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.

[aruanan]

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

I couldn't care less about a face on Mars. The fact is that the earth accelerates toward the actual position of the sun, not toward the apparent position as it would if gravity propagated at the speed of light. This isn't an opinion or an interpretation but a demonstrated phenomenon. It can't be explained by a speed of gravity that doesn't exceed the speed of light.

[rboatman]

Of course there is always "Boatmans Law of Gravity" to consider.

"Gravity does not exist, the world sux!"

[aruanan]

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)...

Thanks for demonstrating a. your inability to understand what was said or b. your unwillingness to acknowledge what was said.

[PFKEY]

Perhaps there is a minimum distance -- a quantum of space -- and a minimum amount of time -- a quantum of time. There is a speed limit on light because it cannot take less than one quantum of time to travel across one quantum of space. Do I know what I'm talking about here? Absolutely not. But I don't think anyboody else does, either.

Sounds like you have a firm grasp and a excellent wit.

[js1138]

Brane death.

[js1138]

The fact is that the earth accelerates toward the actual position of the sun, not toward the apparent position as it would if gravity propagated at the speed of light. This isn't an opinion or an interpretation but a demonstrated phenomenon. It can't be explained by a speed of gravity that doesn't exceed the speed of light.

You done messed with the Man. I feel a heat wave comin' on.

[MHGinTN]

The fact is that the earth accelerates toward the actual position of the sun, not toward the apparent position as it would if gravity propagated at the speed of light. Pardon this intrusion, but what you've asserted would be based on a quantum exchange or interaction, at whatever speed. Have you considered the possibility that mass acts directly upon spacetime (or, space or time) instead of other masses?... Perhaps the favored analogy is more accurate than what it is trying to convey as the apparent effect, ie, the rubber sheet and mass dimpling, with the rubber sheet being spacetime and the dimpler being mass. [Excuse me for this inanity, but I have been trying to get someone to consider that every mass is a bit of dimension time, a pinch of dimension space, and energy, thus mass having a bit of time and a bit of space inherent is 'displacing' the background spacetime field. So far, I've done such a bad job of trying to illustrate this concept that Physicist has blown my notion off as inane. Perhaps it is, but it represents a new perspective.]

[Doctor Stochastic]

JPL doesn't assume instantenous gravity here.

[Dan Day]

"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.

You're *way* off base here. First, try rereading the portion of my post which you quoted above until you begin to understand it.

They examined the manner in which the incoming radio waves were bent by Jupiter's gravity. The manner of the bending would vary in characteristic ways if Jupiter's gravity field moved with Jupiter instantaneously, versus propagating at the speed of light (or any other velocity).

The manner of the bending was consistent with a gravity propagation speed of between 0.70-1.20 times the speed of light. QED.

And contrary to your amusing claim that a .25c margin of error is "far too large to measure speeds drasticly greater than C", you demonstrate a wildly simplistic view of how margins of error are calculated. When they say that the "actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25", it specifically means that while there were uncertainties in the measurements (as is always the case), they were such that even taking into account the necessary amount of fuzziness in the measurements, they were still good enough to conclusively *exclude* any results less than 0.70c or greater than 1.20c. That's what margin of error *means*, son -- it means that the measurements were good enough to exclude the possibility of results outside the given range. So by definition, the possible error *was* good enough to "measure" (and subsequently rule out) "speeds drasticly greater than C".

If you have any objection to the *actual* methodology used, feel free to present it. But so far you haven't even demonstrated that you understand the nature of the actual measurements and calculations made.

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.

What box of Cracker Jacks did you get your understanding of physics from?

Please explain how, exactly, you believe that the speed of propagation of light and/or gravity in any way relates to whether one would "bend" the other. This ought to be amusing.

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.

Except that it isn't, unless you can explain in good detail how you managed to pull that novel assertion out of your hind end, *and* provide sufficient evidence for it.

Thus, I'll go with Newton and speculate on a much faster speed of Gravity, along the lines of the Speed of Light squared.

We await your experimental evidence.

All that this experiment measured was the speed of radio waves as they bent around Jupiter.

No, as a matter of fact, it did not. It measured the deformation of incoming radio waves as a gravitation source (Jupiter) moved across it, and showed that the manner of the deformation is consistent only with the scenario where the gravitational field propagates outward from the source at near the speed of light. QED.

Deal with it.

You act as if you think they somehow timed the incoming radio waves with a stopwatch and mistook that speed for the "speed of gravity". That quite simply is not the case, and it only reveals your own poor understanding of what was actually done. The actual methodology in no way could mistake the speed of radio waves for the speed of the gravitational field whose effects were being examined.

[Aracelis]

I always assumed that gravity was an instantaneous thing.

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

[seams2me]

as I recall from my Chilbearing Years, the word for pregnant is very similar: gravid. I remember thinking that gravity traveled much faster than the speed of light in those days! LOL

[seams2me]

brain-melting-and-dripping-out-my-ears BUMP

[seams2me]

So maybe my husband was right, when he was trying to convince us that we actually travel faster driving southwest than we do northeast, on our summer vacations from Texas to Michigan? His theory involves the rotation of the earth, combined with the fact that going south is downhill.

[Dan Day]

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.

I hate to be the one to break the news to you, but C^2 doesn't represent a velocity. It can't. The units don't work. Nor does it represent a velocity in "E=MC^2" -- it's a conversion factor which manages to mate up the units of mass (g) with those of energy (cm^2*g/sec^2).

So is it *really* your contention that gravity propagates at a "velocity" of C^2 = 3.47x10^10 square miles per second squared, despite the fact that "square miles per second squared" isn't a velocity, it's a numeric hash? Or, if you prefer, how about 2,220x10^10 acres per second squared? Fascinating...

