Physicist Disputes Speed of Gravity Claim

A experiment showing gravitational lensing by the planet Jupiter early this year was originally interpreted as providing a measurement of the speed of gravity, although the conclusion was controversial from the outset.

At the APS April meeting, Clifford Will of Washington University in St. Louis, Missouri, a leading theorist in the interpretation of general relativity, presented his own analysis, disputing the earlier claims.

On September 8, 2002, Jupiter passed within 3.7 arcminutes of quasar J0842+1835, the center of a distant galaxy and a strong source of radio waves.

Ed Fomalont, a researcher at the National Radio Astronomy Observatory in Charlottesville, VA, used atomic clocks and the "Very Long Baseline Array" of radio telescopes to measure the brief length of time by which radiation from a quasar was delayed as it passed by the planet Jupiter.

Fomalont's measurement showed that the gravitational influence of the moving planet delayed the radio waves by about 5 trillionths of a second, or bent the waves by less than 15 billionths of a degree.

According to the general theory of relativity, gravity must be propagated at the same speed as light: 186,000 miles per second. Therefore, measuring the speed of gravity would test Einstein's theory.

Using Fomalont's data, Sergei Kopeikin (University of Missouri, Columbia) inferred that the speed of gravity is indeed the same as that of light, although the margin of error was 20%

"They obtained a very beautiful experimental result, and I have no quarrel with that," said Will. "The issue is the interpretation of the measurement. I don't think this result says anything about the speed of gravity."

In a paper recently accepted for publication by the Astrophysical Journal, Will claims that although the experiment is capable of measuring the speed of gravity, the effect is too small to measure, and that the value presented by Kopeikin and Fomalont as the speed of gravity is actually the speed of light.

"When I did a detailed calculation that put gravity's speed at any value, the result for the delay of light was independent of gravity's speed," said Will. "It depended only on the speed of light. So it's not possible to determine the speed of gravity from these light-delay observations." That measurement will have to wait for the LIGO observatories to begin regularly detecting gravitational waves.

Will also criticized the press for prematurely reporting the result and, to some extent, magnifying a simple scientific debate into a controversy.

"The press jumped on this in a rather uncritical way," he said. "Experimentally, it's really a tour de force measurement, which will be diminished somewhat by the controversy."