First speed of gravity measurement revealed

NewScientist.com ^ | 01/07/2003 | Ed Fomalont and Sergei Kopeikin

[forsnax5]

The speed of gravity has been measured for the first time. The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.

Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri in Columbia made the measurement, with the help of the planet Jupiter.

"We became the first two people to know the speed of gravity, one of the fundamental constants of nature," the scientists say, in an article in New Scientist print edition. One important consequence of the result is that it places constraints on theories of "brane worlds", which suggest the Universe has more spatial dimensions than the familiar three.

John Baez, a physicist from the University of California at Riverside, comments: "Einstein wins yet again." He adds that any other result would have come as a shock.

You can read Fomalont and Kopeikin's account of their unique experiment in an exclusive, full-length feature in the next issue of New Scientist print edition, on sale from 9 January.

Isaac Newton thought the influence of gravity was instantaneous, but Einstein assumed it travelled at the speed of light and built this into his 1915 general theory of relativity.

Light-speed gravity means that if the Sun suddenly disappeared from the centre of the Solar System, the Earth would remain in orbit for about 8.3 minutes - the time it takes light to travel from the Sun to the Earth. Then, suddenly feeling no gravity, Earth would shoot off into space in a straight line.

But the assumption of light-speed gravity has come under pressure from brane world theories, which suggest there are extra spatial dimensions rolled up very small. Gravity could take a short cut through these extra dimensions and so appear to travel faster than the speed of light - without violating the equations of general relativity.

But how can you measure the speed of gravity? One way would be to detect gravitational waves, little ripples in space-time that propagate out from accelerating masses. But no one has yet managed to do this.

Measuring the speed of gravity

Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity.

The opportunity to do this arose in September 2002, when Jupiter passed in front of a quasar that emits bright radio waves. Fomalont and Kopeikin combined observations from a series of radio telescopes across the Earth to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves.

From that they worked out that gravity does move at the same speed as light. Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.

Their result, announced on Tuesday at a meeting of the American Astronomical Society meeting in Seattle, should help narrow down the possible number of extra dimensions and their sizes.

But experts say the indirect evidence that gravity propagates at the speed of light was already overwhelming. "It would be revolutionary if gravity were measured not to propagate at the speed of light - we were virtually certain that it must," says Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio.

[forsnax5]

Ping for Gravity fans!

[Fiddlstix]

BTTT

[Excuse_My_Bellicosity]

Pretty wild. I always assumed that gravity was an instantaneous thing.

[aruanan]

Ping for Gravity fans!

Except that if gravity propagated at the speed of light, the earth would long ago have ceased to orbit the sun. The calculation of orbital trajectories, in order to work, have to be plotted treating the speed of gravity as virtually instantaneous.

[Southack]

"But how can you measure the speed of gravity? One way would be to detect gravitational waves, little ripples in space-time that propagate out from accelerating masses. But no one has yet managed to do this.

Measuring the speed of gravity Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity. The opportunity to do this arose in September 2002, when Jupiter passed in front of a quasar that emits bright radio waves. Fomalont and Kopeikin combined observations from a series of radio telescopes across the Earth to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves."

Then again, perhaps all that they really measured was the speed of the radio waves bending around Jupiter...

[Excuse_My_Bellicosity]

It's amazing that Einstein's theories, unprovable when he was alive, are still being shown to be right.

I remember in 1995 when they discovered that Einstein-Bose Condensate was just like Einstein (and Bose) predicted it would be in 1925.

[forsnax5]

I always assumed that gravity was an instantaneous thing.

You're in good company. So did Isaac Newton...

[blam]

Gravity particles bump.

[conservativemusician]

Fascinating stuff.

[MonroeDNA]

Geeze, we all knew that. It HAS to.

*burp*

[NEWwoman]

What Gravitas!

[Jhoffa_]

Ping for Gravity fans!

Of course you realize this ping is entirely redundant..

[aruanan]

You're in good company. So did Isaac Newton...

I would think that measurements of earth's acceleration toward the sun that show a direction that is 8.3 minutes ahead of the apparent position of the sun in the sky also demonstrate a propagation speed that is virtually (at these distances) instantaneous. That is, the earth is not accelerating toward where "gravity waves" are supposedly reaching the earth together with the photons that left the sun 8.3 minutes previously but toward where the sun actually is.

[supercat]

Pretty wild. I always assumed that gravity was an instantaneous thing.

What, then, would prevent the use of gravitational waves as a faster-than-light communications medium?

[Libertarianize the GOP]

http://www.freerepublic.com/perl/bump-list

[supercat]

[thoughts]: (1) Do photons interact gravitationally with each other? (2) Do gravity waves have mass?

[Nick Danger]

Perhaps there is a minimum distance -- a quantum of space -- and a minimum amount of time -- a quantum of time. There is a speed limit on light because it cannot take less than one quantum of time to travel across one quantum of space. Do I know what I'm talking about here? Absolutely not. But I don't think anyboody else does, either.

[VadeRetro]

You got it nonetheless. The "Planck" distance and the "Planck" time. Values very small in each case. Everything is grainy when you get small enough.

[rface]

I love gravity!

Columbia, Missouri (actually Ashland, Missouri)