And no, you can't just fudge the units. It doesn't work that way.

[chuckles]

1 sure fire way to tell is to blow up the sun and see if we hurl into deep space instantly, wait for a period of time under 8 1/3 minutes, or go 8 1/3 minutes or longer, to check the speed. If gravity travels, it should have mass. Light has photons, and eletricity has electrons( or holes if you took electronics in college). I always assumed gravity was a force, not a substance. Substance is limited to the speed of light, at least in the definable diminsions. Gravity can be tested in space by hurling an object at a large object, such as a star. The object may accelerate, but will never reach light speed. Ergo the force of gravity is light speed or slower. I guess you could argue it reached the speed of light when it burned up near the surface of the star, but then it would not still be the original object. A conversion would be neccesary. Once it burned up, you could no longer measure its speed, only the flash of light seen at conversion.

I'll go with the speed of light for the speed of gravity. Magnetic waves and electromagnetic waves are close enough to identical and electromagnetic waves are light speed, ie, radio waves.

[Dan Day]

Thanks for demonstrating a. your inability to understand what was said or b. your unwillingness to acknowledge what was said.

I'm sorry, son, the part of your post where you actually supported your slurs by pointing out what I have allegedly misunderstood seems to have gotten lost in transit. Do try again.

After all, I'm sure you're not the sort of person who would just resort to empty insults in an attempt to distract attention from the fact that he was caught saying something unwise (especially when I was perfectly polite in my original reply to you).

You know, like mistaking the *apparent* movement of the Sun due to the Earth's rotation for an *actual* change in position of the Sun, as you did in your original post...

But hey, I'm open-minded -- please explain how it's somehow *my* mistake when you wrote:

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.

The mistake here, as I already pointed out, is that the Sun isn't *actually* moving, even if the rotation of the Earth makes it look like it might be. Therefore, there's no difference at all between the Earth's vector of gravitational acceleration towards the sun EITHER WAY (i.e. whether gravity propagates at light speed, or instantaneously). EITHER WAY, the Sun's gravity well maintains the same "shape" and "position" while the Earth cruises around it.

And the "apparent position of the Sun in the sky" matters not a whit either way, since the Sun would reside in the *SAME* "apparent position" whether gravity (or even light) travelled at lightspeed, at an infinite speed, or even at a highway speed limit of 55mph.

Since the Sun ISN'T ACTUALLY MOVING, its gravity signature, and even its visual appearance via light, REMAIN THE SAME whether we're seeing it as it was a nanosecond ago, 8.3 minutes ago, or six months ago -- from *any* vantage point.

Furthermore, the "apparent position" of the Sun in the sky is due to the Earth's rotation, which is a LOCAL situation, and independent of any delay in light/gravity propagation from the Sun -- in simple terms, the only thing that would make the Sun look 8.3 minutes ahead or behind of its apparent position in the sky would be a change in the Earth's rotation itself, *not* any delay in light coming from the Sun.

So yes, please, tell me what *I've* missed...

I also eagerly await your supporting evidence for your amazing claim that there are "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".

[Southack]

"Please explain how, exactly, you believe that the speed of propagation of light and/or gravity in any way relates to whether one would "bend" the other."

It's rather easy, actually. Objects emit more Gravitons the faster they travel. Thus, at speed, Light can be bent by Gravity. Likewise, when Light is slowed down, Gravity affects it less and less.

This was experimentally verified in the lab by the recent demonstrations wherein Light was reduced in speed to under 30 miles per hour. At that speed, Gravity no longer bent it appreciably. Yet speed it up to its natural rate and once again Gravity bends it.

Heck, you can see that Gravity bends Light by simply holding your thumb between your eye and a light source and looking at the edges, however, this phenomenon doesn't happen when Light is traveling at slower speeds because so few Gravitons are being emitted.

[Southack]

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.

"Except that it isn't, unless you can explain in good detail how you managed to pull that novel assertion out of your hind end, *and* provide sufficient evidence for it." - Dan Day

There are really only two possibilities being discussed here:

1. That G = C^2
2. That G = C

I.e. either Gravity travels at the speed of light (C) or else it travels at the speed of light squared (C^2).

Now, given the two above assumptions, lets work with one equation with which neither of us will argue, that E=MC^2.

Now, this means that our two above possibilities would work out to this by substituting the two possible values for G:

Postulate 1. E=MG ---> Where G=C^2
Postulate 2. E=MCG ---> Where G=C

Fair enough. Are you with me so far?

Now, if postulate #1 is correct, then G = C^2 = E/M.

If postulate #2 is correct, then G = E/(MC) = C.

[Southack]

"Since the Sun ISN'T ACTUALLY MOVING, its gravity signature, and even its visual appearance via light, REMAIN THE SAME whether we're seeing it as it was a nanosecond ago, 8.3 minutes ago, or six months ago -- from *any* vantage point."

Except, our Sun IS moving. Our entire Solar System is hurtling through space. The planets simply happen to be doing all of their hurtling at the precise same speed as our Sun, even as our planets revolve around said Sun. Moreover, our entire Galaxy is moving through our universe.

The Sun simply does not sit still, contrary to your claim above.

[Physicist]

Our entire Solar System is hurtling through space. The planets simply happen to be doing all of their hurtling at the precise same speed as our Sun, even as our planets revolve around said Sun. Moreover, our entire Galaxy is moving through our universe.

The Sun simply does not sit still, contrary to your claim above.

Uh, oh! Looks like somebody's forgotten that all motion is relative.