredestroyed soon after Hypatia’s death. It was as if the entire civilization had undergone some
self-inflicted brain surgery, and most of its memories, discoveries, ideas and passions wereextinguished irrevocably. The loss was incalculable. In some cases, we know only thetantalizing titles of the works that were destroyed. In most cases, we know neither the titles
nor the authors. We do know that of the 123 plays of Sophocles in the Library, only sevensurvived. One of those seven is Oedipus Rex. Similar numbers apply to the works ofAeschylus and Euripides. It is a little as if the only surviving works of a man named
William Shakespeare were Coriolanus and A Winter’s Tale, but we had heard that he had
written certain other plays, unknown to us but apparently prized in his time, works entitledHamlet, Macbeth, Julius Caesar, King Lear, Romeo and Juliet.
Of the physical contents of that glorious Library not a single scroll remains. In
modern Alexandria few people have a keen appreciation, much less a detailed knowledge,of the Alexandrian Library or of the great Egyptian civilization that preceded it for
thousands of years. More recent events, other cultural imperatives have taken precedence.
The same is true all over the world. We have only the most tenuous contact with our past.
And yet just a stone’s throw from the remains of the Serapaeum are reminders of many
civilizations: enigmatic sphinxes from pharaonic Egypt; a great column erected to the
Roman Emperor Diocletian by a provincial flunky for not altogether permitting the citizensof Alexandria to starve to death; a Christian church; many minarets; and the hallmarks ofmodern industrial civilization - apartment houses, automobiles, streetcars, urban slums, a
microwave relay tower. There are a million threads from the past intertwined to make theropes and cables of the modern world.
Our achievements rest on the accomplishments of 40,000 generations of our humanpredecessors, all but a tiny fraction of whom are nameless and forgotten. Every now andthen we stumble on a major civilization, such as the ancient culture of Ebla, whichflourished only a few millennia ago and about which we knew nothing. How ignorant weare of our own past! Inscriptions, papyruses, books time-bind the human species and permit
us to hear those few voices and faint cries of our brothers and sisters, our ancestors. And
what a joy of recognition when we realize how like us they were!
We have in this book devoted attention to some of our ancestors whose names havenot been lost: Eratosthenes, Democritus, Aristarchus, Hypatia, Leonardo, Kepler, Newton,
Huygens, Champollion, Humason, Goddard, Einstein - all from Western culture because
the emerging scientific civilization on our planet is mainly a Western civilization; but every
culture - China, India, West Africa, Mesoamerica - has made its major contributions to ourglobal society and had its seminal thinkers. Through technological advances incommunication our planet is in the final stages of being bound up at breakneck pace into asingle global society. If we can accomplish the integration of the Earth without obliterating
cultural differences or destroying ourselves, we will have accomplished a great thing.
Near the site of the Alexandrian Library there is today a headless sphinx sculpted in
the time of the pharaoh Horemheb, in the Eighteenth Dynasty, a millennium before
Alexander. Within easy view of that leonine body is a modern microwave relay tower.
Between them runs an unbroken thread in the history of the human species. From sphinx totower is an instant of cosmic time - a moment in the fifteen or so billion years that haveelapsed since the Big Bang. Almost all record of the passage of the universe from then to

now has been scattered by the winds of time. The evidence of cosmic evolution has been
more thoroughly ravaged than all the papyrus scrolls in the Alexandrian Library. And yet
through daring and intelligence we have stolen a few glimpses of that winding path along
which our ancestors and we have traveled.
