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Final chapter published in decades-long Gravity Probe B project

Final chapter published in decades-long Gravity Probe B project

quartz spheres with Einstein photo

A grueling but ultimately successful effort to test Einstein’s 100-year-old general theory of relativity has come to a close more than half a century after it began. Twenty-one papers published online November 17 in Classical and Quantum Gravity present a detailed summation of Gravity Probe B, a satellite that in 2011 confirmed Einstein’s prediction that Earth dents and whips up the spacetime around it.

“It’s very exciting,” says principal investigator Francis Everitt of Stanford University. “It’s been quite exhausting.”

Mission scientists managed to deliver relatively precise measurements of two general relativity phenomena despite several mishaps that threatened to render the data useless. “I think the Gravity Probe B team is the most heroic bunch of scientists I’ve ever been affiliated with,” says Peter Saulson, a physicist at Syracuse University in New York who monitored the mission as part of a NASA-organized advisory committee. Besides delivering another stamp of approval for general relativity, Gravity Probe B’s enduring legacy may be pioneering technology that enables future discoveries.

General relativity first earned credibility through Einstein’s explanation of Mercury’s orbit and measurements of solar eclipses (SN: 10/17/15, p. 16). In the early 1960s, Everitt began a quest to test some of the theory’s harder-to-test predictions. He planned to measure how much the Earth (and by extension, all objects with mass) warps spacetime, a phenomenon known as the geodetic effect. Everitt also wanted to measure the even feebler frame-dragging effect, in which the spinning Earth should yank and twist the surrounding spacetime.

CURVED SPACE Because of its mass Earth warps the spacetime around it. Gyroscopes inside the Gravity Probe B satellite were able to measure two phenomena caused by this curvature.



After many delays and false starts, Gravity Probe B was finally launched in April 2004. It tested both effects with four gyroscopes consisting of spinning quartz spheres coated with the metal niobium. Under Newton’s laws, the axis of a gyroscope totally isolated from external forces would point in the same direction forever. But because of the geodetic and frame-dragging effects, general relativity predicts that our rotating 6-septillion-kilogram planet should reorient a gyroscope’s axis ever so slightly.

Unfortunately, eliminating outside forces is a difficult task, even in space. Researchers noticed that the ping-pong ball–sized gyroscopes were wobbling in unexpected ways. At other times the axis of a gyroscope would suddenly shift and point in a new direction. Initially, Everitt’s team didn’t know what was causing the deviations, which were tens to hundreds of times larger than the gravity-driven effects the researchers hoped to measure.

Over five years of intense data analysis, the scientists identified issues such as electron interactions between the spheres and casings and subtracted those forces from the measurements. In May 2011, the team announced values for the geodetic and frame-dragging effects that are consistent with general relativity’s predictions (SN: 5/21/11, p. 5). Confirming frame dragging, which has been measured with great precision by only one other experiment (SN: 11/27/04, p. 348), rules out some proposed modifications of general relativity and helps physicists predict the conditions around rapidly spinning black holes. “I think we all consider the mission a success,” says John Conklin, a mission scientist and aerospace engineer at the University of Florida in Gainesville.

For Saulson, some of the most interesting material in the new papers discusses instrument design. That’s because the 40-kilogram quartz pendulums in his current experiment, Advanced LIGO, were built using a bonding technique developed by the Gravity Probe B team. The experiment’s two L-shaped detectors, which just started collecting data in September, are searching for gravitational waves: ripples in spacetime also predicted by Einstein’s theory.

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