A Brief History of Time - Stephen Hawking

Chapter 1 - Our Picture of the Universe
Chapter 2 - Space and Time
Chapter 3 - The Expanding Universe
Chapter 4 - The Uncertainty Principle
Chapter 5 - Elementary Particles and the Forces of Nature
Chapter 6 - Black Holes
Chapter 7 - Black Holes Ain't So Black
Chapter 8 - The Origin and Fate of the Universe
Chapter 9 - The Arrow of Time
Chapter 10 - Wormholes and Time Travel
Chapter 11 - The Unification of Physics
Chapter 12 - Conclusion
Glossary
Acknowledgments & About The Author

FOREWARD
I didn’t write a foreword to the original edition of A Brief History of Time. That was done by Carl Sagan. Instead,
I wrote a short piece titled “Acknowledgments” in which I was advised to thank everyone. Some of thefoundations that had given me support weren’t too pleased to have been mentioned, however, because it led toa great increase in applications.
I don’t think anyone, my publishers, my agent, or myself, expected the book to do anything like as well as it did.
It was in the London Sunday Times best-seller list for 237 weeks, longer than any other book (apparently, theBible and Shakespeare aren’t counted). It has been translated into something like forty languages and has soldabout one copy for every 750 men, women, and children in the world. As Nathan Myhrvold of Microsoft (aformer post-doc of mine) remarked: I have sold more books on physics than Madonna has on sex.
The success of A Brief History indicates that there is widespread interest in the big questions like: Where did
we come from? And why is the universe the way it is?
I have taken the opportunity to update the book and include new theoretical and observational results obtainedsince the book was first published (on April Fools’ Day, 1988). I have included a new chapter on wormholes
and time travel. Einstein’s General Theory of Relativity seems to offer the possibility that we could create andmaintain wormholes, little tubes that connect different regions of space-time. If so, we might be able to usethem for rapid travel around the galaxy or travel back in time. Of course, we have not seen anyone from the
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future (or have we?) but I discuss a possible explanation for this.
I also describe the progress that has been made recently in finding “dualities” or correspondences between
apparently different theories of physics. These correspondences are a strong indication that there is a complete
unified theory of physics, but they also suggest that it may not be possible to express this theory in a single
fundamental formulation. Instead, we may have to use different reflections of the underlying theory in different
situations. It might be like our being unable to represent the surface of the earth on a single map and having touse different maps in different regions. This would be a revolution in our view of the unification of the laws ofscience but it would not change the most important point: that the universe is governed by a set of rational lawsthat we can discover and understand.
On the observational side, by far the most important development has been the measurement of fluctuations in
the cosmic microwave background radiation by COBE (the Cosmic Background Explorer satellite) and othercollaborations. These fluctuations are the finger-prints of creation, tiny initial irregularities in the otherwisesmooth and uniform early universe that later grew into galaxies, stars, and all the structures we see around us.
Their form agrees with the predictions of the proposal that the universe has no boundaries or edges in theimaginary time direction; but further observations will be necessary to distinguish this proposal from otherpossible explanations for the fluctuations in the background. However, within a few years we should knowwhether we can believe that we live in a universe that is completely self-contained and without beginning or
end.
Stephen Hawking

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CHAPTER 1
OUR PICTURE OF THE UNIVERSE

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He
described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast
collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up andsaid: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant
tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on.” “You’re very
clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”
Most people would find the picture of our universe as an infinite tower of tortoises rather ridiculous, but why do
we think we know better? What do we know about the universe, and how do we know it? Where did theuniverse come from, and where is it going? Did the universe have a beginning, and if so, what happened before
then? What is the nature of time? Will it ever come to an end? Can we go back in time? Recent breakthroughs
in physics, made possible in part by fantastic new technologies, suggest answers to some of theselongstanding questions. Someday these answers may seem as obvious to us as the earth orbiting the sun – orperhaps as ridiculous as a tower of tortoises. Only time (whatever that may be) will tell.
As long ago as 340 BC the Greek philosopher Aristotle, in his book On the Heavens, was able to put forward
two good arguments for believing that the earth was a round sphere rather than a Hat plate. First, he realizedthat eclipses of the moon were caused by the earth coming between the sun and the moon. The earth’s
shadow on the moon was always round, which would be true only if the earth was spherical. If the earth had
been a flat disk, the shadow would have been elongated and elliptical, unless the eclipse always occurred at atime when the sun was directly under the center of the disk. Second, the Greeks knew from their travels that
the North Star appeared lower in the sky when viewed in the south than it did in more northerly regions. (Since
the North Star lies over the North Pole, it appears to be directly above an observer at the North Pole, but tosomeone looking from the equator, it appears to lie just at the horizon. From the difference in the apparent
position of the North Star in Egypt and Greece, Aristotle even quoted an estimate that the distance around theearth was 400,000 stadia. It is not known exactly what length a stadium was, but it may have been about 200
yards, which would make Aristotle’s estimate about twice the currently accepted figure. The Greeks even had athird argument that the earth must be round, for why else does one first see the sails of a ship coming over thehorizon, and only later see the hull?
Aristotle thought the earth was stationary and that the sun, the moon, the planets, and the stars moved incircular orbits about the earth. He believed this because he felt, for mystical reasons, that the earth was thecenter of the universe, and that circular motion was the most perfect. This idea was elaborated by Ptolemy inthe second century AD into a complete cosmological model. The earth stood at the center, surrounded by eightspheres that carried the moon, the sun, the stars, and the five planets known at the time, Mercury, Venus,
Mars, Jupiter, and Saturn.

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Figure 1:1
The planets themselves moved on smaller circles attached to their respective spheres in order to account for
their rather complicated observed paths in the sky. The outermost sphere carried the so-called fixed stars,
which always stay in the same positions relative to each other but which rotate together across the sky. What
lay beyond the last sphere was never made very clear, but it certainly was not part of mankind’s observable
universe.
Ptolemy’s model provided a reasonably accurate system for predicting the positions of heavenly bodies in thesky. But in order to predict these positions correctly, Ptolemy had to make an assumption that the moonfollowed a path that sometimes brought it twice as close to the earth as at other times. And that meant that themoon ought sometimes to appear twice as big as at other times! Ptolemy recognized this flaw, but neverthelesshis model was generally, although not universally, accepted. It was adopted by the Christian church as the
picture of the universe that was in accordance with Scripture, for it had the great advantage that it left lots ofroom outside the sphere of fixed stars for heaven and hell.
A simpler model, however, was proposed in 1514 by a Polish priest, Nicholas Copernicus. (At first, perhaps forfear of being branded a heretic by his church, Copernicus circulated his model anonymously.) His idea was thatthe sun was stationary at the center and that the earth and the planets moved in circular orbits around the sun.
Nearly a century passed before this idea was taken seriously. Then two astronomers – the German, Johannes
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Kepler, and the Italian, Galileo Galilei – started publicly to support the Copernican theory, despite the fact thatthe orbits it predicted did not quite match the ones observed. The death blow to the Aristotelian/Ptolemaic
theory came in 1609. In that year, Galileo started observing the night sky with a telescope, which had just been
invented. When he looked at the planet Jupiter, Galileo found that it was accompanied by several small
satellites or moons that orbited around it. This implied that everything did not have to orbit directly around theearth, as Aristotle and Ptolemy had thought. (It was, of course, still possible to believe that the earth wasstationary at the center of the universe and that the moons of Jupiter moved on extremely complicated paths
around the earth, giving the appearance that they orbited Jupiter. However, Copernicus’s theory was much
simpler.) At the same time, Johannes Kepler had modified Copernicus’s theory, suggesting that the planets
moved not in circles but in ellipses (an ellipse is an elongated circle). The predictions now finally matched theobservations.
As far as Kepler was concerned, elliptical orbits were merely an ad hoc hypothesis, and a rather repugnant one
at that, because ellipses were clearly less perfect than circles. Having discovered almost by accident that
elliptical orbits fit the observations well, he could not reconcile them with his idea that the planets were made toorbit the sun by magnetic forces. An explanation was provided only much later, in 1687, when Sir Isaac Newton
published his Philosophiae Naturalis Principia Mathematica, probably the most important single work everpublished in the physical sciences. In it Newton not only put forward a theory of how bodies move in space andtime, but he also developed the complicated mathematics needed to analyze those motions. In addition,
Newton postulated a law of universal gravitation according to which each body in the universe was attracted
toward every other body by a force that was stronger the more massive the bodies and the closer they were toeach other. It was this same force that caused objects to fall to the ground. (The story that Newton was inspiredby an apple hitting his head is almost certainly apocryphal. All Newton himself ever said was that the idea ofgravity came to him as he sat “in a contemplative mood” and “was occasioned by the fall of an apple.”) Newton
went on to show that, according to his law, gravity causes the moon to move in an elliptical orbit around theearth and causes the earth and the planets to follow elliptical paths around the sun.
The Copernican model got rid of Ptolemy’s celestial spheres, and with them, the idea that the universe had anatural boundary. Since “fixed stars” did not appear to change their positions apart from a rotation across thesky caused by the earth spinning on its axis, it became natural to suppose that the fixed stars were objects likeour sun but very much farther away.
Newton realized that, according to his theory of gravity, the stars should attract each other, so it seemed they
could not remain essentially motionless. Would they not all fall together at some point? In a letter in 1691 to
Richard Bentley, another leading thinker of his day, Newton argued that this would indeed happen if there were
only a finite number of stars distributed over a finite region of space. But he reasoned that if, on the other hand,
there were an infinite number of stars, distributed more or less uniformly over infinite space, this would not
happen, because there would not be any central point for them to fall to.
This argument is an instance of the pitfalls that you can encounter in talking about infinity. In an infinite
universe, every point can be regarded as the center, because every point has an infinite number of stars oneach side of it. The correct approach, it was realized only much later, is to consider the finite situation, in which
the stars all fall in on each other, and then to ask how things change if one adds more stars roughly uniformly
distributed outside this region. According to Newton’s law, the extra stars would make no difference at all to theoriginal ones on average, so the stars would fall in just as fast. We can add as many stars as we like, but they
will still always collapse in on themselves. We now know it is impossible to have an infinite static model of theuniverse in which gravity is always attractive.
It is an interesting reflection on the general climate of thought before the twentieth century that no one hadsuggested that the universe was expanding or contracting. It was generally accepted that either the universe
had existed forever in an unchanging state, or that it had been created at a finite time in the past more or lessas we observe it today. In part this may have been due to people’s tendency to believe in eternal truths, as wellas the comfort they found in the thought that even though they may grow old and die, the universe is eternaland unchanging.
Even those who realized that Newton’s theory of gravity showed that the universe could not be static did notthink to suggest that it might be expanding. Instead, they attempted to modify the theory by making the

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gravitational force repulsive at very large distances. This did not significantly affect their predictions of the
motions of the planets, but it allowed an infinite distribution of stars to remain in equilibrium – with the attractive
forces between nearby stars balanced by the repulsive forces from those that were farther away. However, wenow believe such an equilibrium would be unstable: if the stars in some region got only slightly nearer eachother, the attractive forces between them would become stronger and dominate over the repulsive forces sothat the stars would continue to fall toward each other. On the other hand, if the stars got a bit farther awayfrom each other, the repulsive forces would dominate and drive them farther apart.
Another objection to an infinite static universe is normally ascribed to the German philosopher Heinrich Olbers,
who wrote about this theory in 1823. In fact, various contemporaries of Newton had raised the problem, and theOlbers article was not even the first to contain plausible arguments against it. It was, however, the first to be
widely noted. The difficulty is that in an infinite static universe nearly every line of sight would end on thesurface of a star. Thus one would expect that the whole sky would be as bright as the sun, even at night.
Olbers’ counter-argument was that the light from distant stars would be dimmed by absorption by intervening
matter. However, if that happened the intervening matter would eventually heat up until it glowed as brightly as
the stars. The only way of avoiding the conclusion that the whole of the night sky should be as bright as thesurface of the sun would be to assume that the stars had not been shining forever but had turned on at somefinite time in the past. In that case the absorbing matter might not have heated up yet or the light from distantstars might not yet have reached us. And that brings us to the question of what could have caused the stars tohave turned on in the first place.
The beginning of the universe had, of course, been discussed long before this. According to a number of early
cosmologies and the Jewish/Christian/Muslim tradition, the universe started at a finite, and not very distant,time in the past. One argument for such a beginning was the feeling that it was necessary to have “First Cause”
to explain the existence of the universe. (Within the universe, you always explained one event as being causedby some earlier event, but the existence of the universe itself could be explained in this way only if it had somebeginning.) Another argument was put forward by St. Augustine in his book The City of God. He pointed outthat civilization is progressing and we remember who performed this deed or developed that technique. Thusman, and so also perhaps the universe, could not have been around all that long. St. Augustine accepted adate of about 5000 BC for the Creation of the universe according to the book of Genesis. (It is interesting that
this is not so far from the end of the last Ice Age, about 10,000 BC, which is when archaeologists tell us thatcivilization really began.)
Aristotle, and most of the other Greek philosophers, on the other hand, did not like the idea of a creation
because it smacked too much of divine intervention. They believed, therefore, that the human race and theworld around it had existed, and would exist, forever. The ancients had already considered the argument aboutprogress described above, and answered it by saying that there had been periodic floods or other disasters that
repeatedly set the human race right back to the beginning of civilization.
The questions of whether the universe had a beginning in time and whether it is limited in space were later
extensively examined by the philosopher Immanuel Kant in his monumental (and very obscure) work Critique ofPure Reason, published in 1781. He called these questions antinomies (that is, contradictions) of pure reason
because he felt that there were equally compelling arguments for believing the thesis, that the universe had abeginning, and the antithesis, that it had existed forever. His argument for the thesis was that if the universe did
not have a beginning, there would be an infinite period of time before any event, which he considered absurd.
The argument for the antithesis was that if the universe had a beginning, there would be an infinite period oftime before it, so why should the universe begin at any one particular time? In fact, his cases for both the thesis
and the antithesis are really the same argument. They are both based on his unspoken assumption that time
continues back forever, whether or not the universe had existed forever. As we shall see, the concept of time
has no meaning before the beginning of the universe. This was first pointed out by St. Augustine. When asked:“What did God do before he created the universe?” Augustine didn’t reply: “He was preparing Hell for people
who asked such questions.” Instead, he said that time was a property of the universe that God created, andthat time did not exist before the beginning of the universe.
When most people believed in an essentially static and unchanging universe, the question of whether or not it
had a beginning was really one of metaphysics or theology. One could account for what was observed equallywell on the theory that the universe had existed forever or on the theory that it was set in motion at some finite
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time in such a manner as to look as though it had existed forever. But in 1929, Edwin Hubble made thelandmark observation that wherever you look, distant galaxies are moving rapidly away from us. In other words,
the universe is expanding. This means that at earlier times objects would have been closer together. In fact, it
seemed that there was a time, about ten or twenty thousand million years ago, when they were all at exactly
the same place and when, therefore, the density of the universe was infinite. This discovery finally brought thequestion of the beginning of the universe into the realm of science.
Hubble’s observations suggested that there was a time, called the big bang, when the universe wasinfinitesimally small and infinitely dense. Under such conditions all the laws of science, and therefore all ability
to predict the future, would break down. If there were events earlier than this time, then they could not affectwhat happens at the present time. Their existence can be ignored because it would have no observational
consequences. One may say that time had a beginning at the big bang, in the sense that earlier times simply
would not be defined. It should be emphasized that this beginning in time is very different from those that had
been considered previously. In an unchanging universe a beginning in time is something that has to be
imposed by some being outside the universe; there is no physical necessity for a beginning. One can imagine
that God created the universe at literally any time in the past. On the other hand, if the universe is expanding,there may be physical reasons why there had to be a beginning. One could still imagine that God created theuniverse at the instant of the big bang, or even afterwards in just such a way as to make it look as though there
had been a big bang, but it would be meaningless to suppose that it was created before the big bang. An
expanding universe does not preclude a creator, but it does place limits on when he might have carried out hisjob!
In order to talk about the nature of the universe and to discuss questions such as whether it has a beginning or
an end, you have to be clear about what a scientific theory is. I shall take the simpleminded view that a theory
is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model toobservations that we make. It exists only in our minds and does not have any other reality (whatever that mightmean). A theory is a good theory if it satisfies two requirements. It must accurately describe a large class ofobservations on the basis of a model that contains only a few arbitrary elements, and it must make definite
predictions about the results of future observations. For example, Aristotle believed Empedocles’s theory thateverything was made out of four elements, earth, air, fire, and water. This was simple enough, but did not makeany definite predictions. On the other hand, Newton’s theory of gravity was based on an even simpler model, in
which bodies attracted each other with a force that was proportional to a quantity called their mass andinversely proportional to the square of the distance between them. Yet it predicts the motions of the sun, themoon, and the planets to a high degree of accuracy.
Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No
matter how many times the results of experiments agree with some theory, you can never be sure that the nexttime the result will not contradict the theory. On the other hand, you can disprove a theory by finding even asingle observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper hasemphasized, a good theory is characterized by the fact that it makes a number of predictions that could in
principle be disproved or falsified by observation. Each time new experiments are observed to agree with thepredictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to
disagree, we have to abandon or modify the theory.
At least that is what is supposed to happen, but you can always question the competence of the person who
carried out the observation.
In practice, what often happens is that a new theory is devised that is really an extension of the previous theory.
For example, very accurate observations of the planet Mercury revealed a small difference between its motion
and the predictions of Newton’s theory of gravity. Einstein’s general theory of relativity predicted a slightly
different motion from Newton’s theory. The fact that Einstein’s predictions matched what was seen, while
Newton’s did not, was one of the crucial confirmations of the new theory. However, we still use Newton’s theory
for all practical purposes because the difference between its predictions and those of general relativity is verysmall in the situations that we normally deal with. (Newton’s theory also has the great advantage that it is much
simpler to work with than Einstein’s!)
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approach most scientists actually follow is to separate the problem into two parts. First, there are the laws thattell us how the universe changes with time. (If we know what the universe is like at any one time, these physicallaws tell us how it will look at any later time.) Second, there is the question of the initial state of the universe.
Some people feel that science should be concerned with only the first part; they regard the question of theinitial situation as a matter for metaphysics or religion. They would say that God, being omnipotent, could havestarted the universe off any way he wanted. That may be so, but in that case he also could have made it
develop in a completely arbitrary way. Yet it appears that he chose to make it evolve in a very regular way
according to certain laws. It therefore seems equally reasonable to suppose that there are also laws governing
the initial state.
It turns out to be very difficult to devise a theory to describe the universe all in one go. Instead, we break the
problem up into bits and invent a number of partial theories. Each of these partial theories describes andpredicts a certain limited class of observations, neglecting the effects of other quantities, or representing themby simple sets of numbers. It may be that this approach is completely wrong. If everything in the universe
depends on everything else in a fundamental way, it might be impossible to get close to a full solution by
investigating parts of the problem in isolation. Nevertheless, it is certainly the way that we have made progressin the past. The classic example again is the Newtonian theory of gravity, which tells us that the gravitational
force between two bodies depends only on one number associated with each body, its mass, but is otherwise
independent of what the bodies are made of. Thus one does not need to have a theory of the structure andconstitution of the sun and the planets in order to calculate their orbits.
Today scientists describe the universe in terms of two basic partial theories – the general theory of relativity
and quantum mechanics. They are the great intellectual achievements of the first half of this century. The
general theory of relativity describes the force of gravity and the large-scale structure of the universe, that is,
the structure on scales from only a few miles to as large as a million million million million (1 with twenty-fourzeros after it) miles, the size of the observable universe. Quantum mechanics, on the other hand, deals withphenomena on extremely small scales, such as a millionth of a millionth of an inch. Unfortunately, however,these two theories are known to be inconsistent with each other – they cannot both be correct. One of the
major endeavors in physics today, and the major theme of this book, is the search for a new theory that will
incorporate them both – a quantum theory of gravity. We do not yet have such a theory, and we may still be along way from having one, but we do already know many of the properties that it must have. And we shall see,in later chapters, that we already know a fair amount about the predications a quantum theory of gravity must
make.
Now, if you believe that the universe is not arbitrary, but is governed by definite laws, you ultimately have to
combine the partial theories into a complete unified theory that will describe everything in the universe. But
there is a fundamental paradox in the search for such a complete unified theory. The ideas about scientifictheories outlined above assume we are rational beings who are free to observe the universe as we want and to
draw logical deductions from what we see.
In such a scheme it is reasonable to suppose that we might progress ever closer toward the laws that govern
our universe. Yet if there really is a complete unified theory, it would also presumably determine our actions.
And so the theory itself would determine the outcome of our search for it! And why should it determine that we
come to the right conclusions from the evidence? Might it not equally well determine that we draw the wrong
conclusion.? Or no conclusion at all?
The only answer that I can give to this problem is based on Darwin’s principle of natural selection. The idea isthat in any population of self-reproducing organisms, there will be variations in the genetic material andupbringing that different individuals have. These differences will mean that some individuals are better able
than others to draw the right conclusions about the world around them and to act accordingly. These individuals
will be more likely to survive and reproduce and so their pattern of behavior and thought will come to dominate.
It has certainly been true in the past that what we call intelligence and scientific discovery have conveyed asurvival advantage. It is not so clear that this is still the case: our scientific discoveries may well destroy us all,and even if they don’t, a complete unified theory may not make much difference to our chances of survival.
However, provided the universe has evolved in a regular way, we might expect that the reasoning abilities thatnatural selection has given us would be valid also in our search for a complete unified theory, and so would notlead us to the wrong conclusions.
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Because the partial theories that we already have are sufficient to make accurate predictions in all but the mostextreme situations, the search for the ultimate theory of the universe seems difficult to justify on practicalgrounds. (It is worth noting, though, that similar arguments could have been used against both relativity andquantum mechanics, and these theories have given us both nuclear energy and the microelectronics
revolution!) The discovery of a complete unified theory, therefore, may not aid the survival of our species. It
may not even affect our lifestyle. But ever since the dawn of civilization, people have not been content to see
events as unconnected and inexplicable. They have craved an understanding of the underlying order in theworld. Today we still yearn to know why we are here and where we came from. Humanity’s deepest desire forknowledge is justification enough for our continuing quest. And our goal is nothing less than a complete
description of the universe we live in.

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CHAPTER 2SPACE AND TIME

Our present ideas about the motion of bodies date back to Galileo and Newton. Before them people believed
Aristotle, who said that the natural state of a body was to be at rest and that it moved only if driven by a force or
impulse. It followed that a heavy body should fall faster than a light one, because it would have a greater pull
toward the earth.
The Aristotelian tradition also held that one could work out all the laws that govern the universe by purethought: it was not necessary to check by observation. So no one until Galileo bothered to see whether bodies
of different weight did in fact fall at different speeds. It is said that Galileo demonstrated that Aristotle’s belief
was false by dropping weights from the leaning tower of Pisa. The story is almost certainly untrue, but Galileo
did do something equivalent: he rolled balls of different weights down a smooth slope. The situation is similar tothat of heavy bodies falling vertically, but it is easier to observe because the Speeds are smaller. Galileo’s
measurements indicated that each body increased its speed at the same rate, no matter what its weight. For
example, if you let go of a ball on a slope that drops by one meter for every ten meters you go along, the ballwill be traveling down the slope at a speed of about one meter per second after one second, two meters per
second after two seconds, and so on, however heavy the ball. Of course a lead weight would fall faster than afeather, but that is only because a feather is slowed down by air resistance. If one drops two bodies that don’t
have much air resistance, such as two different lead weights, they fall at the same rate. On the moon, where
there is no air to slow things down, the astronaut David R. Scott performed the feather and lead weight
experiment and found that indeed they did hit the ground at the same time.
Galileo’s measurements were used by Newton as the basis of his laws of motion. In Galileo’s experiments, as abody rolled down the slope it was always acted on by the same force (its weight), and the effect was to make it
constantly speed up. This showed that the real effect of a force is always to change the speed of a body, rather
than just to set it moving, as was previously thought. It also meant that whenever a body is not acted on by any
force, it will keep on moving in a straight line at the same speed. This idea was first stated explicitly in Newton’s
Principia Mathematica, published in 1687, and is known as Newton’s first law. What happens to a body when aforce does act on it is given by Newton’s second law. This states that the body will accelerate, or change its
speed, at a rate that is proportional to the force. (For example, the acceleration is twice as great if the force istwice as great.) The acceleration is also smaller the greater the mass (or quantity of matter) of the body. (Thesame force acting on a body of twice the mass will produce half the acceleration.) A familiar example isprovided by a car: the more powerful the engine, the greater the acceleration, but the heavier the car, thesmaller the acceleration for the same engine. In addition to his laws of motion, Newton discovered a law todescribe the force of gravity, which states that every body attracts every other body with a force that is
proportional to the mass of each body. Thus the force between two bodies would be twice as strong if one of
the bodies (say, body A) had its mass doubled. This is what you might expect because one could think of the
new body A as being made of two bodies with the original mass. Each would attract body B with the originalforce. Thus the total force between A and B would be twice the original force. And if, say, one of the bodies had
twice the mass, and the other had three times the mass, then the force would be six times as strong. One can
now see why all bodies fall at the same rate: a body of twice the weight will have twice the force of gravity
pulling it down, but it will also have twice the mass. According to Newton’s second law, these two effects will
exactly cancel each other, so the acceleration will be the same in all cases.
Newton’s law of gravity also tells us that the farther apart the bodies, the smaller the force. Newton’s law ofgravity says that the gravitational attraction of a star is exactly one quarter that of a similar star at half the
distance. This law predicts the orbits of the earth, the moon, and the planets with great accuracy. If the law
were that the gravitational attraction of a star went down faster or increased more rapidly with distance, theorbits of the planets would not be elliptical, they would either spiral in to the sun or escape from the sun.
The big difference between the ideas of Aristotle and those of Galileo and Newton is that Aristotle believed in apreferred state of rest, which any body would take up if it were not driven by some force Or impulse. In
particular, he thought that the earth was at rest. But it follows from Newton’s laws that there is no unique

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standard of rest. One could equally well say that body A was at rest and body B was moving at constant speedwith respect to body A, or that body B was at rest and body A was moving. For example, if one sets aside for a
moment the rotation of the earth and its orbit round the sun, one could say that the earth was at rest and that a
train on it was traveling north at ninety miles per hour or that the train was at rest and the earth was moving
south at ninety miles per hour. If one carried out experiments with moving bodies on the train, all Newton’s laws
would still hold. For instance, playing Ping-Pong on the train, one would find that the ball obeyed Newton’s laws
just like a ball on a table by the track. So there is no way to tell whether it is the train or the earth that is moving.
The lack of an absolute standard of rest meant that one could not determine whether two events that took place
at different times occurred in the same position in space. For example, suppose our Ping-Pong ball on the train
bounces straight up and down, hitting the table twice on the same spot one second apart. To someone on the
track, the two bounces would seem to take place about forty meters apart, because the train would have
traveled that far down the track between the bounces. The nonexistence of absolute rest therefore meant that
one could not give an event an absolute position in space, as Aristotle had believed. The positions of eventsand the distances between them would be different for a person on the train and one on the track, and there
would be no reason to prefer one person’s position to the other’s.
Newton was very worried by this lack of absolute position, or absolute space, as it was called, because it did
not accord with his idea of an absolute God. In fact, he refused to accept lack of absolute space, even though it
was implied by his laws. He was severely criticized for this irrational belief by many people, most notably by
Bishop Berkeley, a philosopher who believed that all material objects and space and time are an illusion. Whenthe famous Dr. Johnson was told of Berkeley’s opinion, he cried, “I refute it thus!” and stubbed his toe on alarge stone.
Both Aristotle and Newton believed in absolute time. That is, they believed that one could unambiguously
measure the interval of time between two events, and that this time would be the same whoever measured it,
provided they used a good clock. Time was completely separate from and independent of space. This is whatmost people would take to be the commonsense view. However, we have had to change our ideas about space
and time. Although our apparently commonsense notions work well when dealing with things like apples, orplanets that travel comparatively slowly, they don’t work at all for things moving at or near the speed of light.
The fact that light travels at a finite, but very high, speed was first discovered in 1676 by the Danish astronomer
Ole Christensen Roemer. He observed that the times at which the moons of Jupiter appeared to pass behind
Jupiter were not evenly spaced, as one would expect if the moons went round Jupiter at a constant rate. As theearth and Jupiter orbit around the sun, the distance between them varies. Roemer noticed that eclipses ofJupiter’s moons appeared later the farther we were from Jupiter. He argued that this was because the light fromthe moons took longer to reach us when we were farther away. His measurements of the variations in the
distance of the earth from Jupiter were, however, not very accurate, and so his value for the speed of light was
140,000 miles per second, compared to the modern value of 186,000 miles per second. Nevertheless,Roemer’s achievement, in not only proving that light travels at a finite speed, but also in measuring that speed,
was remarkable – coming as it did eleven years before Newton’s publication of Principia Mathematica. A proper
theory of the propagation of light didn’t come until 1865, when the British physicist James Clerk Maxwell
succeeded in unifying the partial theories that up to then had been used to describe the forces of electricity andmagnetism. Maxwell’s equations predicted that there could be wavelike disturbances in the combined
electromagnetic field, and that these would travel at a fixed speed, like ripples on a pond. If the wavelength ofthese waves (the distance between one wave crest and the next) is a meter or more, they are what we now callradio waves. Shorter wavelengths are known as microwaves (a few centimeters) or infrared (more than aten-thousandth of a centimeter). Visible light has a wavelength of between only forty and eighty millionths of a
centimeter. Even shorter wavelengths are known as ultraviolet, X rays, and gamma rays.
Maxwell’s theory predicted that radio or light waves should travel at a certain fixed speed. But Newton’s theory
had got rid of the idea of absolute rest, so if light was supposed to travel at a fixed speed, one would have tosay what that fixed speed was to be measured relative to.
It was therefore suggested that there was a substance called the "ether" that was present everywhere, even in"empty" space. Light waves should travel through the ether as sound waves travel through air, and their speed
should therefore be relative to the ether. Different observers, moving relative to the ether, would see light

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coming toward them at different speeds, but light's speed relative to the ether would remain fixed. In particular,
as the earth was moving through the ether on its orbit round the sun, the speed of light measured in the
direction of the earth's motion through the ether (when we were moving toward the source of the light) should
be higher than the speed of light at right angles to that motion (when we are not moving toward the source). In
1887Albert Michelson (who later became the first American to receive the Nobel Prize for physics) and Edward
Morley carried out a very careful experiment at the Case School of Applied Science in Cleveland. They
compared the speed of light in the direction of the earth's motion with that at right angles to the earth's motion.
To their great surprise, they found they were exactly the same!
Between 1887 and 1905 there were several attempts, most notably by the Dutch physicist Hendrik Lorentz, toexplain the result of the Michelson-Morley experiment in terms of objects contracting and clocks slowing downwhen they moved through the ether. However, in a famous paper in 1905, a hitherto unknown clerk in the
Swiss patent office, Albert Einstein, pointed out that the whole idea of an ether was unnecessary, providing one
was willing to abandon the idea of absolute time. A similar point was made a few weeks later by a leading
French mathematician, Henri Poincare. Einstein’s arguments were closer to physics than those of Poincare,
who regarded this problem as mathematical. Einstein is usually given the credit for the new theory, but
Poincare is remembered by having his name attached to an important part of it.
The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be
the same for all freely moving observers, no matter what their speed. This was true for Newton’s laws ofmotion, but now the idea was extended to include Maxwell’s theory and the speed of light: all observers shouldmeasure the same speed of light, no matter how fast they are moving. This simple idea has some remarkableconsequences. Perhaps the best known are the equivalence of mass and energy, summed up in Einstein’s
famous equation E=mc2 (where E is energy, m is mass, and c is the speed of light), and the law that nothing
may travel faster than the speed of light. Because of the equivalence of energy and mass, the energy which an
object has due to its motion will add to its mass. In other words, it will make it harder to increase its speed. Thiseffect is only really significant for objects moving at speeds close to the speed of light. For example, at 10
percent of the speed of light an object’s mass is only 0.5 percent more than normal, while at 90 percent of the
speed of light it would be more than twice its normal mass. As an object approaches the speed of light, its massrises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the
speed of light, because by then its mass would have become infinite, and by the equivalence of mass andenergy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object isforever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves thathave no intrinsic mass, can move at the speed of light.
An equally remarkable consequence of relativity is the way it has revolutionized our ideas of space and time. In
Newton’s theory, if a pulse of light is sent from one place to another, different observers would agree on thetime that the journey took (since time is absolute), but will not always agree on how far the light traveled (since
space is not absolute). Since the speed of the light is just the distance it has traveled divided by the time it has
taken, different observers would measure different speeds for the light. In relativity, on the other hand, allobservers must agree on how fast light travels. They still, however, do not agree on the distance the light has
traveled, so they must therefore now also disagree over the time it has taken. (The time taken is the distancethe light has traveled – which the observers do not agree on – divided by the light’s speed – which they doagree on.) In other words, the theory of relativity put an end to the idea of absolute time! It appeared that eachobserver must have his own measure of time, as recorded by a clock carried with him, and that identical clocks
carried by different observers would not necessarily agree.
Each observer could use radar to say where and when an event took place by sending out a pulse of light orradio waves. Part of the pulse is reflected back at the event and the observer measures the time at which hereceives the echo. The time of the event is then said to be the time halfway between when the pulse was sent
and the time when the reflection was received back: the distance of the event is half the time taken for thisround trip, multiplied by the speed of light. (An event, in this sense, is something that takes place at a single
point in space, at a specified point in time.) This idea is shown here, which is an example of a space-time
diagram...

