Gravity Probe B Finds Frame-Dragging...

Image Credit: New Scientist

The beleaguered Gravity Probe B mission has finally measured a subtle effect of general relativity called frame dragging. The result comes nearly six years after it finished making measurements and years after other experiments measured the effect to greater precision.

Frame dragging is distortions in the fabric of the spacetime metric, making the orbit of a nearby particle precess: the slow, conical motion of the axis of rotation, caused by the gravitational attraction of a mass object (sun-to-earth, earth-to-moon). It "drags the frame" of spacetime with the influenced particle/planet/astronaut/etc.

Gravity Probe B (<--web site) was conceived as a physics experiment by NASA and Standford University. From the site:

"Gravity Probe B (GP-B) is a NASA physics mission to experimentally investigate Albert Einstein's 1916 general theory of relativity—his theory of gravity. GP-B used four spherical gyroscopes and a telescope, housed in a satellite orbiting 642 km (400 mi) above the Earth, to measure in a new way, and with unprecedented accuracy, two extraordinary effects predicted by the general theory of relativity (the second effect having never before been directly measured):


"1.The geodetic effect—the amount by which the Earth warps the local spacetime in which it resides.


"2.The frame-dragging effect—the amount by which the rotating Earth drags its local spacetime around with it.

"The GP-B experiment tests these two effects by precisely measuring the displacement angles of the spin axes of the four gyros over the course of a year and comparing these experimental results with predictions from Einstein's theory.


"GP-B is actually the second dedicated NASA physics experiment to test aspects of general relativity. The first, Gravity Probe A, was led in 1976 by Dr. Robert Vessot of the Smithsonian Astrophysical Observatory. Gravity Probe A compared elapsed time in three identical hydrogen maser clocks—two on the ground and the third traveling for two hours in a rocket, and confirmed the Einstein redshift prediction to 1.4 parts in 10^4."