Gravity Probe B has finally measured an effect called frame dragging (Image: Gravity Probe B/Stanford)

The beleaguered Gravity Probe B mission has finally measured a subtle effect of general relativity called frame dragging. The result comes nearly six years after it finished making measurements and years after other experiments measured the effect to greater precision.

NASA launched the $750 million mission in 2004 and it finished collecting data in September 2005. Its goal was to test Einstein’s general theory of relativity, the currently accepted theory of gravity, by measuring subtle distortions in the fabric of space-time due to the Earth’s gravitational field.

To achieve this, the Gravity Probe B spacecraft contained four superconducting niobium spheres about the size of ping pong balls. They were set spinning, and it was expected that their spin axis would change slightly over time as a result of these distortions.


But the data was much noisier than expected, making it initially difficult to detect these effects.

In April 2007, after more than a year of data analysis, the team reported detecting one such phenomenon, called the geodetic effect, which is due to the dent the Earth’s gravity makes in space-time.

The second effect the mission was meant to measure proved much more elusive. As the Earth rotates, it drags the surrounding space around with it – a phenomenon known as frame dragging or the Lense-Thirring effect.

Swirling honey

“Imagine the Earth as if it were immersed in honey,” says Francis Everitt of Stanford University in California, the mission’s chief scientist. “As the planet rotates, the honey around it would swirl, and it’s the same with space and time.”

A 2008 NASA review was pessimistic about the prospects for detecting frame dragging in Gravity Probe B’s noisy data. But data analysis continued with private funding, some arranged by the Saudi royal family.

Now, after further analysis of the data, Gravity Probe B scientists say they have detected frame-dragging with a precision of about 20 per cent.

Earlier results

“We have managed to test two of the most profound effects of general relativity and to do so in a new way,” Everitt said in a NASA press conference on Wednesday.

This is the first time frame dragging has been measured in this way. But it was measured previously in 2004 to about 10 per cent precision by its effects on the orbits of the LAGEOS I and II satellites. Tracking the motion of the moon with lasers has also measured frame dragging to a precision of 0.1 per cent.

Given these earlier results, questions are likely to remain about the value of Gravity Probe B’s contribution, but Everitt defended the mission’s value. “The great beauty of it is that we have complementary tests of general relativity,” he said.”We completed this landmark experiment testing Einstein’s universe … and Einstein survives.”

Journal reference: Physical Review Letters (forthcoming)