Clocking gravity

Peter Weiss

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: http://www.sciencenews.org/articles/20021221/bob9.asp). 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.

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Letters:

You are usually pretty good at explaining things to those of us outside the hard sciences, but the speed of gravity? Speed of light OK, but what is the speed of gravity?

Robert L. Schrag
North Carolina State University
Raleigh, NC

An object with mass creates a gravitational field around itself. Imagine that the object moves; then, the strength of its field everywhere in space changes. The speed at which that change ripples through space is the speed of gravity.—P. Weiss

It would seem to me that the shift in the apparent position of the quasar was due to the bending of light by Jupiter's gravity. I would conclude that it is proof of Einstein's proposal that gravity is due to the bending of space around a massive object, but I fail to see how this can be construed as an indication of the speed of gravity.

Stephen Moody
Richmond, VA

References:

Kopeikin, S., and E.B. Fomalont. 2003. Measuring the speed of propagation of gravity. 201st Meeting of the American Astronomical Society. Jan. 8. Seattle. Abstract available at http://www.aas.org/publications/baas/v34n4/aas201/133.htm.

Further Readings:

Weiss, P. 2002. Getting warped. Science News 162 (Dec. 21&28):394. Available at http://www.sciencenews.org/articles/20021221/bob9.asp.

Sources:

Edward B. Fomalont
National Radio Astronomical Observatory
520 Edgemont Road
Charlottesville, VA 22903

Sergei M. Kopeikin
Department of Physics and Astronomy
University of Missouri, Columbia
Physics Building
Columbia, MO 65211

Clifford M. Will
McDonnell Center for the Space Sciences
Department of Physics
Washington University
One Brookings Drive
St. Louis, MO 63130

From Science News, Volume 163, No. 4, January 25, 2003, p. 61.