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Figure 2:1
Using this procedure, observers who are moving relative to each other will assign different times and positions
to the same event. No particular observer’s measurements are any more correct than any other observer’s, but
all the measurements are related. Any observer can work out precisely what time and position any otherobserver will assign to an event, provided he knows the other observer’s relative velocity.
Nowadays we use just this method to measure distances precisely, because we can measure time moreaccurately than length. In effect, the meter is defined to be the distance traveled by light in
0.000000003335640952 second, as measured by a cesium clock. (The reason for that particular number is thatit corresponds to the historical definition of the meter – in terms of two marks on a particular platinum bar kept
in Paris.) Equally, we can use a more convenient, new unit of length called a light-second. This is simply
defined as the distance that light travels in one second. In the theory of relativity, we now define distance in
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terms of time and the speed of light, so it follows automatically that every observer will measure light to havethe same speed (by definition, 1 meter per 0.000000003335640952 second). There is no need to introduce theidea of an ether, whose presence anyway cannot be detected, as the Michelson-Morley experiment showed.
The theory of relativity does, however, force us to change fundamentally our ideas of space and time. We must
accept that time is not completely separate from and independent of space, but is combined with it to form an
object called space-time.
It is a matter of common experience that one can describe the position of a point in space by three numbers, orcoordinates. For instance, one can say that a point in a room is seven feet from one wall, three feet fromanother, and five feet above the floor. Or one could specify that a point was at a certain latitude and longitude
and a certain height above sea level. One is free to use any three suitable coordinates, although they have only
a limited range of validity. One would not specify the position of the moon in terms of miles north and miles
west of Piccadilly Circus and feet above sea level. Instead, one might describe it in terms of distance from thesun, distance from the plane of the orbits of the planets, and the angle between the line joining the moon to thesun and the line joining the sun to a nearby star such as Alpha Centauri. Even these coordinates would not beof much use in describing the position of the sun in our galaxy or the position of our galaxy in the local group ofgalaxies. In fact, one may describe the whole universe in terms of a collection of overlapping patches. In eachpatch, one can use a different set of three coordinates to specify the position of a point.
An event is something that happens at a particular point in space and at a particular time. So one can specify it
by four numbers or coordinates. Again, the choice of coordinates is arbitrary; one can use any threewell-defined spatial coordinates and any measure of time. In relativity, there is no real distinction between thespace and time coordinates, just as there is no real difference between any two space coordinates. One could
choose a new set of coordinates in which, say, the first space coordinate was a combination of the old first andsecond space coordinates. For instance, instead of measuring the position of a point on the earth in miles northof Piccadilly and miles west of Piccadilly, one could use miles northeast of Piccadilly, and miles north-west of
Piccadilly. Similarly, in relativity, one could use a new time coordinate that was the old time (in seconds) plus
the distance (in light-seconds) north of Piccadilly.
It is often helpful to think of the four coordinates of an event as specifying its position in a four-dimensional
space called space-time. It is impossible to imagine a four-dimensional space. I personally find it hard enoughto visualize three-dimensional space! However, it is easy to draw diagrams of two-dimensional spaces, such as
the surface of the earth. (The surface of the earth is two-dimensional because the position of a point can be
specified by two coordinates, latitude and longitude.) I shall generally use diagrams in which time increases
upward and one of the spatial dimensions is shown horizontally. The other two spatial dimensions are ignoredor, sometimes, one of them is indicated by perspective. (These are called space-time diagrams, like Figure
2:1.)

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Figure 2:2
For example, in Figure 2:2 time is measured upward in years and the distance along the line from the sun to
Alpha Centauri is measured horizontally in miles. The paths of the sun and of Alpha Centauri throughspace-time are shown as the vertical lines on the left and right of the diagram. A ray of light from the sunfollows the diagonal line, and takes four years to get from the sun to Alpha Centauri.
As we have seen, Maxwell’s equations predicted that the speed of light should be the same whatever the
speed of the source, and this has been confirmed by accurate measurements. It follows from this that if a pulseof light is emitted at a particular time at a particular point in space, then as time goes on it will spread out as asphere of light whose size and position are independent of the speed of the source. After one millionth of asecond the light will have spread out to form a sphere with a radius of 300 meters; after two millionths of asecond, the radius will be 600 meters; and so on. It will be like the ripples that spread out on the surface of apond when a stone is thrown in. The ripples spread out as a circle that gets bigger as time goes on. If one
stacks snapshots of the ripples at different times one above the other, the expanding circle of ripples will markout a cone whose tip is at the place and time at which the stone hit the water Figure 2:3.

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Figure 2:3
Similarly, the light spreading out from an event forms a (three-dimensional) cone in (the four-dimensional)
space-time. This cone is called the future light cone of the event. In the same way we can draw another cone,
called the past light cone, which is the set of events from which a pulse of light is able to reach the given event
Figure 2:4.

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Figure 2:4
Given an event P, one can divide the other events in the universe into three classes. Those events that can bereached from the event P by a particle or wave traveling at or below the speed of light are said to be in the
future of P. They will lie within or on the expanding sphere of light emitted from the event P. Thus they will lie
within or on the future light cone of P in the space-time diagram. Only events in the future of P can be affectedby what happens at P because nothing can travel faster than light.
Similarly, the past of P can be defined as the set of all events from which it is possible to reach the event P
traveling at or below the speed of light. It is thus the set of events that can affect what happens at P. The
events that do not lie in the future or past of P are said to lie in the elsewhere of P Figure 2:5.

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Figure 2:5
What happens at such events can neither affect nor be affected by what happens at P. For example, if the sun
were to cease to shine at this very moment, it would not affect things on earth at the present time because theywould be in the elsewhere of the event when the sun went out Figure 2:6.

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Figure 2:6
We would know about it only after eight minutes, the time it takes light to reach us from the sun. Only thenwould events on earth lie in the future light cone of the event at which the sun went out. Similarly, we do notknow what is happening at the moment farther away in the universe: the light that we see from distant galaxies
left them millions of years ago, and in the case of the most distant object that we have seen, the light left someeight thousand million years ago. Thus, when we look at the universe, we are seeing it as it was in the past.
If one neglects gravitational effects, as Einstein and Poincare did in 1905, one has what is called the special
theory of relativity. For every event in space-time we may construct a light cone (the set of all possible paths oflight in space-time emitted at that event), and since the speed of light is the same at every event and in every
direction, all the light cones will be identical and will all point in the same direction. The theory also tells us that
nothing can travel faster than light. This means that the path of any object through space and time must be
represented by a line that lies within the light cone at each event on it (Fig. 2.7). The special theory of relativity
was very successful in explaining that the speed of light appears the same to all observers (as shown by theMichelson-Morley experiment) and in describing what happens when things move at speeds close to the speedof light. However, it was inconsistent with the Newtonian theory of gravity, which said that objects attracted
each other with a force that depended on the distance between them. This meant that if one moved one of theobjects, the force on the other one would change instantaneously. Or in other gravitational effects should travelwith infinite velocity, instead of at or below the speed of light, as the special theory of relativity required.
Einstein made a number of unsuccessful attempts between 1908 and 1914 to find a theory of gravity that wasconsistent with special relativity. Finally, in 1915, he proposed what we now call the general theory of relativity.
Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence ofthe fact that space-time is not flat, as had been previously assumed: it is curved, or “warped,” by the distribution

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A Brief History of Time - Stephen Hawking... Chapter 2of mass and energy in it. Bodies like the earth are not made to move on curved orbits by a force called gravity;
instead, they follow the nearest thing to a straight path in a curved space, which is called a geodesic. A
geodesic is the shortest (or longest) path between two nearby points. For example, the surface of the earth is a
two-dimensional curved space. A geodesic on the earth is called a great circle, and is the shortest route
between two points (Fig. 2.8). As the geodesic is the shortest path between any two airports, this is the routean airline navigator will tell the pilot to fly along. In general relativity, bodies always follow straight lines infour-dimensional space-time, but they nevertheless appear to us to move along curved paths in our
three-dimensional space. (This is rather like watching an airplane flying over hilly ground. Although it follows astraight line in three-dimensional space, its shadow follows a curved path on the two-dimensional ground.)The mass of the sun curves space-time in such a way that although the earth follows a straight path in
four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.
Fact, the orbits of the planets predicted by general relativity are almost exactly the same as those predicted by
the Newtonian theory of gravity. However, in the case of Mercury, which, being the nearest planet to the sun,feels the strongest gravitational effects, and has a rather elongated orbit, general relativity predicts that the longaxis of the ellipse should rotate about the sun at a rate of about one degree in ten thousand years. Small
though this effect is, it had been noticed before 1915 and served as one of the first confirmations of Einstein’s
theory. In recent years the even smaller deviations of the orbits of the other planets from the Newtonianpredictions have been measured by radar and found to agree with the predictions of general relativity.
Light rays too must follow geodesics in space-time. Again, the fact that space is curved means that light nolonger appears to travel in straight lines in space. So general relativity predicts that light should be bent bygravitational fields. For example, the theory predicts that the light cones of points near the sun would be slightly
bent inward, on account of the mass of the sun. This means that light from a distant star that happened to passnear the sun would be deflected through a small angle, causing the star to appear in a different position to an
observer on the earth (Fig. 2.9). Of course, if the light from the star always passed close to the sun, we would
not be able to tell whether the light was being deflected or if instead the star was really where we see it.
However, as the earth orbits around the sun, different stars appear to pass behind the sun and have their lightdeflected. They therefore change their apparent position relative to other stars. It is normally very difficult to see
this effect, because the light from the sun makes it impossible to observe stars that appear near to the sun thesky. However, it is possible to do so during an eclipse of the sun, when the sun’s light is blocked out by themoon. Einstein’s prediction of light deflection could not be tested immediately in 1915, because the First World
War was in progress, and it was not until 1919 that a British expedition, observing an eclipse from West Africa,
showed that light was indeed deflected by the sun, just as predicted by the theory. This proof of a German
theory by British scientists was hailed as a great act of reconciliation between the two countries after the war. It
is ionic, therefore, that later examination of the photographs taken on that expedition showed the errors were as
great as the effect they were trying to measure. Their measurement had been sheer luck, or a case of knowing
the result they wanted to get, not an uncommon occurrence in science. The light deflection has, however, been
accurately confirmed by a number of later observations.
Another prediction of general relativity is that time should appear to slower near a massive body like the earth.
This is because there is a relation between the energy of light and its frequency (that is, the number of waves oflight per second): the greater the energy, the higher frequency. As light travels upward in the earth’s
gravitational field, it loses energy, and so its frequency goes down. (This means that the length of time between
one wave crest and the next goes up.) To someone high up, it would appear that everything down below was
making longer to happen. This prediction was tested in 1962, using a pair of very accurate clocks mounted at
the top and bottom of a water tower. The clock at the bottom, which was nearer the earth, was found to run
slower, in exact agreement with general relativity. The difference in the speed of clocks at different heights
above the earth is now of considerable practical importance, with the advent of very accurate navigation
systems based on signals from satellites. If one ignored the predictions of general relativity, the position that
one calculated would be wrong by several miles!
Newton’s laws of motion put an end to the idea of absolute position in space. The theory of relativity gets rid of
absolute time. Consider a pair of twins. Suppose that one twin goes to live on the top of a mountain while theother stays at sea level. The first twin would age faster than the second. Thus, if they met again, one would beolder than the other. In this case, the difference in ages would be very small, but it would be much larger if one
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A Brief History of Time - Stephen Hawking... Chapter 2of the twins went for a long trip in a spaceship at nearly the speed of light. When he returned, he would bemuch younger than the one who stayed on earth. This is known as the twins paradox, but it is a paradox only if
one has the idea of absolute time at the back of one’s mind. In the theory of relativity there is no uniqueabsolute time, but instead each individual has his own personal measure of time that depends on where he isand how he is moving.
Before 1915, space and time were thought of as a fixed arena in which events took place, but which was notaffected by what happened in it. This was true even of the special theory of relativity. Bodies moved, forces
attracted and repelled, but time and space simply continued, unaffected. It was natural to think that space andtime went on forever.
The situation, however, is quite different in the general theory of relativity. Space and time are now dynamic
quantities: when a body moves, or a force acts, it affects the curvature of space and time – and in turn the
structure of space-time affects the way in which bodies move and forces act. Space and time not only affect but
also are affected by everything that happens in the universe. Just as one cannot talk about events in theuniverse without the notions of space and time, so in general relativity it became meaningless to talk about
space and time outside the limits of the universe.
In the following decades this new understanding of space and time was to revolutionize our view of theuniverse. The old idea of an essentially unchanging universe that could have existed, and could continue toexist, forever was replaced by the notion of a dynamic, expanding universe that seemed to have begun a finite
time ago, and that might end at a finite time in the future. That revolution forms the subject of the next chapter.
And years later, it was also to be the starting point for my work in theoretical physics. Roger Penrose and I
showed that Einstein’s general theory of relativity implied that the universe must have a beginning and,possibly, an end.

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A Brief History of Time - Stephen Hawking... Chapter 3

CHAPTER 3
THE EXPANDING UNIVERSE

If one looks at the sky on a clear, moonless night, the brightest objects one sees are likely to be the planets
Venus, Mars, Jupiter, and Saturn. There will also be a very large number of stars, which are just like our own
sun but much farther from us. Some of these fixed stars do, in fact, appear to change very slightly theirpositions relative to each other as earth orbits around the sun: they are not really fixed at all! This is because
they are comparatively near to us. As the earth goes round the sun, we see them from different positions
against the background of more distant stars. This is fortunate, because it enables us to measure directly the
distance of these stars from us: the nearer they are, the more they appear to move. The nearest star, called
Proxima Centauri, is found to be about four light-years away (the light from it takes about four years to reachearth), or about twenty-three million million miles. Most of the other stars that are visible to the naked eye lie
within a few hundred light-years of us. Our sun, for comparison, is a mere light-minutes away! The visible stars
appear spread all over the night sky, but are particularly concentrated in one band, which we call the MilkyWay. As long ago as 1750, some astronomers were suggesting that the appearance of the Milky Way could be
explained if most of the visible stars lie in a single disklike configuration, one example of what we now call aspiral galaxy. Only a few decades later, the astronomer Sir William Herschel confirmed this idea by
painstakingly cataloging the positions and distances of vast numbers of stars. Even so, the idea gainedcomplete acceptance only early this century.
Our modern picture of the universe dates back to only 1924, when the American astronomer Edwin Hubble
demonstrated that ours was not the only galaxy. There were in fact many others, with vast tracts of empty
space between them. In order to prove this, he needed to determine the distances to these other galaxies,
which are so far away that, unlike nearby stars, they really do appear fixed. Hubble was forced, therefore, touse indirect methods to measure the distances. Now, the apparent brightness of a star depends on two factors:
how much light it radiates (its luminosity), and how far it is from us. For nearby stars, we can measure theirapparent brightness and their distance, and so we can work out their luminosity. Conversely, if we knew theluminosity of stars in other galaxies, we could work out their distance by measuring their apparent brightness.
Hubble noted that certain types of stars always have the same luminosity when they are near enough for us tomeasure; therefore, he argued, if we found such stars in another galaxy, we could assume that they had thesame luminosity – and so calculate the distance to that galaxy. If we could do this for a number of stars in thesame galaxy, and our calculations always gave the same distance, we could be fairly confident of our estimate.
In this way, Edwin Hubble worked out the distances to nine different galaxies. We now know that our galaxy is
only one of some hundred thousand million that can be seen using modern telescopes, each galaxy itself
containing some hundred thousand million stars. Figure 3:1 shows a picture of one spiral galaxy that is similar
to what we think ours must look like to someone living in another galaxy.

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Figure 3:1
We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in its
spiral arms orbit around its center about once every several hundred million years. Our sun is just an ordinary,
average-sized, yellow star, near the inner edge of one of the spiral arms. We have certainly come a long way
since Aristotle and Ptolemy, when thought that the earth was the center of the universe!
Stars are so far away that they appear to us to be just pinpoints of light. We cannot see their size or shape. So
how can we tell different types of stars apart? For the vast majority of stars, there is only one characteristicfeature that we can observe – the color of their light. Newton discovered that if light from the sun passes
through a triangular-shaped piece of glass, called a prism, it breaks up into its component colors (its spectrum)as in a rainbow. By focusing a telescope on an individual star or galaxy, one can similarly observe the spectrumof the light from that star or galaxy. Different stars have different spectra, but the relative brightness of thedifferent colors is always exactly what one would expect to find in the light emitted by an object that is glowing
red hot. (In fact, the light emitted by any opaque object that is glowing red hot has a characteristic spectrum
that depends only on its temperature – a thermal spectrum. This means that we can tell a star’s temperaturefrom the spectrum of its light.) Moreover, we find that certain very specific colors are missing from stars’
spectra, and these missing colors may vary from star to star. Since we know that each chemical element
absorbs a characteristic set of very specific colors, by matching these to those that are missing from a star’s
spectrum, we can determine exactly which elements are present in the star’s atmosphere.
In the 1920s, when astronomers began to look at the spectra of stars in other galaxies, they found something
most peculiar: there were the same characteristic sets of missing colors as for stars in our own galaxy, but they
were all shifted by the same relative amount toward the red end of the spectrum. To understand theimplications of this, we must first understand the Doppler effect. As we have seen, visible light consists offluctuations, or waves, in the electromagnetic field. The wavelength (or distance from one wave crest to thenext) of light is extremely small, ranging from four to seven ten-millionths of a meter. The different wavelengthsof light are what the human eye sees as different colors, with the longest wavelengths appearing at the red endof the spectrum and the shortest wavelengths at the blue end. Now imagine a source of light at a constantdistance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of

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the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of thegalaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward
us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will
be smaller than when the star was stationary. This means that the wavelength of the waves we receive isshorter than when the star was stationary. Correspondingly, if the source is moving away from us, thewavelength of the waves we receive will be longer. In the case of light, therefore, means that stars moving
away from us will have their spectra shifted toward the red end of the spectrum (red-shifted) and those moving
toward us will have their spectra blue-shifted. This relationship between wavelength and speed, which is called
the Doppler effect, is an everyday experience. Listen to a car passing on the road: as the car is approaching, its
engine sounds at a higher pitch (corresponding to a shorter wavelength and higher frequency of sound waves),
and when it passes and goes away, it sounds at a lower pitch. The behavior of light or radio waves is similar.
Indeed, the police make use of the Doppler effect to measure the speed of cars by measuring the wavelengthof pulses of radio waves reflected off them.
ln the years following his proof of the existence of other galaxies, Rubble spent his time cataloging theirdistances and observing their spectra. At that time most people expected the galaxies to be moving aroundquite randomly, and so expected to find as many blue-shifted spectra as red-shifted ones. It was quite asurprise, therefore, to find that most galaxies appeared red-shifted: nearly all were moving away from us! Moresurprising still was the finding that Hubble published in 1929: even the size of a galaxy’s red shift is not random,
but is directly proportional to the galaxy’s distance from us. Or, in other words, the farther a galaxy is, the faster
it is moving away! And that meant that the universe could not be static, as everyone previously had thought, isin fact expanding; the distance between the different galaxies is changing all the time.
The discovery that the universe is expanding was one of the great intellectual revolutions of the twentiethcentury. With hindsight, it is easy wonder why no one had thought of it before. Newton, and others should haverealized that a static universe would soon start to contract under the influence of gravity. But suppose insteadthat the universe is expanding. If it was expanding fairly slowly, the force of gravity would cause it eventually tostop expanding and then to start contracting. However, if it was expanding at more than a certain critical rate,
gravity would never be strong enough to stop it, and the universe would continue to expand forever. This is a bit
like what happens when one fires a rocket upward from the surface of the earth. If it has a fairly low speed,
gravity will eventually stop the rocket and it will start falling back. On the other hand, if the rocket has more thana certain critical speed (about seven miles per second), gravity will not be strong enough to pull it back, so it willkeep going away from the earth forever. This behavior of the universe could have been predicted from
Newton’s theory of gravity at any time in the nineteenth, the eighteenth, or even the late seventeenth century.
Yet so strong was the belief in a static universe that it persisted into the early twentieth century. Even Einstein,when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static that
he modified his theory to make this possible, introducing a so-called cosmological constant into his equations.
Einstein introduced a new “antigravity” force, which, unlike other forces, did not come from any particularsource but was built into the very fabric of space-time. He claimed that space-time had an inbuilt tendency to
expand, and this could be made to balance exactly the attraction of all the matter in the universe, so that astatic universe would result. Only one man, it seems, was willing to take general relativity at face value, andwhile Einstein and other physicists were looking for ways of avoiding general relativity’s prediction of anonstatic universe, the Russian physicist and mathematician Alexander Friedmann instead set about explaining
it.
Friedmann made two very simple assumptions about the universe: that the universe looks identical inwhichever direction we look, and that this would also be true if we were observing the universe from anywhere
else. From these two ideas alone, Friedmann showed that we should not expect the universe to be static. In
fact, in 1922, several years before Edwin Hubble’s discovery, Friedmann predicted exactly what Hubble found!
The assumption that the universe looks the same in every direction is clearly not true in reality. For example, as
we have seen, the other stars in our galaxy form a distinct band of light across the night sky, called the MilkyWay. But if we look at distant galaxies, there seems to be more or less the same number of them. So theuniverse does seem to be roughly the same in every direction, provided one views it on a large scale comparedto the distance between galaxies, and ignores the differences on small scales. For a long time, this was
sufficient justification for Friedmann’s assumption – as a rough approximation to the real universe. But more
recently a lucky accident uncovered the fact that Friedmann’s assumption is in fact a remarkably accurate
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description of our universe.
In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, Arno Penzias and Robert
Wilson, were testing a very sensitive microwave detector. (Microwaves are just like light waves, but with awavelength of around a centimeter.) Penzias and Wilson were worried when they found that their detector was
picking up more noise than it ought to. The noise did not appear to be coming from any particular direction.
First they discovered bird droppings in their detector and checked for other possible malfunctions, but soon
ruled these out. They knew that any noise from within the atmosphere would be stronger when the detector
was not pointing straight up than when it was, because light rays travel through much more atmosphere when
received from near the horizon than when received from directly overhead. The extra noise was the same
whichever direction the detector was pointed, so it must come from outside the atmosphere. It was also thesame day and night and throughout the year, even though the earth was rotating on its axis and orbiting aroundthe sun. This showed that the radiation must come from beyond the Solar System, and even from beyond thegalaxy, as otherwise it would vary as the movement of earth pointed the detector in different directions.
In fact, we know that the radiation must have traveled to us across most of the observable universe, and since
it appears to be the same in different directions, the universe must also be the same in every direction, if only
on a large scale. We now know that whichever direction we look, this noise never varies by more than a tiny
fraction: so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation ofFriedmann’s first assumption. However, because the universe is not exactly the same in every direction, but
only on average on a large scale, the microwaves cannot be exactly the same in every direction either. There
have to be slight variations between different directions. These were first detected in 1992 by the Cosmic
Background Explorer satellite, or COBE, at a level of about one part in a hundred thousand. Small though thesevariations are, they are very important, as will be explained in Chapter 8.
At roughly the same time as Penzias and Wilson were investigating noise in their detector, two American
physicists at nearby Princeton University, Bob Dicke and Jim Peebles, were also taking an interest in
microwaves. They were working on a suggestion, made by George Gamow (once a student of AlexanderFriedmann), that the early universe should have been very hot and dense, glowing white hot. Dicke andPeebles argued that we should still be able to see the glow of the early universe, because light from very
distant parts of it would only just be reaching us now. However, the expansion of the universe meant that thislight should be so greatly red-shifted that it would appear to us now as microwave radiation. Dicke and Peebles
were preparing to look for this radiation when Penzias and Wilson heard about their work and realized that they
had already found it. For this, Penzias and Wilson were awarded the Nobel Prize in 1978 (which seems a bit
hard on Dicke and Peebles, not to mention Gamow!).
Now at first sight, all this evidence that the universe looks the same whichever direction we look in might seemto suggest there is something special about our place in the universe. In particular, it might seem that if we
observe all other galaxies to be moving away from us, then we must be at the center of the universe. There is,however, an alternate explanation: the universe might look the same in every direction as seen from any othergalaxy too. This, as we have seen, was Friedmann’s second assumption. We have no scientific evidence for, or
against, this assumption. We believe it only on grounds of modesty: it would be most remarkable if the universe
looked the same in every direction around us, but not around other points in the universe! In Friedmann’smodel, all the galaxies are moving directly away from each other. The situation is rather like a balloon with anumber of spots painted on it being steadily blown up. As the balloon expands, the distance between any two
spots increases, but there is no spot that can be said to be the center of the expansion. Moreover, the farther
apart the spots are, the faster they will be moving apart. Similarly, in Friedmann’s model the speed at which any
two galaxies are moving apart is proportional to the distance between them. So it predicted that the red shift ofa galaxy should be directly proportional to its distance from us, exactly as Hubble found. Despite the success of
his model and his prediction of Hubble’s observations, Friedmann’s work remained largely unknown in the Westuntil similar models were discovered in 1935 by the American physicist Howard Robertson and the British
mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.
Although Friedmann found only one, there are in fact three different kinds of models that obey Friedmann’s two
fundamental assumptions. In the first kind (which Friedmann found) the universe is expanding sufficiently
slowly that the gravitational attraction between the different galaxies causes the expansion to slow down andeventually to stop. The galaxies then start to move toward each other and the universe contracts.
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Figure 3:2
Figure 3:2 shows how the distance between two neighboring galaxies changes as time increases. It starts atzero, increases to a maximum, and then decreases to zero again. In the second kind of solution, the universe isexpanding so rapidly that the gravitational attraction can never stop it, though it does slow it down a bit.

Figure 3:3
Figure 3:3 Shows the Separation between neighboring galaxies in this model. It starts at zero and eventually
the galaxies are moving apart at a steady speed. Finally, there is a third kind of solution, in which the universeis expanding only just fast enough to avoid recollapse.

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Figure 3:4
In this case the separation, shown in Figure 3:4, also starts at zero and increases forever. However, the speed
at which the galaxies are moving apart gets smaller and smaller, although it never quite reaches zero.
A remarkable feature of the first kind of Friedmann model is that in it the universe is not infinite in space, butneither does space have any boundary. Gravity is so strong that space is bent round onto itself, making it rather
like the surface of the earth. If one keeps traveling in a certain direction on the surface of the earth, one nevercomes up against an impassable barrier or falls over the edge, but eventually comes back to where one
started.
In the first kind of Friedmann model, space is just like this, but with three dimensions instead of two for theearth’s surface. The fourth dimension, time, is also finite in extent, but it is like a line with two ends orboundaries, a beginning and an end. We shall see later that when one combines general relativity with theuncertainty principle of quantum mechanics, it is possible for both space and time to be finite without any edges
or boundaries.
The idea that one could go right round the universe and end up where one started makes good science fiction,
but it doesn’t have much practical significance, because it can be shown that the universe would recollapse tozero size before one could get round. You would need to travel faster than light in order to end up where you
started before the universe came to an end – and that is not allowed!
In the first kind of Friedmann model, which expands and recollapses, space is bent in on itself, like the surfaceof the earth. It is therefore finite in extent. In the second kind of model, which expands forever, space is bent
the other way, like the surface of a saddle. So in this case space is infinite. Finally, in the third kind ofFriedmann model, with just the critical rate of expansion, space is flat (and therefore is also infinite).
But which Friedmann model describes our universe? Will the universe eventually stop expanding and start
contracting, or will it expand forever? To answer this question we need to know the present rate of expansion of
the universe and its present average density. If the density is less than a certain critical value, determined by
the rate of expansion, the gravitational attraction will be too weak to halt the expansion. If the density is greaterthan the critical value, gravity will stop the expansion at some time in the future and cause the universe torecollapse.
We can determine the present rate of expansion by measuring the velocities at which other galaxies are
moving away from us, using the Doppler effect. This can be done very accurately. However, the distances to
the galaxies are not very well known because we can only measure them indirectly. So all we know is that theuniverse is expanding by between 5 percent and 10 percent every thousand million years. However, ouruncertainty about the present average density of the universe is even greater. If we add up the masses of all
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the stars that we can see in our galaxy and other galaxies, the total is less than one hundredth of the amountrequired to halt the expansion of the universe, even for the lowest estimate of the rate of expansion. Our galaxy
and other galaxies, however, must contain a large amount of “dark matter” that we cannot see directly, butwhich we know must be there because of the influence of its gravitational attraction on the orbits of stars in thegalaxies. Moreover, most galaxies are found in clusters, and we can similarly infer the presence of yet moredark matter in between the galaxies in these clusters by its effect on the motion of the galaxies. When we add
up all this dark matter, we still get only about one tenth of the amount required to halt the expansion. However,
we cannot exclude the possibility that there might be some other form of matter, distributed almost uniformly
throughout the universe, that we have not yet detected and that might still raise the average density of theuniverse up to the critical value needed to halt the expansion. The present evidence therefore suggests that theuniverse will probably expand forever, but all we can really be sure of is that even if the universe is going torecollapse, it won’t do so for at least another ten thousand million years, since it has already been expanding
for at least that long. This should not unduly worry us: by that time, unless we have colonized beyond the Solar
System, mankind will long since have died out, extinguished along with our sun!
All of the Friedmann solutions have the feature that at some time in the past (between ten and twenty thousand
million years ago) the distance between neighboring galaxies must have been zero. At that time, which we callthe big bang, the density of the universe and the curvature of space-time would have been infinite. Because
mathematics cannot really handle infinite numbers, this means that the general theory of relativity (on whichFriedmann’s solutions are based) predicts that there is a point in the universe where the theory itself breaks
down. Such a point is an example of what mathematicians call a singularity. In fact, all our theories of science
are formulated on the assumption that space-time is smooth and nearly fiat, so they break down at the big bang
singularity, where the curvature of space-time is infinite. This means that even if there were events before thebig bang, one could not use them to determine what would happen afterward, because predictability would
break down at the big bang.
Correspondingly, if, as is the case, we know only what has happened since the big bang, we could not
determine what happened beforehand. As far as we are concerned, events before the big bang can have noconsequences, so they should not form part of a scientific model of the universe. We should therefore cut themout of the model and say that time had a beginning at the big bang.
Many people do not like the idea that time has a beginning, probably because it smacks of divine intervention.
(The Catholic Church, on the other hand, seized on the big bang model and in 1951officially pronounced it to
be in accordance with the Bible.) There were therefore a number of attempts to avoid the conclusion that there
had been a big bang. The proposal that gained widest support was called the steady state theory. It wassuggested in 1948 by two refugees from Nazi-occupied Austria, Hermann Bondi and Thomas Gold, together
with a Briton, Fred Hoyle, who had worked with them on the development of radar during the war. The idea wasthat as the galaxies moved away from each other, new galaxies were continually forming in the gaps in
between, from new matter that was being continually created. The universe would therefore look roughly thesame at all times as well as at all points of space. The steady state theory required a modification of generalrelativity to allow for the continual creation of matter, but the rate that was involved was so low (about one
particle per cubic kilometer per year) that it was not in conflict with experiment. The theory was a good scientifictheory, in the sense described in Chapter 1: it was simple and it made definite predictions that could be tested
by observation. One of these predictions was that the number of galaxies or similar objects in any given volumeof space should be the same wherever and whenever we look in the universe. In the late 1950s and early
1960s a survey of sources of radio waves from outer space was carried out at Cambridge by a group of
astronomers led by Martin Ryle (who had also worked with Bondi, Gold, and Hoyle on radar during the war).
The Cambridge group showed that most of these radio sources must lie outside our galaxy (indeed many ofthem could be identified with other galaxies) and also that there were many more weak sources than strong
ones. They interpreted the weak sources as being the more distant ones, and the stronger ones as being
nearer. Then there appeared to be less common sources per unit volume of space for the nearby sources thanfor the distant ones. This could mean that we are at the center of a great region in the universe in which thesources are fewer than elsewhere. Alternatively, it could mean that the sources were more numerous in thepast, at the time that the radio waves left on their journey to us, than they are now. Either explanation
contradicted the predictions of the steady state theory. Moreover, the discovery of the microwave radiation by
Penzias and Wilson in 1965 also indicated that the universe must have been much denser in the past. The
steady state theory therefore had to be abandoned.
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Another attempt to avoid the conclusion that there must have been a big bang, and therefore a beginning oftime, was made by two Russian scientists, Evgenii Lifshitz and Isaac Khalatnikov, in 1963. They suggested thatthe big bang might be a peculiarity of Friedmann’s models alone, which after all were only approximations to
the real universe. Perhaps, of all the models that were roughly like the real universe, only Friedmann’s would
contain a big bang singularity. In Friedmann’s models, the galaxies are all moving directly away from eachother – so it is not surprising that at some time in the past they were all at the same place. In the real universe,
however, the galaxies are not just moving directly away from each other – they also have small sideways
velocities. So in reality they need never have been all at exactly the same place, only very close together.
Perhaps then the current expanding universe resulted not from a big bang singularity, but from an earlier
contracting phase; as the universe had collapsed the particles in it might not have all collided, but had flown
past and then away from each other, producing the present expansion of the the universe that were roughly likeFriedmann’s models but took account of the irregularities and random velocities of galaxies in the real universe.
They showed that such models could start with a big bang, even though the galaxies were no longer always
moving directly away from each other, but they claimed that this was still only possible in certain exceptionalmodels in which the galaxies were all moving in just the right way. They argued that since there seemed to be
infinitely more Friedmann-like models without a big bang singularity than there were with one, we should
conclude that there had not in reality been a big bang. They later realized, however, that there was a much
more general class of Friedmann-like models that did have singularities, and in which the galaxies did not haveto be moving any special way. They therefore withdrew their claim in 1970.
The work of Lifshitz and Khalatnikov was valuable because it showed that the universe could have had asingularity, a big bang, if the general theory of relativity was correct. However, it did not resolve the crucial
question: Does general relativity predict that our universe should have had a big bang, a beginning of time?
The answer to this carne out of a completely different approach introduced by a British mathematician andphysicist, Roger Penrose, in 1965. Using the way light cones behave in general relativity, together with the factthat gravity is always attractive, he showed that a star collapsing under its own gravity is trapped in a region
whose surface eventually shrinks to zero size. And, since the surface of the region shrinks to zero, so too must
its volume. All the matter in the star will be compressed into a region of zero volume, so the density of matterand the curvature of space-time become infinite. In other words, one has a singularity contained within a regionof space-time known as a black hole.
At first sight, Penrose’s result applied only to stars; it didn’t have anything to say about the question of whether
the entire universe had a big bang singularity in its past. However, at the time that Penrose produced his
theorem, I was a research student desperately looking for a problem with which to complete my Ph.D. thesis.
Two years before, I had been diagnosed as suffering from ALS, commonly known as Lou Gehrig’s disease, or
motor neuron disease, and given to understand that I had only one or two more years to live. In thesecircumstances there had not seemed much point in working on my Ph.D.– I did not expect to survive that long.
Yet two years had gone by and I was not that much worse. In fact, things were going rather well for me and I
had gotten engaged to a very nice girl, Jane Wilde. But in order to get married, I needed a job, and in order to
get a job, I needed a Ph.D.
In 1965 I read about Penrose’s theorem that any body undergoing gravitational collapse must eventually form asingularity. I soon realized that if one reversed the direction of time in Penrose’s theorem, so that the collapse
became an expansion, the conditions of his theorem would still hold, provided the universe were roughly like a
Friedmann model on large scales at the present time. Penrose’s theorem had shown that any collapsing starmust end in a singularity; the time-reversed argument showed that any Friedmann-like expanding universe
must have begun with a singularity. For technical reasons, Penrose’s theorem required that the universe be
infinite in space. So I could in fact, use it to prove that there should be a singularity only if the universe was
expanding fast enough to avoid collapsing again (since only those Friedmann models were infinite in space).
During the next few years I developed new mathematical techniques to remove this and other technicalconditions from the theorems that proved that singularities must occur. The final result was a joint paper byPenrose and myself in 1970, which at last proved that there must have been a big bang singularity provided
only that general relativity is correct and the universe contains as much matter as we observe. There was a lotof opposition to our work, partly from the Russians because of their Marxist belief in scientific determinism, andpartly from people who felt that the whole idea of singularities was repugnant and spoiled the beauty ofEinstein’s theory. However, one cannot really argue with a mathematical theorem. So in the end our work
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became generally accepted and nowadays nearly everyone assumes that the universe started with a big bang
singularity. It is perhaps ironic that, having changed my mind, I am now trying to convince other physicists that
there was in fact no singularity at the beginning of the universe – as we shall see later, it can disappear once
quantum effects are taken into account.
We have seen in this chapter how, in less than half a century, man’s view of the universe formed over millennia
has been transformed. Hubble’s discovery that the universe was expanding, and the realization of theinsignificance of our own planet in the vastness of the universe, were just the starting point. As experimental
and theoretical evidence mounted, it became more and more clear that the universe must have had abeginning in time, until in 1970 this was finally proved by Penrose and myself, on the basis of Einstein’s general
theory of relativity. That proof showed that general relativity is only an incomplete theory: it cannot tell us how
the universe started off, because it predicts that all physical theories, including itself, break down at the
beginning of the universe. However, general relativity claims to be only a partial theory, so what the singularity
theorems really show is that there must have been a time in the very early universe when the universe was so
small that one could no longer ignore the small-scale effects of the other great partial theory of the twentiethcentury, quantum mechanics. At the start of the 1970s, then, we were forced to turn our search for anunderstanding of the universe from our theory of the extraordinarily vast to our theory of the extraordinarily tiny.
That theory, quantum mechanics, will be described next, before we turn to the efforts to combine the two partial
theories into a single quantum theory of gravity.

