Sergei Kopeikin, other scientists measure the speed of gravity

For the first time, scientists have measured the speed of gravity, one of the fundamental constants of Albert Einstein’s 1916 general theory of relativity. Led by Sergei Kopeikin, associate professor of Physics at the University of Missouri-Columbia, a team of scientists took advantage of a rare cosmic alignment on Sept. 8, 2002, to test Einstein’s assumption that gravity moves at the speed of light.

"Newton thought that gravity’s force was instantaneous," Kopeikin said. "Einstein assumed that it moved at the speed of light, but until now, no one had measured it."

Kopeikin worked with Ed Fomalont, a radio astronomer with the National Science Foundation’s National Radio Astronomy Observatory (NRAO) in Charlottesville, Va. On Sept. 8, the scientists measured the shift of a quasar, a celestial object that resembles a star. Jupiter’s gravitational force caused the radio waves from the quasar to shift as Jupiter passed by it closely.

"We have determined that gravity’s propagation speed is equal to the speed of light within an accuracy of 20 percent," Fomalont said.

The scientists presented their findings to the American Astronomical Society's meeting in Seattle, Wash. The landmark measurement is important to physicists working on field theories that attempt to combine particle physics with Einstein's general theory of relativity and electromagnetic theory.

To conduct the experiment, the scientists used the National Science Foundation's Very Long Baseline Array (VLBA), a continent-wide, radio-telescope system, and a 100-meter radio telescope in Effelsberg, Germany, to make an extremely precise observation when Jupiter passed in front of the quasar. The observation recorded a slight "bending" of the radio waves coming from the quasar because of the gravitational effect of Jupiter. The bending resulted in a small change in the quasar’s apparent position in the sky in addition to the deflection of light calculated by Einstein in 1915.

"Because Jupiter is moving around the sun, the precise amount of the bending depends slightly on the speed at which gravity propagates from Jupiter," Kopeikin said. "Since the effect is very small, Einstein neglected it in his calculations."

In 1999, Kopeikin extended Einstein’s theory for light propagation to include the gravitational effects of a moving body on light and radio waves. Prior to this study, no one had tried to measure the speed of gravity because most physicists had assumed that the only way to do so was to detect gravitational waves, Kopeikin said. The MU scientist realized that if Jupiter moved closely in front of a star or radio source, he could test his theory.

The VBLA system is a result of a general radio technique known as Very Long Baseline Interferometry (VLBI).

"I believe this experiment sheds new light on fundamentals of general relativity and represents the first of many more studies and observations of gravitation available presently with existing VLBI technique," Kopeikin said. "We have a lot more to learn about this intriguing cosmic force and its relationship to the other forces in nature."

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Sergei Kopeikin  
Physics and Astronomy Department

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