Atomic Clocks and Additional Improved Atomic Instruments for Navigating
Interstellar Space

(1372 words, minus poem)

In John Masefield\'s poem, \"Sea Fever,\" all he needs to roam the seas
is \"a tall ship and a star to steer her by.\" At the time he wrote the
poem, at the beginning of the 20th century, navigating the open seas by
the stars had been made much more accurate by the invention of the
chronometer in 1735.

Some day humankind will want to navigate between the stars as we advance
to inter-stellar travel. To find position in open space, space travelers
will need even more accurate tracking instruments. For example, much
like the chronometer helped sailors navigate the open sea, atomic clocks
may provide the reference time for space navigators to calculate their
position in interstellar space. Studies of laser-cooled atoms in
low-temperature physics should help us design more accurate instruments
for navigation.

For centuries sailors guided their ships across the oceans by finding
their position through observing the sun, moon, stars and planets.
Because of the Earth\'s rotation, the celestial bodies appear to change
their paths across the sky and to rise and set at new points along the
horizon. To figure out position, a sailor must calculate it from
observing the movement of the positions of the stars. The navigator
measures the altitudes of two or more celestial bodies with a sextant
and then uses astronomical tables---that document the position of the
stars for every second of every day---to calculate his or her own
position relative to the unchanging stars.

To accurately navigate even today a sailor has to know his time relative
to the astronomical tables to the nearest second. The instrument that
made timekeeping of this accuracy possible on the open sea was the
chronometer. The chronometer was an extremely accurate clock kept set to
Greenwich mean time, which is the time used in the astronomical tables.

Today radio signals can help a navigator find position in the open sea.
But before the advent of radio time signals, the chronometer was the
only device that provided the time accurately enough for a ship at sea
to determine its longitude. The first chronometers were mechanical,
built like a massive clock with a spring-driven timekeeper. Devices in
the chronometer compensated for changes in the tension of the spring
caused by changes in temperature to make them more accurate. Modern
chronometers are electronic, using the vibrations of a quartz crystal to
keep time.

Space travelers will have to navigate four dimensions: the three
dimensions of space as well as the fourth dimension of time. In
Einstein\'s theory of general relativity, space and time are intertwined
and affect each other in relation with each other. The problem faced by
space travelers is that in space, time changes with gravitational
effects (because of gravity\'s relation to space-time).

The fabric of space-time is a stretch fabric. In Einstein\'s theory,
massive objects bend the space-time fabric, channeling the way planets
and stars can move. When a massive object distorts space-time, other
objects tend to fall toward the resulting valley, which we call
gravitational attraction, an effect of space-time.

Cartesian geometry provides an analogy. We can understand the familiar
shapes of countries and continents on Earth in flat, two-dimensional
maps, but they don\'t reflect the curvature of the Earth. Cartesian maps
translate the curvature of the Earth onto a flat, two-dimensional map,
making the familiar shapes of countries look distorted. In the same way,
massive spinning objects distort space-time so it is no longer a linear
three-dimensional place. Indeed gravity does not just cause a warp or
depression in the stretch fabric of space-time, but a whirling
depression like an upside-down tornado that bends space-time and drags
the frame of reference---time and spatial dimensions---into a new state.
This hypothesis was supported by studies of black holes and neutron
stars in 1997.

Finding position in space, like navigating on the open sea, will still
rely on navigating by the stars. Given the challenges of space-time,
calculating a position in space will require very accurate time relative
to standard time on Earth for reference. Fortunately, we have much more
accurate clocks today. The most accurate clocks are atomic clocks.

In an atomic clock, the steady \"tick\" of the clock, supplied by an
oscillator is calibrated against the natural frequency of an atom. As
accurate as the natural frequency of an atom is, as we have seen, time
and matter are affected by phenomenon in space. To keep very accurate
time in space, we need to better understand gravitational effects on
atoms. Experiments on slowed, cooled atoms provide this opportunity.
Physicist hope to use the knowledge gained to improve two instruments
that rely on atoms that are important for space navigation, both atomic
clocks and atomic interferometers, which measure gravity and rotation.