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CHAPTER 4
THE UNCERTAINTY PRINCIPLE

The success of scientific theories, particularly Newton’s theory of gravity, led the French scientist the Marquis de Laplaceat the beginning of the nineteenth century to argue that the universe was completely deterministic. Laplace suggested
that there should be a set of scientific laws that would allow us to predict everything that would happen in the universe, if
only we knew the complete state of the universe at one time. For example, if we knew the positions and speeds of thesun and the planets at one time, then we could use Newton’s laws to calculate the state of the Solar System at any othertime. Determinism seems fairly obvious in this case, but Laplace went further to assume that there were similar laws
governing everything else, including human behavior.
The doctrine of scientific determinism was strongly resisted by many people, who felt that it infringed God’s freedom tointervene in the world, but it remained the standard assumption of science until the early years of this century. One of thefirst indications that this belief would have to be abandoned came when calculations by the British scientists Lord
Rayleigh and Sir James Jeans suggested that a hot object, or body, such as a star, must radiate energy at an infinite
rate. According to the laws we believed at the time, a hot body ought to give off electromagnetic waves (such as radio
waves, visible light, or X rays) equally at all frequencies. For example, a hot body should radiate the same amount ofenergy in waves with frequencies between one and two million million waves a second as in waves with frequencies
between two and three million million waves a second. Now since the number of waves a second is unlimited, this would
mean that the total energy radiated would be infinite.
In order to avoid this obviously ridiculous result, the German scientist Max Planck suggested in 1900 that light, X rays,
and other waves could not be emitted at an arbitrary rate, but only in certain packets that he called quanta. Moreover,each quantum had a certain amount of energy that was greater the higher the frequency of the waves, so at a highenough frequency the emission of a single quantum would require more energy than was available. Thus the radiation athigh frequencies would be reduced, and so the rate at which the body lost energy would be finite.
The quantum hypothesis explained the observed rate of emission of radiation from hot bodies very well, but its
implications for determinism were not realized until 1926, when another German scientist, Werner Heisenberg,
formulated his famous uncertainty principle. In order to predict the future position and velocity of a particle, one has to beable to measure its present position and velocity accurately. The obvious way to do this is to shine light on the particle.
Some of the waves of light will be scattered by the particle and this will indicate its position. However, one will not be able
to determine the position of the particle more accurately than the distance between the wave crests of light, so one needs
to use light of a short wavelength in order to measure the position of the particle precisely. Now, by Planck’s quantum
hypothesis, one cannot use an arbitrarily small amount of light; one has to use at least one quantum. This quantum will
disturb the particle and change its velocity in a way that cannot be predicted. moreover, the more accurately one
measures the position, the shorter the wavelength of the light that one needs and hence the higher the energy of a single
quantum. So the velocity of the particle will be disturbed by a larger amount. In other words, the more accurately you try
to measure the position of the particle, the less accurately you can measure its speed, and vice versa. Heisenberg
showed that the uncertainty in the position of the particle times the uncertainty in its velocity times the mass of theparticle can never be smaller than a certain quantity, which is known as Planck’s constant. Moreover, this limit does notdepend on the way in which one tries to measure the position or velocity of the particle, or on the type of particle:
Heisenberg’s uncertainty principle is a fundamental, inescapable property of the world.
The uncertainty principle had profound implications for the way in which we view the world. Even after more than seventy
years they have not been fully appreciated by many philosophers, and are still the subject of much controversy. The
uncertainty principle signaled an end to Laplace’s dream of a theory of science, a model of the universe that would becompletely deterministic: one certainly cannot predict future events exactly if one cannot even measure the present stateof the universe precisely! We could still imagine that there is a set of laws that determine events completely for somesupernatural being, who could observe the present state of the universe without disturbing it. However, such models of
the universe are not of much interest to us ordinary mortals. It seems better to employ the principle of economy known as
Occam’s razor and cut out all the features of the theory that cannot be observed. This approach led Heisenberg, Erwin
Schrodinger, and Paul Dirac in the 1920s to reformulate mechanics into a new theory called quantum mechanics, basedon the uncertainty principle. In this theory particles no longer had separate, well-defined positions and velocities that
could not be observed, Instead, they had a quantum state, which was a combination of position and velocity.
In general, quantum mechanics does not predict a single definite result for an observation. Instead, it predicts a numberof different possible outcomes and tells us how likely each of these is. That is to say, if one made the same measurement
on a large number of similar systems, each of which started off in the same way, one would find that the result of the

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measurement would be A in a certain number of cases, B in a different number, and so on. One could predict theapproximate number of times that the result would be A or B, but one could not predict the specific result of an individual
measurement. Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness intoscience. Einstein objected to this very strongly, despite the important role he had played in the development of theseideas. Einstein was awarded the Nobel Prize for his contribution to quantum theory. Nevertheless, Einstein neveraccepted that the universe was governed by chance; his feelings were summed up in his famous statement “God does
not play dice.” Most other scientists, however, were willing to accept quantum mechanics because it agreed perfectly withexperiment. Indeed, it has been an outstandingly successful theory and underlies nearly all of modern science andtechnology. It governs the behavior of transistors and integrated circuits, which are the essential components ofelectronic devices such as televisions and computers, and is also the basis of modern chemistry and biology. The only
areas of physical science into which quantum mechanics has not yet been properly incorporated are gravity and thelarge-scale structure of the universe.
Although light is made up of waves, Planck’s quantum hypothesis tells us that in some ways it behaves as if it werecomposed of particles: it can be emitted or absorbed only in packets, or quanta. Equally, Heisenberg’s uncertainty
principle implies that particles behave in some respects like waves: they do not have a definite position but are “smearedout” with a certain probability distribution. The theory of quantum mechanics is based on an entirely new type ofmathematics that no longer describes the real world in terms of particles and waves; it is only the observations of theworld that may be described in those terms. There is thus a duality between waves and particles in quantum mechanics:
for some purposes it is helpful to think of particles as waves and for other purposes it is better to think of waves as
particles. An important consequence of this is that one can observe what is called interference between two sets ofwaves or particles. That is to say, the crests of one set of waves may coincide with the troughs of the other set. The two
sets of waves then cancel each other out rather than adding up to a stronger wave as one might expect Figure 4:1.

Figure 4:1
A familiar example of interference in the case of light is the colors that are often seen in soap bubbles. These are causedby reflection of light from the two sides of the thin film of water forming the bubble. White light consists of light waves ofall different wavelengths, or colors, For certain wavelengths the crests of the waves reflected from one side of the soap
film coincide with the troughs reflected from the other side. The colors corresponding to these wavelengths are absent
from the reflected light, which therefore appears to be colored. Interference can also occur for particles, because of the
duality introduced by quantum mechanics. A famous example is the so-called two-slit experiment Figure 4:2.

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Figure 4:2
Consider a partition with two narrow parallel slits in it. On one side of the partition one places a source of fight of aparticular color (that is, of a particular wavelength). Most of the light will hit the partition, but a small amount will go
through the slits. Now suppose one places a screen on the far side of the partition from the light. Any point on the screen
will receive waves from the two slits. However, in general, the distance the light has to travel from the source to thescreen via the two slits will be different. This will mean that the waves from the slits will not be in phase with each otherwhen they arrive at the screen: in some places the waves will cancel each other out, and in others they will reinforce
each other. The result is a characteristic pattern of light and dark fringes.
The remarkable thing is that one gets exactly the same kind of fringes if one replaces the source of light by a source ofparticles such as electrons with a definite speed (this means that the corresponding waves have a definite length). It
seems the more peculiar because if one only has one slit, one does not get any fringes, just a uniform distribution ofelectrons across the screen. One might therefore think that opening another slit would just increase the number ofelectrons hitting each point of the screen, but, because of interference, it actually decreases it in some places. Ifelectrons are sent through the slits one at a time, one would expect each to pass through one slit or the other, and sobehave just as if the slit it passed through were the only one there – giving a uniform distribution on the screen. In reality,
however, even when the electrons are sent one at a time, the fringes still appear. Each electron, therefore, must be

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passing through both slits at the same time!
The phenomenon of interference between particles has been crucial to our understanding of the structure of atoms, thebasic units of chemistry and biology and the building blocks out of which we, and everything around us, are made. At the
beginning of this century it was thought that atoms were rather like the planets orbiting the sun, with electrons (particlesof negative electricity) orbiting around a central nucleus, which carried positive electricity. The attraction between thepositive and negative electricity was supposed to keep the electrons in their orbits in the same way that the gravitational
attraction between the sun and the planets keeps the planets in their orbits. The trouble with this was that the laws of
mechanics and electricity, before quantum mechanics, predicted that the electrons would lose energy and so spiralinward until they collided with the nucleus. This would mean that the atom, and indeed all matter, should rapidly collapse
to a state of very high density. A partial solution to this problem was found by the Danish scientist Niels Bohr in 1913. Hesuggested that maybe the electrons were not able to orbit at just any distance from the central nucleus but only at certain
specified distances. If one also supposed that only one or two electrons could orbit at any one of these distances, thiswould solve the problem of the collapse of the atom, because the electrons could not spiral in any farther than to fill up
the orbits with e least distances and energies.
This model explained quite well the structure of the simplest atom, hydrogen, which has only one electron orbiting aroundthe nucleus. But it was not clear how one ought to extend it to more complicated atoms. Moreover, the idea of a limited
set of allowed orbits seemed very arbitrary. The new theory of quantum mechanics resolved this difficulty. It revealed that
an electron orbiting around the nucleus could be thought of as a wave, with a wavelength that depended on its velocity.
For certain orbits, the length of the orbit would correspond to a whole number (as opposed to a fractional number) ofwavelengths of the electron. For these orbits the wave crest would be in the same position each time round, so thewaves would add up: these orbits would correspond to Bohr’s allowed orbits. However, for orbits whose lengths were nota whole number of wavelengths, each wave crest would eventually be canceled out by a trough as the electrons went
round; these orbits would not be allowed.
A nice way of visualizing the wave/particle duality is the so-called sum over histories introduced by the American scientist
Richard Feynman. In this approach the particle is not supposed to have a single history or path in space-time, as it wouldin a classical, nonquantum theory. Instead it is supposed to go from A to B by every possible path. With each path there
are associated a couple of numbers: one represents the size of a wave and the other represents the position in the cycle(i.e., whether it is at a crest or a trough). The probability of going from A to B is found by adding up the waves for all thepaths. In general, if one compares a set of neighboring paths, the phases or positions in the cycle will differ greatly. Thismeans that the waves associated with these paths will almost exactly cancel each other out. However, for some sets ofneighboring paths the phase will not vary much between paths. The waves for these paths will not cancel out Such paths
correspond to Bohr’s allowed orbits.
With these ideas, in concrete mathematical form, it was relatively straightforward to calculate the allowed orbits in more
complicated atoms and even in molecules, which are made up of a number of atoms held together by electrons in orbitsthat go round more than one nucleus. Since the structure of molecules and their reactions with each other underlie all ofchemistry and biology, quantum mechanics allows us in principle to predict nearly everything we see around us, within
the limits set by the uncertainty principle. (In practice, however, the calculations required for systems containing morethan a few electrons are so complicated that we cannot do them.)
Einstein’s general theory of relativity seems to govern the large-scale structure of the universe. It is what is called aclassical theory; that is, it does not take account of the uncertainty principle of quantum mechanics, as it should forconsistency with other theories. The reason that this does not lead to any discrepancy with observation is that all thegravitational fields that we normally experience are very weak. How-ever, the singularity theorems discussed earlier
indicate that the gravitational field should get very strong in at least two situations, black holes and the big bang. In suchstrong fields the effects of quantum mechanics should be important. Thus, in a sense, classical general relativity, by
predicting points of infinite density, predicts its own downfall, just as classical (that is, nonquantum) mechanics predicted
its downfall by suggesting that atoms should collapse to infinite density. We do not yet have a complete consistent theory
that unifies general relativity and quantum mechanics, but we do know a number of the features it should have. The
consequences that these would have for black holes and the big bang will be described in later chapters. For the
moment, however, we shall turn to the recent attempts to bring together our understanding of the other forces of nature
into a single, unified quantum theory.

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CHAPTER 5
ELEMENTARY PARTICLES AND THE FORCES OF NATURE

Aristotle believed that all the matter in the universe was made up of four basic elements – earth, air, fire, and water.
These elements were acted on by two forces: gravity, the tendency for earth and water to sink, and levity, the tendency
for air and fire to rise. This division of the contents of the universe into matter and forces is still used today. Aristotle
believed that matter was continuous, that is, one could divide a piece of matter into smaller and smaller bits without any
limit: one never came up against a grain of matter that could not be divided further. A few Greeks, however, such as
Democritus, held that matter was inherently grainy and that everything was made up of large numbers of various different
kinds of atoms. (The word atom means “indivisible” in Greek.) For centuries the argument continued without any real
evidence on either side, but in 1803 the British chemist and physicist John Dalton pointed out that the fact that chemicalcompounds always combined in certain proportions could be explained by the grouping together of atoms to form units
called molecules. However, the argument between the two schools of thought was not finally settled in favor of theatomists until the early years of this century. One of the important pieces of physical evidence was provided by Einstein.
In a paper written in 1905, a few weeks before the famous paper on special relativity, Einstein pointed out that what wascalled Brownian motion – the irregular, random motion of small particles of dust suspended in a liquid – could be
explained as the effect of atoms of the liquid colliding with the dust particles.
By this time there were already suspicions that these atoms were not, after all, indivisible. Several years previously afellow of Trinity College, Cambridge, J. J. Thomson, had demonstrated the existence of a particle of matter, called theelectron, that had a mass less than one thousandth of that of the lightest atom. He used a setup rather like a modern TV
picture tube: a red-hot metal filament gave off the electrons, and because these have a negative electric charge, anelectric field could be used to accelerate them toward a phosphor-coated screen. When they hit the screen, flashes oflight were generated. Soon it was realized that these electrons must be coming from within the atoms themselves, and in1911 the New Zealand physicist Ernest Rutherford finally showed that the atoms of matter do have internal structure:they are made up of an extremely tiny, positively charged nucleus, around which a number of electrons orbit. He deduced
this by analyzing the way in which alpha-particles, which are positively charged particles given off by radioactive atoms,
are deflected when they collide with atoms.
At first it was thought that the nucleus of the atom was made up of electrons and different numbers of a positively
charged particle called the proton, from the Greek word meaning “first,” because it was believed to be the fundamental
unit from which matter was made. However, in 1932 a colleague of Rutherford’s at Cambridge, James Chadwick,
discovered that the nucleus contained another particle, called the neutron, which had almost the same mass as a proton
but no electrical charge. Chadwick received the Nobel Prize for his discovery, and was elected Master of Gonville andCaius College, Cambridge (the college of which I am now a fellow). He later resigned as Master because of
disagreements with the Fellows. There had been a bitter dispute in the college ever since a group of young Fellows
returning after the war had voted many of the old Fellows out of the college offices they had held for a long time. Thiswas before my time; I joined the college in 1965 at the tail end of the bitterness, when similar disagreements forced
another Nobel Prize – winning Master, Sir Nevill Mott, to resign.
Up to about thirty years ago, it was thought that protons and neutrons were “elementary” particles, but experiments inwhich protons were collided with other protons or electrons at high speeds indicated that they were in fact made up ofsmaller particles. These particles were named quarks by the Caltech physicist Murray Gell-Mann, who won the Nobel
Prize in 1969 for his work on them. The origin of the name is an enigmatic quotation from James Joyce: “Three quarks for
Muster Mark!” The word quark is supposed to be pronounced like quart, but with a k at the end instead of a t, but is
usually pronounced to rhyme with lark.
There are a number of different varieties of quarks: there are six “flavors,” which we call up, down, strange, charmed,
bottom, and top. The first three flavors had been known since the 1960s but the charmed quark was discovered only in
1974, the bottom in 1977, and the top in 1995. Each flavor comes in three “colors,” red, green, and blue. (It should beemphasized that these terms are just labels: quarks are much smaller than the wavelength of visible light and so do nothave any color in the normal sense. It is just that modern physicists seem to have more imaginative ways of naming newparticles and phenomena – they no longer restrict themselves to Greek!) A proton or neutron is made up of three quarks,
one of each color. A proton contains two up quarks and one down quark; a neutron contains two down and one up. We
can create particles made up of the other quarks (strange, charmed, bottom, and top), but these all have a much greatermass and decay very rapidly into protons and neutrons.
We now know that neither the atoms nor the protons and neutrons within them are indivisible. So the question is: what
are the truly elementary particles, the basic building blocks from which everything is made? Since the wavelength of light

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is much larger than the size of an atom, we cannot hope to “look” at the parts of an atom in the ordinary way. We need touse something with a much smaller wave-length. As we saw in the last chapter, quantum mechanics tells us that all
particles are in fact waves, and that the higher the energy of a particle, the smaller the wavelength of the corresponding
wave. So the best answer we can give to our question depends on how high a particle energy we have at our disposal,
because this determines on how small a length scale we can look. These particle energies are usually measured in units
called electron volts. (In Thomson’s experiments with electrons, we saw that he used an electric field to accelerate theelectrons. The energy that an electron gains from an electric field of one volt is what is known as an electron volt.) In thenineteenth century, when the only particle energies that people knew how to use were the low energies of a few electron
volts generated by chemical reactions such as burning, it was thought that atoms were the smallest unit. In Rutherford’s
experiment, the alpha-particles had energies of millions of electron volts. More recently, we have learned how to use
electromagnetic fields to give particles energies of at first millions and then thousands of millions of electron volts. And so
we know that particles that were thought to be “elementary” thirty years ago are, in fact, made up of smaller particles.
May these, as we go to still higher energies, in turn be found to be made from still smaller particles? This is certainly
possible, but we do have some theoretical reasons for believing that we have, or are very near to, a knowledge of theultimate building blocks of nature.
Using the wave/particle duality discussed in the last chapter, every-thing in the universe, including light and gravity, canbe described in terms of particles. These particles have a property called spin. One way of thinking of spin is to imagine
the particles as little tops spinning about an axis. However, this can be misleading, because quantum mechanics tells usthat the particles do not have any well-defined axis. What the spin of a particle really tells us is what the particle looks likefrom different directions. A particle of spin 0 is like a dot: it looks the same from every direction Figure 5:1-i. On the other
hand, a particle of spin 1 is like an arrow: it looks different from different directions Figure 5:1-ii. Only if one turns it rounda complete revolution (360 degrees) does the particle look the same. A particle of spin 2 is like a double-headed arrow
Figure 5:1-iii: it looks the same if one turns it round half a revolution (180 degrees). Similarly, higher spin particles look
the same if one turns them through smaller fractions of a complete revolution. All this seems fairly straightforward, but theremark-able fact is that there are particles that do not look the same if one turns them through just one revolution: you
have to turn them through two complete revolutions! Such particles are said to have spin ½.

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Figure 5:1
All the known particles in the universe can be divided into two groups: particles of spin ½, which make up the matter inthe universe, and particles of spin 0, 1, and 2, which, as we shall see, give rise to forces between the matter particles.
The matter particles obey what is called Pauli’s exclusion principle. This was discovered in 1925 by an Austrian physicist,
Wolfgang Pauli – for which he received the Nobel Prize in 1945. He was the archetypal theoretical physicist: it was saidof him that even his presence in the same town would make experiments go wrong! Pauli’s exclusion principle says that
two similar particles can-not exist in the same state; that is, they cannot have both the same position and the same
velocity, within the limits given by the uncertainty principle. The exclusion principle is crucial because it explains whymatter particles do not collapse to a state of very high density under the influence of the forces produced by the particlesof spin 0, 1, and 2: if the matter particles have very nearly the same positions, they must have different velocities, whichmeans that they will not stay in the same position for long. If the world had been created without the exclusion principle,
quarks would not form separate, well-defined protons and neutrons. Nor would these, together with electrons, form
separate, well-defined atoms. They would all collapse to form a roughly uniform, dense “soup.”
A proper understanding of the electron and other spin-½ particles did not come until 1928, when a theory was proposed
by Paul Dirac, who later was elected to the Lucasian Professorship of Mathematics at Cambridge (the sameprofessorship that Newton had once held and that I now hold). Dirac’s theory was the first of its kind that was consistent
with both quantum mechanics and the special theory of relativity. It explained mathematically why the electron had
spin-½; that is, why it didn’t look the same if you turned it through only one complete revolution, but did if you turned it
through two revolutions. It also predicted that the electron should have a partner: an anti-electron, or positron. The
discovery of the positron in 1932 confirmed Dirac’s theory and led to his being awarded the Nobel Prize for physics in1933. We now know that every particle has an antiparticle, with which it can annihilate. (In the case of the force-carrying
particles, the antiparticles are the same as the particles themselves.) There could be whole antiworlds and antipeople
made out of antiparticles. However, if you meet your antiself, don’t shake hands! You would both vanish in a great flashof light. The question of why there seem to be so many more particles than antiparticles around us is extremely

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important, and I shall return to it later in the chapter.
In quantum mechanics, the forces or interactions between matter particles are all supposed to be carried by particles ofinteger spin – 0, 1, or 2. What happens is that a matter particle, such as an electron or a quark, emits a force-carrying
particle. The recoil from this emission changes the velocity of the matter particle. The force-carrying particle then collides
with another matter particle and is absorbed. This collision changes the velocity of the second particle, just as if there had
been a force between the two matter particles. It is an important property of ' the force-carrying particles that they do notobey the exclusion principle. This means that there is no limit to the number that can be exchanged, and so they can giverise to a strong force. However, if the force-carrying particles have a high mass, it will be difficult to produce andexchange them over a large distance. So the forces that they carry will have only a short range. On the other hand, if theforce-carrying particles have no mass of their own, the forces will be long range. The force-carrying particles exchanged
between matter particles are said to be virtual particles because, unlike “real” particles, they cannot be directly detectedby a particle detector. We know they exist, however, because they do have a measurable effect: they give rise to forces
between matter particles. Particles of spin 0, 1, or 2 do also exist in some circumstances as real particles, when they canbe directly detected. They then appear to us as what a classical physicist would call waves, such as waves of light orgravitational waves. They may sometimes be emitted when matter particles interact with each other by exchanging virtual
force-carrying particles. (For example, the electric repulsive force between two electrons is due to the exchange of virtualphotons, which can never be directly detected; but if one electron moves past another, real photons may be given off,
which we detect as light waves.)
Force-carrying particles can be grouped into four categories according to the strength of the force that they carry and theparticles with which they interact. It should be emphasized that this division into four classes is man-made; it isconvenient for the construction of partial theories, but it may not correspond to anything deeper. Ultimately, mostphysicists hope to find a unified theory that will explain all four forces as different aspects of a single force. Indeed, many
would say this is the prime goal of physics today. Recently, successful attempts have been made to unify three of thefour categories of force – and I shall describe these in this chapter. The question of the unification of the remaining
category, gravity, we shall leave till later.
The first category is the gravitational force. This force is universal, that is, every particle feels the force of gravity,
according to its mass or energy. Gravity is the weakest of the four forces by a long way; it is so weak that we would not
notice it at all were it not for two special properties that it has: it can act over large distances, and it is always attractive.
This means that the very weak gravitational forces between the individual particles in two large bodies, such as the earth
and the sun, can all add up to produce a significant force. The other three forces are either short range, or are sometimes
attractive and some-times repulsive, so they tend to cancel out. In the quantum mechanical way of looking at thegravitational field, the force between two matter particles is pictured as being carried by a particle of spin 2 called thegraviton. This has no mass of its own, so the force that it carries is long range. The gravitational force between the sun
and the earth is ascribed to the exchange of gravitons between the particles that make up these two bodies. Although theexchanged particles are virtual, they certainly do produce a measurable effect – they make the earth orbit the sun! Realgravitons make up what classical physicists would call gravitational waves, which are very weak – and so difficult to
detect that they have not yet been observed.
The next category is the electromagnetic force, which interacts with electrically charged particles like electrons andquarks, but not with uncharged particles such as gravitons. It is much stronger than the gravitational force: theelectromagnetic force between two electrons is about a million million million million million million million (1 with forty-two
zeros after it) times bigger than the gravitational force. However, there are two kinds of electric charge, positive andnegative. The force between two positive charges is repulsive, as is the force between two negative charges, but theforce is attractive between a positive and a negative charge. A large body, such as the earth or the sun, contains nearly
equal numbers of positive and negative charges. Thus the attractive and repulsive forces between the individual particles
nearly cancel each other out, and there is very little net electromagnetic force. However, on the small scales of atoms
and molecules, electromagnetic forces dominate. The electromagnetic attraction between negatively charged electronsand positively charged protons in the nucleus causes the electrons to orbit the nucleus of the atom, just as gravitational
attraction causes the earth to orbit the sun. The electromagnetic attraction is pictured as being caused by the exchangeof large numbers of virtual massless particles of spin 1, called photons. Again, the photons that are exchanged are virtual
particles. However, when an electron changes from one allowed orbit to another one nearer to the nucleus, energy isreleased and a real photon is emitted – which can be observed as visible light by the human eye, if it has the rightwave-length, or by a photon detector such as photographic film. Equally, if a real photon collides with an atom, it may
move an electron from an orbit nearer the nucleus to one farther away. This uses up the energy of the photon, so it isabsorbed.
The third category is called the weak nuclear force, which is responsible for radioactivity and which acts on all matterparticles of spin-½, but not on particles of spin 0, 1, or 2, such as photons and gravitons. The weak nuclear force was notwell understood until 1967, when Abdus Salam at Imperial College, London, and Steven Weinberg at Harvard both
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proposed theories that unified this interaction with the electromagnetic force, just as Maxwell had unified electricity andmagnetism about a hundred years earlier. They suggested that in addition to the photon, there were three other spin-1
particles, known collectively as massive vector bosons, that carried the weak force. These were called W+ (pronounced
W plus), W- (pronounced W minus), and Zº (pronounced Z naught), and each had a mass of around 100 GeV (GeV
stands for gigaelectron-volt, or one thousand million electron volts). The Weinberg-Salam theory exhibits a property
known as spontaneous symmetry breaking. This means that what appear to be a number of completely different particles
at low energies are in fact found to be all the same type of particle, only in different states. At high energies all theseparticles behave similarly. The effect is rather like the behavior of a roulette ball on a roulette wheel. At high energies(when the wheel is spun quickly) the ball behaves in essentially only one way – it rolls round and round. But as the wheelslows, the energy of the ball decreases, and eventually the ball drops into one of the thirty-seven slots in the wheel. In
other words, at low energies there are thirty-seven different states in which the ball can exist. If, for some reason, we
could only observe the ball at low energies, we would then think that there were thirty-seven different types of ball!
In the Weinberg-Salam theory, at energies much greater than 100 GeV, the three new particles and the photon would all
behave in a similar manner. But at the lower particle energies that occur in most normal situations, this symmetry
between the particles would be broken. WE, W, and Zº would acquire large masses, making the forces they carry have avery short range. At the time that Salam and Weinberg proposed their theory, few people believed them, and particle
accelerators were not powerful enough to reach the energies of 100 GeV required to produce real W+, W-, or Zº particles.
However, over the next ten years or so, the other predictions of the theory at lower energies agreed so well withexperiment that, in 1979, Salam and Weinberg were awarded the Nobel Prize for physics, together with Sheldon
Glashow, also at Harvard, who had suggested similar unified theories of the electromagnetic and weak nuclear forces.
The Nobel committee was spared the embarrassment of having made a mistake by the discovery in 1983 at CERN(European Centre for Nuclear Research) of the three massive partners of the photon, with the correct predicted masses
and other properties. Carlo Rubbia, who led the team of several hundred physicists that made the discovery, received theNobel Prize in 1984, along with Simon van der Meer, the CERNengineer who developed the antimatter storage system
employed. (It is very difficult to make a mark in experimental physics these days unless you are already at the top! )The fourth category is the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the
protons and neutrons together in the nucleus of an atom. It is believed that this force is carried by another spin-1 particle,
called the gluon, which interacts only with itself and with the quarks. The strong nuclear force has a curious property
called confinement: it always binds particles together into combinations that have no color. One cannot have a single
quark on its own because it would have a color (red, green, or blue). Instead, a red quark has to be joined to a green anda blue quark by a “string” of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron. Anotherpossibility is a pair consisting of a quark and an antiquark (red + antired, or green + antigreen, or blue + antiblue = white).
Such combinations make up the particles known as mesons, which are unstable because the quark and antiquark canannihilate each other, producing electrons and other particles. Similarly, confinement prevents one having a single gluon
on its own, because gluons also have color. Instead, one has to have a collection of gluons whose colors add up to white.
Such a collection forms an unstable particle called a glueball.
The fact that confinement prevents one from observing an isolated quark or gluon might seem to make the whole notionof quarks and gluons as particles somewhat metaphysical. However, there is another property of the strong nuclear
force, called asymptotic freedom, that makes the concept of quarks and gluons well defined. At normal energies, thestrong nuclear force is indeed strong, and it binds the quarks tightly together. However, experiments with large particle
accelerators indicate that at high energies the strong force becomes much weaker, and the quarks and gluons behave
almost like free particles.

