Gravity moves at speed of light

By DAN FALK, UPI Science News

SEATTLE, Jan. 7 (UPI) -- Two U.S. astronomers claimed Tuesday to have measured the speed of gravity for the first time and discovered it travels at the same speed as light.

The finding confirms what most physicists had assumed, but which until now no one had been able to measure directly, the astronomers said.


"Gravity is one of the most important forces in the universe, but it's not understood particularly well," Edward Fomalont of the National Radio Astronomical Observatory in Green Bank, W.Va., told United Press International. "Many people assumed it propagates at the speed of light. We thought, 'we can measure it, so let's measure it.'"

Fomalont, who works with NRAO in Charlottesville, Va., and colleague Sergei Kopeikin, of the University of Missouri in Columbia, presented their research at the 201st meeting of the American Astronomical Society.

They described their experiment as involving technical skill and plain luck as they carefully measured the way light from a distant quasar, or quasi-stellar object -- among the most powerful energy sources in the universe -- was deflected as it passed behind the planet Jupiter during a rare chance alignment of the two objects last September.


The measurement was carried out using NRAO's array of 25-meter radio telescopes, as well as a 100-meter instrument in Effelsberg, Germany. The displacements involved were tiny -- shifts in the quasar's position had to be measured to within 50 millionths of an arc-second -- equivalent to detecting an object the size of a silver dollar on the moon, the astronomers said.

"We measured exactly what we expected, so I'm relieved," Kopeikin told UPI, "and I'm happy that we were able to conduct such a high-precision experiment."

Gravity always was known to propagate at a high speed. In the 17th century, Isaac Newton assumed it moved infinitely fast. In more modern times, however, theorists have assumed gravitational forces move at the speed of light. This was one of the assumptions at the heart of Einstein's theory of relativity, which he published in 1915. However, testing that assumption has been painstakingly difficult.

Despite the precise measurement, Fomalont and Kopeikin, who have submitted their paper for publication in the Astrophysical Journal, said their margin of error remains at about 20 percent.

Steven Carlip, a physics professor at the University of California, Davis, called the experiment was "a nice demonstration" of Einstein's principle.


"There were a couple of possible outcomes," Carlip told UPI. "The prediction of general relativity was that the result would be the speed of light exactly. There are some other extensions of general relativity theory that have been proposed that would give values very slightly different, but nobody expected this experiment to make distinctions that accurately."

He explained that the first measurements of deflection of light by the sun were less accurate than that.

"The other possibility," he said, which would be remote, "was that they could have a speed of propagation much faster than the speed of light. In Newtonian gravity, the speed of propagation is infinite, or nearly infinite. While no physicist working in the field seriously believed that's what they would see, it had to be checked."

Carlip added that future measurements of gravitational propagational speed should be more accurate. A number of gravitational wave interferometers have come on line in recent months that should be able to detect gravitational waves directly for the first time, and thereby measure the speed.

Physicist Phil Schewe, a spokesman for the American Institute of Physics in College Park, Md., said Fomalont and Kopeikin's experiment is relevant because the force of gravity is so fundamental in our universe.


"It's important to measure something as central as the speed of gravity," Schewe told UPI. "It's there in Einstein's equations, but he implicitly took it to be equal to the speed of light. We shouldn't just assume it; we should measure it."


(With additional reporting by Dan Whipple, UPI Science News, in Broomfield, Colo.)

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