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...

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