Clocking gravity. (Physics).

Albert Einstein's general theory of relativity predicts that the speed of gravity equals that of light, but no one's ever been able to measure gravity's speed.

Now, a research team says it has done just that and found evidence that Einstein was right. However, skeptics are questioning whether the experiment measured what it was supposed to.

More than 80 years ago, Einstein worked out the equations that describe how a stationary mass bends light with its gravitational field (SN: 12/21&28/02, p. 394). Recently, Sergei M. Kopeikin of the University of Missouri in Columbia extended those equations to the changing gravitational fields around moving bodies, such as Jupiter.

As Jupiter moves, its gravity can shift the apparent positions of quasars whose powerful radio emissions bypass the planet on their way toward Earth. Kopeikin calculated that the amount of shift depends on the speed of gravity.

Last September, when Jupiter passed near the line of sight to quasar J0842+1835, Kopeikin and Edward B. Fomalont of the National Radio Astronomy Observatory in Charlottesville, Va., used radiotelescopes in the United States and Germany to track the quasar's apparent position.

Earlier this month, at the American Astronomical Society meeting in Seattle, Kopeikin announced that the new observations indicate that the speed of gravity is 1.06 times the speed of light, plus or minus 20 percent.

That interpretation of the observations is getting a chilly reception from some relativity specialists, including Clifford M. Will of Washington University in St. Louis. Will argues that any indication of gravity's speed would be too small to be detected by the current technique. Kopeikin counters that Will and other critics have erred in their calculations. It's too soon to tell what verdict will emerge from further analysis of both claims.--P.W.