For unknown ages after the explosive outpouring of matter and energy of the Big
Bang, the Cosmos was without form. There were no galaxies, no planets, no life. Deep,
impenetrable darkness was everywhere, hydrogen atoms in the void. Here and there denser
accumulations of gas were imperceptibly growing, globes of matter were condensing hydrogen raindrops more massive than suns. Within these globes of gas was first kindled
the nuclear fire latent in matter. A first generation of stars was born, flooding the Cosmos
with light. There were in those times not yet any planets to receive the light, no living
creatures to admire the radiance of the heavens. Deep in the stellar furnaces the alchemy of
nuclear fusion created heavy elements, the ashes of hydrogen burning, the atomic building
materials of future planets and lifeforms. Massive stars soon exhausted their stores ofnuclear fuel. Rocked by colossal explosions, they returned most of their substance back into
the thin gas from which they had once condensed. Here in the dark lush clouds between thestars, new raindrops made of many elements were forming, later generations of stars beingborn. Nearby, smaller raindrops grew, bodies far too little to ignite the nuclear fire, dropletsin the interstellar mist on their way to form the planets. Among them was a small world ofstone and iron, the early Earth.
Congealing and warming, the Earth released the methane, ammonia, water andhydrogen gases that had been trapped within, forming the primitive atmosphere and thefirst oceans. Starlight from the Sun bathed and warmed the primeval Earth, drove storms,
generated lightning and thunder. Volcanoes overflowed with lava. These processes
disrupted molecules of the primitive atmosphere; the fragments fell back together again
into more and more complex forms, which dissolved in the early oceans. After a time theseas achieved the consistency of a warm, dilute soup. Molecules were organized, andcomplex chemical reactions driven, on the surface of clays. And one day a molecule arosethat quite by accident was able to make crude copies of itself out of the other molecules inthe broth. As time passed, more elaborate and more accurate self-replicating molecules
arose. Those combinations best suited to further replication were favored by the sieve of
natural selection. Those that copied better produced more copies. And the primitive oceanicbroth gradually grew thin as it was consumed by and transformed into complex
condensations of self-replicating organic molecules. Gradually, imperceptibly, life had
begun.
Single-celled plants evolved, and life began to generate its own food.
Photosynthesis transformed the atmosphere. Sex was invented. Once free-living forms
banded together to make a complex cell with specialized functions. Chemical receptors
evolved, and the Cosmos could taste and smell. One-celled organisms evolved into
multicellular colonies, elaborating their various parts into specialized organ systems. Eyesand ears evolved, and now the Cosmos could see and hear. Plants and animals discoveredthat the land could support life. Organisms buzzed, crawled, scuttled, lumbered, glided,
flapped, shimmied, climbed and soared. Colossal beasts thundered through the steaming
jungles. Small creatures emerged, born live instead of in hard-shelled containers, with afluid like the early oceans coursing through their veins. They survived by swiftness andcunning. And then, only a moment ago, some small arboreal animals scampered down fromthe trees. They became upright and taught themselves the use of tools, domesticated otheranimals, plants and fire, and devised language. The ash of stellar alchemy was now
emerging into consciousness. At an ever-accelerating pace, it invented writing, cities, artand science, and sent spaceships to the planets and the stars. These are some of the things
that hydrogen atoms do, given fifteen billion years of cosmic evolution.
It has the sound of epic myth, and rightly. But it is simply a description of cosmic
evolution as revealed by the science of our time. We are difficult to come by and a danger
to ourselves. But any account of cosmic evolution makes it clear that all the creatures of the
Earth, the latest manufactures of the galactic hydrogen industry, are beings to be cherished.
Elsewhere there may be other equally astonishing transmutations of matter, so wistfully welisten for a humming in the sky.
We have held the peculiar notion that a person or society that is a little different
from us, whoever we are, is somehow strange or bizarre, to be distrusted or loathed. Thinkof the negative connotations of words like alien or outlandish. And yet the monuments andcultures of each of our civilizations merely represent different ways of being human. An
extraterrestrial visitor, looking at the differences among human beings and their societies,
would find those differences trivial compared to the similarities. The Cosmos may bedensely populated with intelligent beings. But the Darwinian lesson is clear: There will be
no humans elsewhere. Only here. Only on this small planet. We are a rare as well as anendangered species. Every one of us is, in the cosmic perspective, precious. If a humandisagrees with you, let him live. In a hundred billion galaxies, you will not find another.