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Figure 5:2
Figure 5:2 shows a photograph of a collision between a high-energy proton and antiproton. The success of the unificationof the electromagnetic and weak nuclear forces led to a number of attempts to combine these two forces with the strong
nuclear force into what is called a grand unified theory (or GUT). This title is rather an exaggeration: the resultant theories
are not all that grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories,
because they contain a number of parameters whose values cannot be predicted from the theory but have to be chosen
to fit in with experiment. Nevertheless, they may be a step toward a complete, fully unified theory. The basic idea of
GUTs is as follows: as was mentioned above, the strong nuclear force gets weaker at high energies. On the other hand,the electromagnetic and weak forces, which are not asymptotically free, get stronger at high energies. At some very highenergy, called the grand unification energy, these three forces would all have the same strength and so could just be
different aspects of a single force. The GUTs also predict that at this energy the different spin-½ matter particles, likequarks and electrons, would also all be essentially the same, thus achieving another unification.
The value of the grand unification energy is not very well known, but it would probably have to be at least a thousand
million million GeV. The present generation of particle accelerators can collide particles at energies of about one hundred
GeV, and machines are planned that would raise this to a few thousand GeV. But a machine that was powerful enough toaccelerate particles to the grand unification energy would have to be as big as the Solar System – and would be unlikely
to be funded in the present economic climate. Thus it is impossible to test grand unified theories directly in the laboratory.
However, just as in the case of the electromagnetic and weak unified theory, there are low-energy consequences of thetheory that can be tested.
The most interesting of these is the prediction that protons, which make up much of the mass of ordinary matter, canspontaneously decay into lighter particles such as antielectrons. The reason this is possible is that at the grandunification energy there is no essential difference between a quark and an antielectron. The three quarks inside a proton
normally do not have enough energy to change into antielectrons, but very occasionally one of them may acquire

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sufficient energy to make the transition because the uncertainty principle means that the energy of the quarks inside theproton cannot be fixed exactly. The proton would then decay. The probability of a quark gaining sufficient energy is so
low that one is likely to have to wait at least a million million million million million years (1 followed by thirty zeros). This ismuch longer than the time since the big bang, which is a mere ten thousand million years or so (1 followed by ten zeros).
Thus one might think that the possibility of spontaneous proton decay could not be tested experimentally. However, one
can increase one’s chances of detecting a decay by observing a large amount of matter containing a very large numberof protons. (If, for example, one observed a number of protons equal to 1 followed by thirty-one zeros for a period of one
year, one would expect, according to the simplest GUT, to observe more than one proton decay.)
A number of such experiments have been carried out, but none have yielded definite evidence of proton or neutron
decay. One experiment used eight thousand tons of water and was performed in the Morton Salt Mine in Ohio (to avoid
other events taking place, caused by cosmic rays, that might be confused with proton decay). Since no spontaneous
proton decay had been observed during the experiment, one can calculate that the probable life of the proton must be
greater than ten million million million million million years (1 with thirty-one zeros). This is longer than the lifetime
predicted by the simplest grand unified theory, but there are more elaborate theories in which the predicted lifetimes are
longer. Still more sensitive experiments involving even larger quantities of matter will be needed to test them.
Even though it is very difficult to observe spontaneous proton decay, it may be that our very existence is a consequenceof the reverse process, the production of protons, or more simply, of quarks, from an initial situation in which there were
no more quarks than antiquarks, which is the most natural way to imagine the universe starting out. Matter on the earth ismade up mainly of protons and neutrons, which in turn are made up of quarks. There are no antiprotons or antineutrons,
made up from antiquarks, except for a few that physicists produce in large particle accelerators. We have evidence fromcosmic rays that the same is true for all the matter in our galaxy: there are no antiprotons or antineutrons apart from asmall number that are produced as particle/ antiparticle pairs in high-energy collisions. If there were large regions of
antimatter in our galaxy, we would expect to observe large quantities of radiation from the borders between the regions ofmatter and antimatter, where many particles would be colliding with their anti-particles, annihilating each other and giving
off high-energy radiation.
We have no direct evidence as to whether the matter in other galaxies is made up of protons and neutrons or antiprotons
and anti-neutrons, but it must be one or the other: there cannot be a mixture in a single galaxy because in that case wewould again observe a lot of radiation from annihilations. We therefore believe that all galaxies are composed of quarks
rather than antiquarks; it seems implausible that some galaxies should be matter and some antimatter.
Why should there be so many more quarks than antiquarks? Why are there not equal numbers of each? It is certainly
fortunate for us that the numbers are unequal because, if they had been the same, nearly all the quarks and antiquarks
would have annihilated each other in the early universe and left a universe filled with radiation but hardly any matter.
There would then have been no galaxies, stars, or planets on which human life could have developed. Luckily, grandunified theories may provide an explanation of why the universe should now contain more quarks than antiquarks, even if
it started out with equal numbers of each. As we have seen, GUTs allow quarks to change into antielectrons at highenergy. They also allow the reverse processes, antiquarks turning into electrons, and electrons and antielectrons turning
into antiquarks and quarks. There was a time in the very early universe when it was so hot that the particle energies
would have been high enough for these transformations to take place. But why should that lead to more quarks than
antiquarks? The reason is that the laws of physics are not quite the same for particles and antiparticles.
Up to 1956 it was believed that the laws of physics obeyed each of three separate symmetries called C, P, and T. The
symmetry C means that the laws are the same for particles and antiparticles. The symmetry P means that the laws are
the same for any situation and its mirror image (the mirror image of a particle spinning in a right-handed direction is one
spinning in a left-handed direction). The symmetry T means that if you reverse the direction of motion of all particles andantiparticles, the system should go back to what it was at earlier times; in other words, the laws are the same in theforward and backward directions of time. In 1956 two American physicists, Tsung-Dao Lee and Chen Ning Yang,suggested that the weak force does not in fact obey the symmetry P. In other words, the weak force would make theuniverse develop in a different way from the way in which the mirror image of the universe would develop. The same
year, a colleague, Chien-Shiung Wu, proved their prediction correct. She did this by lining up the nuclei of radioactive
atoms in a magnetic field, so that they were all spinning in the same direction, and showed that the electrons were given
off more in one direction than another. The following year, Lee and Yang received the Nobel Prize for their idea. It wasalso found that the weak force did not obey the symmetry C. That is, it would cause a universe composed of antiparticles
to behave differently from our universe. Nevertheless, it seemed that the weak force did obey the combined symmetry
CP. That is, the universe would develop in the same way as its mirror image if, in addition, every particle was swappedwith its antiparticle! However, in 1964 two more Americans, J. W. Cronin and Val Fitch, discovered that even the CP
symmetry was not obeyed in the decay of certain particles called K-mesons. Cronin and Fitch eventually received theNobel Prize for their work in 1980. (A lot of prizes have been awarded for showing that the universe is not as simple as
we might have thought!)
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There is a mathematical theorem that says that any theory that obeys quantum mechanics and relativity must always
obey the combined symmetry CPT. In other words, the universe would have to behave the same if one replaced particles
by antiparticles, took the mirror image, and also reversed the direction of time. But Cronin and Fitch showed that if one
replaces particles by antiparticles and takes the mirror image, but does not reverse the direction of time, then theuniverse does not behave the same. The laws of physics, therefore, must change if one reverses the direction of time –they do not obey the symmetry T.
Certainly the early universe does not obey the symmetry T: as time runs forward the universe expands – if it ran
backward, the universe would be contracting. And since there are forces that do not obey the symmetry T, it follows thatas the universe expands, these forces could cause more antielectrons to turn into quarks than electrons into antiquarks.
Then, as the universe expanded and cooled, the antiquarks would annihilate with the quarks, but since there would be
more quarks than antiquarks, a small excess of quarks would remain. It is these that make up the matter we see today
and out of which we ourselves are made. Thus our very existence could be regarded as a confirmation of grand unified
theories, though a qualitative one only; the uncertainties are such that one cannot predict the numbers of quarks that willbe left after the annihilation, or even whether it would be quarks or antiquarks that would remain. (Had it been an excess
of antiquarks, however, we would simply have named antiquarks quarks, and quarks antiquarks.)
Grand unified theories do not include the force of gravity. This does not matter too much, because gravity is such a weakforce that its effects can usually be neglected when we are dealing with elementary particles or atoms. However, the factthat it is both long range and always attractive means that its effects all add up. So for a sufficiently large number ofmatter particles, gravitational forces can dominate over all other forces. This is why it is gravity that determines theevolution of the universe. Even for objects the size of stars, the attractive force of gravity can win over all the other forces
and cause the star to collapse. My work in the 1970s focused on the black holes that can result from such stellar collapse
and the intense gravitational fields around them. It was this that led to the first hints of how the theories of quantum
mechanics and general relativity might affect each other – a glimpse of the shape of a quantum theory of gravity yet tocome.

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A Brief History of Time - Stephen Hawking... Chapter 6

CHAPTER 6
BLACK HOLES

The term black hole is of very recent origin. It was coined in 1969 by the American scientist John Wheeler as a graphic
description of an idea that goes back at least two hundred years, to a time when there were two theories about light:
one, which Newton favored, was that it was composed of particles; the other was that it was made of waves. We now
know that really both theories are correct. By the wave/particle duality of quantum mechanics, light can be regarded asboth a wave and a particle. Under the theory that light is made up of waves, it was not clear how it would respond to
gravity. But if light is composed of particles, one might expect them to be affected by gravity in the same way thatcannonballs, rockets, and planets are. At first people thought that particles of light traveled infinitely fast, so gravity
would not have been able to slow them down, but the discovery by Roemer that light travels at a finite speed meant thatgravity might have an important effect.
On this assumption, a Cambridge don, John Michell, wrote a paper in 1783 in the Philosophical Transactions of theRoyal Society of London in which he pointed out that a star that was sufficiently massive and compact would have sucha strong gravitational field that light could not escape: any light emitted from the surface of the star would be draggedback by the star’s gravitational attraction before it could get very far. Michell suggested that there might be a large
number of stars like this. Although we would not be able to see them because the light from them would not reach us,
we would still feel their gravitational attraction. Such objects are what we now call black holes, because that is what
they are: black voids in space. A similar suggestion was made a few years later by the French scientist the Marquis de
Laplace, apparently independently of Michell. Interestingly enough, Laplace included it in only the first and second
editions of his book The System of the World, and left it out of later editions; perhaps he decided that it was a crazyidea. (Also, the particle theory of light went out of favor during the nineteenth century; it seemed that everything could
be explained by the wave theory, and according to the wave theory, it was not clear that light would be affected bygravity at all.)In fact, it is not really consistent to treat light like cannonballs in Newton’s theory of gravity because the speed of light isfixed. (A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; aphoton, however, must continue upward at a constant speed. How then can Newtonian grav-ity affect light?) A
consistent theory of how gravity affects light did not come along until Einstein proposed general relativity in 1915. Andeven then it was a long time before the implications of the theory for massive stars were understood.
To understand how a black hole might be formed, we first need an understanding of the life cycle of a star. A star isformed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As
it contracts, the atoms of the gas collide with each other more and more frequently and at greater and greater speeds –the gas heats up. Eventually, the gas will be so hot that when the hydrogen atoms collide they no longer bounce off
each other, but instead coalesce to form helium. The heat released in this reaction, which is like a controlled hydrogenbomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it issufficient to balance the gravitational attraction, and the gas stops contracting. It is a bit like a balloon – there is abalance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in therubber, which is trying to make the balloon smaller. Stars will remain stable like this for a long time, with heat from thenuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen andother nuclear fuels. Paradoxically, the more fuel a star starts off with, the sooner it runs out. This is because the more
massive the star is, the hotter it needs to be to balance its gravitational attraction. And the hotter it is, the faster it will
use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive
stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When a starruns out of fuel, it starts to cool off and so to contract. What might happen to it then was first understood only at the endof the 1920s.
In 1928 an Indian graduate student, Subrahmanyan Chandrasekhar, set sail for England to study at Cambridge with the
British astronomer Sir Arthur Eddington, an expert on general relativity. (According to some accounts, a journalist told
Eddington in the early 1920s that he had heard there were only three people in the world who understood generalrelativity. Eddington paused, then replied, “I am trying to think who the third person is.”) During his voyage from India,
Chandrasekhar worked out how big a star could be and still support itself against its own gravity after it had used up allits fuel. The idea was this: when the star becomes small, the matter particles get very near each other, and so
according to the Pauli exclusion principle, they must have very different velocities. This makes them move away fromeach other and so tends to make the star expand. A star can therefore maintain itself at a constant radius by a balance
between the attraction of gravity and the repulsion that arises from the exclusion principle, just as earlier in its life

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gravity was balanced by the heat.
Chandrasekhar realized, however, that there is a limit to the repulsion that the exclusion principle can provide. The
theory of relativity limits the maximum difference in the velocities of the matter particles in the star to the speed of light.
This means that when the star got sufficiently dense, the repulsion caused by the exclusion principle would be less than
the attraction of gravity. Chandrasekhar calculated that a cold star of more than about one and a half times the mass ofthe sun would not be able to support itself against its own gravity. (This mass is now known as the Chandrasekhar
limit.) A similar discovery was made about the same time by the Russian scientist Lev Davidovich Landau.
This had serious implications for the ultimate fate of massive stars. If a star’s mass is less than the Chandrasekhar limit,
it can eventually stop contracting and settle down to a possible final state as a “white dwarf” with a radius of a few
thousand miles and a density of hundreds of tons per cubic inch. A white dwarf is supported by the exclusion principle
repulsion between the electrons in its matter. We observe a large number of these white dwarf stars. One of the first to
be discovered is a star that is orbiting around Sirius, the brightest star in the night sky.
Landau pointed out that there was another possible final state for a star, also with a limiting mass of about one or twotimes the mass of the sun but much smaller even than a white dwarf. These stars would be supported by the exclusion
principle repulsion between neutrons and protons, rather than between electrons. They were therefore called neutronstars. They would have a radius of only ten miles or so and a density of hundreds of millions of tons per cubic inch. At
the time they were first predicted, there was no way that neutron stars could be observed. They were not actuallydetected until much later.
Stars with masses above the Chandrasekhar limit, on the other hand, have a big problem when they come to the end of
their fuel. In some cases they may explode or manage to throw off enough matter to reduce their mass below the limit
and so avoid catastrophic gravitational collapse, but it was difficult to believe that this always happened, no matter how
big the star. How would it know that it had to lose weight? And even if every star managed to lose enough mass to
avoid collapse, what would happen if you added more mass to a white dwarf 'or neutron star to take it over the limit?
Would it collapse to infinite density? Eddington was shocked by that implication, and he refused to believe
Chandrasekhar’s result. Eddington thought it was simply not possible that a star could collapse to a point. This was theview of most scientists: Einstein himself wrote a paper in which he claimed that stars would not shrink to zero size. The
hostility of other scientists, particularly Eddington, his former teacher and the leading authority on the structure of stars,
persuaded Chandrasekhar to abandon this line of work and turn instead to other problems in astronomy, such as the
motion of star clusters. However, when he was awarded the Nobel Prize in 1983, it was, at least in part, for his early
work on the limiting mass of cold stars.
Chandrasekhar had shown that the exclusion principle could not halt the collapse of a star more massive than the
Chandrasekhar limit, but the problem of understanding what would happen to such a star, according to generalrelativity, was first solved by a young American, Robert Oppenheimer, in 1939. His result, however, suggested that
there would be no observational consequences that could be detected by the telescopes of the day. Then World War IIintervened and Oppenheimer himself became closely involved in the atom bomb project. After the war the problem ofgravitational collapse was largely forgotten as most scientists became caught up in what happens on the scale of theatom and its nucleus. In the 1960s, however, interest in the large-scale problems of astronomy and cosmology was
revived by a great increase in the number and range of astronomical observations brought about by the application ofmodern technology. Oppenheimer’s work was then rediscovered and extended by a number of people.
The picture that we now have from Oppenheimer’s work is as follows. The gravitational field of the star changes thepaths of light rays in space-time from what they would have been had the star not been present. The light cones, whichindicate the paths followed in space and time by flashes of light emitted from their tips, are bent slightly inward near thesurface of the star. This can be seen in the bending of light from distant stars observed during an eclipse of the sun. As
the star contracts, the gravitational field at its surface gets stronger and the light cones get bent inward more. This
makes it more difficult for light from the star to escape, and the light appears dimmer and redder to an observer at a
distance. Eventually, when the star has shrunk to a certain critical radius, the gravitational field at the surface becomesso strong that the light cones are bent inward so much that light can no longer escape Figure 6:1.

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Figure 6:1
According to the theory of relativity, nothing can travel faster than light. Thus if light cannot escape, neither can anythingelse; everything is dragged back by the gravitational field. So one has a set of events, a region of space-time, fromwhich it is not possible to escape to reach a distant observer. This region is what we now call a black hole. Its boundaryis called the event horizon and it coincides with the paths of light rays that just fail to escape from the black hole.
In order to understand what you would see if you were watching a star collapse to form a black hole, one has toremember that in the theory of relativity there is no absolute time. Each observer has his own measure of time. The time
for someone on a star will be different from that for someone at a distance, because of the gravitational field of the star.

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Suppose an intrepid astronaut on the surface of the collapsing star, collapsing inward with it, sent a signal every
second, according to his watch, to his spaceship orbiting about the star. At some time on his watch, say 11:00, the star
would shrink below the critical radius at which the gravitational field becomes so strong nothing can escape, and his
signals would no longer reach the spaceship. As 11:00 approached his companions watching from the spaceship wouldfind the intervals between successive signals from the astronaut getting longer and longer, but this effect would be verysmall before 10:59:59. They would have to wait only very slightly more than a second between the astronaut’s 10:59:58
signal and the one that he sent when his watch read 10:59:59, but they would have to wait forever for the 11:00 signal.
The light waves emitted from the surface of the star between 10:59:59 and 11:00, by the astronaut’s watch, would bespread out over an infinite period of time, as seen from the spaceship. The time interval between the arrival of
successive waves at the spaceship would get longer and longer, so the light from the star would appear redder andredder and fainter and fainter. Eventually, the star would be so dim that it could no longer be seen from the spaceship:
all that would be left would be a black hole in space. The star would, however, continue to exert the same gravitational
force on the spaceship, which would continue to orbit the black hole. This scenario is not entirely realistic, however,
because of the following problem. Gravity gets weaker the farther you are from the star, so the gravitational force on our
intrepid astronaut’s feet would always be greater than the force on his head. This difference in the forces would stretch
our astronaut out like spaghetti or tear him apart before the star had contracted to the critical radius at which the event
horizon formed! However, we believe that there are much larger objects in the universe, like the central regions ofgalaxies, that can also undergo gravitational collapse to produce black holes; an astronaut on one of these would not betorn apart before the black hole formed. He would not, in fact, feel anything special as he reached the critical radius,and could pass the point of no return without noticing it However, within just a few hours, as the region continued tocollapse, the difference in the gravitational forces on his head and his feet would become so strong that again it would
tear him apart.
The work that Roger Penrose and I did between 1965 and 1970 showed that, according to general relativity, there must
be a singularity of infinite density and space-time curvature within a black hole. This is rather like the big bang at the
beginning of time, only it would be an end of time for the collapsing body and the astronaut. At this singularity the lawsof science and our ability to predict the future would break down. However, any observer who remained outside theblack hole would not be affected by this failure of predictability, because neither light nor any other signal could reachhim from the singularity. This remarkable fact led Roger Penrose to propose the cosmic censorship hypothesis, whichmight be paraphrased as “God abhors a naked singularity.” In other words, the singularities produced by gravitational
collapse occur only in places, like black holes, where they are decently hidden from outside view by an event horizon.
Strictly, this is what is known as the weak cosmic censorship hypothesis: it protects observers who remain outside theblack hole from the consequences of the breakdown of predictability that occurs at the singularity, but it does nothing at
all for the poor unfortunate astronaut who falls into the hole.
There are some solutions of the equations of general relativity in which it is possible for our astronaut to see a naked
singularity: he may be able to avoid hitting the singularity and instead fall through a "wormhole” and come out in anotherregion of the universe. This would offer great possibilities for travel in space and time, but unfortunately it seems thatthese solutions may all be highly unstable; the least disturbance, such as the presence of an astronaut, may change
them so that the astronaut could not see the singularity until he hit it and his time came to an end. In other words, thesingularity would always lie in his future and never in his past. The strong version of the cosmic censorship hypothesis
states that in a realistic solution, the singularities would always lie either entirely in the future (like the singularities ofgravitational collapse) or entirely in the past (like the , big bang). I strongly believe in cosmic censorship so I bet Kip
Thorne and John Preskill of Cal Tech that it would always hold. I lost the bet on a technicality because examples were
produced of solutions with a singularity that was visible from a long way away. So I had to pay up, which according to
the terms of the bet meant I had to clothe their nakedness. But I can claim a moral victory. The naked singularities were
unstable: the least disturbance would cause them either to disappear or to be hidden behind an event horizon. So theywould not occur in realistic situations.
The event horizon, the boundary of the region of space-time from which it is not possible to escape, acts rather like a
one-way membrane around the black hole: objects, such as unwary astronauts, can fall through the event horizon into
the black hole, but nothing can ever get out of the black hole through the event horizon. (Remember that the event
horizon is the path in space-time of light that is trying to escape from the black hole, and nothing can travel faster thanlight.) One could well say of the event horizon what the poet Dante said of the entrance to Hell: “All hope abandon, ye
who enter here.” Anything or anyone who falls through the event horizon will soon reach the region of infinite density
and the end of time.
General relativity predicts that heavy objects that are moving will cause the emission of gravitational waves, ripples in
the curvature of space that travel at the speed of light. These are similar to light waves, which are ripples of theelectromagnetic field, but they are much harder to detect. They can be observed by the very slight change in separation
they produce between neighboring freely moving objects. A number of detectors are being built in the United States,

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Europe, and Japan that will measure displacements of one part in a thousand million million million (1 with twenty-one
zeros after it), or less than the nucleus of an atom over a distance of ten miles.
Like light, gravitational waves carry energy away from the objects that emit them. One would therefore expect a system
of massive objects to settle down eventually to a stationary state, because the energy in any movement would be
carried away by the emission of gravitational waves. (It is rather like dropping a cork into water: at first it bobs up anddown a great deal, but as the ripples carry away its energy, it eventually settles down to a stationary state.) For
example, the movement of the earth in its orbit round the sun produces gravitational waves. The effect of the energy
loss will be to change the orbit of the earth so that gradually it gets nearer and nearer to the sun, eventually collides withit, and settles down to a stationary state. The rate of energy loss in the case of the earth and the sun is very low – about
enough to run a small electric heater. This means it will take about a thousand million million million million years for theearth to run into the sun, so there’s no immediate cause for worry! The change in the orbit of the earth is too slow to beobserved, but this same effect has been observed over the past few years occurring in the system called PSR 1913 +
16 (PSR stands for “pulsar,” a special type of neutron star that emits regular pulses of radio waves). This system
contains two neutron stars orbiting each other, and the energy they are losing by the emission of gravitational waves iscausing them to spiral in toward each other. This confirmation of general relativity won J. H. Taylor and R. A. Hulse theNobel Prize in 1993. It will take about three hundred million . years for them to collide. Just before they do, they will beorbiting so fast that they will emit enough gravitational waves for detectors like LIGO to pick up.
During the gravitational collapse of a star to form a black hole, the movements would be much more rapid, so the rate at
which energy is carried away would be much higher. It would therefore not be too long ' before it settled down to astationary state. What would this final stage look like? One might suppose that it would depend on all the complex
features of the star from which it had formed – not only its mass and rate of rotation, but also the different densities ofvarious parts of the star, and the complicated movements of the gases within the star. And if black holes were as varied
as the objects that collapsed to form them, it might be very difficult to make any predictions about black holes ingeneral.
In 1967, however, the study of black holes was revolutionized by Werner Israel, a Canadian scientist (who was born inBerlin, brought up in South Africa, and took his doctoral degree in Ireland). Israel showed that, according to generalrelativity, non-rotating black holes must be very simple; they were perfectly spherical, their size depended only on theirmass, and any two such black holes with the same mass were identical. They could, in fact, be described by aparticular solution of Einstein’s equations that had been known since 1917, found by Karl Schwarzschild shortly after
the discovery of general relativity. At first many people, including Israel himself, argued that since black holes had to be
perfectly spherical, a black hole could only form from the collapse of a perfectly spherical object. Any real star – which
would never be perfectly spherical – could therefore only collapse to form a naked singularity.
There was, however, a different interpretation of Israel’s result, which was advocated by Roger Penrose and John
Wheeler in particular. They argued that the rapid movements involved in a star’s collapse would mean that thegravitational waves it gave off would make it ever more spherical, and by the time it had settled down to a stationary
state, it would be precisely spherical. According to this view, any non-rotating star, however complicated its shape andinternal structure, would end up after gravitational collapse as a perfectly spherical black hole, whose size would
depend only on its mass. Further calculations supported this view, and it soon came to be adopted generally.
Israel’s result dealt with the case of black holes formed from non-rotating bodies only. In 1963, Roy Kerr, a New
Zealander, found a set of solutions of the equations of general relativity that described rotating black holes. These“Kerr” black holes rotate at a constant rate, their size and shape depending only on their mass and rate of rotation. If therotation is zero, the black hole is perfectly round and the solution is identical to the Schwarzschild solution. If therotation is non-zero, the black hole bulges outward near its equator (just as the earth or the sun bulge due to theirrotation), and the faster it rotates, the more it bulges. So, to extend Israel’s result to include rotating bodies, it was
conjectured that any rotating body that collapsed to form a black hole would eventually settle down to a stationary state
described by the Kerr solution. In 1970 a colleague and fellow research student of mine at Cambridge, Brandon Carter,
took the first step toward proving this conjecture. He showed that, provided a stationary rotating black hole had an axisof symmetry, like a spinning top, its size and shape would depend only on its mass and rate of rotation. Then, in 1971, I
proved that any stationary rotating black hole would indeed have such an axis of symmetry. Finally, in 1973, David
Robinson at Kings College, London, used Carter’s and my results to show that the conjecture had been correct: such a
black hole had indeed to be the Kerr solution. So after gravitational collapse a black hole must settle down into a statein which it could be rotating, but not pulsating. Moreover, its size and shape would depend only on its mass and rate ofrotation, and not on the nature of the body that had collapsed to form it. This result became known by the maxim: “Ablack hole has no hair.” The “no hair” theorem is of great practical importance, because it so greatly restricts thepossible types of black holes. One can therefore make detailed models of objects that might contain black holes andcompare the predictions of the models with observations. It also means that a very large amount of information about

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the body that has collapsed must be lost when a black hole is formed, because afterward all we can possibly measure
about the body is its mass and rate of rotation. The significance of this will be seen in the next chapter.
Black holes are one of only a fairly small number of cases in the history of science in which a theory was developed ingreat detail as a mathematical model before there was any evidence from observations that it was correct. Indeed, thisused to be the main argument of opponents of black holes: how could one believe in objects for which the only
evidence was calculations based on the dubious theory of general relativity? In 1963, however, Maarten Schmidt, anastronomer at the Palomar Observatory in California, measured the red shift of a faint starlike object in the direction of
the source of radio waves called 3C273 (that is, source number 273 in the third Cambridge catalogue of radio sources).
He found it was too large to be caused by a gravitational field: if it had been a gravitational red shift, the object would
have to be so massive and so near to us that it would disturb the orbits of planets in the Solar System. This suggested
that the red shift was instead caused by the expansion of the universe, which, in turn, meant that the object was a very
long distance away. And to be visible at such a great distance, the object must be very bright, must, in other words, beemitting a huge amount of energy. The only mechanism that people could think of that would produce such large
quantities of energy seemed to be the gravitational collapse not just of a star but of a whole central region of a galaxy. A
number of other similar “quasi-stellar objects,” or quasars, have been discovered, all with large red shifts. But they are
all too far away and therefore too difficult to observe to provide conclusive evidence of black holes.
Further encouragement for the existence of black holes came in 1967 with the discovery by a research student at
Cambridge, Jocelyn Bell-Burnell, of objects in the sky that were emitting regular pulses of radio waves. At first Bell andher supervisor, Antony Hewish, thought they might have made contact with an alien civilization in the galaxy! Indeed, atthe seminar at which they announced their discovery, I remember that they called the first four sources to be foundLGM 1 – 4, LGM standing for “Little Green Men.” In the end, however, they and everyone else came to the lessromantic conclusion that these objects, which were given the name pulsars, were in fact rotating neutron stars that were
emitting pulses of radio waves because of a complicated interaction between their magnetic fields and surrounding
matter. This was bad news for writers of space westerns, but very hopeful for the small number of us who believed inblack holes at that time: it was the first positive evidence that neutron stars existed. A neutron star has a radius of about
ten miles, only a few times the critical radius at which a star becomes a black hole. If a star could collapse to such asmall size, it is not unreasonable to expect that other stars could collapse to even smaller size and become black holes.
How could we hope to detect a black hole, as by its very definition it does not emit any light? It might seem a bit likelooking for a black cat in a coal cellar. Fortunately, there is a way. As John Michell pointed out in his pioneering paper in
1783, a black hole still exerts a gravitational fierce on nearby objects. Astronomers have observed many systems inwhich two stars orbit around each other, attracted toward each other by gravity. They also observe systems in which
there is only one visible star that is orbiting around some unseen companion. One cannot, of course, immediately
conclude that the companion is a black hole: it might merely be a star that is too faint to be seen. However, some ofthese systems, like the one called Cygnus X-1 Figure 6:2, are also strong sources of X-rays.

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Figure 6:2
The best explanation for this phenomenon is that matter has been blown off the surface of the visible star. As it falls
toward the unseen companion, it develops a spiral motion (rather like water running out of a bath), and it gets very hot,emitting X-rays Figure 6:3.

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Figure 6:3
For this mechanism to work, the unseen object has to be very small, like a white dwarf, neutron star, or black hole.
From the observed orbit of the visible star, one can determine the lowest possible mass of the unseen object. In thecase of Cygnus X-l, this is about six times the mass of the sun, which, according to Chandrasekhar’r result, is too great
for the unseen object to be a white dwarf. It is also too large a mass to be a neutron star. It seems, therefore, that it
must be a black hole.
There are other models to explain Cygnus X-1 that do not include a black hole, but they are all rather far-fetched. A
black hole seems to be the only really natural explanation of the observations. Despite this, I had a bet with Kip Thorneof the California Institute of Technology that in fact Cygnus X-1 does not contain a black hole! This was a form f
insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black
holes do not exist. But in that case, I would have the consolation of winning my bet, which would bring me four years ofthe magazine Private Eye. In fact, although the situation with Cygnus X-1 has not changed much since we made the bet
in 1975, there is now so much other observational evidence in favor of black holes that I have conceded the bet. I paid
the specified penalty, which was a one-year subscription to Penthouse, to the outrage of Kip’s liberated wife.
We also now have evidence for several other black holes in systems like Cygnus X-1 in our galaxy and in two
neighboring galaxies called the Magellanic Clouds. The number of black holes, however, is almost certainly very muchhigher; in the long history of the universe, many stars must have burned all their nuclear fuel and have had to collapse.
The number of black holes may well be greater even than the number of visible stars, which totals about a hundred
thousand million in our galaxy alone. The extra gravitational attraction of such a large number of black holes could
explain why our galaxy rotates at the rate it does: the mass of the visible stars is insufficient to account for this. We also

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have some evidence that there is a much larger black hole, with a mass of about a hundred thousand times that of thesun, at the center of our galaxy. Stars in the galaxy that come too near this black hole will be torn apart by thedifference in the gravitational forces on their near and far sides. Their remains and gas that is thrown off other stars, will
fall toward the black hole. As in the case of Cygnus X-l, the gas will spiral inward and will heat up, though not as muchas in that case. It will not get hot enough to emit X rays, but it could account for the very compact source of radio waves
and infrared rays that is observed at the galactic center.
It is thought that similar but even larger black holes, with masses of about a hundred million times the mass of the sun,occur at the centers of quasars. For example, observations with the Hubble telescope of the galaxy known as M87
reveal that it contains a disk of gas 130 light-years across rotating about a central object two thousand million times themass of the sun. This can only be a black hole. Matter falling into such a supermassive black hole would provide theonly source of power great enough to explain the enormous amounts of energy that these objects are emitting. As thematter spirals into the black hole, it would make the black hole rotate in the same direction, causing it to develop amagnetic field rather like that of the earth. Very high-energy particles would be generated near the black hole by the
in-falling matter. The magnetic field would be so strong that it could focus these particles into jets ejected outward along
the axis of rotation of the black hole, that is, in the directions of its north and south poles. Such jets are indeed observedin a number of galaxies and quasars. One can also consider the possibility that there might be black holes with masses
much less than that of the sun. Such black holes could not be formed by gravitational collapse, because their masses
are below the Chandrasekhar mass limit: stars of this low mass can support themselves against the force of gravity
even when they have exhausted their nuclear fuel. Low-mass black holes could form only if matter was compressed toenormous densities by very large external pressures. Such conditions could occur in a very big hydrogen bomb: thephysicist John Wheeler once calculated that if one took all the heavy water in all the oceans of the world, one could
build a hydrogen bomb that would compress matter at the center so much that a black hole would be created. (Ofcourse, there would be no one left to observe it!) A more practical possibility is that such low-mass black holes might
have been formed in the high temperatures and pressures of the very early universe. Black holes would have been
formed only if the early universe had not been perfectly smooth and uniform, because only a small region that wasdenser than average could be compressed in this way to form a black hole. But we know that there must have beensome irregularities, because otherwise the matter in the universe would still be perfectly uniformly distributed at thepresent epoch, instead of being clumped together in stars and galaxies.
Whether the irregularities required to account for stars and galaxies would have led to the formation of a significant
number of “primordial” black holes clearly depends on the details of the conditions in the early universe. So if we could
determine how many primordial black holes there are now, we would learn a lot about the very early stages of theuniverse. Primordial black holes with masses more than a thousand million tons (the mass of a large mountain) could
be detected only by their gravitational influence on other, visible matter or on the expansion of the universe. However,as we shall learn in the next chapter, black holes are not really black after all: they glow like a hot body, and the smaller
they are, the more they glow. So, paradoxically, smaller black holes might actually turn out to be easier to detect thanlarge ones!

