More Evidence for General Relativity

In April 2004, NASA launched a science satellite called Gravity Probe B; it contained an experiment to test two predictions of Einstein’s theory of General Relativity.   The theory predicts that a massive object, such as the Earth, warps the four-dimensional space-time around it; this is sometimes referred to as the geodetic effect.  A common analogy is placing a bowling ball on a trampoline or water bed — the mass of the ball causes the surface to be distorted.   A second prediction is the rotation of an object, such as the Earth, will cause the space-time fabric around it to be “dragged” along with the rotation, an effect called frame dragging.  An analogy here might be to put a rotating ball into a viscous liquid; the rotation of the ball will cause the liquid to swirl in the same direction.   (Though these analogies can be helpful, it is important not to stretch them too far.  In the real case, we are talking about the effect of a three-dimensional object on four-dimensional space-time.)

The probe contained four high-precision gyroscopes, quartz spheres coated with a film of niobium [Nb, atomic number 41]. The spinning spheres were cooled with liquid helium to keep them at a temperature of 2.3 K (about -455.5 F), at which temperature niobium becomes a superconductor.  A telescope system kept the satellite oriented toward a guide star, and the instrumentation measured tiny deviations in the gyroscopes’ axes of rotation.   (The mission site has a page explaining the technology in more detail.)   Data was collected for about twelve months, and then analyzed; unfortunately, the data was much noisier than had been anticipated.

However, an article on the “Wired Science” blog at Wired reports that the analysis of the data has finally been completed.  The results, which will be published in the journal Physical Review Letters,  provide confirmation of the theoretical predictions, despite the tiny size of the measured effect.

The pointer shifted by just 6,000 milliarcseconds — the width of a human hair as seen from 10 miles away — over the course of a year, Everitt said. Despite the difficulty in detecting such a small tilt, the physicists were able to confirm the geodetic effect to an accuracy of 0.28 percent, and frame-dragging to within 20 percent.

The mission site’s status page has much more detailed and complete results, and promises to have a copy [PDF] of the final paper soon.

Other experiments have also confirmed these effects, in some cases more precisely.  As always in science, though, more confirmatory evidence, especially when obtained by a different technique, bolsters our confidence in the theory we’ve built.

Update Thursday, 26 May, 23:10 EDT

The abstract and a free download of the preliminary paper are now available at the arXiv.org site.

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