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.
Don't congratulate yourself too much. It's called the tidal force, and scientists and sailors are aware of it.
Einstein's example was for a uniform (i.e. divergence-free) gravitational field. In principle, you can create a uniform field by constructing a large, flat, massive sheet; near the center of the sheet, the field will be uniform. Or, you can achieve an arbitrary uniformity it by examining a sufficiently small volume of space.
The point of the equivalence principle is not that it isn't possible in practice to tell whether you're on the surface of a planet, but that accelerations and gravitational fields are physically the same phenomenon. That's a pretty radical notion.
Ice is cold and hard while steam is hot and gasseous, but that doesn't alter the fact that they are fundamentally the same stuff.
My physics books all say that the two were indistinguishable, and gave the elevator example. It always bugged my profs to no end when I pointed out that they are always distinguishable. Frowns, bad grades, "troublemaker," always followed.
I'm so scarred!!!! LOL!
Yes, of course a "uniform gravitaional field" would be equivelent. But there ain't no such thing, unless someone can give me an example (then I'll go back to the class and shut up). Never has been, never will be.
The elevator thing taught to all budding physicists should have a disclaimer:
Caution: What you are about to hear is false, even reduced to a simplification."
Or am I wrong?
All interrelated and subject to the constant speed of light.
Nowhere is stated the Calabi-Yao. A deficit article, imo.
If you simplify your version of the example and assume that it's just you in the elevator (no pendulums, no instruments, just you, personally, as the solitary observer), can you tell the difference?
Is something wrong with the large, massive sheet?
And don't forget, for almost any conceivable gravitational field and any given level of sensitivity, there is a calculable scale below which the field is indistinguishable from a uniform field. That means that this problem is not irrelevant to the laboratory.
Or am I wrong?
You're missing the point. You might as relevantly have pointed out that the situation where someone is on a rocketship without knowing it is unlikely ever to arise. The equivalence principle is a far-from-obvious statement about the physical nature of gravity and inertia, not merely a practical limitation on figuring out whether you're travelling on a rocketship. Instead of pettifogging about the practical details of a given example, why don't you try to focus on the principle that the example was meant to illustrate?
The mass of the Sun 'attracts' the mass of our planet ... not directly, but by creating a field in which the motion of our planet is effected. Is that correct?
If that is a correct notion, the question then becomes, what about mass causes this 'field' influencing other masses? ... Don't physicists define the 'thing' causing the influence, creating the field, 'gravitons'? And if there are gravitons, doesn't it appear that these gravitons are actually influencing the spacetime of the universe (the background field in which masses exist), and that affected spacetime is that which then acts upon the other mass(es)?
"We now know that the speed of gravity is probably equal to the speed of light," Fomalont said. "And we can confidently exclude any speed for gravity that is over twice that of light."space.comThat gravity works instantaneously is almost impossible, according to the study.
The results have been submitted for publication in a peer-reviewed scientific journal, but the publication has been held up because of criticisms of the work leveled by some researchers.
*T. Van Flandern and J.P. Vigier (2002), “Experimental Repeal of the Speed Limit for Gravitational, Electrodynamic, and Quantum Field Interactions”, Found.Phys. 32, 1031-1068.
T. Van Flandern (1998) , “The speed of gravity – What the experiments say”, Phys.Lett.A ,/em>250, 1-11.
Here's what Chris Hillman has to say about this result (from here)
..."In a paper remarkable chiefly for the extraordinary number of obvious errors it contained (see above), Tom Van Flandern, (``The speed of gravity-- what the experiments say'' Phys.Lett.A 250 (1998) 1-11), stated: the Global Positioning System (GPS) showed the remarkable fact that all atomic clocks on board orbiting satellites moving at high speeds in different directions could be simultaneously and continuously synchronized with each other and with all ground clocks. No "relativity of simultaneity" corrections, as required by SR, were needed. This too seemed initially to falsify SR. But on further inspection, continually changing synchronization corrections for each clock exist such that the predictions of SR are fulfilled for any local co-moving frame. To avoid the embarrassment of that complexity, GPS analysis is now done exclusively in the Earth-centered inertial frame (the local gravity field). And the pre-launch adjustment of clock rates to compensate for relativistic effects then hides the fact that all orbiting satellite clocks would be seen to tick slower than ground clocks if not rate-compensated for their orbital motion, and that no reciprocity would exist when satellites view ground clocks.
At first glance, Van Flandern here appears to be claiming that the fact that the GPS continues to operate with great accuracy has in fact disproven the predictions of str concerning moving clocks (Van Flandern doesn't mention the gtr effects, but they are also significant). On careful reading, in this paper he actually appears to be saying in effect that anything that can be explained using str can be explained just as well using the Lorentz ether theory (let), a theory which he has never specified but which is usually taken to be mathematically equivalent to str, but with a different interpretation of Lorentz transformations, one which most physicists since Lorentz's day have found implausible. However, more recently, in postings to sci.physics.relativity, Van Flandern has clearly stated that he believes that changes in electrostatic and gravititostatic potentials are transmitted instantly (literally!), just as if electromagnetism and gravity were truly governed by the Poisson equation, a viewpoint which is mathematically utterly inconsistent with both str and gtr, contrary to his claims in an earlier (and also wildly erroneous) paper, ``Possible new properties of gravity'', Astrophysics and Space Science 244 (1996)...."
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There are also several archive preprints covering his approach: here, and here.
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Frankly, anyone who believes in an artificial face on Mars has issues.
The problem can be set up any way you want.
For example: calculate the force of gravity from a uniform sphere at a distance two radii from the center of mass.
Or, calculate the force of gravity 1 mile above a mass shaped like a brick 1000 miles long, 300 miles wide, and 200 miles thick.
Or, calculate the force of gravity 1 mile above a mass that is one mile thick and infinitely wide and infinitely long.
Like that.
Why the constraint?
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