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A Brief History of Time - Stephen Hawking... Chapter 7

CHAPTER 7
BLACK HOLES AIN’T SO BLACK

Before 1970, my research on general relativity had concentrated mainly on the question of whether or not there had
been a big bang singularity. However, one evening in November that year, shortly after the birth of my daughter, Lucy,I started to think about black holes as I was getting into bed. My disability makes this rather a slow process, so I had
plenty of time. At that date there was no precise definition of which points in space-time lay inside a black hole andwhich lay outside. I had already discussed with Roger Penrose the idea of defining a black hole as the set of eventsfrom which it was not possible to escape to a large distance, which is now the generally accepted definition. It meansthat the boundary of the black hole, the event horizon, is formed by the light rays that just fail to escape from the blackhole, hovering forever just on the edge Figure 7:1. It is a bit like running away from the police and just managing tokeep one step ahead but not being able to get clear away!

Figure 7:1
Suddenly I realized that the paths of these light rays could never approach one another. If they did they must
eventually run into one another. It would be like meeting someone else running away from the police in the opposite
direction – you would both be caught! (Or, in this case, fall into a black hole.) But if these light rays were swallowed upby the black hole, then they could not have been on the boundary of the black hole. So the paths of light rays in the

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event horizon had always to be moving parallel to, or away from, each other. Another way of seeing this is that theevent horizon, the boundary of the black hole, is like the edge of a shadow – the shadow of impending doom. If you
look at the shadow cast by a source at a great distance, such as the sun, you will see that the rays of light in the edge
are not approaching each other.
If the rays of light that form the event horizon, the boundary of the black hole, can never approach each other, the areaof the event horizon might stay the same or increase with time, but it could never decrease because that would meanthat at least some of the rays of light in the boundary would have to be approaching each other. In fact, the area would
increase whenever matter or radiation fell into the black hole Figure 7:2.

Figures 7:2 & 7:3
Or if two black holes collided and merged together to form a single black hole, the area of the event horizon of the final
black hole would be greater than or equal to the sum of the areas of the event horizons of the original black holes
Figure 7:3. This nondecreasing property of the event horizon’s area placed an important restriction on the possible
behavior of black holes. I was so excited with my discovery that I did not get much sleep that night. The next day I rang
up Roger Penrose. He agreed with me. I think, in fact, that he had been aware of this property of the area. However,
he had been using a slightly different definition of a black hole. He had not realized that the boundaries of the black
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hole according to the two definitions would be the same, and hence so would their areas, provided the black hole had
settled down to a state in which it was not changing with time.
The nondecreasing behavior of a black hole’s area was very reminiscent of the behavior of a physical quantity called
entropy, which measures the degree of disorder of a system. It is a matter of common experience that disorder willtend to increase if things are left to themselves. (One has only to stop making repairs around the house to see that!)
One can create order out of disorder (for example, one can paint the house), but that requires expenditure of effort orenergy and so decreases the amount of ordered energy available.
A precise statement of this idea is known as the second law of thermodynamics. It states that the entropy of an isolated
system always increases, and that when two systems are joined together, the entropy of the combined system isgreater than the sum of the entropies of the individual systems. For example, consider a system of gas molecules in abox. The molecules can be thought of as little billiard balls continually colliding with each other and bouncing off thewalls of the box. The higher the temperature of the gas, the faster the molecules move, and so the more frequently andharder they collide with the walls of the box and the greater the outward pressure they exert on the walls. Suppose that
initially the molecules are all confined to the left-hand side of the box by a partition. If the partition is then removed, themolecules will tend to spread out and occupy both halves of the box. At some later time they could, by chance, all be in
the right half or back in the left half, but it is overwhelmingly more probable that there will be roughly equal numbers in
the two halves. Such a state is less ordered, or more disordered, than the original state in which all the molecules werein one half. One therefore says that the entropy of the gas has gone up. Similarly, suppose one starts with two boxes,
one containing oxygen molecules and the other containing nitrogen molecules. If one joins the boxes together andremoves the intervening wall, the oxygen and the nitrogen molecules will start to mix. At a later time the most probablestate would be a fairly uniform mixture of oxygen and nitrogen molecules throughout the two boxes. This state would
be less ordered, and hence have more entropy, than the initial state of two separate boxes.
The second law of thermodynamics has a rather different status than that of other laws of science, such as Newton's
law of gravity, for example, because it does not hold always, just in the vast majority of cases. The probability of all thegas molecules in our first box
found in one half of the box at a later time is many millions of millions to one, but it can happen. However, if one has a
black hole around there seems to be a rather easier way of violating the second law: just throw some matter with a lotof entropy such as a box of gas, down the black hole. The total entropy of matter outside the black hole would go
down. One could, of course, still say that the total entropy, including the entropy inside the black hole, has not gone
down - but since there is no way to look inside the black hole, we cannot see how much entropy the matter inside it
has. It would be nice, then, if there was some feature of the black hole by which observers outside the black hole could
tell its entropy, and which would increase whenever matter carrying entropy fell into the black hole. Following thediscovery, described above, that the area of the event horizon increased whenever matter fell into a black hole, a
research student at Princeton named Jacob Bekenstein suggested that the area of the event horizon was a measure of
the entropy of the black hole. As matter carrying entropy fell into a black hole, the area of its event horizon would go
up, so that the sum of the entropy of matter outside black holes and the area of the horizons would never go down.
This suggestion seemed to prevent the second law of thermodynamics from being violated in most situations.
However, there was one fatal flaw. If a black hole has entropy, then it ought to also have a temperature. But a bodywith a particular temperature must emit radiation at a certain rate. It is a matter of common experience that if one heats
up a poker in a fire it glows red hot and emits radiation, but bodies at lower temperatures emit radiation too; one justdoes not normally notice it because the amount is fairly small. This radiation is required in order to prevent violation of
the second law. So black holes ought to emit radiation. But by their very definition, black holes are objects that are notsupposed to emit anything. It therefore seemed that the area of the event horizon of a black hole could not be regarded
as its entropy. In 1972 I wrote a paper with Brandon Carter and an American colleague, Jim Bardeen, in which we
pointed out that although there were many similarities between entropy and the area of the event horizon, there wasthis apparently fatal difficulty. I must admit that in writing this paper I was motivated partly by irritation with Bekenstein,
who, I felt, had misused my discovery of the increase of the area of the event horizon. However, it turned out in the end
that he was basically correct, though in a manner he had certainly not expected.
In September 1973, while I was visiting Moscow, I discussed black holes with two leading Soviet experts, Yakov
Zeldovich and Alexander Starobinsky. They convinced me that, according to the quantum mechanical uncertainty
principle, rotating black holes should create and emit particles. I believed their arguments on physical grounds, but I did
not like the mathematical way in which they calculated the emission. I therefore set about devising a better
mathematical treatment, which I described at an informal seminar in Oxford at the end of November 1973. At that time I
had not done the calculations to find out how much would actually be emitted. I was expecting to discover just theradiation that Zeldovich and Starobinsky had predicted from rotating black holes. However, when I did the calculation, I
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found, to my surprise and annoyance, that even non-rotating black holes should apparently create and emit particles at
a steady rate. At first I thought that this emission indicated that one of the approximations I had used was not valid. I
was afraid that if Bekenstein found out about it, he would use it as a further argument to support his ideas about theentropy of black holes, which I still did not like. However, the more I thought about it, the more it seemed that theapproximations really ought to hold. But what finally convinced me that the emission was real was that the spectrum of
the emitted particles was exactly that which would be emitted by a hot body, and that the black hole was emitting
particles at exactly the correct rate to prevent violations of the second law. Since then the calculations have been
repeated in a number of different forms by other people. They all confirm that a black hole ought to emit particles andradiation as if it were a hot body with a temperature that depends only on the black hole’s mass: the higher the mass,
the lower the temperature.
How is it possible that a black hole appears to emit particles when we know that nothing can escape from within its
event horizon? The answer, quantum theory tells us, is that the particles do not come from within the black hole, butfrom the “empty” space just outside the black hole’s event horizon! We can understand this in the following way: what
we think of as “empty” space cannot be completely empty because that would mean that all the fields, such as thegravitational and electromagnetic fields, would have to be exactly zero. However, the value of a field and its rate ofchange with time are like the position and velocity of a particle: the uncertainty principle implies that the moreaccurately one knows one of these quantities, the less accurately one can know the other. So in empty space the fieldcannot be fixed at exactly zero, because then it would have both a precise value (zero) and a precise rate of change(also zero). There must be a certain minimum amount of uncertainty, or quantum fluctuations, in the value of the field.
One can think of these fluctuations as pairs of particles of light or gravity that appear together at some time, moveapart, and then come together again and annihilate each other. These particles are virtual particles like the particles
that carry the gravitational force of the sun: unlike real particles, they cannot be observed directly with a particle
detector. However, their indirect effects, such as small changes in the energy of electron orbits in atoms, can bemeasured and agree with the theoretical predictions to a remarkable degree of accuracy. The uncertainty principle also
predicts that there will be similar virtual pairs of matter particles, such as electrons or quarks. In this case, however,one member of the pair will be a particle and the other an antiparticle (the antiparticles of light and gravity are the sameas the particles).
Because energy cannot be created out of nothing, one of the partners in a particle/antiparticle pair will have positiveenergy, and the other partner negative energy. The one with negative energy is condemned to be a short-lived virtual
particle because real particles always have positive energy in normal situations. It must therefore seek out its partner
and annihilate with it. However, a real particle close to a massive body has less energy than if it were far away,
because it would take energy to lift it far away against the gravitational attraction of the body. Normally, the energy ofthe particle is still positive, but the gravitational field inside a black hole is so strong that even a real particle can havenegative energy there. It is therefore possible, if a black hole is present, for the virtual particle with negative energy tofall into the black hole and become a real particle or antiparticle. In this case it no longer has to annihilate with its
partner. Its forsaken partner may fall into the black hole as well. Or, having positive energy, it might also escape fromthe vicinity of the black hole as a real particle or antiparticle Figure 7:4.

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Figure 7:4
To an observer at a distance, it will appear to have been emitted from the black hole. The smaller the black hole, theshorter the distance the particle with negative energy will have to go before it becomes a real particle, and thus thegreater the rate of emission, and the apparent temperature, of the black hole.
The positive energy of the outgoing radiation would be balanced by a flow of negative energy particles into the black
hole. By Einstein’s equation E = mc2 (where E is energy, m is mass, and c is the speed of light), energy is proportional
to mass. A flow of negative energy into the black hole therefore reduces its mass. As the black hole loses mass, thearea of its event horizon gets smaller, but this decrease in the entropy of the black hole is more than compensated forby the entropy of the emitted radiation, so the second law is never violated.
Moreover, the lower the mass of the black hole, the higher its temperature. So as the black hole loses mass, its
temperature and rate of emission increase, so it loses mass more quickly. What happens when the mass of the blackhole eventually becomes extremely small is not quite clear, but the most reasonable guess is that it would disappear
completely in a tremendous final burst of emission, equivalent to the explosion of millions of H-bombs.
A black hole with a mass a few times that of the sun would have a temperature of only one ten millionth of a degree
above absolute zero. This is much less than the temperature of the microwave radiation that fills the universe (about
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2.7º above absolute zero), so such black holes would emit even less than they absorb. If the universe is destined to go
on expanding forever, the temperature of the microwave radiation will eventually decrease to less than that of such a
black hole, which will then begin to lose mass. But, even then, its temperature would be so low that it would take about
a million million million million million million million million million million million years (1 with sixty-six zeros after it) toevaporate completely. This is much longer than the age of the universe, which is only about ten or twenty thousand
million years (1 or 2 with ten zeros after it). On the other hand, as mentioned in Chapter 6, there might be primordial
black holes with a very much smaller mass that were made by the collapse of irregularities in the very early stages of
the universe. Such black holes would have a much higher temperature and would be emitting radiation at a muchgreater rate. A primordial black hole with an initial mass of a thousand million tons would have a lifetime roughly equal
to the age of the universe. Primordial black holes with initial masses less than this figure would already havecompletely evaporated, but those with slightly greater masses would still be emitting radiation in the form of X rays andgamma rays. These X rays and gamma rays are like waves of light, but with a much shorter wavelength. Such holes
hardly deserve the epithet black: they really are white hot and are emitting energy at a rate of about ten thousand
megawatts.
One such black hole could run ten large power stations, if only we could harness its power. This would be rather
difficult, however: the black hole would have the mass of a mountain compressed into less than a million millionth of aninch, the size of the nucleus of an atom! If you had one of these black holes on the surface of the earth, there would be
no way to stop it from falling through the floor to the center of the earth. It would oscillate through the earth and back,until eventually it settled down at the center. So the only place to put such a black hole, in which one might use theenergy that it emitted, would be in orbit around the earth – and the only way that one could get it to orbit the earth
would be to attract it there by towing a large mass in front of it, rather like a carrot in front of a donkey. This does notsound like a very practical proposition, at least not in the immediate future.
But even if we cannot harness the emission from these primordial black holes, what are our chances of observing
them? We could look for the gamma rays that the primordial black holes emit during most of their lifetime. Although theradiation from most would be very weak because they are far away, the total from all of them might be detectable. We
do observe such a background of gamma rays: Figure 7:5 shows how the observed intensity differs at different
frequencies (the number of waves per second). However, this background could have been, and probably was,generated by processes other than primordial black holes. The dotted line in Figure 7:5 shows how the intensity should
vary with frequency for gamma rays given off by primordial black holes, if there were on average 300 per cubic
light-year. One can therefore say that the observations of the gamma ray background do not provide any positive
evidence for primordial black holes, but they do tell us that on average there cannot be more than 300 in every cubic
light-year in the universe. This limit means that primordial black holes could make up at most one millionth of thematter in the universe.

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Figure 7:5
With primordial black holes being so scarce, it might seem unlikely that there would be one near enough for us toobserve as an individual source of gamma rays. But since gravity would draw primordial black holes toward any matter,they should be much more common in and around galaxies. So although the gamma ray background tells us that there
can be no more than 300 primordial black holes per cubic light-year on average, it tells us nothing about how common
they might be in our own galaxy. If they were, say, a million times more common than this, then the nearest black hole
to us would probably be at a distance of about a thousand million kilometers, or about as far away as Pluto, the farthest
known planet. At this distance it would still be very difficult to detect the steady emission of a black hole, even if it was
ten thousand megawatts. In order to observe a primordial black hole one would have to detect several gamma ray
quanta coming from the same direction within a reasonable space of time, such as a week. Otherwise, they might
simply be part of the background. But Planck’s quantum principle tells us that each gamma ray quantum has a very
high energy, because gamma rays have a very high frequency, so it would not take many quanta to radiate even ten
thousand megawatts. And to observe these few coming from the distance of Pluto would require a larger gamma ray
detector than any that have been constructed so far. Moreover, the detector would have to be in space, because
gamma rays cannot penetrate the atmosphere.
Of course, if a black hole as close as Pluto were to reach the end of its life and blow up, it would be easy to detect the
final burst of emission. But if the black hole has been emitting for the last ten or twenty thousand million years, thechance of it reaching the end of its life within the next few years, rather than several million years in the past or future,is really rather small! So in order to have a reasonable chance of seeing an explosion before your research grant ran
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gamma rays from space have been detected by satellites originally constructed to look for violations of the Test Ban
Treaty. These seem to occur about sixteen times a month and to be roughly uniformly distributed in direction acrossthe sky. This indicates that they come from outside the Solar System since otherwise we would expect them to beconcentrated toward the plane of the orbits of the planets. The uniform distribution also indicates that the sources areeither fairly near to us in our galaxy or right outside it at cosmological distances because otherwise, again, they would
be concentrated toward the plane of the galaxy. In the latter case, the energy required to account for the bursts would
be far too high to have been produced by tiny black holes, but if the sources were close in galactic terms, it might bepossible that they were exploding black holes. I would very much like this to be the case but I have to recognize that
there are other possible explanations for the gamma ray bursts, such as colliding neutron stars. New observations in
the next few years, particularly by gravitational wave detectors like LIGO, should enable us to discover the origin of the
gamma ray bursts.
Even if the search for primordial black holes proves negative, as it seems it may, it will still give us important
information about the very early stages of the universe. If the early universe had been chaotic or irregular, or if thepressure of matter had been low, one would have expected it to produce many more primordial black holes than thelimit already set by our observations of the gamma ray background. Only if the early universe was very smooth anduniform, with a high pressure, can one explain the absence of observable numbers of primordial black holes.
The idea of radiation from black holes was the first example of a prediction that depended in an essential way on both
the great theories of this century, general relativity and quantum mechanics. It aroused a lot of opposition initiallybecause it upset the existing viewpoint: “How can a black hole emit anything?” When I first announced the results of
my calculations at a conference at the Rutherford-Appleton Laboratory near Oxford, I was greeted with generalincredulity. At the end of my talk the chairman of the session, John G. Taylor from Kings College, London, claimed it
was all nonsense. He even wrote a paper to that effect. However, in the end most people, including John Taylor, havecome to the conclusion that black holes must radiate like hot bodies if our other ideas about general relativity andquantum mechanics are correct. Thus, even though we have not yet managed to find a primordial black hole, there isfairly general agreement that if we did, it would have to be emitting a lot of gamma rays and X rays.
The existence of radiation from black holes seems to imply that gravitational collapse is not as final and irreversible as
we once thought. If an astronaut falls into a black hole, its mass will increase, but eventually the energy equivalent ofthat extra mass will be returned to the universe in the form of radiation. Thus, in a sense, the astronaut will be“recycled.” It would be a poor sort of immortality, however, because any personal concept of time for the astronaut
would almost certainly come to an end as he was torn apart inside the black hole! Even the types of particles that were
eventually emitted by the black hole would in general be different from those that made up the astronaut: the only
feature of the astronaut that would survive would be his mass or energy.
The approximations I used to derive the emission from black holes should work well when the black hole has a mass
greater than a fraction of a gram. However, they will break down at the end of the black hole’s life when its mass getsvery small. The most likely outcome seems to be that the black hole will just disappear, at least from our region of theuniverse, taking with it the astronaut and any singularity there might be inside it, if indeed there is one. This was thefirst indication that quantum mechanics might remove the singularities that were predicted by general relativity.
However, the methods that I and other people were using in 1974 were not able to answer questions such as whether
singularities would occur in quantum gravity. From 1975 onward I therefore started to develop a more powerfulapproach to quantum gravity based on Richard Feynrnan’s idea of a sum over histories. The answers that thisapproach suggests for the origin and fate of the universe and its contents, such as astronauts, will be de-scribed in thenext two chapters. We shall see that although the uncertainty principle places limitations on the accuracy of all our
predictions, it may at the same time remove the fundamental unpredictability that occurs at a space-time singularity.

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CHAPTER 8
THE ORIGIN AND FATE OF THE UNIVERSE

Einstein’s general theory of relativity, on its own, predicted that space-time began at the big bang singularity andwould come to an end either at the big crunch singularity (if the whole universe recollapsed), or at a singularity inside
a black hole (if a local region, such as a star, were to collapse). Any matter that fell into the hole would be destroyed
at the singularity, and only the gravitational effect of its mass would continue to be felt outside. On the other hand,when quantum effects were taken into account, it seemed that the mass or energy of the matter would eventually be
returned to the rest of the universe, and that the black hole, along with any singularity inside it, would evaporateaway and finally disappear. Could quantum mechanics have an equally dramatic effect on the big bang and big
crunch singularities? What really happens during the very early or late stages of the universe, when gravitational
fields are so strong that quantum effects cannot be ignored? Does the universe in fact have a beginning or an end?
And if so, what are they like?
Throughout the 1970s I had been mainly studying black holes, but in 1981 my interest in questions about the origin
and fate of the universe was reawakened when I attended a conference on cosmology organized by the Jesuits inthe Vatican. The Catholic Church had made a bad mistake with Galileo when it tried to lay down the law on aquestion of science, declaring that the sun went round the earth. Now, centuries later, it had decided to invite anumber of experts to advise it on cosmology. At the end of the conference the participants were granted an audience
with the Pope. He told us that it was all right to study the evolution of the universe after the big bang, but we should
not inquire into the big bang itself because that was the moment of Creation and therefore the work of God. I was
glad then that he did not know the subject of the talk I had just given at the conference – the possibility thatspace-time was finite but had no boundary, which means that it had no beginning, no moment of Creation. I had nodesire to share the fate of Galileo, with whom I feel a strong sense of identity, partly because of the coincidence of
having been born exactly 300 years after his death!
In order to explain the ideas that I and other people have had about how quantum mechanics may affect the origin
and fate of the universe, it is necessary first to understand the generally accepted history of the universe, according
to what is known as the “hot big bang model.” This assumes that the universe is described by a Friedmann model,
right back to the big bang. In such models one finds that as the universe expands, any matter or radiation in it getscooler. (When the universe doubles in size, its temperature falls by half.) Since temperature is simply a measure of
the average energy – or speed – of the particles, this cooling of the universe would have a major effect on the matterin it. At very high temperatures, particles would be moving around so fast that they could escape any attraction
toward each other due to nuclear or electromagnetic forces, but as they cooled off one would expect particles that
attract each other to start to clump together. Moreover, even the types of particles that exist in the universe would
depend on the temperature. At high enough temperatures, particles have so much energy that whenever they collide
many different particle/antiparticle pairs would be produced – and although some of these particles would annihilate
on hitting antiparticles, they would be produced more rap-idly than they could annihilate. At lower temperatures,
however, when colliding particles have less energy, particle/antiparticle pairs would be produced less quickly – andannihilation would become faster than production.
At the big bang itself the universe is thought to have had zero size, and so to have been infinitely hot. But as theuniverse expanded, the temperature of the radiation decreased. One second after the big bang, it would have fallen
to about ten thousand million degrees. This is about a thousand times the temperature at the center of the sun, buttemperatures as high as this are reached in H-bomb explosions. At this time the universe would have contained
mostly photons, electrons, and neutrinos (extremely light particles that are affected only by the weak force andgravity) and their antiparticles, together with some protons and neutrons. As the universe continued to expand andthe temperature to drop, the rate at which electron/antielectron pairs were being produced in collisions would havefallen below the rate at which they were being destroyed by annihilation. So most of the electrons and antielectrons
would have annihilated with each other to produce more photons, leaving only a few electrons left over. The
neutrinos and antineutrinos, however, would not have annihilated with each other, because these particles interact
with themselves and with other particles only very weakly. So they should still be around today. If we could observe
them, it would provide a good test of this picture of a very hot early stage of the universe. Unfortunately, theirenergies nowadays would be too low for us to observe them directly. However, if neutrinos are not massless, buthave a small mass of their own, as suggested by some recent experiments, we might be able to detect them
indirectly: they could be a form of “dark matter,” like that mentioned earlier, with sufficient gravitational attraction tostop the expansion of the universe and cause it to collapse again.
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About one hundred seconds after the big bang, the temperature would have fallen to one thousand million degrees,
the temperature inside the hottest stars. At this temperature protons and neutrons would no longer have sufficientenergy to escape the attraction of the strong nuclear force, and would have started to combine together to produce
the nuclei of atoms of deuterium (heavy hydrogen), which contain one proton and one neutron. The deuterium nuclei
would then have combined with more protons and neutrons to make helium nuclei, which contain two protons andtwo neutrons, and also small amounts of a couple of heavier elements, lithium and beryllium. One can calculate thatin the hot big bang model about a quarter of the protons and neutrons would have been converted into helium nuclei,
along with a small amount of heavy hydrogen and other elements. The remaining neutrons would have decayed into
protons, which are the nuclei of ordinary hydrogen atoms.
This picture of a hot early stage of the universe was first put forward by the scientist George Gamow in a famous
paper written in 1948 with a student of his, Ralph Alpher. Gamow had quite a sense of humor – he persuaded thenuclear scientist Hans Bethe to add his name to the paper to make the list of authors “Alpher, Bethe, Gamow,” likethe first three letters of the Greek alphabet, alpha, beta, gamma: particularly appropriate for a paper on the beginningof the universe! In this paper they made the remarkable prediction that radiation (in the form of photons) from thevery hot early stages of the universe should still be around today, but with its temperature reduced to only a few
degrees above absolute zero (–273ºC). It was this radiation that Penzias and Wilson found in 1965. At the time that
Alpher, Bethe, and Gamow wrote their paper, not much was known about the nuclear reactions of protons andneutrons. Predictions made for the proportions of various elements in the early universe were therefore rather
inaccurate, but these calculations have been repeated in the light of better knowledge and now agree very well withwhat we observe. It is, moreover, very difficult to explain in any other way why there should be so much helium in theuniverse. We are therefore fairly confident that we have the right picture, at least back to about one second after the
big bang.
Within only a few hours of the big bang, the production of helium and other elements would have stopped. And after
that, for the next million years or so, the universe would have just continued expanding, without anything muchhappening. Eventually, once the temperature had dropped to a few thousand degrees, and electrons and nuclei nolonger had enough energy to overcome the electromagnetic attraction between them, they would have started
combining to form atoms. The universe as a whole would have continued expanding and cooling, but in regions thatwere slightly denser than average, the expansion would have been slowed down by the extra gravitational attraction.
This would eventually stop expansion in some regions and cause them to start to recollapse. As they werecollapsing, the gravitational pull of matter outside these regions might start them rotating slightly. As the collapsingregion got smaller, it would spin faster – just as skaters spinning on ice spin faster as they draw in their arms.
Eventually, when the region got small enough, it would be spinning fast enough to balance the attraction of gravity,and in this way disklike rotating galaxies were born. Other regions, which did not happen to pick up a rotation, would
become oval-shaped objects called elliptical galaxies. In these, the region would stop collapsing because individual
parts of the galaxy would be orbiting stably round its center, but the galaxy would have no overall rotation.
As time went on, the hydrogen and helium gas in the galaxies would break up into smaller clouds that would collapse
under their own gravity. As these contracted, and the atoms within them collided with one another, the temperatureof the gas would increase, until eventually it became hot enough to start nuclear fusion reactions. These would
convert the hydrogen into more helium, and the heat given off would raise the pressure, and so stop the clouds fromcontracting any further. They would remain stable in this state for a long time as stars like our sun, burning hydrogen
into helium and radiating the resulting energy as heat and light. More massive stars would need to be hotter tobalance their stronger gravitational attraction, making the nuclear fusion reactions proceed so much more rapidly thatthey would use up their hydrogen in as little as a hundred million years. They would then contract slightly, and as
they heated up further, would start to convert helium into heavier elements like carbon or oxygen. This, however,would not release much more energy, so a crisis would occur, as was described in the chapter on black holes. What
happens next is not completely clear, but it seems likely that the central regions of the star would collapse to a very
dense state, such as a neutron star or black hole. The outer regions of the star may sometimes get blown off in atremendous explosion called a supernova, which would outshine all the other stars in its galaxy. Some of the heavier
elements produced near the end of the star’s life would be flung back into the gas in the galaxy, and would provide
some of the raw material for the next generation of stars. Our own sun contains about 2 percent of these heavier
elements, because it is a second- or third-generation star, formed some five thousand million years ago out of acloud of rotating gas containing the debris of earlier supernovas. Most of the gas in that cloud went to form the sun or
got blown away, but a small amount of the heavier elements collected together to form the bodies that now orbit thesun as planets like the earth.
The earth was initially very hot and without an atmosphere. In the course of time it cooled and acquired an
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atmosphere from the emission of gases from the rocks. This early atmosphere was not one in which we could havesurvived. It contained no oxygen, but a lot of other gases that are poisonous to us, such as hydrogen sulfide (the gasthat gives rotten eggs their smell). There are, however, other primitive forms of life that can flourish under suchconditions. It is thought that they developed in the oceans, possibly as a result of chance combinations of atoms intolarge structures, called macromolecules, which were capable of assembling other atoms in the ocean into similar
structures. They would thus have reproduced themselves and multiplied. In some cases there would be errors in the
reproduction. Mostly these errors would have been such that the new macromolecule could not reproduce itself andeventually would have been destroyed. However, a few of the errors would have produced new macromolecules thatwere even better at reproducing themselves. They would have therefore had an advantage and would have tended
to replace the original macromolecules. In this way a process of evolution was started that led to the development of
more and more complicated, self-reproducing organisms. The first primitive forms of life consumed various materials,
including hydrogen sulfide, and released oxygen. This gradually changed the atmosphere to the composition that it
has today, and allowed the development of higher forms of life such as fish, reptiles, mammals, and ultimately thehuman race.
This picture of a universe that started off very hot and cooled as it expanded is in agreement with all theobservational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered:
1. Why was the early universe so hot?
2. Why is the universe so uniform on a large scale? Why does it look the same at all points of space and in all
directions? In particular, why is the temperature of the microwave back-ground radiation so nearly the same when we
look in different directions? It is a bit like asking a number of students an exam question. If they all give exactly thesame answer, you can be pretty sure they have communicated with each other. Yet, in the model described above,
there would not have been time since the big bang for light to get from one distant region to another, even though theregions were close together in the early universe. According to the theory of relativity, if light cannot get from one
region to another, no other information can. So there would be no way in which different regions in the early universe
could have come to have the same temperature as each other, unless for some unexplained reason they happened
to start out with the same temperature.
3. Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse
from those that go on expanding forever, that even now, ten thousand million years later, it is still expanding at nearly
the critical rate? If the rate of expansion one second after the big bang had been smaller by even one part in ahundred thousand million million, the universe would have recollapsed before it ever reached its present size.
4. Despite the fact that the universe is so uniform and homogeneous on a large scale, it contains local irregularities,
such as stars and galaxies. These are thought to have developed from small differences in the density of the early
universe from one region to another. What was the origin of these density fluctuations?
The general theory of relativity, on its own, cannot explain these features or answer these questions because of its
prediction that the universe started off with infinite density at the big bang singularity. At the singularity, generalrelativity and all other physical laws would break down: one couldn’t predict what would come out of the singularity.
As explained before, this means that one might as well cut the big bang, and any events before it, out of the theory,
because they can have no effect on what we observe. Space-time would have a boundary – a beginning at the bigbang.
Science seems to have uncovered a set of laws that, within the limits set by the uncertainty principle, tell us how theuniverse will develop with time, if we know its state at any one time. These laws may have originally been decreed byGod, but it appears that he has since left the universe to evolve according to them and does not now intervene in it.
But how did he choose the initial state or configuration of the universe? What were the “boundary conditions” at the
beginning of time?
One possible answer is to say that God chose the initial configuration of the universe for reasons that we cannot
hope to understand. This would certainly have been within the power of an omnipotent being, but if he had started it
off in such an incomprehensible way, why did he choose to let it evolve according to laws that we could understand?
The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, butthat they reflect a certain underlying order, which may or may not be divinely inspired. It would be only natural tosuppose that this order should apply not only to the laws, but also to the conditions at the boundary of space-timethat specify the initial state of the universe. There may be a large number of models of the universe with different
initial conditions that all obey the laws. There ought to be some principle that picks out one initial state, and hence

