First gravity measurement this weekend

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Scientists jump at the chance to test Einstein's general theory of relativity.

There might not be a similar opportunity to determine the speed of gravity for another decade Credit: © Getty Images

This weekend, astronomers hope to measure the speed of gravity directly for the first time. They plan to capitalize on a lucky alignment of Earth, Jupiter and a group of distant stars on 7 and 8 September.

Measuring how fast gravity waves - such as those caused by the motion of the planets or the explosion of a star - move through space is central to confirming Einstein's general theory of relativity. This and many other theories of physics rest upon the speed of light and gravity being equal.

These theories will be put to the test by measurements of radio waves from a distant star-like astronomical body called a quasar. The waves will be collected by a string of US National Radio Astronomy Observatory (NRAO) telescopes throughout the Virgin Islands and Hawaii, in tandem with a single radio telescope in Effelsberg, Germany.

When Jupiter passes between the quasar and Earth this weekend, its gravitational field will bend the quasar's radio waves. Astronomers will use the amount by which the radio waves are bent to infer the speed of gravity. There might not be a similar opportunity to determine the speed of gravity for another decade.

“There is no theoretical hint that some surprises might come from these experiments Giovanni Amelino-Camelia , Rome University”

If all goes well, the final results should be released in around two months' time. But researchers aren't expecting any shocks. "There is no theoretical hint that some surprises might come from these experiments," says astrophysicist Giovanni Amelino-Camelia of the University of Rome. "But these experiments are absolutely necessary, even so."

In the unlikely event that this weekend's recordings do find a mismatch between the speed of light and that of gravity, "the impact would be very significant", Amelino-Camelia adds.

Physicist Sergei Kopeikin of the University of Missouri-Columbia had the idea for this experiment about two years ago. He presented his models relating to the experiment at a conference in 2001, and soon afterwards the wheels were set in motion. "Everybody got excited and said we have to do this," he says.

The accuracy needed to verify Kopeikin's calculations is challenging. The telescopes must pick up angular differences of less than a third of a billionth of a degree. But Kopeikin is confident that his techniques, developed in collaboration with Ed Fomalont of the NRAO, are sensitive to discrepancies 100 times smaller than that - just enough to confirm Einstein's predictions.

Ed Gerstner is the editor of Nature's Physics and Materials Portals

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Gerstner, E. First gravity measurement this weekend. Nature (2002) doi:10.1038/news020902-13

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