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



 
 
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Old February 22nd 07, 05:05 AM posted to sci.space.news
Andrew Yee[_1_]
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Default Steering atoms toward better navigation, physicists test Newton and Einstein along the way (Forwarded)

News Service
Stanford University
Stanford, California

Contact:
Dawn Levy, News Service
(650) 725-1944

Comment:
Mark Kasevich, Physics
(650) 723-4356

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.]

-30-

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...vich_mark.html
* American Association for the Advancement of Science
http://www.aaas.org/
 




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