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one model, to represent our universe.
One such possibility is what are called chaotic boundary conditions. These implicitly assume either that the universeis spatially infinite or that there are infinitely many universes. Under chaotic boundary conditions, the probability offinding any particular region of space in any given configuration just after the big bang is the same, in some sense,as the probability of finding it in any other configuration: the initial state of the universe is chosen purely randomly.
This would mean that the early universe would have probably been very chaotic and irregular because there aremany more chaotic and disordered configurations for the universe than there are smooth and ordered ones. (If eachconfiguration is equally probable, it is likely that the universe started out in a chaotic and disordered state, simply
because there are so many more of them.) It is difficult to see how such chaotic initial conditions could have givenrise to a universe that is so smooth and regular on a large scale as ours is today. One would also have expected thedensity fluctuations in such a model to have led to the formation of many more primordial black holes than the upper
limit that has been set by observations of the gamma ray background.
If the universe is indeed spatially infinite, or if there are infinitely many universes, there would probably be some large
regions somewhere that started out in a smooth and uniform manner. It is a bit like the well-known horde of monkeys
hammering away on typewriters – most of what they write will be garbage, but very occasionally by pure chance they
will type out one of Shakespeare’s sonnets. Similarly, in the case of the universe, could it be that we are living in aregion that just happens by chance to be smooth and uniform? At first sight this might seem very improbable,
because such smooth regions would be heavily outnumbered by chaotic and irregular regions. However, supposethat only in the smooth regions were galaxies and stars formed and were conditions right for the development ofcomplicated self-replicating organisms like ourselves who were capable of asking the question: why is the universe
so smooth.? This is an example of the application of what is known as the anthropic principle, which can beparaphrased as “We see the universe the way it is because we exist.”
There are two versions of the anthropic principle, the weak and the strong. The weak anthropic principle states thatin a universe that is large or infinite in space and/or time, the conditions necessary for the development of intelligent
life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions
should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that arenecessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty.
One example of the use of the weak anthropic principle is to “explain” why the big bang occurred about ten thousand
million years ago – it takes about that long for intelligent beings to evolve. As explained above, an early generation ofstars first had to form. These stars converted some of the original hydrogen and helium into elements like carbon andoxygen, out of which we are made. The stars then exploded as supernovas, and their debris went to form other stars
and planets, among them those of our Solar System, which is about five thousand million years old. The first one or
two thousand million years of the earth’s existence were too hot for the development of anything complicated. The
remaining three thousand million years or so have been taken up by the slow process of biological evolution, which
has led from the simplest organisms to beings who are capable of measuring time back to the big bang.
Few people would quarrel with the validity or utility of the weak anthropic principle. Some, however, go much further
and propose a strong version of the principle. According to this theory, there are either many different universes or
many different regions of a single universe, each with its own initial configuration and, perhaps, with its own set oflaws of science. In most of these universes the conditions would not be right for the development of complicatedorganisms; only in the few universes that are like ours would intelligent beings develop and ask the question, “Why is
the universe the way we see it?” The answer is then simple: if it had been different, we would not be here!
The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electriccharge of the electron and the ratio of the masses of the proton and the electron. We cannot, at the moment at least,
predict the values of these numbers from theory – we have to find them by observation. It may be that one day weshall discover a complete unified theory that predicts them all, but it is also possible that some or all of them varyfrom universe to universe or within a single universe. The remarkable fact is that the values of these numbers seemto have been very finely adjusted to make possible the development of life. For example, if the electric charge of theelectron had been only slightly different, stars either would have been unable to burn hydrogen and helium, or elsethey would not have exploded. Of course, there might be other forms of intelligent life, not dreamed of even by
writers of science fiction, that did not require the light of a star like the sun or the heavier chemical elements that are
made in stars and are flung back into space when the stars explode. Nevertheless, it seems clear that there are
relatively few ranges of values for the numbers that would allow the development of any form of intelligent life. Most
sets of values would give rise to universes that, although they might be very beautiful, would contain no one able towonder at that beauty. One can take this either as evidence of a divine purpose in Creation and the choice of the
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laws of science or as support for the strong anthropic principle.
There are a number of objections that one can raise to the strong anthropic principle as an explanation of theobserved state of the universe. First, in what sense can all these different universes be said to exist? If they arereally separate from each other, what happens in another universe can have no observable consequences in ourown universe. We should therefore use the principle of economy and cut them out of the theory. If, on the other
hand, they are just different regions of a single universe, the laws of science would have to be the same in eachregion, because otherwise one could not move continuously from one region to another. In this case the only
difference between the regions would be their initial configurations and so the strong anthropic principle would
reduce to the weak one.
A second objection to the strong anthropic principle is that it runs against the tide of the whole history of science. We
have developed from the geocentric cosmologies of Ptolemy and his forebears, through the heliocentric cosmology
of Copernicus and Galileo, to the modern picture in which the earth is a medium-sized planet orbiting around an
average star in the outer suburbs of an ordinary spiral galaxy, which is itself only one of about a million million
galaxies in the observable universe. Yet the strong anthropic principle would claim that this whole vast construction
exists simply for our sake. This is very hard to believe. Our Solar System is certainly a prerequisite for our existence,
hand one might extend this to the whole of our galaxy to allow for an earlier generation of stars that created theheavier elements. But there does not seem to be any need for all those other galaxies, nor for the universe to be so
uniform and similar in every direction on the large scale.
One would feel happier about the anthropic principle, at least in its weak version, if one could show that quite anumber of different initial configurations for the universe would have evolved to produce a universe like the one weobserve. If this is the case, a universe that developed from some sort of random initial conditions should contain anumber of regions that are smooth and uniform and are suitable for the evolution of intelligent life. On the other hand,
if the initial state of the universe had to be chosen extremely carefully to lead to something like what we see aroundus, the universe would be unlikely to contain any region in which life would appear. In the hot big bang model
described above, there was not enough time in the early universe for heat to have flowed from one region to another.
This means that the initial state of the universe would have to have had exactly the same temperature everywhere inorder to account for the fact that the microwave back-ground has the same temperature in every direction we look.
The initial rate of expansion also would have had to be chosen very precisely for the rate of expansion still to be so
close to the critical rate needed to avoid recollapse. This means that the initial state of the universe must have beenvery carefully chosen indeed if the hot big bang model was correct right back to the beginning of time. It would bevery difficult to explain why the universe should have begun in just this way, except as the act of a God who intended
to create beings like us.
In an attempt to find a model of the universe in which many different initial configurations could have evolved tosomething like the present universe, a scientist at the Massachusetts Institute of Technology, Alan Guth, suggested
that the early universe might have gone through a period of very rapid expansion. This expansion is said to be“inflationary,” meaning that the universe at one time expanded at an increasing rate rather than the decreasing ratethat it does today. According to Guth, the radius of the universe increased by a million million million million million (1with thirty zeros after it) times in only a tiny fraction of a second.
Guth suggested that the universe started out from the big bang in a very hot, but rather chaotic, state. These hightemperatures would have meant that the particles in the universe would be moving very fast and would have highenergies. As we discussed earlier, one would expect that at such high temperatures the strong and weak nuclear
forces and the electromagnetic force would all be unified into a single force. As the universe expanded, it would cool,and particle energies would go down. Eventually there would be what is called a phase transition and the symmetry
between the forces would be broken: the strong force would become different from the weak and electromagnetic
forces. One common example of a phase transition is the freezing of water when you cool it down. Liquid water issymmetrical, the same at every point and in every direction. However, when ice crystals form, they will have definite
positions and will be lined up in some direction. This breaks water’s symmetry.
In the case of water, if one is careful, one can “supercool” it: that is, one can reduce the temperature below thefreezing point (OºC) without ice forming. Guth suggested that the universe might behave in a similar way: thetemperature might drop below the critical value without the symmetry between the forces being broken. If thishappened, the universe would be in an unstable state, with more energy than if the symmetry had been broken. This
special extra energy can be shown to have an antigravitational effect: it would have acted just like the cosmologicalconstant that Einstein introduced into general relativity when he was trying to construct a static model of theuniverse. Since the universe would already be expanding just as in the hot big bang model, the repulsive effect of
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this cosmological constant would therefore have made the universe expand at an ever-increasing rate. Even inregions where there were more matter particles than average, the gravitational attraction of the matter would havebeen outweighed by the repulsion of the effective cosmological constant. Thus these regions would also expand inan accelerating inflationary manner. As they expanded and the matter particles got farther apart, one would be left
with an expanding universe that contained hardly any particles and was still in the supercooled state. Any
irregularities in the universe would simply have been smoothed out by the expansion, as the wrinkles in a balloon aresmoothed away when you blow it up. Thus the present smooth and uniform state of the universe could have evolved
from many different non-uniform initial states.
In such a universe, in which the expansion was accelerated by a cosmological constant rather than slowed down by
the gravitational attraction of matter, there would be enough time for light to travel from one region to another in theearly universe. This could provide a solution to the problem, raised earlier, of why different regions in the early
universe have the same properties. Moreover, the rate of expansion of the universe would automatically become
very close to the critical rate determined by the energy density of the universe. This could then explain why the rateof expansion is still so close to the critical rate, without having to assume that the initial rate of expansion of theuniverse was very carefully chosen.
The idea of inflation could also explain why there is so much matter in the universe. There are something like ten
million million million million million million million million million million million million million million (1 with eighty
zeros after it) particles in the region of the universe that we can observe. Where did they all come from? The answer
is that, in quantum theory, particles can be created out of energy in the form of particle/antiparticle pairs. But that justraises the question of where the energy came from. The answer is that the total energy of the universe is exactly
zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity.
Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart,
because you have to expend energy to separate them against the gravitational force that is pulling them together.
Thus, in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform
in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by thematter. So the total energy of the universe is zero.
Now twice zero is also zero. Thus the universe can double the amount of positive matter energy and also double thenegative gravitational energy without violation of the conservation of energy. This does not happen in the normal
expansion of the universe in which the matter energy density goes down as the universe gets bigger. It does happen,
however, in the inflationary expansion because the energy density of the supercooled state remains constant while
the universe expands: when the universe doubles in size, the positive matter energy and the negative gravitationalenergy both double, so the total energy remains zero. During the inflationary phase, the universe increases its sizeby a very large amount. Thus the total amount of energy available to make particles becomes very large. As Guthhas remarked, “It is said that there’s no such thing as a free lunch. But the universe is the ultimate free lunch.”
The universe is not expanding in an inflationary way today. Thus there has to be some mechanism that would
eliminate the very large effective cosmological constant and so change the rate of expansion from an accelerated
one to one that is slowed down by gravity, as we have today. In the inflationary expansion one might expect thateventually the symmetry between the forces would be broken, just as super-cooled water always freezes in the end.
The extra energy of the unbroken symmetry state would then be released and would reheat the universe to atemperature just below the critical temperature for symmetry between the forces. The universe would then go on to
expand and cool just like the hot big bang model, but there would now be an explanation of why the universe was
expanding at exactly the critical rate and why different regions had the same temperature.
In Guth’s original proposal the phase transition was supposed to occur suddenly, rather like the appearance of ice
crystals in very cold water. The idea was that “bubbles” of the new phase of broken symmetry would have formed in
the old phase, like bubbles of steam surrounded by boiling water. The bubbles were supposed to expand and meetup with each other until the whole universe was in the new phase. The trouble was, as I and several other people
pointed out, that the universe was expanding so fast that even if the bubbles grew at the speed of light, they would
be moving away from each other and so could not join up. The universe would be left in a very non-uniform state,with some regions still having symmetry between the different forces. Such a model of the universe would notcorrespond to what we see.
In October 1981, I went to Moscow for a conference on quantum gravity. After the conference I gave a seminar on
the inflationary model and its problems at the Sternberg Astronomical Institute. Before this, I had got someone else
to give my lectures for me, because most people could not understand my voice. But there was not time to prepare
this seminar, so I gave it myself, with one of my graduate students repeating my words. It worked well, and gave me
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much more contact with my audience. In the audience was a young Russian, Andrei Linde, from the Lebedev
Institute in Moscow. He said that the difficulty with the bubbles not joining up could be avoided if the bubbles were so
big that our region of the universe is all contained inside a single bubble. In order for this to work, the change fromsymmetry to broken symmetry must have taken place very slowly inside the bubble, but this is quite possible
according to grand unified theories. Linde’s idea of a slow breaking of symmetry was very good, but I later realizedthat his bubbles would have to have been bigger than the size of the universe at the time! I showed that instead thesymmetry would have broken everywhere at the same time, rather than just inside bubbles. This would lead to a
uniform universe, as we observe. I was very excited by this idea and discussed it with one of my students, Ian Moss.
As a friend of Linde’s, I was rather embarrassed, however, when I was later sent his paper by a scientific journal andasked whether it was suitable for publication. I replied that there was this flaw about the bubbles being bigger thanthe universe, but that the basic idea of a slow breaking of symmetry was very good. I recommended that the paper ¿published as it was because it would take Linde several months to correct it, since anything he sent to the Westwould have to be passed by Soviet censorship, which was neither very skillful nor very quick with scientific papers.
Instead, I wrote a short paper with Ian Moss in the same journal in which we pointed out this problem with the bubble
and showed how it could be resolved.
The day after I got back from Moscow I set out for Philadelphia, where I was due to receive a medal from theFranklin Institute. My secretary, Judy Fella, had used her not inconsiderable charm to persuade British Airways togive herself and me free seats on a Concorde as a publicity venture. However, I .was held up on my way to the
airport by heavy rain and I missed the plane. Nevertheless, I got to Philadelphia in the end and received my medal. I
was then asked to give a seminar on the inflationary universe at Drexel University in Philadelphia. I gave the same
seminar about the problems of the inflationary universe, just as in Moscow.
A very similar idea to Linde’s was put forth independently a few months later by Paul Steinhardt and Andreas
Albrecht of the University of Pennsylvania. They are now given joint credit with Linde for what is called “the new
inflationary model,” based on the idea of a slow breaking of symmetry. (The old inflationary model was Guth’s
original suggestion of fast symmetry breaking with the formation of bubbles.)The new inflationary model was a good attempt to explain why the universe is the way it is. However, I and several
other people showed that, at least in its original form, it predicted much greater variations in the temperature of the
microwave background radiation than are observed. Later work has also cast doubt on whether there could be aphase transition in the very early universe of the kind required. In my personal opinion, the new inflationary model is
now dead as a scientific theory, although a lot of people do not seem to have heard of its demise and are still writing
papers as if it were viable. A better model, called the chaotic inflationary model, was put forward by Linde in 1983. In
this there is no phase transition or supercooling. Instead, there is a spin 0 field, which, because of quantum
fluctuations, would have large values in some regions of the early universe. The energy of the field in those regions
would behave like a cosmological constant. It would have a repulsive gravitational effect, and thus make thoseregions expand in an inflationary manner. As they expanded, the energy of the field in them would slowly decrease
until the inflationary expansion changed to an expansion like that in the hot big bang model. One of these regions
would become what we now see as the observable universe. This model has all the advantages of the earlier
inflationary models, but it does not depend on a dubious phase transition, and it can moreover give a reasonable sizefor the fluctuations in the temperature of the microwave background that agrees with observation.
This work on inflationary models showed that the present state of the universe could have arisen from quite a large
number of different initial configurations. This is important, because it shows that the initial state of the part of theuniverse that we inhabit did not have to be chosen with great care. So we may, if we wish, use the weak anthropic
principle to explain why the universe looks the way it does now. It cannot be the case, however, that every initialconfiguration would have led to a universe like the one we observe. One can show this by considering a very
different state for the universe at the present time, say, a very lumpy and irregular one. One could use the laws ofscience to evolve the universe back in time to determine its configuration at earlier times. According to the singularity
theorems of classical general relativity, there would still have been a big bang singularity. If you evolve such auniverse forward in time according to the laws of science, you will end up with the lumpy and irregular state you
started with. Thus there must have been initial configurations that would not have given rise to a universe like theone we see today. So even the inflationary model does not tell us why the initial configuration was not such as toproduce something very different from what we observe. Must we turn to the anthropic principle for an explanation?
Was it all just a lucky chance? That would seem a counsel of despair, a negation of all our hopes of understanding
the underlying order of the universe.
In order to predict how the universe should have started off, one needs laws that hold at the beginning of time. If theclassical theory of general relativity was correct, the singularity theorems that Roger Penrose and I proved show that
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the beginning of time would have been a point of infinite density and infinite curvature of space-time. All the known
laws of science would break down at such a point. One might suppose that there were new laws that held at
singularities, but it would be very difficult even to formulate such laws at such badly behaved points, and we would
have no guide from observations as to what those laws might be. However, what the singularity theorems reallyindicate is that the gravitational field becomes so strong that quantum gravitational effects become important:classical theory is no longer a good description of the universe. So one has to use a quantum theory of gravity to
discuss the very early stages of the universe. As we shall see, it is possible in the quantum theory for the ordinary
laws of science to hold everywhere, including at the beginning of time: it is not necessary to postulate new laws for
singularities, because there need not be any singularities in the quantum theory.
We don’t yet have a complete and consistent theory that combines quantum mechanics and gravity. However, weare fairly certain of some features that such a unified theory should have. One is that it should incorporate
Feynman’s proposal to formulate quantum theory in terms of a sum over histories. In this approach, a particle does
not have just a single history, as it would in a classical theory. Instead, it is supposed to follow every possible path inspace-time, and with each of these histories there are associated a couple of numbers, one represent-ing the size ofa wave and the other representing its position in the cycle (its phase). The probability that the particle, say, passes
through some particular point is found by adding up the waves associated with every possible history that passes
through that point. When one actually tries to perform these sums, however, one runs into severe technicalproblems. The only way around these is the following peculiar prescription: one must add up the waves for particle
histories that are not in the “real” time that you and I experience but take place in what is called imaginary time.
Imaginary time may sound like science fiction but it is in fact a well-defined mathematical concept. If we take any
ordinary (or “real”) number and multiply it by itself, the result is a positive number. (For example, 2 times 2 is 4, but
so is – 2 times – 2.) There are, however, special numbers (called imaginary numbers) that give negative numbers
when multiplied by themselves. (The one called i, when multiplied by itself, gives – 1, 2i multiplied by itself gives – 4,
and so on.)One can picture real and imaginary numbers in the following way: The real numbers can be represented by a line
going from left to right, with zero in the middle, negative numbers like – 1, – 2, etc. on the left, and positive numbers,
1, 2, etc. on the right. Then imaginary numbers are represented by a line going up and down the page, with i, 2i, etc.
above the middle, and – i, – 2i, etc. below. Thus imaginary numbers are in a sense numbers at right angles toordinary real numbers.
To avoid the technical difficulties with Feynman’s sum over histories, one must use imaginary time. That is to say, forthe purposes of the calculation one must measure time using imaginary numbers, rather than real ones. This has aninteresting effect on space-time: the distinction between time and space disappears completely. A space-time inwhich events have imaginary values of the time coordinate is said to be Euclidean, after the ancient Greek Euclid,
who founded the study of the geometry of two-dimensional surfaces. What we now call Euclidean space-time is very
similar except that it has four dimensions instead of two. In Euclidean space-time there is no difference between thetime direction and directions in space. On the other hand, in real space-time, in which events are labeled by ordinary,real values of the time coordinate, it is easy to tell the difference – the time direction at all points lies within the light
cone, and space directions lie outside. In any case, as far as everyday quantum mechanics is concerned, we may
regard our use of imaginary time and Euclidean space-time as merely a mathematical device (or trick) to calculate
answers about real space-time.
A second feature that we believe must be part of any ultimate theory is Einstein’s idea that the gravitational field isrepresented by curved space-time: particles try to follow the nearest thing to a straight path in a curved space, but
because space-time is not flat their paths appear to be bent, as if by a gravitational field. When we apply Feynman’ssum over histories to Einstein’s view of gravity, the analogue of the history of a particle is now a complete curved
space-time that represents the history of the whole universe. To avoid the technical difficulties in actually performing
the sum over histories, these curved space-times must be taken to be Euclidean. That is, time is imaginary and is
indistinguishable from directions in space. To calculate the probability of finding a real space-time with some certain
property, such as looking the same at every point and in every direction, one adds up the waves associated with all
the histories that have that property.
In the classical theory of general relativity, there are many different possible curved space-times, each corresponding
to a different initial state of the universe. If we knew the initial state of our universe, we would know its entire history.
Similarly, in the quantum theory of gravity, there are many different possible quantum states for the universe. Again,
if we knew how the Euclidean curved space-times in the sum over histories behaved at early times, we would know
the quantum state of the universe.

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In the classical theory of gravity, which is based on real space-time, there are only two possible ways the universe
can behave: either it has existed for an infinite time, or else it had a beginning at a singularity at some finite time in
the past. In the quantum theory of gravity, on the other hand, a third possibility arises. Because one is using
Euclidean space-times, in which the time direction is on the same footing as directions in space, it is possible forspace-time to be finite in extent and yet to have no singularities that formed a boundary or edge. Space-time would
be like the surface of the earth, only with two more dimensions. The surface of the earth is finite in extent but it
doesn’t have a boundary or edge: if you sail off into the sunset, you don’t fall off the edge or run into a singularity. (Iknow, because I have been round the world!)
If Euclidean space-time stretches back to infinite imaginary time, or else starts at a singularity in imaginary time, wehave the same problem as in the classical theory of specifying the initial state of the universe: God may know how
the universe began, but we cannot give any particular reason for thinking it began one way rather than another. On
the other hand, the quantum theory of gravity has opened up a new possibility, in which there would be no boundary
to space-time and so there would be no need to specify the behavior at the boundary. There would be no
singularities at which the laws of science broke down, and no edge of space-time at which one would have to appeal
to God or some new law to set the boundary conditions for space-time. One could say: “The boundary condition of
the universe is that it has no boundary.” The universe would be completely self-contained and not affected byanything outside itself. It would neither be created nor destroyed, It would just BE.
It was at the conference in the Vatican mentioned earlier that I first put forward the suggestion that maybe time andspace together formed a surface that was finite in size but did not have any boundary or edge. My paper was rather
mathematical, however, so its implications for the role of God in the creation of the universe were not generally
recognized at the time (just as well for me). At the time of the Vatican conference, I did not know how to use the “noboundary” idea to make predictions about the universe. However, I spent the following sum-mer at the University ofCalifornia, Santa Barbara. There a friend and colleague of mine, Jim Hartle, worked out with me what conditions theuniverse must satisfy if space-time had no boundary. When I returned to Cambridge, I continued this work with two of
my research students, Julian Luttrel and Jonathan Halliwell.
I’d like to emphasize that this idea that time and space should be finite “without boundary” is just a proposal: it cannot
be deduced from some other principle. Like any other scientific theory, it may initially be put forward for aesthetic or
metaphysical reasons, but the real test is whether it makes predictions that agree with observation. This, how-ever, isdifficult to determine in the case of quantum gravity, for two reasons. First, as will be explained in Chapter 11, we are
not yet sure exactly which theory successfully combines general relativity and quantum mechanics, though we know
quite a lot about the form such a theory must have. Second, any model that described the whole universe in detailwould be much too complicated mathematically for us to be able to calculate exact predictions. One therefore has tomake simplifying assumptions and approximations – and even then, the problem of extracting predictions remains aformidable one.
Each history in the sum over histories will describe not only the space-time but everything in it as well, including any
complicated organisms like human beings who can observe the history of the universe. This may provide anotherjustification for the anthropic principle, for if all the histories are possible, then so long as we exist in one of thehistories, we may use the anthropic principle to explain why the universe is found to be the way it is. Exactly whatmeaning can be attached to the other histories, in which we do not exist, is not clear. This view of a quantum theoryof gravity would be much more satisfactory, however, if one could show that, using the sum over histories, ouruniverse is not just one of the possible histories but one of the most probable ones. To do this, we must perform thesum over histories for all possible Euclidean space-times that have no boundary.
Under the “no boundary” proposal one learns that the chance of the universe being found to be following most of thepossible histories is negligible, but there is a particular family of histories that are much more probable than theothers. These histories may be pictured as being like the surface of the earth, with the distance from the North Pole
representing imaginary time and the size of a circle of constant distance from the North Pole representing the spatial
size of the universe. The universe starts at the North Pole as a single point. As one moves south, the circles oflatitude at constant distance from the North Pole get bigger, corresponding to the universe expanding with imaginary
time Figure 8:1. The universe would reach a maximum size at the equator and would contract with increasing
imaginary time to a single point at the South Pole. Ever though the universe would have zero size at the North andSouth Poles, these points would not be singularities, any more than the North aid South Poles on the earth are
singular. The laws of science will hold at them, just as they do at the North and South Poles on the earth.

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Figure 8:1
The history of the universe in real time, however, would look very different. At about ten or twenty thousand million
years ago, it would have a minimum size, which was equal to the maximum radius of the history in imaginary time. At
later real times, the universe would expand like the chaotic inflationary model proposed by Linde (but one would not
now have to assume that the universe was created somehow in the right sort of state). The universe would expand toa very large size Figure 8:1 and eventually it would collapse again into what looks like a singularity in real time. Thus,
in a sense, we are still all doomed, even if we keep away from black holes. Only if we could picture the universe interms of imaginary time would there be no singularities.
If the universe really is in such a quantum state, there would be no singularities in the history of the universe inimaginary time. It might seem therefore that my more recent work had completely undone the results of my earlier
work on singularities. But, as indicated above, the real importance of the singularity theorems was that they showedthat the gravitational field must become so strong that quantum gravitational effects could not be ignored. This in turnled to the idea that the universe could be finite in imaginary time but without boundaries or singularities. When one
goes back to the real time in which we live, however, there will still appear to be singularities. The poor astronaut
who falls into a black hole will still come to a sticky end; only if he lived in imaginary time would he encounter nosingularities.
This might suggest that the so-called imaginary time is really the real time, and that what we call real time is just a
figment of our imaginations. In real time, the universe has a beginning and an end at singularities that form aboundary to space-time and at which the laws of science break down. But in imaginary time, there are no
singularities or boundaries. So maybe what we call imaginary time is really more basic, and what we call real is justan idea that we invent to help us describe what we think the universe is like. But according to the approach I
described in Chapter 1, a scientific theory is just a mathematical model we make to describe our observations: itexists only in our minds. So it is meaningless to ask: which is real, “real” or “imaginary” time? It is simply a matter ofwhich is the more useful description.
One can also use the sum over histories, along with the no boundary proposal, to find which properties of theuniverse are likely to occur together. For example, one can calculate the probability that the universe is expanding at
nearly the same rate in all different directions at a time when the density of the universe has its present value. In thesimplified models that have been examined so far, this probability turns out to be high; that is, the proposed noboundary condition leads to the prediction that it is extremely probable that the present rate of expansion of theuniverse is almost the same in each direction. This is consistent with the observations of the microwave backgroundradiation, which show that it has almost exactly the same intensity in any direction. If the universe were expanding
faster in some directions than in others, the intensity of the radiation in those directions would be reduced by an

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additional red shift.
Further predictions of the no boundary condition are currently being worked out. A particularly interesting problem isthe size of the small departures from uniform density in the early universe that caused the formation first of thegalaxies, then of stars, and finally of us. The uncertainty principle implies that the early universe cannot have been
completely uniform because there must have been some uncertainties or fluctuations in the positions and velocitiesof the particles. Using the no boundary condition, we find that the universe must in fact have started off with just theminimum possible non-uniformity allowed by the uncertainty principle. The universe would have then undergone a
period of rapid expansion, as in the inflationary models. During this period, the initial non-uniformities would havebeen amplified until they were big enough to explain the origin of the structures we observe around us. In 1992 theCosmic Background Explorer satellite (COBE) first detected very slight variations in the intensity of the microwave
background with direction. The way these non-uniformities depend on direction seems to agree with the predictionsof the inflationary model and the no boundary proposal. Thus the no boundary proposal is a good scientific theory inthe sense of Karl Popper: it could have been falsified by observations but instead its predictions have been
confirmed. In an expanding universe in which the density of matter varied slightly from place to place, gravity would
have caused the denser regions to slow down their expansion and start contracting. This would lead to the formationof galaxies, stars, and eventually even insignificant creatures like ourselves. Thus all the complicated structures that
we see in the universe might be explained by the no boundary condition for the universe together with the uncertainty
principle of quantum mechanics.
The idea that space and time may form a closed surface without boundary also has profound implications for the roleof God in the affairs of the universe. With the success of scientific theories in describing events, most people havecome to believe that God allows the universe to evolve according to a set of laws and does not intervene in theuniverse to break these laws. However, the laws do not tell us what the universe should have looked like when it
started – it would still be up to God to wind up the clockwork and choose how to start it off. So long as the universe
had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having noboundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator?

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A Brief History of Time - Stephen Hawking... Chapter 9

CHAPTER 9
THE ARROW OF TIME

In previous chapters we have seen how our views of the nature of time have changed over the years. Up to thebeginning of this century people believed in an absolute time. That is, each event could be labeled by a number
called “time” in a unique way, and all good clocks would agree on the time interval between two events.
However, the discovery that the speed of light appeared the same to every observer, no matter how he wasmoving, led to the theory of relativity – and in that one had to abandon the idea that there was a uniqueabsolute time. Instead, each observer would have his own measure of time as recorded by a clock that he
carried: clocks carried by different observers would not necessarily agree. Thus time became a more personal
concept, relative to the observer who measured it.
When one tried to unify gravity with quantum mechanics, one had to introduce the idea of “imaginary” time.
Imaginary time is indistinguishable from directions in space. If one can go north, one can turn around and headsouth; equally, if one can go forward in imaginary time, one ought to be able to turn round and go backward.
This means that there can be no important difference between the forward and backward directions ofimaginary time. On the other hand, when one looks at “real” time, there’s a very big difference between theforward and backward directions, as we all know. Where does this difference between the past and the futurecome from? Why do we remember the past but not the future?
The laws of science do not distinguish between the past and the future. More precisely, as explained earlier,
the laws of science are unchanged under the combination of operations (or symmetries) known as C, P, and T.
(C means changing particles for antiparticles. P means taking the mirror image, so left and right areinterchanged. And T means reversing the direction of motion of all particles: in effect, running the motion
backward.) The laws of science that govern the behavior of matter under all normal situations are unchanged
under the combination of the two operations C and P on their own. In other words, life would be just the samefor the inhabitants of another planet who were both mirror images of us and who were made of antimatter,rather than matter.
If the laws of science are unchanged by the combination of operations C and P, and also by the combination C,
P, and T, they must also be unchanged under the operation T alone. Yet there is a big difference between theforward and backward directions of real time in ordinary life. Imagine a cup of water falling off a table andbreaking into pieces on the floor. If you take a film of this, you can easily tell whether it is being run forward or
backward. If you run it backward you will see the pieces suddenly gather themselves together off the floor andjump back to form a whole cup on the table. You can tell that the film is being run backward because this kindof behavior is never observed in ordinary life. If it were, crockery manufacturers would go out of business.
The explanation that is usually given as to why we don’t see broken cups gathering themselves together off thefloor and jumping back onto the table is that it is forbidden by the second law of thermodynamics. This says thatin any closed system disorder, or entropy, always increases with time. In other words, it is a form of Murphy’slaw: things always tend to go wrong! An intact cup on the table is a state of high order, but a broken cup on thefloor is a disordered state. One can go readily from the cup on the table in the past to the broken cup on thefloor in the future, but not the other way round.
The increase of disorder or entropy with time is one example of what is called an arrow of time, something that
distinguishes the past from the future, giving a direction to time. There are at least three different arrows oftime. First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy
increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, thedirection in which we remember the past but not the future. Finally, there is the cosmological arrow of time. Thisis the direction of time in which the universe is expanding rather than contracting.
In this chapter I shall argue that the no boundary condition for the universe, together with the weak anthropic
principle, can explain why all three arrows point in the same direction – and moreover, why a well-defined arrowof time should exist at all. I shall argue that the psychological arrow is determined by the thermodynamic arrow,

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and that these two arrows necessarily always point in the same direction. If one assumes the no boundary
condition for the universe, we shall see that there must be well-defined thermodynamic and cosmologicalarrows of time, but they will not point in the same direction for the whole history of the universe. However, I
shall argue that it is only when they do point in the same direction that conditions are suitable for thedevelopment of intelligent beings who can ask the question: why does disorder increase in the same directionof time as that in which the universe expands?
I shall discuss first the thermodynamic arrow of time. The second law of thermodynamics results from the factthat there are always many more disordered states than there are ordered ones. For example, consider thepieces of a jigsaw in a box. There is one, and. only one, arrangement in which the pieces make a complete
picture. On the other hand, there are a very large number of arrangements in which the pieces are disordered
and don’t make a picture.
Suppose a system starts out in one of the small number of ordered states. As time goes by, the system will
evolve according to the laws of science and its state will change. At a later time, it is more probable that thesystem will be in a disordered state than in an ordered one because there are more disordered states. Thus
disorder will tend to increase with time if the system obeys an initial condition of high order.
Suppose the pieces of the jigsaw start off in a box in the ordered arrangement in which they form a picture. If
you shake the box, the pieces will take up another arrangement. This will probably be a disorderedarrangement in which the pieces don’t form a proper picture, simply because there are so many moredisordered arrangements. Some groups of pieces may still form parts of the picture, but the more you shakethe box, the more likely it is that these groups will get broken up and the pieces will be in a completely jumbled
state in which they don’t form any sort of picture. So the disorder of the pieces will probably increase with time if
the pieces obey the initial condition that they start off in a condition of high order.
Suppose, however, that God decided that the universe should finish up in a state of high order but that it didn’t
matter what state it started in. At early times the universe would probably be in a disordered state. This would
mean that disorder would decrease with time. You would see broken cups gathering themselves together andjumping back onto the table. However, any human beings who were observing the cups would be living in auniverse in which disorder decreased with time. I shall argue that such beings would have a psychologicalarrow of time that was backward. That is, they would remember events in the future, and not remember eventsin their past. When the cup was broken, they would remember it being on the table, but when it was on thetable, they would not remember it being on the floor.
It is rather difficult to talk about human memory because we don’t know how the brain works in detail. We do,
however, know all about how computer memories work. I shall therefore discuss the psychological arrow oftime for computers. I think it is reasonable to assume that the arrow for computers is the same as that forhumans. If it were not, one could make a killing on the stock exchange by having a computer that would
remember tomorrow’s prices! A computer memory is basically a device containing elements that can exist ineither of two states. A simple example is an abacus. In its simplest form, this consists of a number of wires; oneach wire there are a number of beads that can be put in one of two positions. Before an item is recorded in acomputer’s memory, the memory is in a disordered state, with equal probabilities for the two possible states.
(The abacus beads are scattered randomly on the wires of the abacus.) After the memory interacts with thesystem to be remembered, it will definitely be in one state or the other, according to the state of the system.
(Each abacus bead will be at either the left or the right of the abacus wire.) So the memory has passed from adisordered state to an ordered one. However, in order to make sure that the memory is in the right state, it isnecessary to use a certain amount of energy (to move the bead or to power the computer, for example). This
energy is dissipated as heat, and increases the amount of disorder in the universe. One can show that thisincrease in disorder is always greater than the increase in the order of the memory itself. Thus the heat
expelled by the computer’s cooling fan means that when a computer records an item in memory, the total
amount of disorder in the universe still goes up. The direction of time in which a computer remembers the pastis the same as that in which disorder increases.
Our subjective sense of the direction of time, the psychological arrow of time, is therefore determined within ourbrain by the thermodynamic arrow of time. Just like a computer, we must remember things in the order in which
entropy increases. This makes the second law of thermodynamics almost trivial. Disorder increases with time

