Preliminary Results from Gravity Probe B Announced at April Meeting
Image courtesy of NASA/Stanford
Gravity Probe B is an orbiting observatory dedicated to testing general relativity. Stanford officials call it “the most sophisticated orbiting laboratory ever created,” but it is also possibly the longest-running, most expensive single experiment in history, experiencing numerous delays and a few unexpected complications in the data collection that made the subsequent analysis more difficult.
The measurement data indicate that general relativity correctly predicts the geodetic effect–how much the mass of Earth is warping local spacetime–to within around 1%. Once certain unanticipated torques on the gyroscopes are better understood, the GP-B scientists expect the precision of their geodetic measurement to improve to a level of 0.01%.
Still to come is the final analysis of the data on measuring the frame-dragging effect: whether or not, and by what degree, the Earth drags the fabric of spacetime with it as it rotates. However, Everitt, while cautious, is “optimistic” that they have caught “glimpses” of the frame-dragging effect.
That caution translates into another eight months of data analysis to account for the unexpected anomalies, but Everitt is confident that in the end, GP-B’s results will mesh nicely with Einstein’s prediction at the 1% level. (An earlier indirect measurement of frame-dragging by the LAGEOS satellite had a 10-15% uncertainty.)
There is little doubt about the confirmation of the geodetic effect, according to Everitt: “[It] is clearly seen even in the unprocessed scientific data.” Einstein predicted that geodetic warping around Earth would cause the spin axes of each gyroscope to shift by 6.606 arc-seconds per year, or 0.0018 degrees.
But frame-dragging is a much tinier effect; the prediction is that the twisting of Earth’s local spacetime would cause the spin axis to shift by 0.039 arc-seconds per year, or 0.000011 degrees. It is much harder to measure accurately–particularly since the “signal” indicating relativistic effects of gravity around Earth must be extracted from a bunch of background noise. This is where the biggest delays have occurred in terms of analyzing the data.
First, the initial in-flight verification phase of the project took twice as long as expected. Then, as the experiment was running, computer reboots in response to random radiation strikes meant there were interruptions in the data streams.
The GP-B scientists also overlooked a tiny electrostatic “patch” effect in the gyroscopes. These patches can cause the gyroscope to “wobble” a bit as it spins, much like a football that isn’t thrown in a perfect spiral. The scientists were able to model and predict that wobble. What they didn’t expect was that the pattern would subtly shift over time. They accounted for electrostatic patches on the rotor, said Everitt, but forgot about the housing.
Those same electrostatic patches also caused small torques in the gyroscopes’ spin axes, and the resulting slight changes in orientation could be mistaken for the relativity “signal” that GP-B is designed to measure.
Gravity Probe B was first conceived in 1959 by two scientists named George Pugh and Leonard Schiff to precisely measure the displacement angles of the spin axes of four different gyroscopes in space over the course of a year and then compare that data with Einstein’s predictions. But the instrumentation and associated technologies didn’t exist at the time.
It has taken several decades for science to advance sufficiently to make GP-B feasible. For instance, they needed wobble-free gyroscopes; one way to measure the geodetic effect is through the perturbative influence of massive bodies on nearby gyroscopes. This was achieved by creating the world’s smoothest, most perfect spheres, only surpassed in their perfect roundness by very dense neutron stars.
The four GP-B gyroscopes 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. Covered with niobium and reposing at a temperature of just 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.
GP-B scientists also needed very sensitive and precise sensors capable of measuring an effect on a par with observing an object roughly the width of a human hair from about a mile away. The distortion of space caused by Earth’s rotation around its axis should only deflect the spinning axis of the gyroscope by the tiniest of angles–so small that it would take more than a million years for the gyroscope to turn in a full circle. The invention and subsequent development of Superconducting-Quantum Interference-Device-(SQUID)-based sensors made it possible to measure those tiny magnetic variations.
Other necessary advancements included the Global Positioning System and a suspension system capable of keeping the gyroscopes’ spinning rotors from making contact with the walls.
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