TWO weeks ago, a pair of scientists made the sensational announcement that they had measured the speed of gravity for the first time. But the claim has sparked a passionate debate among physicists over whether the researchers have measured anything other than the speed of light.
While Newton thought gravity acts instantaneously, Einstein鈥檚 theory of general relativity assumes that the force of gravity propagates at the speed of light. This means that if the Sun were suddenly plucked out of the Solar System, the Earth would stay in orbit for 8.3 minutes 鈥� the same time as it takes light to travel between the Sun and Earth. Then, feeling no gravity, Earth would fly off into space.
Measuring the speed of gravity seemed impossible with current technology. But by reworking Einstein鈥檚 equations, Sergei Kopeikin of the University of Missouri-Columbia came up with a possible way. Kopeikin argued that the time delay of a light signal passing through the gravitational field of a moving planet 鈥� Jupiter, for instance 鈥� will depend on the speed of gravity. What鈥檚 more, he said, it should be possible to measure the effect, and hence gravity鈥檚 speed, by watching how Jupiter鈥檚 gravity bends background light.
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In September last year, Kopeikin and his colleague Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, did just that using radio telescopes around the world. They concluded that gravity does indeed move at the speed of light (New 杏吧原创, 11 January, p 32).
But Clifford Will of Washington University in St Louis, an expert on experimental tests of general relativity, says Kopeikin鈥檚 reasoning is flawed. Will鈥檚 own calculations suggest that the speed of gravity would not have any measurable effect on radio signals passing Jupiter. All Kopeikin measured was the speed of light. 鈥淭his actually conforms to intuition, but you can鈥檛 know for sure until you sweat the details,鈥� he says. His analysis is at .
Kopeikin is sticking to his guns. In a letter to Will and The Astrophysical Journal he says Will is mistaken in using a static approximation for Jupiter鈥檚 changing gravitational field. Although the approximation is accepted by most physicists, Kopeikin says his own dynamical approximation is more exact, and proves the experiment was sensitive to the speed of gravity after all.
鈥淲ill鈥檚 arguments appear to make Kopeikin and Fomalont鈥檚 claim of a measurement of the speed of gravity invalid, or in the very least in serious doubt,鈥� says Caltech鈥檚 Barry Barish, director of the LIGO experiment, which aims to detect gravitational waves. 鈥淗owever, in all fairness, we all need to wait and see if Kopeikin and Fomalont have some response that counters Will鈥檚 critique and conclusions.鈥