Einstein’s general relativity is probably one of the most successful theories in modern physics. So far, it has passed every observational and experimental test. Now, two more predictions of Einstein’s theory have been confirmed.

The Gravity Probe B (GP-B) experiment used four ultra-precise gyroscopes to measure two aspects of Einstein’s theory about gravity. The first one, the geodetic effect, describes the warping of space and time around a gravitational body. The second one, called frame-dragging, describes how a spinning object pulls space and time with it as it rotates.


© Stanford University

The project was initiated 52 years ago, making it one of NASA’s longest-running projects. While in a polar orbit around Earth, the satellite pointed at a single star, IM Pegasi, measuring the two effects with unprecedented precision. Stanford and NASA researchers observed a change in the direction of the gyroscopes’ spin as they were pulled by Earth’s gravity. If gravity did not affect space and time, the gyroscopes would have pointed in the same direction forever while in orbit.

The findings are online in the journal Physical Review Letters.

“Imagine the Earth as if it were immersed in honey. As the planet rotated its axis and orbited the Sun, the honey around it would warp and swirl, and it’s the same with space and time,” said Francis Everitt, a Stanford physicist and principal investigator for Gravity Probe B.

GP-B enabled a lot of innovations, used in other missions such as COBE or in GPS technologies to allow airplanes to land unaided. Its development led to groundbraking technologies making a number of Earth-observing satellites possible. It also provided a practical training ground for numerous students and high school students, among which were the first American woman in space, Sally Ride, and the Nobel laureate Eric Cornell.

These results are extremely important and will have a strong impact on theoretical physics. Future attempts to test Einstein’s theories will face a difficult challenge: they will have to be more accurate, and do better than GP-B.


For more information about Gravity Probe B:



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