Steering atoms toward better navigation, physicists test Newton and Einstein along the way (Forwarded)

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February 13, 2007

Steering atoms toward better navigation, physicists test Newton and
Einstein along the way

BY Kristin Abkemeier

Stanford physicist Mark Kasevich has adapted the technology in today's
airplane navigation systems to work with atoms so cold that they almost
stand still. At temperatures scarcely above absolute zero, atoms no longer
behave as particles but rather as de Broglie waves, named for the theorist
who originally posited that all matter behaves as both a light wave and as
a particle. These waves can be configured to add or subtract, or
interfere, with one another in an interferometer -- an instrument that is
used on airplanes to measure very small changes in rotation. Since global
positioning system (GPS) location information is not available everywhere,
airplanes still use inertial navigation systems founded on laser-based
interferometers, even though their accuracy drifts over time. Kasevich's
"atomic interferometer" may form the basis of a next-generation navigation
system that gauges the airplane's location much more accurately.

"Navigation problems -- how to get from point A to point B -- tell us
about space-time," says Kasevich, a professor in the departments of
Physics and Applied Physics who will speak about atomic sensors Feb. 17 in
San Francisco at the annual meeting of the American Association for the
Advancement of Science (AAAS). "When we build these de Broglie wave
navigation sensors, we're also building sensors that can test these
fundamental laws about space-time."

Kasevich's atomic interferometer also is a sensitive detector of gravity
-- by far the weakest of the four fundamental forces of physics. Kasevich
and his research group are using the interferometer to measure the
gravitational constant, G, to greater precision than has ever been reached
in the more than three centuries since Isaac Newton put forward his law of
universal gravitation. Moreover, Kasevich is putting another physics
legend to the test in ongoing research of Einstein's century-old principle
of equivalence, which states it is impossible to tell the difference
between the acceleration of an object due to gravity and the acceleration
of its frame of reference.

The panel in which Kasevich is speaking is titled "What's Hot in Cold."
Other participants include Tom Shachtman, author of the nonfiction book
Absolute Zero and the Conquest of Cold, as well as physicists Heather
Lewandowski of the University of Colorado-Boulder; Steven M. Girvin of
Yale University; Richard Packard of the University of California-Berkeley;
and Moses Chan of Pennsylvania State University-University Park. They will
describe how matter cooled to low temperatures behaves according to the
laws of quantum mechanics, which operate quite differently from the
familiar world of classical physics. Whether gas, liquid or solid, each
system in this ultracool regime proves to be a rich trove of new physics.

Interferometry -- old and new

Navigation technology inspired Kasevich's atomic sensors. Airplanes
monitor their attitude with ring-laser gyroscopes, which use
interferometry to detect rotation. In conventional interferometers, a
single-wavelength beam from a laser is split into two paths and later
recombined so that the final wave exhibits a characteristic pattern. This
interference pattern will differ depending upon the differences in paths
traveled by the two split waves. If the paths are identical, they will
recombine as the original wave. But as the airplane with its gyroscope
turns, rotation of the interferometer inside changes one split wave's path
relative to the other, and the difference causes the recombined wave to
partially dim. With a large enough shift between the split paths, the
recombined wave can vanish entirely in what is known as total destructive
interference.

Kasevich's team applies this principle using not laser light but cesium
atoms. As an atom is cooled to very low temperatures, below minus-459 F,
its velocity slows to zero, and -- due to the principles of quantum
mechanics -- the atom begins to behave like a wave, just as in Louis de
Broglie's Nobel Prize-winning prediction of 1923. The colder and therefore
slower the cesium atom becomes, the longer its wavelength. Ultimately
these wave-like atoms can get so cold that they reach wavelengths
comparable to visible light. And they can be split and made to recombine
just as in a conventional laser interferometer, yielding the atomic
interferometer.

The most bizarre property of the atomic interferometer, Kasevich says, is
that total destructive interference makes atoms seem to disappear.

"Nature lets me take this atom, split it in half and bring it back
together," he says. "The cesium atom is in two places at once, and nature
lets it do that. You can't do that with marbles."

But matter is neither created nor destroyed. "We're manipulating the
probability of where we find the matter in space," Kasevich clarifies.

Substituting an atomic interferometer for a conventional one inside an
airplane's ring-laser gyroscope would yield an atomic gyroscope. The
atomic gyroscope, if it could be produced at a portable size, would be a
desirable replacement for ring-laser gyroscopes because the older
technology loses accuracy in gauging the airplane's location to the tune
of about 1 mile (1852 meters) per hour. By comparison, an atomic sensor
could lead to drifts of around 16 feet (5 meters) per hour -- three
one-thousandths of the error.

G attracts Kasevich's interest

Besides their potential for improving navigation accuracy, Kasevich's
atomic interferometers or sensors also are sensitive enough to detect
changes in the split wave induced by gravity. The level of sensitivity is
fine enough to be able to detect changes in gravity at levels below one
part per billion. Gravity is the longest known of all fundamental physical
forces. Kasevich's group continues to work to refine the atomic sensors in
hopes of measuring Newton's gravitational constant G beyond the level of
precision at which it has been measured -- a figure that has not improved
much since British natural philosopher Henry Cavendish published the first
measurement more than two centuries ago.

"We want to add our voice to the chorus of 'What is G really?' " says
Kasevich.

Another mystery that ultracold atoms may help solve is Einstein's
equivalence principle, which to date hasn't been proved or refuted. In his
equivalence principle, Einstein asserted the gravitation experienced while
standing on a massive body, such as Earth, is the same as the pseudo-force
experienced by an observer in an accelerated frame of reference. Just like
a spinning dancer's body causes her skirt to twirl, the revolving Earth
drags space and time around it, providing the frame of reference from
which we determine positions and movements.

An ongoing experiment to test this principle is set up in a 10-meter-tall
tube installed in the basement of the Varian Physics Building at Stanford.
It employs isotopes -- atoms of a chemical element with the same atomic
number and nearly identical chemical behavior but with different atomic
masses. Two different isotopes of rubidium are cooled to ultralow
temperature and released into free fall. The wave-like atoms fall very
slowly, "like releasing a fistful of sand," Kasevich says. If the two
isotopes, which have slightly different masses, accelerate at differing
rates as measured with atomic interferometry, this means the principle of
equivalence fails.

The implications are profound, Kasevich says. "If Einstein's equivalence
principle doesn't hold, that means that we would have to rethink the law
of physics at a very basic level."

[Kristin Abkemeier is a freelance writer.]

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Editor Note: The symposium will take place Saturday, Feb. 17, from 2 to 5
p.m. at the Hilton San Francisco, 333 O'Farrell St., San Francisco, CA
94102, Continental Ballroom 3.

A photo of Kasevich is available on the web at
http://newsphotos.stanford.edu/.

Relevant Web URLs:

* Mark Kasevich's Webpage
http://www.stanford.edu/dept/physics/people/faculty/kasevich_mark.html
* American Association for the Advancement of Science
http://www.aaas.org/


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