The Speed of Gravity

Can the speed of gravity be measured directly through the observation of gravitational lensing effects? Two scientists who monitored the deflection of quasar light as it passed very near Jupiter argue that they have derived an experimental value for the speed of gravity equal to 1.06 times the speed of light (with an uncertainty of 20%). But two other scientists claim that the lensing experiment only served as a crude measurement of the speed of light itself.

Physicists have long taken for granted that the effect of gravitational force, like the effect of electromagnetic force, is not instantaneous but should travel at a finite velocity. A familiar example of this delay is the fact that when we see the sun, we see it as it was 8 minutes ago. Many believe that gravity also travels at the speed of light. The trouble is, while it is relatively easy to gauge the strength of gravity (one can measure gravity even near a black hole, where orbiting matter emits telltale x rays), it is difficult to study the propagation of gravity.

Although not as heavy as a star, Jupiter still has considerable gravity, and when on September 8, 2002, it swept very near the position of quasar J0842 + 1835, the theory of general relativity suggests that the apparent quasar position on the sky would execute a small loop over the course of several days owing to the lensing of quasar light by the passing planet. Sergei Kopeiken (University of Missouri) and Ed Fomolont (National Radio Astronomy Observatory, or NRAO) have now seen just such a loop, as they reported this week at the meeting of the American Astronomical Society (AAS) in Seattle. For this purpose they employed the Very Long Baseline Array (VLBA) of radio telescopes, a configuration of dish detectors providing an angular resolution of 10 micro-arcseconds. Actually the observed lensing loop was slightly displaced from what one would expect if gravity propagated instantaneously. Kopeiken and Fomolont interpret this slight displacement as providing an experimental handle on the speed of gravity itself, and thereby calculate the value of 1.06 times c.

Other scientists disagree with this interpretation, and say that the radio lensing data can do little more than provide a measurement of the speed of light, not gravity. Two such opinions, by scientists who did not report at the AAS meeting, are as follows: Clifford Will of Washington University in the US (preprint at (www.arxiv.org/abs/astro-ph/0301145) and Hideki Asada of Hirosaki University in Japan (www.arxiv.org/abs/astro-ph/0206266)