Speed Of Gravity

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Einstein proven right on gravity

Albert Einstein

The speed of gravity has been measured for the first time, revealing that it does indeed travel at the speed of light.

It means that Einstein's General Theory of Relativity has passed yet another test with flying colours.

The measurement was made by Ed Fomalont of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri, in Columbia, both US.

Writing in New Scientist magazine, they say: "We became the first two people to know the speed of gravity, one of the fundamental constants of nature."

Higher dimensions

Isaac Newton believed the influence of gravity was instantaneous. Later, Albert Einstein assumed it travelled at the speed of light and built his 1915 General Theory of Relativity around that assumption.

If gravity travelled at the speed of light it would mean that if the Sun suddenly vanished from the Solar System, the Earth would remain in orbit for about eight minutes - the time taken for light to travel from the star to our planet. Then, in the absence of gravity, Earth would move off in a straight line.

Modern researchers say that knowing the speed of gravity is important in the study of branches of cosmology where the Universe has more spatial dimensions than the usual three.

Some of those theories suggest that gravity could take a short cut through higher dimensions and so appear to travel faster than the speed of light.

Jupiter's help

To measure gravity's velocity, Kopeikin determined that it could be determined with the help of the planet Jupiter, if its mass and velocity were known.

The perfect opportunity arose in September 2002, when Jupiter passed in front of a quasar - a distant, very active galaxy - that emits radio waves.

Fomalont and Kopeikin combined observations from a series of radio telescopes to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves.

From the observations the researchers determined that that gravity does indeed move at the same speed as light.

The results of the study have been presented to this weeks meeting of the American Astronomical Society (AAS) in Seattle.

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Critics weigh in over speed of gravity

15:56 17 January 03 

NewScientist.com news service 

Scientists 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's 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's equations, Sergei Kopeikin of the University of Missouri in Columbia came up with an experiment.

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's speed, by watching how Jupiter's 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's reasoning is flawed. Will's 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.

"This actually conforms to intuition, but you can't know for sure until you sweat the details," he says. His analysis can be read here.

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's 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. "I remain convinced that their experiment is not sensitive to the speed of gravity, and that my calculation demonstrates it conclusively," he says. 

Hazel Muir


Copyright 2010 Tim Stouse
Last modified: December 10, 2010
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