Gravity Probe B the orbiting observatory devoted to testing the general theory of relativity, has measured the geodetic effect-the warping of spacetime in the vicinity of and caused by Earth-with a precision of 1%. The basic approach to studying this subtle effect is to monitor the precession of gyroscopes onboard the craft in a polar orbit around the Earth. The observed precession rate, 6.6 arc-seconds per year, is close to that predicted by general relativity.
The geodetic effect can be measured in several ways, including the use of clocks, the deflection of light, and the perturbative influence of massive bodies on nearby gyroscopes. GP-B is of the latter type, and its current precision is as good as or better than previous measurements. And once certain unanticipated torques on the gyroscopes are better understood, GP-B scientists expect the precision of their geodetic measurement to improve to a level of 0.01%.
These first GP-B results were reported at the APS meeting by Francis Everitt (Stanford).
The idea for using gyroscopes to observe the warping of spacetime was proposed almost 50 years ago, and Everitt has been an active proponent and then scientific overseer of the project for much of that subsequent time.
A second major goal of GP-B is to measure frame dragging, a phenomenon which arises from the fact that space is, in the context of general relativity, a viscous fluid rather than the rigid scaffolding Isaac Newton took it to be. When the Earth rotates it partly takes spacetime around with it, and this imposes an additional torque on the gyroscopes.
Thus an extra precession, perpendicular to and 170 times weaker than for the geodetic effect, should be observed. Everitt said that GP-B saw “glimpses” of frame dragging in this early analysis of the data and expects to report an actual detection with a precision at the 1% level by the time of the final presentation of the data, now scheduled for December 2007. (An indirect measurement of frame dragging at the 10-15% uncertainty level was made earlier by the LAGEOS satellite.)
Some of the GP-B equipment is unprecedented.
The onboard telescope used to orient the gyroscopes (by sighting toward a specific star) provided a star-tracking ability better by a factor of 1000 than previous telescopes. The gyroscopes themselves-four of them for redundancy-are the most nearly spherical things ever made: the ping-pong-ball-sized objects are out of round by no more than 10 nm. They are electrostatically held in a small case, spun up to speeds of 4000 rpm by puffs of gas. The gas is then removed, creating a vacuum of 10^-12 torr. Covered with niobium and reposing at a temperature of a few kelvin, the balls are rotating superconductors, and as such they develop a tiny magnetic signature which can be read out to fix the sphere’s instantaneous orientation. (For more information see einstein.stanford.edu)