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Einstein and Beyond
MAY 2005
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Putting Relativity to the Test
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Photo: Gravity Probe B Satellite
Illustration courtesy Gravity Probe B Image Archive at Stanford University

The most controlled experimental test of the general theory of relativity is orbiting the Earth in a three-ton satellite known as Gravity Probe B. First proposed in 1960, this project is a collaboration of Stanford University, NASA, and Lockheed Martin Space Systems.

By Miki Meek

After more than a year of orbiting the Earth, the latest and most technically precise experiment testing Albert Einstein's revolutionary general theory of relativity is in its final phase of data collection. 
Over the next few months scientists from Stanford University will finish gathering data from Gravity Probe B (GP-B), hoping to measure two effects predicted by the 1916 theory, Einstein's description of gravity.
"Almost all physicists would agree that Einstein's relativity is the most intellectually beautiful of all the theories in physics," says Francis Everitt, the principal investigator for GP-B and a professor at Stanford University. "But if you ask the embarrassing question of how many tests do we really have that it's right, then the answer is not very many."
According to relativity, massive bodies like the Earth or a star indent and bend a four-dimensional fabric called space-time. This indentation, known as the geodetic effect, is what we experience as gravity. Think of a bowling ball in the middle of a mattress. The depression created would pull objects with a small mass on the mattress toward the bowling ball, just as lighter objects fall toward Earth. Although the warping of space-time has been observed, no one has ever measured it in such a controlled experiment. 
The general theory of relativity also predicts a second effect, which has never been verified. Known as frame-dragging, the Earth's rotation drags local space-time around. This is similar to the way a baseball spinning in a bowl of honey would drag some of the honey around with it. 
GP-B, orbiting 401 miles (642 kilometers) above Earth, is searching for these effects with four supersensitive gyroscopes. Made of fused quartz, these gyroscopes, each the size of a Ping-Pong ball, can spin 10,000 times a minute and are the most perfectly round objects ever engineered. They are housed inside a huge thermos bottle filled with supercooled liquid helium, which stabilizes the temperature. The gyroscopes are also aligned with a guide star, called IM Pegasi, to give scientists a reference point for measuring adjustments in their spin axes.
Both the geodetic effect and frame-dragging should cause the gyroscopes' spin axes to wobble as GP-B circles the Earth. For the geodetic effect, scientists calculated that the spin axes should change by an angle of 6.6 arcseconds a year, relative to the guide star. The drift for frame-dragging should be 0.41 arcseconds in the direction of the Earth's rotation.
Scientists have allocated at least a year to analyze the completed data before releasing a report. If results match predictions based on Einstein's theory, then the experiment will solidify fundamental assumptions that have been made about the universe and the mathematics of relativity.
It would also help scientists calculate the much greater frame-dragging that should occur near the edge of spinning black holes. Search for evidence of this is a major object of Beyond Einstein, a NASA program dedicated to studying the structure and evolution of the universe.
However, if the data are inconsistent with Einstein's predictions, then scientists may have to start searching for a theory to amend relativity, a cornerstone of modern physics.  
"We know in some sense that Einstein's theory can't be the last word because it's incompatible with quantum physics [a theory that describes atoms and subatomic particles]," says Everitt, who's worked more than 40 years to get GP-B off the ground.
"I'm not saying that GP-B will be the sure provider of clues on this subject. But if we discover something that doesn't fit, then maybe that will give us a clue about the theoretical road we ought to be going down."
Related Links

Gravity Probe B
Read more about the GP-B experiment, find a photo gallery, and get the latest updates.
Beyond Einstein
Find out about a series of astrophysics missions designed to answer three questions: What powered the big bang? What happens to space, time, and matter at the edge of a black hole? And what mysterious dark energy is pulling the universe apart?
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