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Relativity's strongest test begins: if "new physics" lurks beyond Einstein's general relativity, Gravity Probe B may find it.

If "new physics" lurks beyond Einstein's general relativity, Gravity Probe B may find it.

AFTER NEARLY 40 YEARS of engineering and 10 years of lobbying to keep it alive, on April 20th scientists at Stanford University and NASA finally saw Gravity Probe B (GP-B) take flight to carry out the most accurate tests yet of Einstein's general theory of relativity.

The satellite took up a circular orbit passing 400 miles (640 kilometers) above Earth's poles. It will measure two effects on the fabric of space-time predicted by general relativity. One is frame-dragging, the twisting of space caused by Earth's rotation. This effect has never been directly detected, though astronomers think they have found evidence of it near spinning black holes. The other is the "geodetic effect," the curvature of space-time due to the presence of mass, which is already well known but will be measured more accurately than before--in hopes that new physics beyond relativity might start showing up.

Using spherical gyroscopes aligned with the utmost precision to a distant guide star, GP-B will be able to measure directional change caused by the warping of near-Earth space to an accuracy of a ten-thousandth of an arcsecond. To achieve this, engineers had to overcome a host of challenges to provide an undisturbed space-time reference system with extraordinary isolation from all outside forces.

Albert Einstein published his general theory of relativity in 1916, proposing a four-dimensional fabric of space and time that is shaped by the mass and energy within it. In this scheme, an object in free fall travels through space along a "geodesic"--a curve (rather than a straight line) that represents the shortest possible path between two points. In most situations the geodetic effect is extremely small, but it is enough to explain every known property of gravity with perfect precision--so far.

The most accurate measurement of the geodetic effect to date, involving gravitational lensing of radio signals from the Cassini spacecraft, matches Einstein's prediction to about 1 part in 40,000 (when measured in terms of a quantity called gamma; see the January issue, page 18). GP-B should improve that to between 1 part in 100,000 and 1 part in 250,000.

However, the most anticipated observation that GP-B will attempt is of frame-dragging. According to general relativity, the rotation of a massive object twists the space-time around it. This twisting is much weaker than the geodetic effect, but GP-B should measure it to an accuracy of about 1 part in 100.

Rex Geveden, GP-B program manager at NASA's Marshall Space Flight Center, explains some of the technical hurdles the satellite had to overcome. "The spacecraft flies in a drag-free orbit, which means that the gyroscope rotors are literally in free fall about the Earth, and the spacecraft flies around the gyroscopes." The craft maintains its course by detecting any slight deviations from the free-falling gyroscopes and using tiny thrusters to bring the craft back into line with them.

Four spherical gyroscopes of solid quartz are used, two for each effect. Each is 1.5 inches (3.8 centimeters) in diameter and a million times more accurate than the best navigation gyroscopes. These are the most perfect spheres within light-years, deviating from perfection by no more than the thickness of 40 atoms; only neutron stars rival them. The gyroscopes are electrically suspended and spin in a vacuum chamber 9 feet long encased in superfluid liquid helium. The chamber is fused to a 5.6-inch telescope trained on a specially selected guide star to provide an orientation standard good to 0.1 milliarcsecond.

A thin coating of niobium metal on the spheres sets up a slight magnetic field, which is shielded from the outside by superconducting lead foil enclosing the vacuum chamber. The spheres' magnetic field provides a rotation axis measured by a SQUID (Superconducting QUantum Interference Device) that can detect about 0.1 milliarcsecond of drift.

"If general relativity fails, such a failure would have tremendous consequences for fundamental physics," says theorist Kip Thorne of Caltech. "What we are going after in GP-B, in terms of the search for any deviation of frame-dragging from general relativity's prediction, is a search for whether or not the foundations for general relativity are there. Is gravity really a metric theory [based on warpage of space-time], or not? That is, in some sense, what's being probed."
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Author:Johnston, Lisa R.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2004
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