Posted on 01/07/2003 6:23:34 PM PST by 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.
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.
I've explained this to you before. All of these results are in perfect agreement with the predictions of General Relativity. The gravitational field does point to the true position of the sun, and gravity does propagate at the speed of light. There is no disagreement.
The problem here is that you're confusing the field with undulations in the field. Fields don't propagate; only changes in them do.
You know, I had one in a jar on my desk just the other day... Dang, where'd it go? =]
That's heavy, man.
It's very simple. Jupiter's presence makes a gravity lens that is capable of refracting radio waves just like an eyeglass lens refracts light. The physicists checked whether the prescription on the leading edge of Jupiter was the same as the prescription on the trailing edge. It was a slick bit of work, but not hard to understand.
Don't blame me; I voted for Velcro.
[This ping list for the evolution -- not creationism -- side of evolution threads, and sometimes for other science topics. To be included, or dropped, let me know via freepmail.]
Elegant. This work is a classic. If I recall correctly, it was the Jovian system that once was used to measure the speed of light, by observing the delay in the appearance of its moons (when coming from behind Jupiter) at different times of the year, the lag time being due to the earth being at different distances from Jupiter. I may have this backwards, and it could be that lightspeed was then known, so the delay was used to measure the size of earth's orbit. Either way, ol' Jupiter has been very useful.
It keeps the salt in the shaker.
Gravity is great until you hit your mid 30's and have had 3 or 4 kids. Then it sucks!
Summary
- S. Kopeikin misquotes Van Flandern as predicting that cg (Kopeikin’s “speed of gravity”) will be infinity. Van Flandern and Vigier are in print showing that six experiments better than Kopeikin’s already show that the speed of gravity is >> c (c = speed of light). But in posted discussions with Kopeikin and in USENET newsgroups, Van Flandern clearly states that Kopeikin’s “cg” parameter cannot be the speed of gravity and will certainly come out near the value c, as it did.
n Asada followed up Kopeikin’s Astrophysical Journal paper with his own paper in the same journal showing that Kopeikin was simply measuring a quantity that propagated at the speed of light, and was definitely not measuring the speed of gravity.
n Kopeikin has mostly ignored these well-founded corrections, apparently because he justified funding for his experiment by the claim that it would measure the speed of gravity.
n Kopeikin new paper at the Los Alamos archive shockingly revises his protocol, equations, and methodology. Scientific method forbids changing the protocol after the results are in, especially when it is done to avoid an unwanted or unexpected result.
n Kopeikin’s new equations introduce a new factor, c/cg, for time in the Einstein equations. This factor drives time intervals to zero for large values of cg, thereby making large cg results *impossible* for any experimental data regardless of reality.
n Kopeikin’s now-forced results do a great disservice to science in general and the advancement of physics in particular because they no longer represent what his own experiment showed, much less the speed of gravity.
Wait 'till you pass 50. Even your earlobes start to sag!
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