Gravity Probe B Says Einstein was Right. Again.
It’s a sad day for physics crackpots bent on disproving relativity, because once again, it turns out that Einstein was right.
The last bits of data collected several years ago by NASA’s Gravity Probe B satellite have been analyzed, and the result is a resounding confirmation of two critical predictions of the general theory of relativity.
Perhaps you’re thinking, “Wait a minute — NASA has a satellite up there testing the predictions of relativity?” Yes it does! NASA launched Gravity Probe B in April 2004, and these new results are the culmination of more than 50 years of effort.
Per Isaac Newton, the spin axis of a perfect gyroscope orbiting the Earth would remain unchanged for all eternity. But Newton saw gravity as a force between objects. Einstein re-envisioned gravity as arising from the warping of spacetime. That’s the central tenet of General Relativity, and Einstein made several predictions that could be used to test the accuracy of his theory.
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One was confirmed right away: Einstein said that a ray of light passing near the sun would be deflected by a certain degree, because the sun’s mass would warp the surrounding spacetime and the light ray would follow that curvature.
This effect would be observable during a solar eclipse, when the sun’s light was temporarily blocked. In May 1919, two separate scientific expeditions — one in Brazil and another on an island off the coast of West Africa — observed just such a deflection during a solar eclipse.
Einstein also predicted that the Earth’s mass would warp its surrounding spacetime, and that it would “drag” the fabric of spacetime by a certain degree as it rotates. Those last two predictions are the reason Gravity Probe B was first conceived in 1959 by two Stanford scientists named George Pugh and Leonard Schiff.
The idea was 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.
Basically, the gyroscope orbits the Earth in a state of perfect free-fall because the body of the craft shields it from outside disturbances like friction and magnetic fields. But we didn’t have the instrumentation and associated technologies to build such a complicated system for several decades.
After its launch, NASA pointed Gravity Probe B at a single star, IM Pegasi, and collected tons of data to see if the on-board gyroscopes would continue to point in that same direction forever — as Newton expected — or whether there would be tiny changes in the direction of their spin in response to Earth’s gravitational pull.
I last wrote about Gravity Probe B back in 2007, where preliminary results were presented at an American Physical Society Meeting in Jacksonville, Florida. At that meeting, Francis Everitt of Stanford University announced that the data indicated that Einstein’s theory correctly predicts the “geodesic effect” (i.e., how much the mass of the Earth is warping local spacetime) to within around 1 percent.
But the scientists were having a bit more trouble with data analysis to measure the much-tinier “frame-dragging” effect (i.e., whether or not, and by what degree, the Earth drags the fabric of spacetime with it as it rotates). The prediction is that the twisting of earth’s local spacetime would cause the spin axis to shift by 0.039 arc-seconds, or 0.000011 degrees. That’s much harder to measure accurately — particularly since the “signal” must be extracted from a bunch of background noise.
Plus, there were unexpected challenges, most notably when a solar flare knocked out some cells in the on-board computer memory; the system automatically switched to the backup computer. The subsequent computer reboots meant there were interruptions in the data streams, and any physicist can tell you that segmented data is much harder to analyze.
Still, there were “glimpses” of the frame-dragging effect. At the time, everyone expected another eight months to a year of slogging through the data to account for unexpected anomalies and such. Who knew it would end up taking another four years?
Astronomers are lucky, in that they can quickly publish their images from the Hubble Space Telescope and worry about more specific analysis later. It might take years to complete that analysis, but everyone gets to look at pretty pictures in the meantime, so there’s a sense of accomplishment and money well spent.
But Gravity Probe B was looking for two very small numbers given a very small margin for error. “So we’d better not get either one wrong,” Everitt told me back in 2007, when there were discontented rumblings about cost overruns and delays in analyzing the data. “Deliberateness is the name of the game.”
I liked Everitt for saying that, being of a more deliberate bent myself. And I’m sure he wasn’t expecting such a long delay either. He’s been with the mission since at least 1980; Everitt’s in it for the long haul.
And now the long wait is over, and what did they find? Everitt and his colleagues have further refined their measurement of the geodesic effect to an impressive 0.28 percent accuracy. And they can confirm a frame-dragging effect as well, although there the accuracy is nowhere near as precise: it’s confirmed to a 19 percent accuracy. Like I said, it’s a trickier measurement, and there were problems with the data at the outset. But this becomes the record to beat for future experimentalists eager to try their hand at defeating the great Einstein.
There are still rumblings in the scientific community about the long duration and cost of launching a mission to confirm what most physicists think has already been tested to their satisfaction. In an especially budget-conscious era, that’s a legitimate concern, as so many worthy projects lie fallow from lack of available funding.
Detractors of Gravity Probe B no doubt question whether those funds might have been better spent on exploring new physics, rather than confirming Einstein.
It’s a valid point. But the pace of scientific knowledge on that front has been staggering over the last 50 years. Even if the original scientific mission got left in the dust, the technological expertise required spurred considerable innovation. Related R&D probably produced a few helpful spinoff technologies, although NASA’s Website currently informs me that this section is “under construction.”
So, we spent 50 years and $700 million to conclude that Einstein’s general theory of relativity is still right. Only time will tell if it was worth the price.