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because we measure time in the direction in which disorder increases You can’t have a safer bet than that!
But why should the thermodynamic arrow of time exist at all? Or, in other words, why should the universe be ina state of high order at one end of time, the end that we call the past? Why is it not in a state of complete
disorder at all times? After all, this might seem more probable. And why is the direction of time in which
disorder increases the same as that in which the universe expands?
In the classical theory of general relativity one cannot predict how the universe would have begun because all
the known laws of science would have broken down at the big bang singularity. The universe could havestarted out in a very smooth and ordered state. This would have led to well-defined thermodynamic andcosmological arrows of time, as we observe. But it could equally well have started out in a very lumpy anddisordered state. In that case, the universe would already be in a state of complete disorder, so disorder could
not increase with time. It would either stay constant, in which case there would be no well-defined
thermodynamic arrow of time, or it would decrease, in which case the thermodynamic arrow of time would pointin the opposite direction to the cosmological arrow. Neither of these possibilities agrees with what we observe.
However, as we have seen, classical general relativity predicts its own downfall. When the curvature ofspace-time becomes large, quantum gravitational effects will become important and the classical theory willcease to be a good description of the universe. One has to use a quantum theory of gravity to understand how
the universe began.
In a quantum theory of gravity, as we saw in the last chapter, in order to specify the state of the universe one
would still have to say how the possible histories of the universe would behave at the boundary of space-time in
the past. One could avoid this difficulty of having to describe what we do not and cannot know only if thehistories satisfy the no boundary condition: they are finite in extent but have no boundaries, edges, or
singularities. In that case, the beginning of time would be a regular, smooth point of space-time and theuniverse would have begun its expansion in a very smooth and ordered state. It could not have been
completely uniform, because that would violate the uncertainty principle of quantum theory. There had to besmall fluctuations in the density and velocities of particles. The no boundary condition, however, implied thatthese fluctuations were as small as they could be, consistent with the uncertainty principle.
The universe would have started off with a period of exponential or “inflationary” expansion in which it would
have increased its size by a very large factor. During this expansion, the density fluctuations would haveremained small at first, but later would have started to grow. Regions in which the density was slightly higher
than average would have had their expansion slowed down by the gravitational attraction of the extra mass.
Eventually, such regions would stop expanding and collapse to form galaxies, stars, and beings like us. The
universe would have started in a smooth and ordered state, and would become lumpy and disordered as time
went on. This would explain the existence of the thermodynamic arrow of time.
But what would happen if and when the universe stopped expanding and began to contract? Would thethermodynamic arrow reverse and disorder begin to decrease with time? This would lead to all sorts ofscience-fiction-like possibilities for people who survived from the expanding to the contracting phase. Would
they see broken cups gathering themselves together off the floor and jumping back onto the table? Would they
be able to remember tomorrow’s prices and make a fortune on the stock market? It might seem a bit academic
to worry about what will happen when the universe collapses again, as it will not start to contract for at least
another ten thousand million years. But there is a quicker way to find out what will happen: jump into a black
hole. The collapse of a star to form a black hole is rather like the later stages of the collapse of the whole
universe. So if disorder were to decrease in the contracting phase of the universe, one might also expect it todecrease inside a black hole. So perhaps an astronaut who fell into a black hole would be able to make moneyat roulette by remembering where the ball went before he placed his bet. (Unfortunately, however, he would nothave long to play before he was turned to spaghetti. Nor would he be able to let us know about the reversal of
the thermodynamic arrow, or even bank his winnings, because he would be trapped behind the event horizonof the black hole.)
At first, I believed that disorder would decrease when the universe recollapsed. This was because I thought thatthe universe had to return to a smooth and ordered state when it became small again. This would mean thatthe contracting phase would be like the time reverse of the expanding phase. People in the contracting phasewould live their lives backward: they would die before they were born and get younger as the universe

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contracted.
This idea is attractive because it would mean a nice symmetry between the expanding and contracting phases.
However, one cannot adopt it on its own, independent of other ideas about the universe. The question is: is it
implied by the no boundary condition, or is it inconsistent with that condition? As I said, I thought at first that the
no boundary condition did indeed imply that disorder would decrease in the contracting phase. I was misledpartly by the analogy with the surface of the earth. If one took the beginning of the universe to correspond to
the North Pole, then the end of the universe should be similar to the beginning, just as the South Pole is similar
to the North. However, the North and South Poles correspond to the beginning and end of the universe inimaginary time. The beginning and end in real time can be very different from each other. I was also misled bywork I had done on a simple model of the universe in which the collapsing phase looked like the time reverse of
the expanding phase. However, a colleague of mine, Don Page, of Penn State University, pointed out that the
no boundary condition did not require the contracting phase necessarily to be the time reverse of the expanding
phase. Further, one of my students, Raymond Laflamme, found that in a slightly more complicated model, thecollapse of the universe was very different from the expansion. I realized that I had made a mistake: the noboundary condition implied that disorder would in fact continue to increase during the contraction. The
thermodynamic and psychological arrows of time would not reverse when the universe begins to recontract, or
inside black holes.
What should you do when you find you have made a mistake like that? Some people never admit that they are
wrong and continue to find new, and often mutually inconsistent, arguments to support their case – as
Eddington did in opposing black hole theory. Others claim to have never really supported the incorrect view inthe first place or, if they did, it was only to show that it was inconsistent. It seems to me much better and lessconfusing if you admit in print that you were wrong. A good example of this was Einstein, who called thecosmological constant, which he introduced when he was trying to make a static model of the universe, thebiggest mistake of his life.
To return to the arrow of time, there remains the question: why do we observe that the thermodynamic andcosmological arrows point in the same direction? Or in other words, why does disorder increase in the same
direction of time as that in which the universe expands? If one believes that the universe will expand and thencontract again, as the no boundary proposal seems to imply, this becomes a question of why we should be in
the expanding phase rather than the contracting phase.
One can answer this on the basis of the weak anthropic principle. Conditions in the contracting phase would not
be suitable for the existence of intelligent beings who could ask the question: why is disorder increasing in thesame direction of time as that in which the universe is expanding? The inflation in the early stages of theuniverse, which the no boundary proposal predicts, means that the universe must be expanding at very close to
the critical rate at which it would just avoid recollapse, and so will not recollapse for a very long time. By then all
the stars will have burned out and the protons and neutrons in them will probably have decayed into lightparticles and radiation. The universe would be in a state of almost complete disorder. There would be no strong
thermodynamic arrow of time. Disorder couldn’t increase much because the universe would be in a state ofalmost complete disorder already. However, a strong thermodynamic arrow is necessary for intelligent life tooperate. In order to survive, human beings have to consume food, which is an ordered form of energy, andconvert it into heat, which is a disordered form of energy. Thus intelligent life could not exist in the contracting
phase of the universe. This is the explanation of why we observe that the thermodynamic and cosmologicalarrows of time point in the same direction. It is not that the expansion of the universe causes disorder toincrease. Rather, it is that the no boundary condition causes disorder to increase and the conditions to besuitable for intelligent life only in the expanding phase.
To summarize, the laws of science do not distinguish between the forward and backward directions of time.
However, there are at least three arrows of time that do distinguish the past from the future. They are thethermodynamic arrow, the direction of time in which disorder increases; the psychological arrow, the directionof time in which we remember the past and not the future; and the cosmological arrow, the direction of time inwhich the universe expands rather than contracts. I have shown that the psychological arrow is essentially thesame as the thermodynamic arrow, so that the two would always point in the same direction. The no boundary
proposal for the universe predicts the existence of a well-defined thermodynamic arrow of time because theuniverse must start off in a smooth and ordered state. And the reason we observe this thermodynamic arrow to
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A Brief History of Time - Stephen Hawking... Chapter 9agree with the cosmological arrow is that intelligent beings can exist only in the expanding phase. The
contracting phase will be unsuitable because it has no strong thermodynamic arrow of time.
The progress of the human race in understanding the universe has established a small corner of order in an
increasingly disordered universe. If you remember every word in this book, your memory will have recorded
about two million pieces of information: the order in your brain will have increased by about two million units.
However, while you have been reading the book, you will have converted at least a thousand calories ofordered energy, in the form of food, into disordered energy, in the form of heat that you lose to the air aroundyou by convection and sweat. This will increase the disorder of the universe by about twenty million million
million million units – or about ten million million million times the increase in order in your brain – and that’s if
you remember everything in this book. In the next chapter but one I will try to increase the order in our neck of
the woods a little further by explaining how people are trying to fit together the partial theories I have describedto form a complete unified theory that would cover everything in the universe.

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A Brief History of Time - Stephen Hawking... Chapter 10

CHAPTER 10
WORMHOLES AND TIME TRAVELThe last chapter discussed why we see time go forward: why disorder increases and why we remember the
past but not the future. Time was treated as if it were a straight railway line on which one could only go one way
or the other.
But what if the railway line had loops and branches so that a train could keep going forward but come back to astation it had already passed? In other words, might it be possible for someone to travel into the future or thepast?
H. G. Wells in The Time Machine explored these possibilities as have countless other writers of science fiction.
Yet many of the ideas of science fiction, like submarines and travel to the moon, have become matters ofscience fact. So what are the prospects for time travel?
The first indication that the laws of physics might really allow people to travel in time came in 1949 when Kurt
Godel discovered a new space-time allowed by general relativity. Godel was a mathematician who was famousfor proving that it is impossible to prove all true statements, even if you limit yourself to trying to prove all thetrue statements in a subject as apparently cut and dried as arithmetic. Like the uncertainty principle, Godel’s
incompleteness theorem may be a fundamental limitation on our ability to understand and predict the universe,
but so far at least it hasn’t seemed to be an obstacle in our search for a complete unified theory.
Godel got to know about general relativity when he and Einstein spent their later years at the Institute forAdvanced Study in Princeton. His space-time had the curious property that the whole universe was rotating.
One might ask: “Rotating with respect to what?” The answer is that distant matter would be rotating withrespect to directions that little tops or gyroscopes point in.
This had the side effect that it would be possible for someone to go off in a rocket ship and return to earth
before he set out. This property really upset Einstein, who had thought that general relativity wouldn’t allow time
travel. However, given Einstein’s record of ill-founded opposition to gravitational collapse and the uncertainty
principle, maybe this was an encouraging sign. The solution Godel found doesn’t correspond to the universe
we live in because we can show that the universe is not rotating. It also had a non-zero value of thecosmological constant that Einstein introduced when he thought the universe was unchanging. After Hubble
discovered the expansion of the universe, there was no need for a cosmological constant and it is now
generally believed to be zero. However, other more reasonable space-times that are allowed by generalrelativity and which permit travel into the past have since been found. One is in the interior of a rotating black
hole. Another is a space-time that contains two cosmic strings moving past each other at high speed. As theirname suggests, cosmic strings are objects that are like string in that they have length but a tiny cross section.
Actually, they are more like rubber bands because they are under enormous tension, something like a million
million million million tons. A cosmic string attached to the earth could accelerate it from 0 to 60 mph in 1/30thof a second. Cosmic strings may sound like pure science fiction but there are reasons to believe they could
have formed in the early universe as a result of symmetry-breaking of the kind discussed in Chapter 5.
Because they would be under enormous tension and could start in any configuration, they might accelerate tovery high speeds when they straighten out.
The Godel solution and the cosmic string space-time start out so distorted that travel into the past was always
possible. God might have created such a warped universe but we have no reason to believe he did.
Observations of the microwave background and of the abundances of the light elements indicate that the early
universe did not have the kind of curvature required to allow time travel. The same conclusion follows on
theoretical grounds if the no boundary proposal is correct. So the question is: if the universe starts out without
the kind of curvature required for time travel, can we subsequently warp local regions of space-time sufficiently
to allow it?
A closely related problem that is also of concern to writers of science fiction is rapid interstellar or intergalactic

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travel. According to relativity, nothing can travel faster than light. If we therefore sent a spaceship to our nearest
neighboring star, Alpha Centauri, which is about four light-years away, it would take at least eight years before
we could expect the travelers to return and tell us what they had found. If the expedition were to the center ofour galaxy, it would be at least a hundred thousand years before it came back. The theory of relativity does
allow one consolation. This is the so-called twins paradox mentioned in Chapter 2.
Because there is no unique standard of time, but rather observers each have their own time as measured byclocks that they carry with them, it is possible for the journey to seem to be much shorter for the space travelers
than for those who remain on earth. But there would not be much joy in returning from a space voyage a few
years older to find that everyone you had left behind was dead and gone thousands of years ago. So in order tohave any human interest in their stories, science fiction writers had to suppose that we would one day discover
how to travel faster than light. What most of thee authors don’t seem to have realized is that if you can travel
faster than light, the theory of relativity implies you can also travel back in the, as the following limerick says:
There was a young lady of Wight
Who traveled much faster than light.
She departed one day,
In a relative way,
And arrived on the previous night
The point is that the theory of relativity says hat there is no unique measure of time that all observers will agree
on Rather, each observer has his or her own measure of time. If it is possible for a rocket traveling below the
speed of light to get from event A (say, the final of the 100-meter race of the Olympic Games in 202) to event B(say, the opening of the 100,004th meeting of the Congress of Alpha Centauri), then all observers will agree
that event A happened before event B according to their times. Suppose, however, that the spaceship would
have to travel faster than light to carry the news of the race to the Congress. Then observers moving atdifferent speeds can disagree about whether event A occurred before B or vice versa. According to the time ofan observer who is at rest with respect to the earth, it may be that the Congress opened after the race. Thus
this observer would think that a spaceship could get from A to B in time if only it could ignore the speed-of-light
speed limit. However, to an observer at Alpha Centauri moving away from the earth at nearly the speed of light,
it would appear that event B, the opening of the Congress, would occur before event A, the 100-meter race.
The theory of relativity says that the laws of physics appear the same to observers moving at different speeds.
This has been well tested by experiment and is likely to remain a feature even if we find a more advanced
theory to replace relativity Thus the moving observer would say that if faster-than-light travel is possible, it
should be possible to get from event B, the opening of the Congress, to event A, the 100-meter race. If one
went slightly faster, one could even get back before the race and place a bet on it in the sure knowledge that
one would win.
There is a problem with breaking the speed-of-light barrier. The theory of relativity says that the rocket power
needed to accelerate a spaceship gets greater and greater the nearer it gets to the speed of light. We haveexperimental evidence for this, not with spaceships but with elementary particles in particle accelerators likethose at Fermilab or CERN (European Centre for Nuclear Research). We can accelerate particles to 99.99
percent of the speed of light, but however much power we feed in, we can’t get them beyond the speed-of-lightbarrier. Similarly with spaceships: no matter how much rocket power they have, they can’t accelerate beyond
the speed of light.
That might seem to rule out both rapid space travel and travel back in time. However, there is a possible way
out. It might be that one could warp space-time so that there was a shortcut between A and B One way of doing
this would be to create a wormhole between A and B. As its name suggests, a wormhole is a thin tube ofspace-time which can connect two nearly flat regions far apart.
There need be no relation between the distance through the wormhole and the separation of its ends in thenearly Hat background. Thus one could imagine that one could create or find a wormhole that world lead fromthe vicinity of the Solar System to Alpha Centauri. The distance through the wormhole might be only a few
million miles even though earth and Alpha Centauri are twenty million million miles apart in ordinary space. This
would allow news of the 100-meter race to reach the opening of the Congress. But then an observer moving
toward 6e earth should also be able to find another wormhole that would enable him to get from the opening of
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the Congress on Alpha Centauri back to earth before the start of the race. So wormholes, like any otherpossible form of travel faster than light, would allow one to travel into the past.
The idea of wormholes between different regions of space-time was not an invention of science fiction writers
but came from a very respectable source.
In 1935, Einstein and Nathan Rosen wrote a paper in which they showed that general relativity allowed what
they called “bridges,” but which are now known as wormholes. The Einstein-Rosen bridges didn’t last longenough for a spaceship to get through: the ship would run into a singularity as the wormhole pinched off.
However, it has been suggested that it might be possible for an advanced civilization to keep a wormhole open.
To do this, or to warp space-time in any other way so as to permit time travel, one can show that one needs aregion of space-time with negative curvature, like the surface of a saddle. Ordi-nary matter, which has apositive energy density, gives space-time a positive curvature, like the surface of a sphere. So what one needs,in order to warp space-time in a way that will allow travel into the past, is matter with negative energy density.
Energy is a bit like money: if you have a positive balance, you can distribute it in various ways, but according to
the classical laws that were believed at the beginning of the century, you weren’t allowed to be overdrawn. So
these classical laws would have ruled out any possibility of time travel. However, as has been described inearlier chapters, the classical laws were superseded by quantum laws based on the uncertainty principle. The
quantum laws are more liberal and allow you to be overdrawn on one or two accounts provided the totalbalance is positive. In other words, quantum theory allows the energy density to be negative in some places,
provided that this is made up for by positive energy densities in other places, so that the total energy re-mains
positive. An example of how quantum theory can allow negative energy densities is provided by what is called
the Casimir effect. As we saw in Chapter 7, even what we think of as “empty” space is filled with pairs of virtual
particles and antiparticles that appear together, move apart, and come back together and annihilate each other.
Now, suppose one has two parallel metal plates a short distance apart. The plates will act like mirrors for thevirtual photons or particles of light. In fact they will form a cavity between them, a bit like an organ pipe that will
resonate only at certain notes. This means that virtual photons can occur in the space between the plates only
if their wavelengths (the distance between the crest of one wave and the next) fit a whole number of times into
the gap between the plates. If the width of a cavity is a whole number of wavelengths plus a fraction of awave-length, then after some reflections backward and forward between the plates, the crests of one wave will
coincide with the troughs of another and the waves will cancel out.
Because the virtual photons between the plates can have only the resonant wavelengths, there will be slightly
fewer of them than in the region outside the plates where virtual photons can have any wavelength. Thus there
will be slightly fewer virtual photons hitting the inside surfaces of the plates than the outside surfaces. One
would therefore expect a force on the plates, pushing them toward each other. This force has actually been
detected and has the predicted value. Thus we have experimental evidence that virtual particles exist and havereal effects.
The fact that there are fewer virtual photons between the plates means that their energy density will be lessthan elsewhere. But the total energy density in “empty” space far away from the plates must be zero, because
otherwise the energy density would warp the space and it would not be almost flat. So, if the energy density
between the plates is less than the energy density far away, it must be negative.
We thus have experimental evidence both that space-time can be warped (from the bending of light during
eclipses) and that it can be curved in the way necessary to allow time travel (from the Casimir effect). One
might hope therefore that as we advance in science and technology, we would eventually manage to build atime machine. But if so, why hasn’t anyone come back from the future and told us how to do it? There might begood reasons why it would be unwise to give us the secret of time travel at our present primitive state ofdevelopment, but unless human nature changes radically, it is difficult to believe that some visitor from thefuture wouldn’t spill the beans. Of course, some people would claim that sightings of UFOs are evidence that
we are being visited either by aliens or by people from the future. (If the aliens were to get here in reasonable
time, they would need faster-than-light travel, so the two possibilities may be equivalent.)
However, I think that any visit by aliens or people from the future would be much more obvious and, probably,much more unpleasant. If they are going to reveal themselves at all, why do so only to those who are not

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regarded as reliable witnesses? If they are trying to warn us of some great danger, they are not being very
effective.
A possible way to explain the absence of visitors from the future would be to say that the past is fixed because
we have observed it and seen that it does not have the kind of warping needed to allow travel back from thefuture. On the other hand, the future is unknown and open, so it might well have the curvature required. This
would mean that any time travel would be confined to the future. There would be no chance of Captain Kirk andthe Starship Enterprise turning up at the present time.
This might explain why we have not yet been overrun by tourists from the future, but it would not avoid the
problems that would arise if one were able to go back and change history. Suppose, for example, you went
back and killed your great-great-grandfather while he was still a child. There are many versions of this paradox
but they are essentially equivalent: one would get contradictions if one were free to change the past.
There seem to be two possible resolutions to the paradoxes posed by time travel. One I shall call the consistenthistories approach. It says that even if space-time is warped so that it would be possible to travel into the past,
what happens in space-time must be a consistent solution of the laws of physics. According to this viewpoint,
you could not go back in time unless history showed that you had already arrived in the past and, while there,
had not killed your great-great-grandfather or committed any other acts that would conflict with your current
situation in the present. Moreover, when you did go back, you wouldn’t be able to change recorded history.
That means you wouldn’t have free will to do what you wanted. Of course, one could say that free will is an
illusion anyway. If there really is a complete unified theory that governs everything, it presumably also
determines your actions. But it does so in a way that is impossible to calculate for an organism that is ascomplicated as a human being. The reason we say that humans have free will is because we can’t predict what
they will do. However, if the human then goes off in a rocket ship and comes back before he or she set off, wewill be able to predict what he or she will do because it will be part of recorded history. Thus, in that situation,
the time traveler would have no free will.
The other possible way to resolve the paradoxes of time travel might be called the alternative histories
hypothesis. The idea here is that when time travelers go back to the past, they enter alternative histories which
differ from recorded history. Thus they can act freely, without the constraint of consistency with their previous
history. Steven Spiel-berg had fun with this notion in the Back to the Future films: Marty McFly was able to go
back and change his parents’ courtship to a more satisfactory history.
The alternative histories hypothesis sounds rather like Richard Feynman’s way of expressing quantum theory
as a sum over histories, which was described in Chapters 4 and 8. This said that the universe didn’t just have asingle history: rather it had every possible history, each with its own probability. However, there seems to be an
important difference between Feynman’s proposal and alternative histories. In Feynman’s sum, each history
comprises a complete space-time and everything in it. The space-time may be so warped that it is possible totravel in a rocket into the past. But the rocket would remain in the same space-time and therefore the same
history, which would have to be consistent. Thus Feynman’s sum over histories proposal seems to support theconsistent histories hypothesis rather than the alternative histories.
The Feynman sum over histories does allow travel into the past on a microscopic scale. In Chapter 9 we sawthat the laws of science are unchanged by combinations of the operations C, P, and T. This means that an
antiparticle spinning in the anticlockwise direction and moving from A to B can also be viewed as an ordinary
particle spinning clockwise and moving backward in time from B to A. Similarly, an ordinary particle moving
forward in time is equivalent to an antiparticle moving backward in time. As has been discussed in this chapter
and Chapter 7, “empty” space is filled with pairs of virtual particles and antiparticles that appear together, moveapart, and then come back together and annihilate each other.
So, one can regard the pair of particles as a single particle moving on a closed loop in space-time. When thepair is moving forward in time (from the event at which it appears to that at which it annihilates), it is called aparticle. But when the particle is traveling back in time (from the event at which the pair annihilates to that at
which it appears), it is said to be an antiparticle traveling forward in time.
The explanation of how black holes can emit particles and radiation (given in Chapter 7) was that one member

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A Brief History of Time - Stephen Hawking... Chapter 10of a virtual particle/ antiparticle pair (say, the antiparticle) might fall into the black hole, leaving the othermember without a partner with which to annihilate. The forsaken particle might fall into the hole as well, but it
might also escape from the vicinity of the black hole. If so, to an observer at a distance it would appear to be aparticle emitted by the black hole.
One can, however, have a different but equivalent intuitive picture of the mechanism for emission from black
holes. One can regard the member of the virtual pair that fell into the black hole (say, the antiparticle) as aparticle traveling backward in time out of the hole. When it gets to the point at which the virtual
particle/antiparticle pair appeared together, it is scattered by the gravitational field into a particle travelingforward in time and escaping from the black hole. If, instead, it were the particle member of the virtual pair thatfell into the hole, one could regard it as an antiparticle traveling back in time and coming out of the black hole.
Thus the radiation by black holes shows that quantum theory allows travel back in time on a microscopic scale
and that such time travel can produce observable effects.
One can therefore ask: does quantum theory allow time travel on a macroscopic scale, which people could
use? At first sight, it seems it should. The Feynman sum over histories proposal is supposed to be over all
histories. Thus it should include histories in which space-time is so warped that it is possible to travel into thepast. Why then aren’t we in trouble with history? Suppose, for example, someone had gone back and given the
Nazis the secret of the atom bomb?
One would avoid these problems if what I call the chronology protection conjecture holds. This says that thelaws of physics conspire to prevent macroscopic bodies from carrying information into the past. Like the cosmic
censorship conjecture, it has not been proved but there are reasons to believe it is true.
The reason to believe that chronology protection operates is that when space-time is warped enough to maketravel into the past possible, virtual particles moving on closed loops in space-time can become real particles
traveling forward in time at or below the speed of light. As these particles can go round the loop any number oftimes, they pass each point on their route many times. Thus their energy is counted over and over again andthe energy density will become very large. This could give space-time a positive curvature that would not allow
travel into the past. It is not yet clear whether these particles would cause positive or negative curvature orwhether the curvature produced by some kinds of virtual particles might cancel that produced by other kinds.
Thus the possibility of time travel remains open. But I’m not going to bet on it. My opponent might have theunfair advantage of knowing the future.

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A Brief History of Time - Stephen Hawking... Chapter 11

CHAPTER 11
THE UNIFICATION OF PHYSICS

As was explained in the first chapter, it would be very difficult to construct a complete unified theory of
everything in the universe all at one go. So instead we have made progress by finding partial theories that
describe a limited range of happenings and by neglecting other effects or approximating them by certain
numbers. (Chemistry, for example, allows us to calculate the interactions of atoms, without knowing the internal
structure of an atom’s nucleus.) Ultimately, however, one would hope to find a complete, consistent, unified
theory that would include all these partial theories as approximations, and that did not need to be adjusted to fit
the facts by picking the values of certain arbitrary numbers in the theory. The quest for such a theory is known
as “the unification of physics.” Einstein spent most of his later years unsuccessfully searching for a unified
theory, but the time was not ripe: there were partial theories for gravity and the electromagnetic force, but very
little was known about the nuclear forces. Moreover, Einstein refused to believe in the reality of quantum
mechanics, despite the important role he had played in its development. Yet it seems that the uncertainty
principle is a fundamental feature of the universe we live in. A successful unified theory must, therefore,
necessarily incorporate this principle.
As I shall describe, the prospects for finding such a theory seem to be much better now because we know somuch more about the universe. But we must beware of overconfidence – we have had false dawns before! At
the beginning of this century, for example, it was thought that everything could be explained in terms of theproperties of continuous matter, such as elasticity and heat conduction. The discovery of atomic structure andthe uncertainty principle put an emphatic end to that. Then again, in 1928, physicist and Nobel Prize winner
Max Born told a group of visitors to Gottingen University, “Physics, as we know it, will be over in six months.”
His confidence was based on the recent discovery by Dirac of the equation that governed the electron. It wasthought that a similar equation would govern the proton, which was the only other particle known at the time,
and that would be the end of theoretical physics. However, the discovery of the neutron and of nuclear forces
knocked that one on the head too. Having said this, I still believe there are grounds for cautious optimism that
we may now be near the end of the search for the ultimate laws of nature.
In previous chapters I have described general relativity, the partial theory of gravity, and the partial theories thatgovern the weak, the strong, and the electromagnetic forces. The last three may be combined in so-called
grand unified theories, or GUTs, which are not very satisfactory because they do not include gravity andbecause they contain a number of quantities, like the relative masses of different particles, that cannot be
predicted from the theory but have to be chosen to fit observations. The main difficulty in finding a theory thatunifies gravity with the other forces is that general relativity is a “classical” theory; that is, it does not incorporate
the uncertainty principle of quantum mechanics. On the other hand, the other partial theories depend on
quantum mechanics in an essential way. A necessary first step, therefore, is to combine general relativity withthe uncertainty principle. As we have seen, this can produce some remarkable consequences, such as black
holes not being black, and the universe not having any singularities but being completely self-contained andwithout a boundary. The trouble is, as explained in Chapter 7, that the uncertainty principle means that even
“empty” space is filled with pairs of virtual particles and antiparticles. These pairs would have an infinite amountof energy and, therefore, by Einstein’s famous equation E = mc2, they would have an infinite amount of mass.
Their gravitational attraction would thus curve up the universe to infinitely small size.
Rather similar, seemingly absurd infinities occur in the other partial theories, but in all these cases the infinities
can be canceled out by a process called renormalization. This involves canceling the infinities by introducing
other infinities. Although this technique is rather dubious mathematically, it does seem to work in practice, andhas been used with these theories to make predictions that agree with observations to an extraordinary degreeof accuracy. Renormalization, however, does have a serious drawback from the point of view of trying to find acomplete theory, because it means that the actual values of the masses and the strengths of the forces cannot
be predicted from the theory, but have to be chosen to fit the observations.
In attempting to incorporate the uncertainty principle into general relativity, one has only two quantities that canbe adjusted: the strength of gravity and the value of the cosmological constant. But adjusting these is not

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sufficient to remove all the infinities. One therefore has a theory that seems to predict that certain quantities,
such as the curvature of space-time, are really infinite, yet these quantities can be observed and measured tobe perfectly finite! This problem in combining general relativity and the uncertainty principle had beensuspected for some time, but was finally confirmed by detailed calculations in 1972. Four years later, a possible
solution, called “supergravity,” was suggested. The idea was to combine the spin-2 particle called the graviton,
which carries the gravitational force, with certain other particles of spin 3/2, 1, ½, and 0. In a sense, all theseparticles could then be regarded as different aspects of the same “superparticle,” thus unifying the matterparticles with spin ½ and 3/2 with the force-carrying particles of spin 0, 1, and 2. The virtual particle/antiparticle
pairs of spin ½ and 3/2 would have negative energy, and so would tend to cancel out the positive energy of thespin 2, 1, and 0 virtual pairs. This would cause many of the possible infinities to cancel out, but it wassuspected that some infinities might still remain. However, the calculations required to find out whether or notthere were any infinities left uncancelled were so long and difficult that no one was prepared to undertake them.
Even with a computer it was reckoned it would take at least four years, and the chances were very high that
one would make at least one mistake, probably more. So one would know one had the right answer only if
someone else repeated the calculation and got the same answer, and that did not seem very likely!
Despite these problems, and the fact that the particles in the super-gravity theories did not seem to match theobserved particles, most scientists believed that supergravity was probably the right answer to the problem of
the unification of physics. It seemed the best way of unifying gravity with the other forces. However, in 1984
there was a remarkable change of opinion in favor of what are called string theories. In these theories the basic
objects are not particles, which occupy a single point of space, but things that have a length but no otherdimension, like an infinitely thin piece of string. These strings may have ends (the so-called open strings) or
they may be joined up with themselves in closed loops (closed strings) Figure 11:1 and Figure 11:2.

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Figures 11:1 & 11:2
A particle occupies one point of space at each instant of time. Thus its history can be represented by a line inspace-time (the “world-line”). A string, on the other hand, occupies a line in space at each moment of time. So
its history in space-time is a two-dimensional surface called the world-sheet. (Any point on such a world-sheet
can be described by two numbers, one specifying the time and the other the position of the point on the string.)
The world-sheet of an open string is a strip: its edges represent the paths through space-time of the ends of thestring Figure 11:1. The world-sheet of a closed string is a cylinder or tube Figure 11:2: a slice through the tube
is a circle, which represents the position of the string at one particular time.
Two pieces of string can join together to form a single string; in the case of open strings they simply join at theends Figure 11:3, while in the case of closed strings it is like the two legs joining on a pair of trousers Figure
11:4.

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Figure 11:3

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Figure 11:4
Similarly, a single piece of string can divide into two strings. In string theories, what were previously thought ofas particles are now pictured as waves traveling down the string, like waves on a vibrating kite string. The
emission or absorption of one particle by another corresponds to the dividing or joining together of strings. For
example, the gravitational force of the sun on the earth was pictured in particle theories as being caused by theemission of a graviton by a particle in the sun and its absorption by a particle in the earth Figure 11:5.

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Figures 11:5 & 11:6
In string theory, this process corresponds to an H-shaped tube or pipe Figure 11:6 (string theory is rather likeplumbing, in a way). The two vertical sides of the H correspond to the particles in the sun and the earth, and thehorizontal crossbar corresponds to the graviton that travels between them.
String theory has a curious history. It was originally invented in the late 1960s in an attempt to find a theory todescribe the strong force. The idea was that particles like the proton and the neutron could be regarded aswaves on a string. The strong forces between the particles would correspond to pieces of string that went
between other bits of string, as in a spider’s web. For this theory to give the observed value of the strong force
between particles, the strings had to be like rubber bands with a pull of about ten tons.
In 1974 Joel Scherk from Paris and John Schwarz from the California Institute of Technology published a paper
in which they showed that string theory could describe the gravitational force, but only if the tension in the string
were very much higher, about a thousand million million million million million million tons (1 with thirty-ninezeros after it). The predictions of the string theory would be just the same as those of general relativity on
normal length scales, but they would differ at very small distances, less than a thousand million million million
million millionth of a centimeter (a centimeter divided by 1 with thirty-three zeros after it). Their work did notreceive much attention, however, because at just about that time most people abandoned the original string
theory of the strong force in favor of the theory based on quarks and gluons, which seemed to fit much betterwith observations. Scherk died in tragic circumstances (he suffered from diabetes and went into a coma when
no one was around to give him an injection of insulin). So Schwarz was left alone as almost the only supporterof string theory, but now with the much higher proposed value of the string tension.
In 1984 interest in strings suddenly revived, apparently for two reasons. One was that people were not reallymaking much progress toward showing that supergravity was finite or that it could explain the kinds of particles
that we observe. The other was the publication of a paper by John Schwarz and Mike Green of Queen Mary
College, London, that showed that string theory might be able to explain the existence of particles that have abuilt-in left-handedness, like some of the particles that we observe. Whatever the reasons, a large number ofpeople soon began to work on string theory and a new version was developed, the so-called heterotic string,which seemed as if it might be able to explain the types of particles that we observe.
String theories also lead to infinities, but it is thought they will all cancel out in versions like the heterotic string

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A Brief History of Time - Stephen Hawking... Chapter 11(though this is not yet known for certain). String theories, however, have a bigger problem: they seem to beconsistent only if space-time has either ten or twenty-six dimensions, instead of the usual four! Of course, extra
space-time dimensions are a commonplace of science fiction indeed, they provide an ideal way of overcoming
the normal restriction of general relativity that one cannot travel faster than light or back in time (see Chapter
10). The idea is to take a shortcut through the extra dimensions. One can picture this in the following way.
Imagine that the space we live in has only two dimensions and is curved like the surface of an anchor ring or
torus Figure 11:7.

