Einstein Passes Strictest Test: NASA’s Gravity Probe-B Proves General Theory of Relativity Correct

It is the culmination of a five-decade old project. Gravity Probe-B (GP-B) has detected a minuscule, but theoretically expected, tilt in its magnetic needles, essentially proving Einstein correct, yet again.

"Told ya"

The Physics

Einstein’s general theory of relativity (GTR), the most accurate description of gravity known to science, predicts two critical phenomenon, which differentiates it from Newton’s theory of gravity: geodetic effect and frame dragging. The geodetic effect is just a technical term to describe the well known phenomenon of warping of space-time by massive objects. Light, which appears to follow a straight line path, will bend, following the curvature of space-time. Frame dragging refers to the phenomenon in which space-time seems to be ‘dragged’ along with any rotating gravitational body. It’s as if there is friction between the body and surrounding space-time.

Physicist Francis Everitt, of Stanford University, lead researcher in the GP-B experiment, explains it better:

Picture the Earth immersed in honey, and you can imagine the honey would be dragged around with it. That’s what happens to space-time. Earth actually drags space and time around with it.

francis everitt
Francis Everitt

If Einstein’s theory is really true, then these effects should lead to a tiny, but observable, in the orientation of a suspended gyroscopic magnet.

The Project

The project was proposed by theoretical physicist Leonard Schiff in 1959, when he was the head of the Stanford Physics Department. In 1962, Schiff and team recruited Everitt. In 1963, NASA got interested in the project, a full 7 years before the lunar launch.

The experimental confirmation of such tiny effects was a huge challenge. Einstein, himself was of the opinion that

their magnitude is so small that confirmation of them by lab experiments is not to be thought of.

Thankfully, Einstein was wrong. In 2004 (after 41 years), the Gravity Probe-B was launched by NASA.

The spacecraft had four ping-pong sized gyroscopes. These were made of fused quartz spheres (the most spherical spheres ever made) and uniformly covered by a layer of Niobium and cooled by Liquid Helium. At liquid Helium temperatures, Niobium becomes a superconductor, allowing electrical currents to flow without any resistance. Once an electrical current is setup, it doesn’t decay.

One of the four Gyroscopes on Gravity Probe B
gravity probe b
Gravity Probe B

Rotating currents setup a magnetic field pointing in a particular direction. The magnetic pointer was set to point at IM Pegasi, a single star. The prediction is that, if Einstein is right, minute changes in the direction of the magnetic pointer would occur.

The Result

The pointer shifted by 6000 milli arcseconds in one year that’s the breadth of a human hair seen from 10 km away. However, the extent was just as expected, confirming GTR yet again. The geodetic effect was confirmed to within 0.23% accuracy and the frame-dragging to about 20%.

Beyond just Einstein

Even though the primary objective of this project was to verify GTR in the most accurate way possible, GP-B has also, unwittingly, contributed a lot to the development of various technologies, most notably the technology to build better gyroscopes. It has made key contributions to various developments on the COsmic Background Explorer (or COBE), which was instrumental in demonstrating that the Universe is expanding just as the Big Bang Theory claims.


The positive GP-B results will help scientists understand gravity better, especially in extreme cases, like black holes. Many people ask, why test Einstein further? Well, even though he has been thoroughly tested and has passed all tests with flying colors, that’s how science proceeds. The more stringent the test, the more credibility the theory gains when it passes it.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.