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First speed of gravity measurement revealed
NewScientist.com ^ | 01/07/2003 | Ed Fomalont and Sergei Kopeikin

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.

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.


TOPICS: News/Current Events
KEYWORDS: crevolist; gravity; podkletnov; realscience; science; stringtheory; tvf
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To: Libertarianize the GOP
Thanks for the ping!

How did Einstein figure all of this out, just amazing!

21 posted on 01/07/2003 6:57:54 PM PST by Ernest_at_the_Beach
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To: forsnax5
If gravity travels at the same speed as light is it an electromagnetic wave or is it the medium that it travels through that limits the speed of propagation of all forces?
22 posted on 01/07/2003 7:06:03 PM PST by Gary Boldwater
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To: aruanan
You are confusing instantaneous with continuous.

The effect of Gravity as a field is based on a continuous "warping" of space based on the presence of mass. And mass such as the sun does not instantaneously dissappear, as in the thought experiment cited of a vanishing sun. Every "now" is based on the influence of where everything elsewhere "was" at this moment's light-horizon...

If the sun exploded from its center in an emmense explosion such that all the remaining mass of the sun was accelerated into a spherical shell expanding at near light speed from the center of the sun, once this shell of matter passed the earth, what was left of the earth would proceed in an essentially straight line - no longer an orbit - based on the long-recognized zeroing out of the gravitational attraction of matter outside a sphere, modified by the reaction of the collision of the expanding debris impacting the earth, and physically propelling it in a slightly different direction...
23 posted on 01/07/2003 7:09:37 PM PST by muffaletaman
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To: Nick Danger
Perhaps there is a minimum distance -- a quantum of space -- and a minimum amount of time -- a quantum of time.

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...

HEADACHE!


24 posted on 01/07/2003 7:11:34 PM PST by forsnax5
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To: rface
I love gravity!

It keeps the salt in the shaker.

25 posted on 01/07/2003 7:16:36 PM PST by facedown (Armed in the Heartland)
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To: forsnax5
Only one tiny thing they forgot to mention in the article: No one knows what gravity is.
26 posted on 01/07/2003 7:18:50 PM PST by Semper911
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To: forsnax5
........they worked out that gravity does move at the same speed as light.......

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!

27 posted on 01/07/2003 7:20:30 PM PST by DoctorMichael (My brain hurts.)
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To: Southack
perhaps all that they really measured was the speed of the radio waves...

Yes, you are on the right track, since radio waves travel at the speed of light.

Isn't that a co-inky-dink.

28 posted on 01/07/2003 7:21:03 PM PST by Semper911
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To: muffaletaman
Likewise the importance that as near as it can be measured, inertial mass and gravitational mass are exactly equal.

In the supposed experiment measuring the weight of an object suspended over a rapidly spinning superconducting disk, and finding the claimed finding that weight of the object is lessened...

(1) either a repulsive electromagnetic effect is generating a force upward... normal action/reaction is occuring, nothing more...

(2) or a method of shielding the gravity field is being generated, which if true can be engineered into a warp drive someday allowing interstellar travel vis-a-vis star trek (i.e. if gravitational mass and inertial mass can be caused to become different and the difference manipulated, then perhaps an inertial bubble can be created, and the bubble accelerated by a generated polar flux in the universal G-field - one direction of the entire universe attracts, the opposite direction repulses. Thus, though the object within the bubble never exceeds local light speed within the bubble, the bubble itself can be moved at warp speeds, multiples of the speed of light)...

(3) or... (fill in the blank yourself...)
29 posted on 01/07/2003 7:24:29 PM PST by muffaletaman
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To: forsnax5; Southack
Of course I should add my usual disclaimer:

What I don't know about physics could fill a book. (Hundreds of them, actually)

30 posted on 01/07/2003 7:24:51 PM PST by Semper911
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To: PatrickHenry; Physicist; ThinkPlease
gravity speed bump!
31 posted on 01/07/2003 7:26:04 PM PST by longshadow
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To: forsnax5
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.

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?

32 posted on 01/07/2003 7:26:53 PM PST by Sawdring
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To: DoctorMichael
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.

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...

33 posted on 01/07/2003 7:30:29 PM PST by forsnax5
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To: forsnax5
Is there a graviton responsible for propagation of mass attraction? With what does the graviton interact to deform spacetime? If two masses attract each other (deform spacetime in a way to influence each other's relative inertial existence), to what location of a mass or spacetime are the gravitons focused? Could it be that gravity is a temporal force/effect not strictly a spatial effect?... Might mass warp the planar 'smoothness' of present time rather than the volume of spatial spacetime?
34 posted on 01/07/2003 7:31:36 PM PST by MHGinTN (It's obvious I'm no scientists, asking such questions)
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To: forsnax5

35 posted on 01/07/2003 7:32:42 PM PST by null and void (And only one with the nawth end of a southron bound haws...)
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To: forsnax5
Very interesting, especially if all the current proposals that the speed of light is variable is true. The implications are stunning!
36 posted on 01/07/2003 7:33:38 PM PST by vannrox
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To: forsnax5
OK someone has to say it :

Gravity Sux! ;-)

37 posted on 01/07/2003 7:34:32 PM PST by commish
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To: forsnax5
Yes but we won't ever get our anti-gravity boots unless we can find out "why" gravity pulls us down. Maybe if we could creat something upon which the mass of a foreign body like the moon would zero in on, therefor countering the the pull of earth, and give us a lift... kind of like a magnifying glass pulling in the reys of the sun, a device could be created that would magnify the gravity pull of the moon.
38 posted on 01/07/2003 7:43:42 PM PST by Godfollow
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To: DoctorMichael
Gravity, in this model, is the curvature of the sheet (the space/time continuum).

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.

39 posted on 01/07/2003 7:43:58 PM PST by Oberon
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To: forsnax5
Sorry I'm late to the thread! I had to come all the way over from the "I-told-you-so" Department.

;^)
40 posted on 01/07/2003 7:45:36 PM PST by Physicist
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