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The Gravity Probe
By Gary Taubes

If Einstein was right, a spinning planet should twist the fabric of space-time. To see the effect, all we need is a perfect gyroscope. And a perfect telescope. And a perfect vacuum in a perfect chamber in an orbit 400 miles up. After 40 years of planning and half a billion dollars, the test is about to begin.

In late 1959 Leonard Schiff, then a professor of physics at Stanford, noticed a magazine ad for a new kind of gyroscope. The ad prompted a discussion with his colleague William Fairbank, an aficionado of low-temperature physics, who asked what he would do with a perfect gyroscope, should such a device ever be constructed.

Schiff's answer was just what Fairbank had in mind himself: Use it to test Einstein's general theory of relativity. In particular, test one of the theory's more remarkable predictions--that a rotating body in space would pull the very essence of space-time around with it. The effect, known as frame-dragging, would be small enough to serve as a pedagogical illustration of the word infinitesimal, but it should exist. And it would take a close to perfect gyroscope to observe it.

In 1960, Fairbank, Schiff, and Robert Cannon, a Stanford engineer, took this idea and initiated the Stanford Gyroscope Experiment, as they it called at the time. According to local legend, the three were dripping wet when they made their decision; Francis Everitt, the Stanford physicist who has been heading the project since 1971, calls this "the famous story you've heard ad nauseam about the swimming pool and the three naked men." In the pool, they also agreed that if they wanted the gyroscope to be perfect, they would have to put it in a satellite and send the satellite into orbit.

Thirty-seven years later, what began as a seemingly rhetorical question has grown into NASA's longest running astrophysical development program. Gravity Probe B, as it's now called (Gravity Probe A was a 1976 experiment that tested another aspect of relativity), first found support from NASA in November 1963 and has been proceeding not quite apace ever since. Over the years, a series of NASA-instituted review boards have applauded the program for its remarkable technological accomplishments. In the 1980s it was even designated the centerpiece for the agency's gravitational physics program. Yet it has also been canceled, so far, seven times. As of today, Gravity Probe B is scheduled to be launched into orbit some time between December 1999 and October 2000, a mere four decades after its conception.

Forty years may seem like an extended gestation period for a single experiment. But if nothing else, Gravity Probe B will perform the most accurate confirmation ever of Einstein's theory of relativity, and that alone, its proponents insist, would make the mission worth the investment. Then again, it's possible that the probe will measure a phenomenon quantitatively different from the prediction of general relativity. If so, says Everitt, who seems fond of rating physics discoveries on a scale of one to ten, "that would probably be a nine or a ten." But, he adds, "let's not second-guess what Gravity Probe B is going to do."

The study of general relativity has become something of an anachronism in physics. The theory is so deeply rooted in our science and culture that those who study it sometimes seem to have left physics for philosophy. In the universe as Einstein saw it, which he called the four-dimensional space-time, nothing was absolute. The geometry of space would act on matter, telling it how to move. In turn, the presence of matter or energy--the two are equivalent in an Einsteinian universe--would act on space, telling it how to curve. Gravity was no longer a mysterious force acting at a distance but the result of an object trying to travel in a straight line through space curved by the presence of material bodies.

Not only was general relativity conceptually beautiful, but it was checked and confirmed quickly. Among Einstein's predictions was that light from a distant star would appear to bend inward as it passed through the gravitational well of the sun, and therefore when Earth, the sun, and the star were lined up, the star's image would appear to have moved outward from its normal position. All that was needed for the experiment was a good camera and a solar eclipse, which would line up the three bodies while allowing the star to be seen. When nature provided the latter on May 29,1919, photographs confirmed that we lived in a curved four-dimensional space-time.