Our orbiting laboratory on the International Space Station (ISS) can
help improve the accuracy of precise measurements for these experiments.
\"Time will be affected differently in different gravitational
fields---near a neutron star, a black hole, or in space. Near a neutron
star, a second will pass very slowly,\" says NASA research manager Mark
Lee. Einstein predicted in 1915 that a clock would tick slower (clocks
ticked in 1915) in strong gravity than they do in weak gravity. \"So
more accurate atomic clocks will help us better understand the fabric of
space and time,\" says Lee. Two experiments on the space station will
put a cesium and a rubidium atomic clock on the space station for one
year. The cesium clock experiment, called PARCS, will fly first in 2004
or 2005.

Gravity is slightly weaker on the ISS than at the Earth\'s surface. So a
clock aboard the space station will be one second faster every 10,000
years than a clock on Earth. The PARCS experiment will compare the time
kept by the cesium clock on the station with the master atomic cesium
clock of the United States, which is at the National Institute of
Standards and Technology (NIST) in Boulder, Colorado. NIST researchers
will attempt to measure this difference 100 times more accurately than
previously using the most accurate clock ever built, the PARCS clock
that keeps time accurately to within 1 second in 300 million years.

To achieve this high accuracy, PARCS will use atoms that have been
cooled to a temperature of just 1 millionth of a degree above absolute
zero. These lower temperatures allow the natural frequency of the atom
to be measured much more accurately. Also, by performing the experiment
in space, atoms can be observed for a longer time. The space station and
all the objects aboard it are really free-falling in space, which is
what produces the effect of weightlessness on board. Taking away the
effect of gravity, the atoms can be observed for a longer time before
they hit the walls of their container. This longer observation time
allows for a more precise measurement of an energy level.

Building on the PARCS experiment, the RACE experiment will put a
rubidium atomic clock on the space station around 2006. The RACE clock
will keep time so well that if it ran for three billion years it would
lose less than 1 second! Rubidium atoms interact less than cesium atoms,
so they make for more accurate clocks. Future clocks based on the
technology developed for RACE might be used to coordinate all of the
world\'s clocks, as well as for telecommunications and navigation---both
on Earth and in space.

Yale\'s RACE clock will also use a novel \"two clocks in one\"
configuration featuring two back-to-back sets of clock cavities, with a
source of ultra-cold atoms with a temperature of just one-millionth of
degree above absolute zero between them. This design reduces other
sources of error, including vibrations of the space station.

These space-age experiments will give us the ability to compensate or
calculate for the gravitational differences in atomic clocks and other
atomic measuring instruments like interferometers in much the same way a
chronometer compensates for how differences in temperature effect the
inner workings of that clock. Knowing that your clock will only lose one
second in over three billion years should be a comfort for space
travelers light years from home, who can\'t call to check their watch
against the latest time on Earth.

**Link** to Sea Fever at
www.blupete.com/Literature/Poetry/MasefieldSeaFever.htm

Sea Fever

I must go down to the seas again, to the lonely sea and the sky

And all I ask is a tall ship and a star to steer her by;

And the wheel\'s kick and the wind\'s song and the white sail\'s
shaking,

And a grey mist on the sea\'s face, and a grey dawn breaking,

I must go down to the seas again, for the call of the running tide

Is a wild call and a clear call that may not be denied;

And all I ask is a windy day with the white clouds flying,

And the flung spray and the blown spume, and the sea-gulls crying

I must go down to the seas again, to the vagrant gypsy life,

To the gull\'s way and the whale\'s way where the wind\'s like a whetted
knife;

And all I ask is a merry yarn from a laughing fellow-rover,

And quiet sleep and a sweet dream when the long trick\'s over.

By John Masefield (1878-1967).

(English Poet Laureate, 1930-1967.)

Link to \"The six-minute guide to space-time\":

http://whyfiles.org/052einstein/frame_drag4.html