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Underground > Laboratory >

Speed of Gravity = Speed of Light

15:56 / 08.01.03

apparently

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."

...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...

...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.

Okay, it's a dodgy BBC science link so has very little detail, much of which is probably crap anyway. Anyone got anything better?

16:32 / 08.01.03

From Nature:

Einstein's theory of relativity only holds if the force of gravity acts at the same speed as light. Until now this was merely an assumption, says theoretical physicist Sergei Kopeikin of the University of Missouri in Columbia...

The chance to take a reading came on 8 September last year. Jupiter passed in front of a distant quasar called J0842. Quasars are intense sources of radio waves that are thought to be giant black holes at the centres of galaxies...

If the speed of gravity were infinite, Kopeikin predicted that the quasar should have traced a perfect circle in the sky as Jupiter passed. If gravity had some finite speed, this circle would distort into an ellipse.

Groups of telescopes can function like a single huge instrument, through a technique called interferometry. By comparing the differences in the signals received at different places, the team pinpointed the quasar's position with incredible accuracy.

The shape of the quasar's motion gave a speed of gravity nearly identical to that of light, with an error margin of plus or minus 20%. "You can rule out a speed of gravity greater than twice the speed of light with a high degree of confidence," Fomalont told the American Astronomical Society's meeting in Seattle this week.

Looks like Einstein was right again.

22:49 / 19.01.03

Then again, not everyone agrees that the experiment is valid. From New Scientist,

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.

Which is disputed by Kopeikin.

the king of byblos

13:54 / 22.01.03

I am a bit unsure about this generally for a number of reasons:
1)"with an error margin of plus or minus 20%" I wouldn't really call that very accurate considering these are universal constants we are talking about; as such the speed of gravity is somewhere between 239,833,966.4meters per second and 359,750,949.6meters per second, apparently.
2) all the measurements are within the framework of relativity anyway; how would you be able to measure if the speed of gravity was faster than the speed of light using mesuring devices working the electtromagnetic spectrum, and therefore limited by the speed of light?
3) the theory is Kopeikin's developed from G.R. it doesn't mean Einstein falls over if it is wrong;-)

The real test will be if these results can tie in with the search for the Higgs boson/theortical-gravity-particle(s). If one knew the speed of gravity accurately then it would not be impossible to theoretically determine the mass/charge of an associted particle and thereby narrow the hunt considerably?

14:16 / 22.01.03

As I understand it,

1) The point is to disprove the Newtonian notion that gravity acts instantaneously, that is has infinite speed. Hence any finite bound is a breakthrough - I couldn't believe that this hadn't been done yet, but there you are.

2) Not sure that the measuring equipment is all that relevant. I think they measured the gravitational effect in the form of some light distortion. The greater the distortion, the slower the speed. Maybe.

3) True, and this is the basis of the dispute. But it is generally possible to come to an agreement whether some extension of GR is correct (given GR is correct) and hence disprove it. In fact, I thought that the really exciting consequence of the measurement of the speed of gravity would be that it might produce a discrepancy with standard GR - and hence point to some property of the unified theory of quantum mechanics and relativity.