Experiment of the week: Satellite used to study general relativity

Gravity Probe B, a well-endowed NASA/Stanford satellite, is at this moment orbiting the Earth. The satellite’s main feature is its four perfectly spherical, shiny balls. These balls serve as the world’s most perfect gyroscopes, used in an extraordinarily complex and expensive experiment to observe the effects of general relativity.

After Einstein introduced the idea that the fabric of space-time is warped by mass, other scientists used his theory to predict unexpected phenomena. Many general relativity effects have been experimentally observed, such as Sir Arthur Eddington’s 1919 jungle expedition to observe starlight bent by the sun. Yet for many years, experimental confirmation of two additional and important general relativity phenomena was missing. Known as the “frame-dragging effect” and the “geodetic effect,” these two features of the theory involved correction factors so small that it wasn’t until the 21st century that they could be measured.

There is no doubt that technology has produced an incredible instrument in Gravity Probe B due to its small frame-dragging effect. Einstein’s theory predicts that a rotating massive body should slowly “frame-drag” space and time around with it. Over time, this dragging effect should push the gyroscope’s axis of rotation about 40 milliarc-seconds out of alignment. That’s the width of a human hair as seen from 10 miles. The probe intends to measure this to an accuracy of one percent.

At the same time, the gyroscopes should experience a much bigger shift in alignment from the geodetic effect. This effect is again due to the Earth’s gravitational field warping space-time, causing a circular orbit around the Earth to be slightly shorter than pi times the Earth’s diameter. This is the more interesting measurement, since exotic phenomena like hidden extra dimensions and undiscovered forces could cause deviations from the shift value that Einstein predicted: exactly 1.0.

By the project’s end, NASA will have spent over $700 million. Some are saying this was too much to spend on old science. Cheaper, lower-tech experiments have already indirectly confirmed the existence of both effects, taking the wind out of Probe B’s expensive sails. The satellite itself is the product of a 50-year-old idea, and while the end result is an incredibly sophisticated measuring device, its task may perhaps be less than worthy of the effort.

Carnegie Mellon physics professor Richard Holman was blunt: “Frankly, I don’t know why they spent that much money on [the probe]. This project has been going on and off for 30 years, and by this time, there are a lot of other more interesting research projects to sink money into. The chances of discovering new science from this are very slim. We would have seen the effects already. If GR is wrong, we would have already found out.”

Yet the potential for confirming general relativity to such a high degree of accuracy is very exciting. Gravity Probe B’s data collection finished in August 2005, and data analysis is expected to be completed by the end of this summer. Expect Einstein to be proven correct by the end of this year.

Editor’s Note: To learn more about the science and engineering feats involved in Gravity Probe B, visit