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ASTRONOMERS working with the world鈥檚 largest radio telescopes spent last weekend measuring the speed of gravity.

The idea that the pull of gravity is not instantaneous, but instead travels in waves at the speed of light, is a key prediction of Einstein鈥檚 theory of general relativity and underpins much of theoretical physics. But it has never been tested鈥� how can you prove something that may travel at the speed of light has a finite speed?

In 2001, Sergei Kopeikin of the University of Missouri in Columbia thought of a way, based on an effect called 鈥渓ensing鈥�. When a planet or star passes in front of a distant object such as a quasar, the closer object鈥檚 gravity bends radio waves coming from the quasar, focusing them into a ring (see Graphic). But to simplify their calculations, astronomers considering these events usually assume that gravitational fields are static. Kopeikin wondered what effect moving fields would have. He worked out that the gravitational waves would interact with the radio waves, distorting the ring. For gravity waves travelling at the speed of light, the ring should be warped just enough to be detected by Earth鈥檚 best telescopes. 鈥淲hen I first gave a talk about the idea everybody got excited and said we have to do this,鈥� he says.

On 8 September Kopeikin got his chance, when Jupiter passed in front of a distant quasar. Together with Ed Formalont of the National Radio Astronomy Observatory, he collected data on the eclipse from the NRAO鈥檚 Very Long Baseline Array, a series of ten 25-metre telescopes strung out across the US between the Virgin Islands and Hawaii. The data should show the warping of the quasar鈥檚 radio image to an accuracy of better than a millionth of a degree.

But Kopeikin won鈥檛 know if the experiment was successful until he analyses all the data. He hopes to announce the result within a couple of months.