NASA Probe Validates Einstein Within 1%

An anonymous reader writes "Gravity Probe B uses four ultra-precise gyroscopes to measure two effects of Einstein's general relativity theory — the geodetic effect and frame dragging. According to the mission's principal investigator, the data from Gravity Probe B's gyroscopes confirm the Einstein theory's value for the geodetic effect to better than 1%. In a common analogy, the geodetic effect is similar to the shape of the dip created when the ball is placed on to a rubber sheet. If the ball is then rotated, it will start to drag the rubber sheet around with it. In a similar way, the Earth drags local space and time around with it — ever so slightly — as it rotates. Over time, these effects cause the angle of spin of the satellite's gyroscopes to shift by tiny amounts." The investigators will be doing further data analysis over the coming months and expect to release final results late this year.

Comments

oops

[Tom Womack]

Basically, the mission hasn't yet succeeded, and it doesn't seem to be completely certain that it will.

The goal was to measure the frame-dragging effect of the Earth, which is of the order of 40 milli-arcseconds per year; the current calibration (http://einstein.stanford.edu/content/aps_posters/ ExperimentError.pdf) has a one-sigma error of 100 milli-arcseconds per year, significantly larger than the relativistic effect and significantly larger than the effect from the motion of the target star through space. The initial expectation was for an error budget of less than 0.5 mas per year, so there was a lot of work done on measuring the proper motion of the star to that precision.

The problems turn out to be really crazily subtle issues in solid-state physics -- the deposited metal films on the gyroscope and on its housing retain charge in patches large enough that they have to be modelled rather than averaged out -- plus an annoying issue from classical mechanics where the motion of a rigid body around three axes XYZ depends on the ratio of the differences of the moments of inertia X-Z and Y-Z. Whilst the gyroscopes are absurdly precisely made, so the moments of inertia are very close, the ratio of the differences of the moments of inertia is a macroscopic number; this is the same effect, and even a similar cause, to some of the oddities with low-precision floating-point arithmetic.

They'll probably be sorted out, sigma might be reduced by a factor ten after another year's modelling effort, but it seems unlikely that they'll get it down by the factor 200 they were hoping for.

The frame-dragging has already been measured indirectly by looking at the flickers of X-ray sources caused by frame-dragging-induced precession of the accretion discs around black holes, and most of the theories that suggest relativity is wrong would suggest that any oddities would be more pronounced in the huge gravitomagnetic fields near black holes than in the tiny fields near a mass as small, as non-dense and as slowly rotating as Earth.