Gravity Probe B (GP-B)

Gravity Probe B (GP-B) has measured the geodetic effect—the warping of spacetime in the vicinity of and caused by Earth—with a precision of 1%. The tiny effect was observed via the precession of gyroscopes onboard the craft, which is in a polar orbit around Earth. The observed precession rate, 6.6 arcseconds per year, is consistent with the prediction of general relativity and with the only other measurement of the effect, which used the Earth–Moon system orbiting the Sun. Once certain subtle disturbance torques on the gyroscopes are better understood, GP-B scientists expect the precision of their geodetic measurement to improve to 0.01%. The first results were reported at the April meeting of the American Physical Society by Francis Everitt (Stanford University). The other major goal of GP-B is to measure frame dragging: When Earth rotates, general relativity predicts that it drags space and time around with it, causing a different gyroscopic precession, perpendicular to and 1/170 as strong as the geodetic precession. Everitt said that GP-B saw “glimpses” of frame dragging in the early analysis of the data; the researchers expect to report a credible, thoroughly cross-checked measurement at the final results presentation now scheduled for December 2007. The four gyroscopes are the most spherical objects ever made: The ping-pong-ballsized orbs are out of round by no more than 10 nm. They are electrostatically suspended in a small case (shown here) spun up to speeds of 4800 rpm, and held in a vacuum of 10⁻¹⁴ torr. Coated with niobium, the balls are rotating superconductors at a few kelvin, and develop tiny magnetic dipole moments that can be read out to fix the spheres’ instantaneous orientations. (See http://meetings.aps.org/Meeting/APR07/Event/64567 for the abstract.)