Human history can be viewed as a slowly dawning awareness that we are membersof a larger group. Initially our loyalties were to ourselves and our immediate family, next,
to bands of wandering hunter-gatherers, then to tribes, small settlements, city-states,
nations. We have broadened the circle of those we love. We have now organized what aremodestly described as superpowers, which include groups of people from divergent ethnic
and cultural backgrounds working in some sense together - surely a humanizing andcharacter-building experience. If we are to survive, our loyalties must be broadened further,to include the whole human community, the entire planet Earth. Many of those who run thenations will find this idea unpleasant. They will fear the loss of power. We will hear muchabout treason and disloyalty. Rich nation-states will have to share their wealth with poor
ones. But the choice, as H. G. Wells once said in a different context, is clearly the universeor nothing.
A few million years ago there were no humans. Who will be here a few million years
hence? In all the 4.6-billion-year history of our planet, nothing much ever left it. But now,
tiny unmanned exploratory spacecraft from Earth are moving, glistening and elegant,through the solar system. We have made a preliminary reconnaissance of twenty worlds,
among them all the planets visible to the naked eye, all those wandering nocturnal lightsthat stirred our ancestors toward understanding and ecstasy. If we survive, our time will befamous for two reasons: that at this dangerous moment of technological adolescence wemanaged to avoid self-destruction; and because this is the epoch in which we began ourjourney to the stars.
The choice is stark and ironic. The same rocket boosters used to launch probes tothe planets are poised to send nuclear warheads to the nations. The radioactive powersources on Viking and Voyager derive from the same technology that makes nuclear
weapons. The radio and radar techniques employed to track and guide ballistic missiles anddefend against attack are also used to monitor and command the spacecraft on the planetsand to listen for signals from civilizations near other stars. If we use these technologies todestroy ourselves, we surely will venture no more to the planets and the stars. But theconverse is also true. If we continue to the planets and the stars, our chauvinisms will be
shaken further. We will gain a cosmic perspective. We will recognize that our explorations
can be carried out only on behalf of all the people of the planet Earth. We will invest ourenergies in an enterprise devoted not to death but to life: the expansion of ourunderstanding of the Earth and its inhabitants and the search for life elsewhere. Space
exploration - unmanned and manned - uses many of the same technological andorganizational skills and demands the same commitment to valor and daring as does theenterprise of war. Should a time of real disarmament arrive before nuclear war, such
exploration would enable the military-industrial establishments of the major powers toengage at long last in an untainted enterprise. Interests vested in preparations for war canrelatively easily be reinvested in the exploration of the Cosmos.
A reasonable - even an ambitious - program of unmanned exploration of the planetsis inexpensive. The budget for space sciences in the United States is enormous. Comparable
expenditures in the Soviet Union are a few times larger. Together these sums represent theequivalent of two or three nuclear submarines per decade, or the cost overruns on one of themany weapon systems in a single year. In the last quarter of 1979, the program cost of the
U.S. F/A-18 aircraft increased by $5.1 billion, and the F-16 by $3.4 billion. Since their
inceptions, significantly less has been spent on the unmanned planetary programs of boththe United States and the Soviet Union than has been wasted shamefully - for example,
between 1970 and 1975, in the U.S. bombing of Cambodia, an application of national
policy that cost $7 billion. The total cost of a mission such as Viking to Mars, or Voyager
to the outer solar system, is less than that of the 1979-80 Soviet invasion of Afghanistan.
Through technical employment and the stimulation of high technology, money spent on
space exploration has an economic multiplier effect. One study suggests that for every
dollar spent on the planets, seven dollars are returned to the national economy. And yet
there are many important and entirely feasible missions that have not been attempted
because of lack of funds - including roving vehicles to wander across the surface of Mars, acomet rendezvous, Titan entry probes and a full-scale search for radio signals from othercivilizations in space.
