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The Sciences

Relativity Passes Absolute Test

Exacting research finds Einstein was exactly right.

By Stephen OrnesJune 22, 2007 5:00 AM

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According to Albert Einstein’s general theory of relativity, a massive body like Earth should bend the space-time fabric of the universe, causing it to curve and flex like a trampoline supporting a bowling ball.

Nearly three years ago, NASA’s oft-canceled $750 million Gravity Probe B Relativity Mission finally shot into space with one goal—to quantify Einstein’s predictions from Earth’s orbit. Earlier this year, at the meeting of the American Physics Society, principal investigator Francis Everitt delivered the first results: Gravity Probe B has verified Einstein’s theory to within 1 percent.

Four gyroscopes, each the size of a Ping-Pong ball, form the heart of the experiment. The gyroscopes are the most perfectly spherical man-made objects in existence; if inflated to the size of Earth, they would have mountains no more than eight feet high. (Their near-faultless roundness has landed the spheres in the Guinness World Records.) At the beginning of the experiment, the gyroscopes’ axes pointed to a distant star; as the spacecraft moved around Earth for nearly a year, the researchers carefully monitored the position of the axes.

Einstein’s theory predicts that the axes should shift by a tiny amount—0.0018 degree—under the influence of Earth’s pull on space-time. After 18 months of data analysis, Everitt and his team measured the axial shift to within 1 percent of Einstein’s prediction. Everitt, a Stanford physicist who has spent more than 40 years on the project, says the results are sweet indeed. “It’s really extraordinary to look at the output and see Einstein looking back, without any calculations or corrections,” he says. “This measurement is unprecedented in any test of general relativity.”

In addition to claiming that the universe curves around massive bodies, Einstein said that as these bodies rotate, they effectively “drag” space behind them, creating a twist in the cosmic fabric. Everitt says his team plans to announce verification of this “frame dragging” effect later this year.

Gravity Probe B has been a long time in the making. First imagined in 1959, the mission has been canceled at least seven times by NASA. Critics claimed that its precision was not worth the cost or time. “You don’t get to do extremely worthwhile programs without fighting for them,” Everitt says. “And isn’t it nice to just see Einstein right there?”

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