First speed of gravity measurement revealed

NewScientist.com ^ | 01/07/2003 | Ed Fomalont and Sergei Kopeikin

[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.

[Southack]

I don't know that the geometry *IS* different between electromagnetism and Gravity.

But I haven't been able to find a model as large as our Sun and Earth re: Gravity for the electromagnetism argument one way or the other.

You keep trying to put the burden of proving or disproving the analogy of Electromagnetism and Gravity on me, yet it is your argument, not mine, that they must be the same.

And you may very well be correct, but I see no reason why I should be compelled to show it one way or the other.

[aruanan]

The speed of gravity has been measured for the first time.

NOT! The low end for the speed of gravity had been determined many, many decades ago.

[sandydipper]

Einstien was not from planet Earth.

[Physicist]

But I haven't been able to find a model as large as our Sun and Earth re: Gravity for the electromagnetism argument one way or the other.

Irrelevant. Nowhere in your argument do you make any reference to any sort of length scale.

You keep trying to put the burden of proving or disproving the analogy of Electromagnetism and Gravity on me, yet it is your argument, not mine, that they must be the same.

The theoretical motivation for my saying that they are the same under Newtonian physics goes all the way back to Coulomb. I'm trying to understand your theoretical motivation for saying that they're different, if that's what you're saying.

I see no reason why I should be compelled to show it one way or the other.

Because right now your version of physics isn't hanging together. Rescue it if you can. Either gravity and E&M are analogous in your model or they are not. If not, why not?

[DannyTN]

"This is nothing new, mothers have known this for years. Watch a child the next time they spill a bowl of spaghetti...time and gravity both slow down as the bowl falls to the (usually carpeted) floor. LOL "

Time slows down for the Mother as the bowl falls. The child on the other hand experiences a dramatic speed up in time as the Mother suddenly begins to talk very fast and much louder (obviously a doppler effect).

[MHGinTN]

The gravity field of the Sun (and the entire solar system intermixed) is said to 'propagate at the rate of "c". The background in which the gravity expresses is the spacetime field.

It is believed that were the Sun to instantaneously disappear, the 'curvature' the Sun imparts to the spacetime field would take the same time to effect the Earth's inertial field as it would take to suddenly notice a darkness where the Sun had been radiating. Or would it ... would it take "x" minutes to reach the point in spacetime where light from the Sun no longer exists?

It is believed that gravitons and photons exist at a reality rate of 300,000 meters per second in a vacuum, but the spacetime field is hardly a vacuum if fields propagate using it as the medium of propagation ... is that correct?

[longshadow]

simple geometrical placemarker

[Cboldt]

-- bumpity bump --

FWIW, I'm under the impression that light is a form of electromagnetism, and that much of the discussion on this thread has indeed been in the form of comparing electromagnetic waves with gravity.

[Oberon]

Light does not bend Gravity easily.

Says who? You got any data to back up that assertion?

[Oberon]

Sadly, far too many people have trouble understanding that our whole Solar System is moving en masse in one direction even as our planets revolve around our Sun (which happens to be centered in the middle of the very System that is moving through space), and thus, they erroneously conclude that the Sun isn't really moving because people on Earth can observe that it "seems" to be always in the same place (not comprehending that it only seems that way because the Earth and the Sun are maintaining their relative positions to each other as they hurtle through space).

All motion is relative. So you say our solar system is moving, and I might agree so far as you've gone... but you haven't specified... it's moving relative to what?

[Southack]

"All motion is relative."

No, the appearance of something moving is relative.

Reality, however, cares not a whit for appearances. Either something is moving or it isn't, regardless of how it appears in your relative frame of reference.

[Oberon]

Either something is moving or it isn't, regardless of how it appears in your relative frame of reference.

All right, then. If what you say is true, then there might likely be some motionless object. Is anything in the universe motionless? Can you point to any single object and say with certainty, "This object is motionless in space. This object is stationary within the fixed, absolute frame of reference"?

And if what you say is true, and there are things that are moving and at least one thing that isn't, how can you tell which is which?

[Southack]

Who says that we *can* tell?

I'm simply pointing out that Reality cares not a whit for perception.

[Oberon]

Who says that we *can* tell?

Evidently you can, because you said so. I'm asking for your evidence. You say that there can be an object in the universe that is motionless in the absolute sense. I ask you to give me an example.

Please?

[Southack]

Would you be so kind as to show me the Post # where I said what you claimed I said? Thanks.

[Oberon]

Post 251, in which you write (and I quote):

"No, the appearance of something moving is relative.

Reality, however, cares not a whit for appearances. Either something is moving or it isn't, regardless of how it appears in your relative frame of reference."

Because you make this assertion, you also implicitly assert that you know it to be true, i.e. you can tell. I'm asking how. Please share your insight with me, by pointing out how you know that there's an absolute frame of reference. Where's the motionless object that serves as our absolute reference point?

Is merely asking sufficient, or do you require that I get down on my knees and pray?

[Oberon]

Hey Gary... I could use a little help here. Have you ever taught a pig to sing?

[Southack]

Reality, however, cares not a whit for appearances. Either something is moving or it isn't, regardless of how it appears in your relative frame of reference. - Southack

"Because you make this assertion, you also implicitly assert that you know it to be true, i.e. you can tell [that something isn't moving]." - Oberon

No, by making that assertation, I am pointing out that Reality doesn't care whether we can "tell" or not.

[cinFLA]

Except that if gravity propagated at the speed of light, the earth would long ago have ceased to orbit the sun.

Why?

[mdmathis6]

"Sadly, there are many in this forum who have absolutely no idea of the gravity of this discovery."

Yes...'tis a weighty issue indeed!