快猫短视频s who announced a landmark first measurement of the speed of gravity on 7 January are fighting back against criticism that their experiment mistakenly measured only the speed of light.
Einstein鈥檚 general theory of relativity assumes that the force of gravity propagates at the speed of light. However, some exotic modern theories suggest it could travel more quickly.
Measuring the speed of gravity seemed impossible with current technology but, by reworking Einstein鈥檚 equations, Sergei Kopeikin of the University of Missouri in Columbia came up with an experiment.
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He argued that the time delay of a light signal passing through the gravitational field of a moving planet depends on the speed of gravity. What is 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.
In September 2002, Kopeikin and his colleague Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, did that using radio telescopes around the world. Their conclusion was that gravity does indeed move at the speed of light.
Undetectable difference
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 if the speed of gravity was different from the speed of light, by any amount, that would not create a detectable alteration in the time delay for radio signals passing Jupiter.
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 can be read .
But Kopeikin is sticking to his guns. In a letter to Will and The Astrophysical Journal, he says Will is mistaken in using a common static approximation for Jupiter鈥檚 changing gravitational field. Kopeikin says his own dynamical description of the field, developed in 1999 at the University of Jena in Germany, is more exact.
Will counters that Kopeikin and Fomalont misunderstand his criticism. 鈥淚 remain convinced that their experiment is not sensitive to the speed of gravity, and that my calculation demonstrates it conclusively,鈥� he says.