The cost of major ventures into space - permanent bases on the Moon or human
exploration of Mars, say - is so large that they will not, I think, be mustered in the very near
future unless we make dramatic progress in nuclear and ‘conventional’ disarmament. Even
then there are probably more pressing needs here on Earth. But I have no doubt that, if weavoid self-destruction, we will sooner or later perform such missions. It is almost
impossible to maintain a static society. There is a kind of psychological compound interest:
even a small tendency toward retrenchment, a turning away from the Cosmos, adds up overmany generations to a significant decline. And conversely, even a slight commitment toventures beyond the Earth - to what we might call, after Columbus, ‘the enterprise of thestars’ - builds over many generations to a significant human presence on other worlds, arejoicing in our participation in the Cosmos.
Some 3.6 million years ago, in what is now northern Tanzania, a volcano erupted,
the resulting cloud of ash covering the surrounding savannahs. In 1979, thepaleoanthropologist Mary Leakey found in that ash footprints - the footprints, she believes,of an early hominid, perhaps an ancestor of all the people on the Earth today. And 380,000
kilometers away, in a flat dry plain that humans have in a moment of optimism called theSea of Tranquility, there is another footprint, left by the first human to walk another world.
We have come far in 3.6 million years, and in 4.6 billion and in 15 billion.
For we are the local embodiment of a Cosmos grown to self-awareness. We havebegun to contemplate our origins: starstuff pondering the stars; organized assemblages often billion billion billion atoms considering the evolution of atoms; tracing the long journeyby which, here at least, consciousness arose. Our loyalties are to the species and the planet.
We speak for Earth. Our obligation to survive is owed not just to ourselves but also to that
Cosmos, ancient and vast, from which we spring.

APPENDIX I
Reductio ad Absurdum and the Square Root of Two
The original Pythagorean argument on the irrationality of the square root of 2
depended on a kind of argument called reductio ad absurdum, a reduction to absurdity: weassume the truth of a statement, follow its consequences and come upon a contradiction,
thereby establishing its falsity. To take a modern example, consider the aphorism by thegreat twentieth-century physicist, Niels Bohr: ‘The opposite of every great idea is anothergreat idea.’ If the statement were true, its consequences might be at least a little perilous.
For example, consider the opposite of the Golden Rule, or proscriptions against lying or‘Thou shalt not kill.’ So let us consider whether Bohr's aphorism is itself a great idea. If so,then the converse statement, ‘The opposite of every great idea is not a great idea,' must also
be true. Then we have reached a reductio ad absurdum. If the converse statement is false,
the aphorism need not detain us long, since it stands self-confessed as not a great idea.
We present a modern version of the proof of the irrationality of the square root of 2
using a reductio ad absurdum, and simple algebra rather than the exclusively geometrical
proof discovered by the Pythagoreans. The style of argument, the mode of thinking, is atleast as interesting as the conclusion:

Consider a square in which the sides are 1 unit long (1 centimeter, 1 inch, 1 light-year, it
does not matter). The diagonal line BC divides the square into two triangles, eachcontaining a right angle. In such right triangles, the Pythagorean theorem holds: 12+12=x2.
But 12 + 12 = 1 + 1 = 2, so x2 = 2 and we write x = the square root of 2. We assume thesquare root of 2 is a rational number: The square root of 2 = p/q, where p and q are integers,whole numbers. They can be as big as we like and can stand for any integers we like. We
can certainly require that they have no common factors. If we were to claim the square rootof 2 = 14/10, for example, we would of course cancel out the factor 2 and write p = 7 and q= 5, not p = 14, q = 10. Any common factor in numerator or denominator would becanceled out before we start. There are an infinite number of p’s and q’s we can choose.
From the square root of 2 = p/q, by squaring both sides of the equation, we find that 2 =p2/q2, or, by multiplying both sides of the equation by q2, we findp2 = 2q2.
(Equation 1)
p2 is then some number multiplied by 2. Therefore p2 is an even number. But the square of
any odd number is odd (12 = 1, 32 = 9, 52 = 25, 72 = 49, etc.). So p itself must be even, andwe can write p = 2s, where s is some other integer. Substituting for p in Equation (1), we
findp2 = (2s)2 = 4s2 = 2q2
Dividing both sides of the last equality by 2, we findq2 = 2S2
Therefore q2 is also an even number, and, by the same argument as we just used for p, it
follows that q is even too. But if p and q are both even, both divisible by 2, then they havenot been reduced to their lowest common factor, contradicting one of our assumptions.
