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.

[One_who_hopes_to_know]

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

[Physicist]

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

The Face on Mars is wearing a really miffed expression, right about now.

[gonzo]

"...one quantum of time to travel across one quantum of space..."

I had a quantum of Scotch in my cabinet on NYE, but somebody made it empty-space. Them dang FLA-FReepers really know how to party.

Wait-a-sec.....never mind - it was me. Happy new Year, pal...........FRegards

[DoctorMichael]

.........the increased curvature propagates as a wave outward from the place you pull on, at some measurable rate..........

.......like making a wave in the the sheets when you make your bed by injecting a wave of air underneath them. This I knew, but was somehow having a hard time incorporating this into the model.

However, like photons being responsible for the transference of the electromagnetic force, where then are the Gravitons, the particles responsible for this force?

[Grig]

OK, I'm not a physics major, but this kind of thing has always been interesting to me and the article leaves me with these questions...

If the speed of gravity is faster than light, how could it be measured as such?

Although it is not possible to travel AT the speed of light, there is no restriction on going faster than the speed of light. In theroy, there are particles that were ejected from the Big Bang at FTL speeds. From this result, these particles would be not be centered on the area that their gravity effects, and the gravity area would always be trying to catch up. How would this effect things like acceleration, mass, inertia etc.?

Could a particle with FTL speed leave it's gravity effect so far behind that they wind up traveling separate paths (FTL particle shoots past a in a straight line, the gravity 'wave' is diverted by the gravity of the star).

How is it known that the speed of gravity is a constant? How do we know that a supermasive object doesn't have a faster or slower speed, or that other conditions don't have any effect?

The speed of light is not constant, it is constant in a vacuum. Light traveling through a different medium (air, glass) has a different speed. What mediums effect the speed of gravity, if any?

If the center of gravity of an object became displaced from the center of the 'gravity wave' it creates, would the object instantly jump to FTL speed as the gravity wave tried to catch up (Warp factor 5 Mr. Sulu!)

Anyway, if you head hasn't blown up by now, good for you.

[Dan Day]

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?

If the gravitational attractor (say, the Sun) is just sitting there minding its own business, then yes, the gravitational field ("dent" in the space-time rubber sheet) is constant and any other object (e.g. a comet) which wanders by will "feel" the effect of the gravitational curvature "immediately".

But what this discovery describes is how quickly the "rubber sheet" responds to *changes*. For example, if you just plunked the Sun down into a spot of space where it didn't previously reside, the question is how long it would take the resulting "curvature" of the "rubber sheet" to propagate outward. Or alternately, if you start rolling the ball (Sun) north across the sheet, does the rubber sheet instantly adjust as the ball rolls, or does it take a little time for it to "catch up" (i.e., will a spot of curvature 50 yards away instantly feel the change, or will the changing curvature have to "ripple" out there like a crowd doing The Wave?

This is hard to describe will in words, an animation would be ideal, but I don't know of any on the web which show this.

[Dan Day]

I would think that measurements of earth's acceleration toward the sun that show a direction that is 8.3 minutes ahead of the apparent position of the sun in the sky also demonstrate a propagation speed that is virtually (at these distances) instantaneous. That is, the earth is not accelerating toward where "gravity waves" are supposedly reaching the earth together with the photons that left the sun 8.3 minutes previously but toward where the sun actually is.

But where the "sun actually is" doesn't really move that much, relative to the Earth. Don't let the apparent "movement" of the sun across the sky as the Earth rotates fool you.

The Sun pretty much sits in the same spot (i.e., right in the middle of the solar system), so there's no testable difference between the effect its gravity has on our orbit if "instantaneous", versus the effect it would have 8.3 minutes delayed.

If the Sun actually *were* circling the Earth in the way it *appears* to, then yeah, an 8.3 minute difference in gravity would be measurable. But then, it would also be circling us at over two million miles per hour (3% of the speed of light)...

[Lonesome in Massachussets]

Yeah, sure, but what's the speed of dark?

[Dan Day]

What, then, would prevent the use of gravitational waves as a faster-than-light communications medium?

Practical obstacles, mainly.

