The Gravity Probe B Bailout

GP-B scientists soon found, however, that the rotors were not just tracing a well-known Newtonian pattern with tiny sidesteps predicted by Einstein. Rather, those spinning rotors were also heeding a third influence: electromagnetism. The final patch of niobium sprayed onto the rotors had effectively polarized the sphere and left a tiny surplus charge that, when spun up and translated into magnetic fields, added a new layer of wobble to account for. GP-B collected data for 353 days in 2004 and 2005 and then spent an additional 46 days conducting tests on the gyros to deduce precisely where those additional tiny magnetic fields lay.

Fortunately, GP-B’s extra wobble can be computationally simulated and thus subtracted from the signal. But it requires painstaking number crunching to derive the magnetic influence on each gyro at each moment in its 353 days of observation. The results so far have been a confirmation of the geodetic effect as predicted by relativity, with the confidence level the team had hoped for: 1 percent.

Frame dragging, a weaker effect, has been more difficult to extract from the data. Confirming, amending, or disproving this most peculiar prediction of relativity requires a more exacting reduction of the niobium-coating noise. Everitt says that the work his team did through September—and will continue into 2009—involves squeezing one clever twist from the data that they hope will enable them to extract the minuscule frame-dragging signal.

The potential solution arises from an observation from the 18th century. In 1729, a British astronomer named James Bradley discovered that the apparent position of stars in the sky, as seen through a telescope, varied by tiny amounts throughout the course of a year. He discovered something called stellar aberration, a small tweaking of a star’s position produced by the fact that any telescope is moving through space as Earth moves around the sun. And with the star’s light moving at the speed of light, it actually takes about a nanosecond for the light to move from the outer lens of the telescope into the eyepiece or, in the case of GP-B’s telescope observing its reference point IM Pegasi, onto the light-sensitive chip that records the star’s light. During that nanosecond, the telescope will have moved a tiny bit. The direction and amount by which the telescope moves during that nanosecond varies depending on the time of year. So there’s a natural wobble—an aberration, in the technical parlance—of any star’s position throughout the course of the year. On top of the yearly aberration, the orbiting GP-B telescope experienced an additional aberration as the satellite traced out its motion around Earth.

GP-B chief scientist George ”Mac” Keiser realized that the slight orbital and annual aberration of IM Pegasi’s image—when compared with the direction in which the gyroscopes were pointing—would produce a further perceived wobble in the data. But this virtual wobble was a known, well-understood phenomenon and could be simply calculated. And since each of the four gyroscopes was experiencing different electromagnetic effects from its outer niobium coating, the aberration could in fact serve as a ”reference wobble” that would enable GP-B number crunchers to sort out how each gyroscope was being torqued by the niobium coating.

Subtract the niobium effect, Everitt says, and GP-B’s signal should be much closer to the ideal single-digit percentage error bars that his team has been shooting for.

So GP-B’s verdict on frame dragging, one of relativity’s most astonishing predictions, may yet emerge from the 2 terabytes of raw data now shared by the Stanford GP-B analysis team’s 20 Sun Microsystems workstations.

”This is what Bill Fairbank used to call the ’anti-Murphy Law,’” said Everitt. ”When you finally understand what you’re doing, nature comes to help.”

Acknowledgements

Paul S. Wesson is grateful to Francis Everitt and James Overduin of Gravity Probe B for their hospitality, and to NSERC for support.