Putting Einstein to the test

From Jacksonville, Fla., at a meeting of the American Physical Society

NASA’s longest-running mission is accomplished—almost.

Gravity Probe B went into orbit in 2004 after 4 decades of development and seven NASA reviews that often threatened to cancel it. It was meant to test general relativity, Albert Einstein’s theory of gravity (SN: 11/5/05, p. 302: Available to subscribers at A matter of gravity; 11/1/03, p. 280: A Spin through Space-Time).

The $700 million probe incorporated numerous technologies to keep four gyroscopes—near-perfect spinning spheres the size of ping-pong balls—virtually free of external disturbances. Magnetic readouts tracked the gyros’ rotation axes, which Newtonian physics predict would be perfectly stable. However, two distinct relativity effects would make the axes drift.

But unexpected anomalies appeared during the mission. Electrostatic fields due to micron-size irregularities in the metal casings of the gyros may have affected the data, said Stanford University’s Francis Everitt. He and his team are confident that they will weed out the noise, and they plan to release final results at the end of the year. For now, they released preliminary results.

Gravity Probe B confirmed the so-called geodetic effect—in which the curvature of space shortens the length of the craft’s orbit by 1.1 inches—to a precision of 1 percent. The other, even subtler effect, called frame dragging, is the result of Earth’s twisting the fabric of space around itself as it rotates. Frame dragging was expected to tilt the gyros’ axes by just 0.000011 degree over 1 year, but the data are still too noisy to show that signal.

“We have some glimpses of the frame-dragging effect,” Everitt said, adding that his team still hopes to measure it to 1 percent accuracy, as initially planned.

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