Speeds of gravity and light equal, scientists announce

Sept. 8 finding confirms Einstein theory prediction


SEATTLE - Astronomers have measured the speed of gravity for the first time and found, reassuringly, that it conforms to their expectations of the universe.

Gravity travels through space at roughly the same speed as light, scientists announced yesterday.

Until now, they could only assume that the two zipped along at the same pace, as Einstein's general theory of relativity predicts.

Observations made during a rare celestial alignment last September provided the measurement of the speed of gravity, one of physics' fundamental constants.

"The result is Einstein was right, of course," said Edward Fomalont, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va. He and Sergei Kopeikin, of the University of Missouri in Columbia, announced the results in Seattle at a meeting of the American Astronomical Society.

Kopeikin and Fomalont measured the speed of gravity as Jupiter passed close to a bright cosmic object known as a quasar on Sept. 8.

Isaac Newton thought the speed of gravity was infinite, so that gravity would be transmitted instantaneously between two objects. But Einstein and modern scientists have believed that it must equal the speed of light.

The difference, Fomalont said, would become apparent if the sun were to disappear one day. If the speed of gravity were infinite, the Earth would immediately begin flying out into space without the sun's presence to hold it fast. But if the speed of gravity equals the speed of light, the Earth would be spared for 8 1/3 minutes - the time it takes a light beam, or gravity, to fly the intervening 93 million miles.

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