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.
How did Einstein figure all of this out, just amazing!
I gave myself a headache on one rainy saturday thinking about the ramifications of quantum time. It's a very interesting idea.
If you imagine a quantum of space like a square in a checkerboard (and make it a cube so it's 3-dimensional) and a quantum of time as how long it takes a quantum of light to pass from one quantum cube to the next, and then you imagine that space can be deformed, so that one quantum is not the same physical size as the next, and that the time quantum doesn't change with the space quantum, then...
It keeps the salt in the shaker.
Okay, I'm confused.
We are always shown the picture of the Einstein space/time continuum: a marble (representing a planet) rolling around on a cross-hatched sheet, circling a steep central drop-off (representing a black hole). The momentum of the marble keeps it from falling inward.
Gravity, in this model, is the curvature of the sheet (the space/time continuum).
Isn't the curvature "felt" instantaneously by the marble because gravity is embedded in the very "fabric" of the space/time continuum?
Damnit Jim, I'm a biologist, not a physicist!
Yes, you are on the right track, since radio waves travel at the speed of light.
Isn't that a co-inky-dink.
What I don't know about physics could fill a book. (Hundreds of them, actually)
Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.
How fast do the "Brane World" theorists think light might travel? Could gravity travel at 1.20 times the speed of light for them to be correct? Or would it be 10,000 or 100 times the speed of light for them to be correct?
I saw a model of this in the Los Angeles County Museum of Science and Industry, many years ago. At the time, I thought that showing a model that used gravity to demonstrate the concept of gravity was cheating. ;)
Gravity and magnetism are two of my favorite puzzles. They embody "spooky action at a distance" for me...
Gravity Sux! ;-)
Yes. And if you pull the sheet down a little farther in that spot, the curvature won't readjust itself instantaneously across the whole sheet... instead, the increased curvature propagates as a wave outward from the place you pull on, at some measurable rate.
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