Speed of gravity still undetermined

By DAN FALK, UPI Science News   |   April 7, 2003 at 6:50 PM
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PHILADELPHIA, April 7 (UPI) -- Although scientists generally agree the attractive phenomenon of gravity actually travels, its rate of speed remains undetermined, a physicist said Monday.

Clifford Will, of Washington University in St. Louis, has disputed a well-publicized report by two scientists earlier this year that the speed of gravity equals the speed of light: 182,000 miles (about 300,000 kilometers) per second.

The scientists -- Sergei Kopeikin of the University of Missouri in Columbia, and Edward Fomalont of the National Radio Astronomical Observatory in Charlottesville, Va. -- made headlines when they claimed they had measured the speed of gravity for the first time. The pair said it was indeed equal to the speed of light.

"I have no reason to doubt the actual measurements," Will told United Press International. The problem is the measurements have "nothing to do with the speed of gravity," he said.

In their paper -- presented Jan. 7 and reported by UPI -- Kopeikin and Fomalont said the measurements, which they made in 2002 of light from a distant quasar, deflected ever so slightly by the mass of the planet Jupiter, verified gravity's speed matched the speed of light.

The deflection that made the measurement possible was caused by two separate gravitational effects, they explained.

The first was a well-known result of Einstein's theory of general relativity, in which massive objects can bend the path of light. The second -- and smaller -- deflection, which Kopeikin and Fomalont measured for the first time, depended, they said, on the speed at which the effect of gravitation propagates through space.

The necessary measurement was exquisitely small, they said -- shifts in the quasar's position had to be determined within 50-millionths of an arc-second, the equivalent to detecting an object the size of a silver dollar on the moon. Kopeikin and Fomalont used NRAO's 25-meter radio telescope in Green Bank, W.Va., and a separate 100-meter instrument in Effelsberg, Germany.

However, Will, an expert on general relativity, said he conducted a detailed analysis of the twin effects presented by the scientific pair and discovered gravity's speed simply drops out of the equations.

Will presented his findings at a meeting of the American Physical Society.

"It turns out the effect depends only on the velocity of the moving object -- in this case Jupiter," he said. The speed of gravity "can't effect the propagation of the light rays" in such a situation.

Kopeikin and Fomalont continue to defend their work.

"I stand by my January claim," Kopeikin told UPI. "I categorically disagree with Dr. Will's understanding of the problem."

Kopeikin said the original mathematical analysis is correct and it is Will who has misinterpreted the data.

Other physicists remain skeptical, however.

"The opinion of everyone here is that Clifford Will is right," physicist Bob Wagoner of Stanford University in Palo Alto, Calif., told UPI. Lior Burko, of the University of Utah in Salt Lake City agreed, saying Will's argument "seemed pretty convincing."

The dispute now will play out in peer-reviewed scientific journals. Will has submitted his critique to the Astrophysical Journal, where it is scheduled to be published in June. Kopeikin has written a rebuttal, which has been accepted for publication in the journal Physics Letters.

Scientists still regard gravity as propagating at a very high speed, probably in waves similar to light -- although so-called gravity waves have yet to be detected. The only question is whether gravity's speed matches the speed of light exactly or whether it varies due to some as-yet-undiscovered property of physics.

Einstein's theory predicts the two velocities must be equal, Will said, adding although the comparative measurement is so difficult no one yet has been able to test it experimentally.

The first generation of gravitational wave detectors, which soon will begin operation, should settle the matter within a few years, he added.

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