Figure 11:7
If you were on one side of the inside edge of the ring and you wanted to get to a point on the other side, you
would have to go round the inner edge of the ring. However, if you were able to travel in the third dimension,
you could cut straight across.
Why don’t we notice all these extra dimensions, if they are really there? Why do we see only three spacedimensions and one time dimension? The suggestion is that the other dimensions are curved up into a space ofvery small size, something like a million million million million millionth of an inch. This is so small that we just
don’t notice it: we see only one time dimension and three space dimensions, in which space-time is fairly flat. It
is like the surface of a straw. If you look at it closely, you see it is two-dimensional (the position of a point on the
straw is described by two numbers, the length along the straw and the distance round the circular direction).
But if you look at it from a distance, you don’t see the thickness of the straw and it looks one-dimensional (theposition of a point is specified only by the length along the straw). So it is with space-time: on a very small scale
it is ten-dimensional and highly curved, but on bigger scales you don’t see the curvature or the extra
dimensions. If this picture is correct, it spells bad news for would-be space travelers: the extra dimensions
would be far too small to allow a spaceship through. However, it raises another major problem. Why should
some, but not all, of the dimensions be curled up into a small ball? Presumably, in the very early universe all
the dimensions would have been very curved. Why did one time dimension and three space dimensions flatten

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out, while the other dimensions remain tightly curled up?
One possible answer is the anthropic principle. Two space dimensions do not seem to be enough to allow for
the development of complicated beings like us. For example, two-dimensional animals living on a
one-dimensional earth would have to climb over each other in order to get past each other. If a two-dimensional
creature ate something it could not digest completely, it would have to bring up the remains the same way it
swallowed them, because if there were a passage right through its body, it would divide the creature into two
separate halves: our two-dimensional being would fall apart Figure 11:8. Similarly, it is difficult to see how there
could be any circulation of the blood in a two-dimensional creature.

Figure 11:8
There would also be problems with more than three space dimensions. The gravitational force between twobodies would decrease more rapidly with distance than it does in three dimensions. (In three dimensions, thegravitational force drops to 1/4 if one doubles the distance. In four dimensions it would drop to 1/5, in fivedimensions to 1/6, and so on.) The significance of this is that the orbits of planets, like the earth, around the sun
would be unstable: the least disturbance from a circular orbit (such as would be caused by the gravitational
attraction of other planets) would result in the earth spiraling away from or into the sun. We would either freezeor be burned up. In fact, the same behavior of gravity with distance in more than three space dimensions
means that the sun would not be able to exist in a stable state with pressure balancing gravity. It would either
fall apart or it would collapse to form a black hole. In either case, it would not be of much use as a source ofheat and light for life on earth. On a smaller scale, the electrical forces that cause the electrons to orbit roundthe nucleus in an atom would behave in the same way as gravitational forces. Thus the electrons would either
escape from the atom altogether or would spiral into the nucleus. In either case, one could not have atoms as
we know them.
It seems clear then that life, at least as we know it, can exist only in regions of space-time in which one time
dimension and three space dimensions are not curled up small. This would mean that one could appeal to theweak anthropic principle, provided one could show that string theory does at least allow there to be suchregions of the universe – and it seems that indeed string theory does. There may well be other regions of theuniverse, or other universes (whatever that may mean), in which all the dimensions are curled up small or inwhich more than four dimensions are nearly flat, but there would be no intelligent beings in such regions to

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observe the different number of effective dimensions.
Another problem is that there are at least four different string theories (open strings and three different closed
string theories) and millions of ways in which the extra dimensions predicted by string theory could be curled
up. Why should just one string theory and one kind of curling up be picked out? For a time there seemed noanswer, and progress got bogged down. Then, from about 1994, people started discovering what are called
dualities: different string theories and different ways of curling up the extra dimensions could lead to the same
results in four dimensions. Moreover, as well as particles, which occupy a single point of space, and strings,
which are lines, there were found to be other objects called p-branes, which occupied two-dimensional or
higher-dimensional volumes in space. (A particle can be regarded as a 0-brane and a string as a 1-brane but
there were also p-branes for p=2 to p=9.) What this seems to indicate is that there is a sort of democracy
among supergravity, string, and p-brane theories: they seem to fit together but none can be said to be more
fundamental than the others. They appear to be different approximations to some fundamental theory that are
valid in different situations.
People have searched for this underlying theory, but without any success so far. However, I believe there may
not be any single formulation of the fundamental theory any more than, as Godel showed, one could formulatearithmetic in terms of a single set of axioms. Instead it may be like maps – you can’t use a single map todescribe the surface of the earth or an anchor ring: you need at least two maps in the case of the earth and four
for the anchor ring to cover every point. Each map is valid only in a limited region, but different maps will have aregion of overlap. The collection of maps provides a complete description of the surface. Similarly, in physics it
may be necessary to use different formulations in different situations, but two different formulations would agree
in situations where they can both be applied. The whole collection of different formulations could be regarded
as a complete unified theory, though one that could not be expressed in terms of a single set of postulates.
But can there really be such a unified theory? Or are we perhaps just chasing a mirage? There seem to be
three possibilities:
1. There really is a complete unified theory (or a collection of overlapping formulations), which we will someday
discover if we are smart enough.
2. There is no ultimate theory of the universe, just an infinite sequence of theories that describe the universe
more and more accurately.
3. There is no theory of the universe: events cannot be predicted beyond a certain extent but occur in a random
and arbitrary manner.
Some would argue for the third possibility on the grounds that if there were a complete set of laws, that would
infringe God’s freedom to change his mind and intervene in the world. It’s a bit like the old paradox: can God
make a stone so heavy that he can’t lift it? But the idea that God might want to change his mind is an exampleof the fallacy, pointed out by St. Augustine, of imagining God as a being existing in time: time is a property onlyof the universe that God created. Presumably, he knew what he intended when he set it up!
With the advent of quantum mechanics, we have come to recognize that events cannot be predicted withcomplete accuracy but that there is always a degree of uncertainty. If one likes, one could ascribe thisrandomness to the intervention of God, but it would be a very strange kind of intervention: there is no evidence
that it is directed toward any purpose. Indeed, if it were, it would by definition not be random. In modern times,
we have effectively removed the third possibility above by redefining the goal of science: our aim is to formulate
a set of laws that enables us to predict events only up to the limit set by the uncertainty principle.
The second possibility, that there is an infinite sequence of more and more refined theories, is in agreementwith all our experience so far. On many occasions we have increased the sensitivity of our measurements ormade a new class of observations, only to discover new phenomena that were not predicted by the existing
theory, and to account for these we have had to develop a more advanced theory. It would therefore not bevery surprising if the present generation of grand unified theories was wrong in claiming that nothing essentially
new will happen between the electroweak unification energy of about 100 GeV and the grand unification energyof about a thousand million million GeV. We might indeed expect to find several new layers of structure more

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basic than the quarks and electrons that we now regard as “elementary” particles.
However, it seems that gravity may provide a limit to this sequence of “boxes within boxes.” If one had aparticle with an energy above what is called the Planck energy, ten million million million GeV (1 followed bynineteen zeros), its mass would be so concentrated that it would cut itself off from the rest of the universe andform a little black hole. Thus it does seem that the sequence of more and more refined theories should havesome limit as we go to higher and higher energies, so that there should be some ultimate theory of theuniverse. Of course, the Planck energy is a very long way from the energies of around a hundred GeV, which
are the most that we can produce in the laboratory at the present time. We shall not bridge that gap withparticle accelerators in the foreseeable future! The very early stages of the universe, however, are an arena
where such energies must have occurred. I think that there is a good chance that the study of the early
universe and the requirements of mathematical consistency will lead us to a complete unified theory within the
lifetime of some of us who are around today, always presuming we don’t blow ourselves up first.
What would it mean if we actually did discover the ultimate theory of the universe? As was explained in Chapter
1, we could never be quite sure that we had indeed found the correct theory, since theories can’t be proved.
But if the theory was mathematically consistent and always gave predictions that agreed with observations, we
could be reasonably confident that it was the right one. It would bring to an end a long and glorious chapter inthe history of humanity’s intellectual struggle to understand the universe. But it would also revolutionize theordinary person’s understanding of the laws that govern the universe. In Newton’s time it was possible for an
educated person to have a grasp of the whole of human knowledge, at least in outline. But since then, the paceof the development of science has made this impossible. Because theories are always being changed toaccount for new observations, they are never properly digested or simplified so that ordinary people canunderstand them. You have to be a specialist, and even then you can only hope to have a proper grasp of asmall proportion of the scientific theories. Further, the rate of progress is so rapid that what one learns at schoolor university is always a bit out of date. Only a few people can keep up with the rapidly advancing frontier ofknowledge, and they have to devote their whole time to it and specialize in a small area. The rest of the
population has little idea of the advances that are being made or the excitement they are generating. Seventy
years ago, if Eddington is to be believed, only two people understood the general theory of relativity. Nowadays
tens of thousands of university graduates do, and many millions of people are at least familiar with the idea. If acomplete unified theory was discovered, it would only be a matter of time before it was digested and simplified
in the same way and taught in schools, at least in outline. We would then all be able to have someunderstanding of the laws that govern the universe and are responsible for our existence.
Even if we do discover a complete unified theory, it would not mean that we would be able to predict events ingeneral, for two reasons. The first is the limitation that the uncertainty principle of quantum mechanics sets on
our powers of prediction. There is nothing we can do to get around that. In practice, however, this first limitationis less restrictive than the second one. It arises from the fact that we could not solve the equations of the theory
exactly, except in very simple situations. (We cannot even solve exactly for the motion of three bodies in
Newton’s theory of gravity, and the difficulty increases with the number of bodies and the complexity of thetheory.) We already know the laws that govern the behavior of matter under all but the most extreme
conditions. In particular, we know the basic laws that underlie all of chemistry and biology. Yet we havecertainly not reduced these subjects to the status of solved problems: we have, as yet, had little success in
predicting human behavior from mathematical equations! So even if we do find a complete set of basic laws,
there will still be in the years ahead the intellectually challenging task of developing better approximation
methods, so that we can make useful predictions of the probable outcomes in complicated and realistic
situations. A complete, consistent, unified theory is only the first step: our goal is a complete understanding of
the events around us, and of our own existence.

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A Brief History of Time - Stephen Hawking... Chapter 12

CHAPTER 12
CONCLUSION

We find ourselves in a bewildering world. We want to make sense of what we see around us and to ask: Whatis the nature of the universe? What is our place in it and where did it and we come from? Why is it the way it is?
To try to answer these questions we adopt some “world picture.” Just as an infinite tower of tortoises supporting
the fiat earth is such a picture, so is the theory of superstrings. Both are theories of the universe, though thelatter is much more mathematical and precise than the former. Both theories lack observational evidence: noone has ever seen a giant tortoise with the earth on its back, but then, no one has seen a superstring either.
However, the tortoise theory fails to be a good scientific theory because it predicts that people should be able tofall off the edge of the world. This has not been found to agree with experience, unless that turns out to be theexplanation for the people who are supposed to have disappeared in the Bermuda Triangle!
The earliest theoretical attempts to describe and explain the universe involved the idea that events and natural
phenomena were controlled by spirits with human emotions who acted in a very humanlike and unpredictablemanner. These spirits inhabited natural objects, like rivers and mountains, including celestial bodies, like thesun and moon. They had to be placated and their favor sought in order to ensure the fertility of the soil and therotation of the seasons. Gradually, however, it must have been noticed that there were certain regularities: thesun always rose in the east and set in the west, whether or not a sacrifice had been made to the sun god.
Further, the sun, the moon, and the planets followed precise paths across the sky that could be predicted inadvance with considerable accuracy. The sun and the moon might still be gods, but they were gods whoobeyed strict laws, apparently without any exceptions, if one discounts stories like that of the sun stopping for
Joshua.
At first, these regularities and laws were obvious only in astronomy and a few other situations. However, as
civilization developed, and particularly in the last 300 years, more and more regularities and laws werediscovered. The success of these laws led Laplace at the beginning of the nineteenth century to postulatescientific determinism; that is, he suggested that there would be a set of laws that would determine theevolution of the universe precisely, given its configuration at one time.
Laplace’s determinism was incomplete in two ways. It did not say how the laws should be chosen and it did not
specify the initial configuration of the universe. These were left to God. God would choose how the universe
began and what laws it obeyed, but he would not intervene in the universe once it had started. In effect, God
was confined to the areas that nineteenth-century science did not understand.
We now know that Laplace’s hopes of determinism cannot be realized, at least in the terms he had in mind.
The uncertainty principle of quantum mechanics implies that certain pairs of quantities, such as the position andvelocity of a particle, cannot both be predicted with complete accuracy. Quantum mechanics deals with thissituation via a class of quantum theories in which particles don’t have well-defined positions and velocities but
are represented by a wave. These quantum theories are deterministic in the sense that they give laws for theevolution of the wave with time. Thus if one knows the wave at one time, one can calculate it at any other time.
The unpredictable, random element comes in only when we try to interpret the wave in terms of the positionsand velocities of particles. But maybe that is our mistake: maybe there are no particle positions and velocities,
but only waves. It is just that we try to fit the waves to our preconceived ideas of positions and velocities. The
resulting mismatch is the cause of the apparent unpredictability.
In effect, we have redefined the task of science to be the discovery of laws that will enable us to predict events
up to the limits set by the uncertainty principle. The question remains, however: how or why were the laws andthe initial state of the universe chosen?
In this book I have given special prominence to the laws that govern gravity, because it is gravity that shapes
the large-scale structure of the universe, even though it is the weakest of the four categories of forces. The
laws of gravity were incompatible with the view held until quite recently that the universe is unchanging in time:

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the fact that gravity is always attractive implies that the universe must be either expanding or contracting.
According to the general theory of relativity, there must have been a state of infinite density in the past, the bigbang, which would have been an effective beginning of time. Similarly, if the whole universe recollapsed, there
must be another state of infinite density in the future, the big crunch, which would be an end of time. Even if thewhole universe did not recollapse, there would be singularities in any localized regions that collapsed to form
black holes. These singularities would be an end of time for anyone who fell into the black hole. At the big bang
and other singularities, all the laws would have broken down, so God would still have had complete freedom tochoose what happened and how the universe began.
When we combine quantum mechanics with general relativity, there seems to be a new possibility that did notarise before: that space and time together might form a finite, four-dimensional space without singularities orboundaries, like the surface of the earth but with more dimensions. It seems that this idea could explain manyof the observed features of the universe, such as its large-scale uniformity and also the smaller-scale
departures from homogeneity, like galaxies, stars, and even human beings. It could even account for the arrowof time that we observe. But if the universe is completely self-contained, with no singularities or boundaries,
and completely described by a unified theory, that has profound implications for the role of God as Creator.
Einstein once asked the question: “How much choice did God have in constructing the universe?” If the noboundary proposal is correct, he had no freedom at all to choose initial conditions. He would, of course, still
have had the freedom to choose the laws that the universe obeyed. This, however, may not really have been allthat much of a choice; there may well be only one, or a small number, of complete unified theories, such as theheterotic string theory, that are self-consistent and allow the existence of structures as complicated as humanbeings who can investigate the laws of the universe and ask about the nature of God.
Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes
fire into the equations and makes a universe for them to describe? The usual approach of science ofconstructing a mathematical model cannot answer the questions of why there should be a universe for themodel to describe. Why does the universe go to all the bother of existing? Is the unified theory so compellingthat it brings about its own existence? Or does it need a creator, and, if so, does he have any other effect on
the universe? And who created him?
Up to now, most scientists have been too occupied with the development of new theories that describe what
the universe is to ask the question why. On the other hand, the people whose business it is to ask why, thephilosophers, have not been able to keep up with the advance of scientific theories. In the eighteenth century,
philosophers considered the whole of human knowledge, including science, to be their field and discussed
questions such as: did the universe have a beginning? However, in the nineteenth and twentieth centuries,
science became too technical and mathematical for the philosophers, or anyone else except a few specialists.
Philosophers reduced the scope of their inquiries so much that Wittgenstein, the most famous philosopher ofthis century, said, “The sole remaining task for philosophy is the analysis of language.” What a comedown fromthe great tradition of philosophy from Aristotle to Kant!
However, if we do discover a complete theory, it should in time be understandable in broad principle byeveryone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able
to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer tothat, it would be the ultimate triumph of human reason – for then we would know the mind of God.

ALBERT EINSTEIN
Einstein’s connection with the politics of the nuclear bomb is well known: he signed the famous letter to
President Franklin Roosevelt that persuaded the United States to take the idea seriously, and he engaged in
postwar efforts to prevent nuclear war. But these were not just the isolated actions of a scientist dragged into
the world of politics. Einstein’s life was, in fact, to use his own words, “divided between politics and equations.”
Einstein’s earliest political activity came during the First World War, when he was a professor in Berlin.
Sickened by what he saw as the waste of human lives, he became involved in antiwar demonstrations. His

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advocacy of civil disobedience and public encouragement of people to refuse conscription did little to endear
him to his colleagues. Then, following the war, he directed his efforts toward reconciliation and improving
international relations. This too did not make him popular, and soon his politics were making it difficult for him tovisit the United States, even to give lectures.
Einstein’s second great cause was Zionism. Although he was Jewish by descent, Einstein rejected the biblicalidea of God. However, a growing awareness of anti-Semitism, both before and during the First World War, led
him gradually to identify with the Jewish community, and later to become an outspoken supporter of Zionism.
Once more unpopularity did not stop him from speaking his mind. His theories came under attack; ananti-Einstein organization was even set up. One man was convicted of inciting others to murder Einstein (andfined a mere six dollars). But Einstein was phlegmatic. When a book was published entitled 100 Authors
Against Einstein, he retorted, “If I were wrong, then one would have been enough!”
In 1933, Hitler came to power. Einstein was in America, and declared he would not return to Germany. Then,
while Nazi militia raided his house and confiscated his bank account, a Berlin newspaper displayed theheadline “Good News from Einstein – He’s Not Coming Back.” In the face of the Nazi threat, Einsteinrenounced pacifism, and eventually, fearing that German scientists would build a nuclear bomb, proposed that
the United States should develop its own. But even before the first atomic bomb had been detonated, he waspublicly warning of the dangers of nuclear war and proposing international control of nuclear weaponry.
Throughout his life, Einstein’s efforts toward peace probably achieved little that would last – and certainly won
him few friends. His vocal support of the Zionist cause, however, was duly recognized in 1952, when he wasoffered the presidency of Israel. He declined, saying he thought he was too naive in politics. But perhaps hisreal reason was different: to quote him again, “Equations are more important to me, because politics is for thepresent, but an equation is something for eternity.”

GALILEO GALILEI
Galileo, perhaps more than any other single person, was responsible for the birth of modern science. His
renowned conflict with the Catholic Church was central to his philosophy, for Galileo was one of the first toargue that man could hope to understand how the world works, and, moreover, that we could do this byobserving the real world.
Galileo had believed Copernican theory (that the planets orbited the sun) since early on, but it was only when
he found the evidence needed to support the idea that he started to publicly support it. He wrote about
Copernicus’s theory in Italian (not the usual academic Latin), and soon his views became widely supported
outside the universities. This annoyed the Aristotelian professors, who united against him seeking to persuade
the Catholic Church to ban Copernicanism.
Galileo, worried by this, traveled to Rome to speak to the ecclesiastical authorities. He argued that the Bible
was not intended to tell us anything about scientific theories, and that it was usual to assume that, where theBible conflicted with common sense, it was being allegorical. But the Church was afraid of a scandal that mightundermine its fight against Protestantism, and so took repressive measures. It declared Copernicanism “false
and erroneous” in 1616, and commanded Galileo never again to “defend or hold” the doctrine. Galileo
acquiesced.
In 1623, a longtime friend of Galileo’s became the Pope. Immediately Galileo tried to get the 1616 decree
revoked. He failed, but he did manage to get permission to write a book discussing both Aristotelian andCopernican theories, on two conditions: he would not take sides and would come to the conclusion that man
could in any case not determine how the world worked because God could bring about the same effects inways unimagined by man, who could not place restrictions on God’s omnipotence.
The book, Dialogue Concerning the Two Chief World Systems, was completed and published in 1632, with thefull backing of the censors – and was immediately greeted throughout Europe as a literary and philosophicalmasterpiece. Soon the Pope, realizing that people were seeing the book as a convincing argument in favor of

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Copernicanism, regretted having allowed its publication. The Pope argued that although the book had theofficial blessing of the censors, Galileo had nevertheless contravened the 1616 decree. He brought Galileo
before the Inquisition, who sentenced him to house arrest for life and commanded him to publicly renounce
Copernicanism. For a second time, Galileo acquiesced.
Galileo remained a faithful Catholic, but his belief in the independence of science had not been crushed. Four
years before his death in 1642, while he was still under house arrest, the manuscript of his second major bookwas smuggled to a publisher in Holland. It was this work, referred to as Two New Sciences, even more than his
support for Copernicus, that was to be the genesis of modern physics.

ISAAC NEWTON
Isaac Newton was not a pleasant man. His relations with other academics were notorious, with most of his later
life spent embroiled in heated disputes. Following publication of Principia Mathematica – surely the mostinfluential book ever written in physics – Newton had risen rapidly into public prominence. He was appointed
president of the Royal Society and became the first scientist ever to be knighted.
Newton soon clashed with the Astronomer Royal, John Flamsteed, who had earlier provided Newton withmuch-needed data for Principia, but was now withholding information that Newton wanted. Newton would nottake no for an answer: he had himself appointed to the governing body of the Royal Observatory and then tried
to force immediate publication of the data. Eventually he arranged for Flamsteed’s work to be seized andprepared for publication by Flamsteed’s mortal enemy, Edmond Halley. But Flamsteed took the case to court
and, in the nick of time, won a court order preventing distribution of the stolen work. Newton was incensed andsought his revenge by systematically deleting all references to Flamsteed in later editions of Principia.
A more serious dispute arose with the German philosopher Gottfried Leibniz. Both Leibniz and Newton had
independently developed a branch of mathematics called calculus, which underlies most of modern physics.
Although we now know that Newton discovered calculus years before Leibniz, he published his work muchlater. A major row ensued over who had been first, with scientists vigorously defending both contenders. It isremarkable, however, that most of the articles appearing in defense of Newton were originally written by his
own hand – and only published in the name of friends! As the row grew, Leibniz made the mistake of appealing
to the Royal Society to resolve the dispute. Newton, as president, appointed an “impartial” committee toinvestigate, coincidentally consisting entirely of Newton’s friends! But that was not all: Newton then wrote thecommittee’s report himself and had the Royal Society publish it, officially accusing Leibniz of plagiarism. Still
unsatisfied, he then wrote an anonymous review of the report in the Royal Society’s own periodical. Following
the death of Leibniz, Newton is reported to have declared that he had taken great satisfaction in “breaking
Leibniz’s heart.”
During the period of these two disputes, Newton had already left Cambridge and academe. He had been active
in anti-Catholic politics at Cambridge, and later in Parliament, and was rewarded eventually with the lucrative
post of Warden of the Royal Mint. Here he used his talents for deviousness and vitriol in a more sociallyacceptable way, successfully conducting a major campaign against counterfeiting, even sending several men to
their death on the gallows.

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A Brief History of Time - Stephen Hawking... Glossary

GLOSSARY

Absolute zero: The lowest possible temperature, at which substances contain no heat energy.
Acceleration: The rate at which the speed of an object is changing.
Anthropic principle: We see the universe the way it is because if it were different we would not be here toobserve it.
Antiparticle: Each type of matter particle has a corresponding antiparticle. When a particle collides with its
antiparticle, they annihilate, leaving only energy.
Atom: The basic unit of ordinary matter, made up of a tiny nucleus (consisting of protons and neutrons)surrounded by orbiting electrons.
Big bang: The singularity at the beginning of the universe.
Big crunch: The singularity at the end of the universe.
Black hole: A region of space-time from which nothing, not even light, can escape, because gravity is sostrong.
Casimir effect: The attractive pressure between two flat, parallel metal plates placed very near to each other ina vacuum. The pressure is due to a reduction in the usual number of virtual particles in the space between theplates.
Chandrasekhar limit: The maximum possible mass of a stable cold star, above which it must collapse into a
black hole.
Conservation of energy: The law of science that states that energy (or its equivalent in mass) can neither becreated nor destroyed.
Coordinates: Numbers that specify the position of a point in space and time.
Cosmological constant: A mathematical device used by Einstein to give space-time an inbuilt tendency to
expand.
Cosmology: The study of the universe as a whole.
Dark matter: Matter in galaxies, clusters, and possibly between clusters, that can not be observed directly but
can be detected by its gravitational effect. As much as 90 percent of the mass of the universe may be in theform of dark matter.
Duality: A correspondence between apparently different theories that lead to the same physical results.
Einstein-Rosen bridge: A thin tube of space-time linking two black holes. Also see Wormhole.
Electric charge: A property of a particle by which it may repel (or attract) other particles that have a charge ofsimilar (or opposite) sign.
Electromagnetic force: The force that arises between particles with electric charge; the second strongest of
the four fundamental forces.
Electron: A particle with negative electric charge that orbits the nucleus of an atom.
Electroweak unification energy: The energy (around 100 GeV) above which the distinction between theelectromagnetic force and the weak force disappears.
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Elementary particle: A particle that, it is believed, cannot be subdivided.
Event: A point in space-time, specified by its time and place.
Event horizon: The boundary of a black hole.
Exclusion principle: The idea that two identical spin-1/2 particles cannot have (within the limits set by theuncertainty principle) both the same position and the same velocity.
Field: Something that exists throughout space and time, as opposed to a particle that exists at only one point at
a time.
Frequency: For a wave, the number of complete cycles per second.
Gamma rays: Electromagnetic rays of very short wavelength, produced in radio-active decay or by collisions ofelementary particles.
General relativity: Einstein’s theory based on the idea that the laws of science should be the same for allobservers, no matter how they are moving. It explains the force of gravity in terms of the curvature of afour-dimensional space-time.
Geodesic: The shortest (or longest) path between two points.
Grand unification energy: The energy above which, it is believed, the electro-magnetic force, weak force, andstrong force become indistinguishable from each other.
Grand unified theory (GUT): A theory which unifies the electromagnetic, strong, and weak forces.
Imaginary time: Time measured using imaginary numbers.
Light cone: A surface in space-time that marks out the possible directions for light rays passing through agiven event.
Light-second (light-year): The distance traveled by light in one second (year).
Magnetic field: The field responsible for magnetic forces, now incorporated along with the electric field, into theelectromagnetic field.
Mass: The quantity of matter in a body; its inertia, or resistance to acceleration.
Microwave background radiation: The radiation from the glowing of the hot early universe, now so greatly
red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters).
Also see COBE, on page 145.
Naked singularity: A space-time singularity not surrounded by a black hole.
Neutrino: An extremely light (possibly massless) particle that is affected only by the weak force and gravity.
Neutron: An uncharged particle, very similar to the proton, which accounts for roughly half the particles in an
atomic nucleus.
Neutron star: A cold star, supported by the exclusion principle repulsion between neutrons.
No boundary condition: The idea that the universe is finite but has no boundary (in imaginary time).
Nuclear fusion: The process by which two nuclei collide and coalesce to form a single, heavier nucleus.
Nucleus: The central part of an atom, consisting only of protons and neutrons, held together by the strong

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force.
Particle accelerator: A machine that, using electromagnets, can accelerate moving charged particles, giving
them more energy.
Phase: For a wave, the position in its cycle at a specified time: a measure of whether it is at a crest, a trough,or somewhere in between.
Photon: A quantum of light.
Planck’s quantum principle: The idea that light (or any other classical waves) can be emitted or absorbed
only in discrete quanta, whose energy is proportional to their wavelength.
Positron: The (positively charged) antiparticle of the electron.
Primordial black hole: A black hole created in the very early universe.
Proportional: ‘X is proportional to Y’ means that when Y is multiplied by any number, so is X. ‘X is inversely
proportional to Y’ means that when Y is multiplied by any number, X is divided by that number.
Proton: A positively charged particle, very similar to the neutron, that accounts for roughly half the particles in
the nucleus of most atoms.
Pulsar: A rotating neutron star that emits regular pulses of radio waves.
Quantum: The indivisible unit in which waves may be emitted or absorbed.
Quantum chromodynamics (QCD): The theory that describes the interactions of quarks and gluons.
Quantum mechanics: The theory developed from Planck’s quantum principle and Heisenberg’s uncertainty
principle.
Quark: A (charged) elementary particle that feels the strong force. Protons and neutrons are each composed ofthree quarks.
Radar: A system using pulsed radio waves to detect the position of objects by measuring the time it takes asingle pulse to reach the object and be reflected back.
Radioactivity: The spontaneous breakdown of one type of atomic nucleus into another.
Red shift: The reddening of light from a star that is moving away from us, due to the Doppler effect.
Singularity: A point in space-time at which the space-time curvature becomes infinite.
Singularity theorem: A theorem that shows that a singularity must exist under certain circumstances – inparticular, that the universe must have started with a singularity.
Space-time: The four-dimensional space whose points are events.
Spatial dimension: Any of the three dimensions that are spacelike – that is, any except the time dimension.
Special relativity: Einstein’s theory based on the idea that the laws of science should be the same for allobservers, no matter how they are moving, in the absence of gravitational phenomena.
Spectrum: The component frequencies that make up a wave. The visible part of the sun’s spectrum can beseen in a rainbow.
Spin: An internal property of elementary particles, related to, but not identical to, the everyday concept of spin.

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Stationary state: One that is not changing with time: a sphere spinning at a constant rate is stationary because
it looks identical at any given instant.
String theory: A theory of physics in which particles are described as waves on strings. Strings have length but
no other dimension.
Strong force: The strongest of the four fundamental forces, with the shortest range of all. It holds the quarks
together within protons and neutrons, and holds the protons and neutrons together to form atoms.
Uncertainty principle: The principle, formulated by Heisenberg, that one can never be exactly sure of both theposition and the velocity of a particle; the more accurately one knows the one, the less accurately one canknow the other.
Virtual particle: In quantum mechanics, a particle that can never be directly detected, but whose existence
does have measurable effects.
Wave/particle duality: The concept in quantum mechanics that there is no distinction between waves andparticles; particles may sometimes behave like waves, and waves like particles.
Wavelength: For a wave, the distance between two adjacent troughs or two adjacent crests.
Weak force: The second weakest of the four fundamental forces, with a very short range. It affects all matterparticles, but not force-carrying particles.
Weight: The force exerted on a body by a gravitational field. It is proportional to, but not the same as, its mass.
White dwarf: A stable cold star, supported by the exclusion principle repulsion between electrons.
Wormhole: A thin tube of space-time connecting distant regions of the universe. Wormholes might also link toparallel or baby universes and could provide the possibility of time travel.

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A Brief History of Time - Stephen Hawking... Acknowledgments

ACKNOWLEDGMENTS

Many people have helped me in writing this book. My scientific colleagues have without exception been
inspiring. Over the years my principal associates and collaborators were Roger Penrose, Robert Geroch,
Brandon Carter, George Ellis, Gary Gibbons, Don Page, and Jim Hartle. I owe a lot to them, and to myresearch students, who have always given me help when needed.
One of my students, Brian Whitt, gave me a lot of help writing the first edition of this book. My editor at Bantam
Books, Peter Guzzardi, made innumerable comments which improved the book considerably. In addition, forthis edition, I would like to thank Andrew Dunn, who helped me revise the text.
I could not have written this book without my communication system. The software, called Equalizer, wasdonated by Walt Waltosz of Words Plus Inc., in Lancaster, California. My speech synthesizer was donated by
Speech Plus, of Sunnyvale, California. The synthesizer and laptop computer were mounted on my wheelchair
by David Mason, of Cambridge Adaptive Communication Ltd. With this system I can communicate better now
than before I lost my voice.
I have had a number of secretaries and assistants over the years in which I wrote and revised this book. On thesecretarial side, I’m very grateful to Judy Fella, Ann Ralph, Laura Gentry, Cheryl Billington, and Sue Masey. My
assistants have been Colin Williams, David Thomas, and Raymond Laflamme, Nick Phillips, Andrew Dunn,
Stuart Jamieson, Jonathan Brenchley, Tim Hunt, Simon Gill, Jon Rogers, and Tom Kendall. They, my nurses,
colleagues, friends, and family have enabled me to live a very full life and to pursue my research despite mydisability.
Stephen Hawking

ABOUT THE AUTHOR
Stephen Hawking, who was born in 1942 on the anniversary of Galileo’s death, holds Isaac Newton’s chair as
Lucasian Professor of Mathematics at the University of Cambridge. Widely regarded as the most brilliant
theoretical physicist since Einstein, he is also the author of Black Holes and Baby Universes, published in 1993,
as well as numerous scientific papers and books.

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