Reductio ad absurdum. But which assumption? The argument cannot be telling us that
reduction to common factors is forbidden, that 14/10 is permitted and 7/5 is not. So theinitial assumption must be wrong; p and q cannot be whole numbers; and the square root of
2 is irrational. In fact, the square root of 2 = 1.4142135 …
What a stunning and unexpected conclusion! How elegant the proof! But the Pythagoreans
felt compelled to suppress this great discovery.

APPENDIX 2
The Five Pythagorean Solids
A regular polygon (Greek for ‘many-angled’) is a two-dimensional figure with somenumber, n, of equal sides. So n = 3 is an equilateral triangle, n = 4 is a square, n = 5 is apentagon, and so on. A polyhedron (Greek for ‘many-sided’ ) is a three-dimensional figure,all of whose faces are polygons: a cube, for example, with 6 squares for faces. A simple
polyhedron, or regular solid, is one with no holes in it. Fundamental to the work of the
Pythagoreans and of Johannes Kepler was the fact that there can be 5 and only 5 regularsolids. The easiest proof comes from a relationship discovered much later by Descartes andby Leonhard Euler which relates the number of faces, F, the number of edges, E, and thenumber of corners or vertices V of a regular solid:
V-E+F=2(Equation 2)
So for a cube, there are 6 faces (F = 6) and 8 vertices (V = 8), and 8 - E + 6 = 2, 14 - E = 2,
and E = 12; Equation (2) predicts that the cube has 12 edges, as it does. A simple geometricproof of Equation (2) can be found in the book by Courant and Robbins in the
Bibliography. From Equation (2) we can prove that there are only five regular solids.
Every edge of a regular solid is shared by the sides of two adjacent polygons. Thinkagain of the cube, where every edge is a boundary between two squares. If we count up all
the sides of all the faces of a polyhedron, nF, we will have counted every edge twice. So
nF = 2E(Equation 3)
Let r represent how many edges meet at each vertex. For a cube, r = 3. Also, every edge
connects two vertices. If we count up all the vertices, rV, we will similarly have counted
every edge twice. So
rV = 2E(Equation 4)
Substituting for V and F in Equation (2) from Equations (3) and (4), we find

If we divide both sides of this equation by 2E, we have

(Equation 5)

We know that n is 3 or more, since the simplest polygon is the triangle, with three sides.
We also know that r is 3 or more, since at least 3 faces meet at a given vertex in apolyhedron. If both n and r were simultaneously more than 3, the left-hand side of Equation(5) would be less than 2/3 and the equation could not be satisfied for any positive value of
E. Thus, by another reductio ad absurdum argument, either n = 3 and r is 3 or more, or r =3 and n is 3 or more.
If n = 3, Equation (5) becomes(1/3) + (1/r) = (1/2) + (1/E), or(Equation 6)
So in this case r can equal 3, 4, or 5 only. (If E were 6 or more, the equation would beviolated.) Now n = 3, r = 3 designates a solid in which 3 triangles meet at each vertex. By
Equation (6) it has 6 edges; by Equation (3) it has 4 faces; by Equation (4) it has 4 vertices.
Clearly it is the pyramid or tetrahedron; n = 3, r = 4 is a solid with 8 faces in which 4
triangles meet at each vertex, the octahedron; and n = 3, r = 5 represents a solid with 20
faces in which 5 triangles meet at each vertex, the icosahedron.
If r = 3, Equation (5) becomes

and by similar arguments n can equal 3, 4, or 5 only. n = 3 is the tetrahedron again; n = 4 isa solid whose faces are 6 squares, the cube; and n = 5 corresponds to a solid whose faces
are 12 pentagons, the dodecahedron.
There are no other integer values of n and r possible, and therefore there are only 5
regular solids, a conclusion from abstract and beautiful mathematics that has had, as wehave seen, the most profound impact on practical human affairs.