Gravity Probe B (GP-B)

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NASA’s Marshall Space Flight Center and Stanford University have developed a sophisticated experiment, Gravity Probe B (GP-B), to test Einstein’s general theory of relativity. It measured how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it.  Einstein’s theory predicts that space and time are distorted by the presence of massive objects. Launched on April 20, 2004, the GP-B mission is one of NASA’s first to address a question of fundamental physics in the new millennium.

The GP-B experiment contained the world’s most precise gyroscopes. The gyroscopes were specifically developed to measure two distinct effects of general relativity.The first of these, the geodetic effect, should cause the spin axis orientation of a gyroscope, circling the Earth in a polar orbit, to change by a tiny angle of 6.6 arcseconds (0.0018 degrees) in a year, relative to a distant guide star. The second effect, known as frame-dragging, predicts that massive celestial bodies, such as Earth drag their local spacetime around with them—ever so slightly—as they rotate.

The major components of the science instrument (four gyroscopes, the optical telescope, the mounting block) were all made of fused quartz. Quartz is very stable over wide temperature ranges—expanding and contracting very little and uniformly.

The science instrument’s optical telescope had an aperture of 14.0 centimeters (5.5 inches). Throughout the experiment, it was pointed at the center of GP-B’s guide star IM Pegasi (HR 8703), which provided the experiment’s frame-of-reference in space. Technicians at the Marshall Space Flight Center originally built the highly advanced polishing equipment needed to manufacture the gyroscope rotors. Engineers at Stanford developed the thin-film technology for placing a superconductive metal coating of Niobium on the gyroscope rotors.

The gyroscope rotors are perhaps the most spherical objects ever made. If the gyroscope rotors were enlarged to the size of the Earth, the tallest mountain or the deepest ocean ravine would be only 2.4 meters (8.0 feet) in height. These gyroscopes are sufficient to achieve the specified 0.5 milliarcsecond (1.4x10-7 degrees) per year accuracy in their drift rate measurements.

The final results for the GP-B mission were announced in a press event held at NASA Headquarters on May 4, 2011. In this event a distinguished group of panelists discussed the GP-B mission, its results, and its technological and human legacy. Over the course of nearly 50 years, more than 100 Ph.D. theses were published on GP-B technology and science. One GP-B alumnus was the first woman astronaut in space, Dr. Sally Ride. Another alumnus, Dr. Eric Cornell, won a Nobel Prize in Physics for experimental work verifying predictions of another of Einstein's theories, that of Bose-Einstein condensation of matter at very low temperatures.

Gravity Probe B did indeed detect and measure the geodetic effect and the frame-dragging effect predicted by Einstein's General Theory of Relativity. The results were that the GP-B mission measured the geodetic effect to a precision of 0.28% and the frame-dragging effect to a precision of 19%, consistent with the predictions of General Relativity. The results were published in a peer-reviewed article that discusses the analysis of the data in detail (see Physical Review Letters, Volume 106, 221101, 1-5 on 3 June 2011). There is also an article by Professor Clifford Will discussing the results and the history of GP-B and related work. For copies of the articles, a video of the press conference, and additional information about the results, please visit the GP-B Mission website.