First, it would be *really* hard to vibrate a mass large enough to produce any non-trivial amount of gravity.

Second, although it seems a strong force, gravity is actually *incredibly* weak compared to electromagnetism, and very, very hard to detect if you're talking about masses less than the size of a moderately large mountain.

Incidentally, this is exactly why researchers have yet to be able to devise an experiment detecting gravity waves, too. If you *could* detect gravity waves, you could make a communication device using them. But to date no one's yet been able to. It's like trying to pick up the sound of a crash-landing mosquito across the Pacific ocean.

[MHGinTN]

If there are gravitons, then it would appear these quanta effect spacetime directly rather than the mass in spacetime directly.

[DoctorMichael]

.......plunked the Sun down into a spot of space where it didn't previously reside, the question is how long it would take the resulting "curvature" of the "rubber sheet" to propagate outward........

Gotcha.

[Dan Day]

Then again, perhaps all that they really measured was the speed of the radio waves bending around Jupiter...

No, they took that into account (of course).

But the point is that the incoming radio waves would bend around Jupiter in slightly different ways if Jupiter's gravitational field "instantly" followed Jupiter around as Jupiter moved in its orbit, versus whether Jupiter's gravitational field lagged a bit behind it.

If instantaneous, Jupiter's "gravitational lens" (which was bending the incoming waves from the distant star) would be spherical, whereas if gravitation travelled at the speed of light, it would be subtly "conical", like the sonic boom coming off of a supersonic jet. The two different "lens" shapes would result in different kinds of "warping" of the incoming waves from the distant star, thus allowing the experimenters to answer the "is it or isn't it" question they were posing.

Pretty clever.

[DoctorMichael]

.......these quanta effect spacetime directly rather than the mass in spacetime directly.........

Now my brain is REALLY beginning to hurt.

Additionally, Mr. Sandman is tugging at my eyelids so I'll have to take this to the privacy of my bedchamber to ponder. LOL

[Southack]

"But the point is that the incoming radio waves would bend around Jupiter in slightly different ways if Jupiter's gravitational field "instantly" followed Jupiter around as Jupiter moved in its orbit, versus whether Jupiter's gravitational field lagged a bit behind it."

Their admitted margin for error in the experiment was .25 times the speed of light, a figure far too large to measure speeds drasticly greater than C.

My money is on Isaac Newton. The Speed of Gravity is far more likely to be substantially faster than the Speed of Light because gravity easily bends Light while Light does not appreciably bend Gravity.

If E=MC^2, and if Gravity (G) is equal to the Energy of a Mass (i.e. G=E/M), then G=C^2. Thus, I'll go with Newton and speculate on a much faster speed of Gravity, along the lines of the Speed of Light squared.

All that this experiment measured was the speed of radio waves as they bent around Jupiter. Gee (pun intended), that's the speed of light!

[MarkL]

Pretty wild. I always assumed that gravity was an instantaneous thing.

Of course not!!! Haven't you ever seen any of the Roadrunner/Coyote cartoons? Notice how when the Coyote goes over the edge of a cliff, he hangs in mid-air for a short period of time? That's the time it takes for gravity to catch up with him! Chuck Jones was quite a visionary!

Mark

[MarkL]

I love gravity!

Gravity! Not just a good idea; It's the law!

Mark

[Old Professer]

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.

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, I thought the sun couldn't suddenly disappear; I thought it would collapse, thereby retaining its gravitational pull while diminishing in volume which would increase its influence on objects under its "umbrella."

In which case it would proceed toward the incipient "black hole."

Hell, 8.3 minutes isn't even time to check my E-Mail, so why should I worry?

[Doctor Stochastic]

Hey Man, gravity---that's heavy.

[PatrickHenry]

Thanks for the bump. Been away, and it's almost midnight. I'll read this tomorrow.

[Spruce]

All the references to 8.3 just yell DOS at me!
My question is this. Why do we use the "blanket" analogy to describe gravity? Why not picture space/time as a 3/d box? When you take from one sector, you give to another. As you accelerate matter in one spot, you deccelerate in another. But the overall mass and speed of all matter remains constant.