Physicist disputes speed of gravity claim

An astrophysicist at Washington University in St. Louis is disputing the claim that measurements of light bending are useful for determining the speed of gravity. Clifford M. Will, Ph.D., professor of physics in Arts & Sciences, presented his arguments April 7 at the annual meeting of the American Physical Society in Philadelphia and in an upcoming article in The Astrophysical Journal.

The dispute concerns claims made by Sergei Kopeikin, Ph.D., at the University of Missouri-Columbia, and Ed Fomalont, Ph.D., at the National Radio Astronomy Observatory in Charlottesville, Va. Using atomic clocks and the Very Long Baseline Array of radiotelescopes, Fomalont measured the brief length of time by which radiation from a quasar was delayed as it passed by the planet Jupiter.

Clifford Will
Clifford Will

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

On Sept. 8, 2002, Jupiter passed within 3.7 arcminutes of quasar J0842 1835, the center of a distant galaxy and a strong source of radiowaves. Fomalont’s measurements showed that the gravitational influence of the moving planet delayed the radiowaves by about 5 trillionths of a second. Thus, the planet 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, Kopeikin inferred that the speed of gravity is indeed the same as that of light, though the margin of error was 20 percent.

“The trouble is that 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,” Will said. “It depended only on the speed of light. Therefore, it is not possible to determine the speed of gravity from these light-delay observations.”

Will argues that Kopeikin and Fomalont’s experiment could search only for a different prediction of the theory of general relativity: gravitomagnetism. That is the presumed magnetic-like gravitational field generated by a large object moving through space. It is analogous to the magnetic field generated by an electric current moving through a wire, though the objects it attracts do not have to contain iron.

Gravitomagnetism is very difficult to detect because it is an extremely weak force. Consequently, only indirect evidence currently supports the concept. “Unfortunately, the gravitomagnetic effect in the Fomalont-Kopeikin experiment is too small to provide a decent test,” Will said.

A space probe to be launched later this year aims to collect more accurate data. NASA’s Gravity Probe B will carry into space four exquisitely sensitive gyroscopes and a telescope that will point each gyroscope’s axis toward a distant star. If the axes of the gyroscopes stray while the probe is in space, gravitomagnetism will be the most probable cause. Will chairs the NASA Science Advisory Committee for Gravity Probe B.

And what about the speed of gravity? Will said it should be possible to measure it accurately in a few years, once the recently completed array of LIGO (laser interferometry for gravity wave detection) observatories is regularly detecting gravitational waves.

Will CM, 2003. “Propagation speed of gravity and the relativistic time delay”. The Astrophysical Journal. In press.