Can the speed of gravity

Be measured directly through the observation of gravitational lensing effects? Sergei Kopeikin (University of Missouri) and Ed Fomalont (National Radio Astronomy Observatory) used the exquisitely sensitive Very Long Baseline Array of radio telescopes to monitor Jupiter’s gravitational deflection of light from a distant quasar that was nearly aligned with the massive planet on 8 September 2002. At the January meeting of the American Astronomical Society in Seattle, the researchers reported that the apparent position of the quasar traced a small loop over the course of several days. Moreover, they argued, the precise measurement of the loop’s shape allows one to determine the speed of gravity. Their result, that the speed of gravity equals 1.06 ± 0.21 times the speed of light, is consistent with Einstein’s theory of relativity. The experiment is widely regarded as a tour de force. But Washington University’s Clifford Will, and other scientists, argue that the reported data are in no way related to the speed of gravity. (E. B. Fomalont, S. M. Kopeikin, http://arxiv.org/abs/astro-ph/0302294 ; C. M. Will, http://arxiv.org/abs/astro-ph/0301145 .)