# The speed of gravity?

Sorry for the layman question, but it's not my field.

Suppose this thought experiment is performed. Light takes 8 minutes to go from the surface of the Sun to Earth. Imagine the Sun is suddenly removed. Clearly, for the remaining 8 minutes, we won't see any difference.

However, I am wondering about the gravitational effect of the Sun. If the propagation of the gravitational force travels with the speed of light, for 8 minutes the Earth will continue to follow an orbit around nothing. If however, gravity is due to a distortion of spacetime, this distortion will cease to exist as soon as the mass is removed, thus the Earth will leave through the orbit tangent.

What is the state of the art of research for this thought experiment? I am pretty sure this is knowledge that can be inferred from observation.

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I thought this had been asked before, but I did some searching and the closest I found was physics.stackexchange.com/q/703. Since that question is so vague, I'm fine leaving this one open. –  David Z Feb 18 '11 at 23:30
Gravity is due to a distortion in spacetime, and, perturbations of the gravitational field do travel at the speed of light. There is no contradiction between these two aspects as your 3rd para seems to suggest. –  user346 Feb 19 '11 at 15:55
Landau and Lifshits (CTF) show that if you linearize the Einstein equation about flat space-time you get a scalar wave equation with speed c. The speed of light c itself comes from the metric. –  user27777 Aug 16 '13 at 5:50

Since general relativity is a local theory just like any good classical field theory, the Earth will respond to the local curvature which can change only once the information about the disappearance of the Sun has been communicated to the Earth's position (through the propagation of gravitational waves).

So yes, the Earth would continue to orbit what should've been the position of the Sun for 8 minutes before flying off tangentially. But I should add that such a disappearance of mass is unphysical anyway since you can't have mass-energy just poofing away or even disappearing and instantaneously appearing somewhere else. (In the second case, mass-energy would be conserved only in the frame of reference in which the disappearance and appearance are simultaneous - this is all a consequence of GR being a classical field theory).

A more realistic situation would be some mass configuration shifting its shape non-spherically in which case the orbits of satellites would be perturbed but only once there has been enough time for gravitational waves to reach the satellite.

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Let me add: The probability that the entire sun quantum tunnel into a far distant location is too close to $0$. –  user29727 Oct 20 '13 at 11:43

Gravitational influences do propagate at the speed of light, not instantaneously.

The question of what would happen if the Sun instantly disappeared is actually a funny one in general relativity. The equations of general relativity imply as a mathematical consequence that energy must be locally conserved. Therefore, there is no valid solution to the equations that describes the Sun suddenly disappearing (since that scenario violates local energy conservation).

(A similar statement holds in electromagnetism, by the way: charge conservation is a logical consequence of Maxwell's equations, so if someone asks you what the electric field does when a charge suddenly disappears, there is no correct answer.)

But you can sensibly ask what would happen if the Sun suddenly changed its mass distribution -- if it exploded, say, sending its mass in different directions at high speeds. The answer is that the Earth's orbit wouldn't change for 8 minutes.

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Beat you to it with the same answer :) –  dbrane Feb 18 '11 at 22:18
Uh, I don't like this discussion of energy conservation. There is no such implication made by GR. And there is also no problem in using discontinuous stress-energy tensor which would in turn imply discontinuity in curvature (in the very same way as happens at characteristic surfaces of gravitational waves). And the very same stuff can be said about EM. –  Marek Feb 18 '11 at 22:29
I'm talking about local energy conservation, $T^{\mu\nu}_{;\nu}=0$. It follows from the Einstein equation as an identity. There's no solution corresponding to the Sun disappearing, because the Sun's disappearance would violate it. –  Ted Bunn Feb 18 '11 at 23:09
Oh, and @Marek, if you really think there's no problem finding solutions to the field equations with sources that don't satisfy the conservation laws, then tell me this: what is a solution to Maxwell's equations corresponding to a point charge $q$ at the origin, which abruptly disappears at $t=0$ -- that is, $\rho({\bf r},t)=q\delta({\bf r})θ(−t)$, ${\bf J}=0$? –  Ted Bunn Feb 19 '11 at 16:24
A followup: I agree that it would be better to insert "locally" before "conserved" in my answer. I'm aware of the problems with global energy conservation in GR. I'm making that edit now. –  Ted Bunn Feb 19 '11 at 17:23

All observations are consistent with standard GR so far, but I don't think the speed of gravity, in particular, has ever been measured.

Experimental measurements of the speed of gravity was quite a controversy a few years ago when a paper came out claiming that the speed of gravity was very close to $c$ as measured by the Shapiro delay. To see papers on the subject google shapiro+speed+gravity:

Clifford Will is an expert in the area and says that there was no measurement. He has a website on the subject that gives links to the various papers:
http://wugrav.wustl.edu/people/CMW/SpeedofGravity.html

My guess is that the Will side won. But academia means "never having to admit you were wrong". Here's a pair of dueling papers on the subject, published in the same journal at the same time (that date to after Clifford Will last updated his page above):

Class.Quant.Grav. 22 (2005) 5181-5186, Sergei M. Kopeikin, Comment on 'Model-dependence of Shapiro time delay and the "speed of gravity/speed of light" controversy'
http://arxiv.org/abs/gr-qc/0510048

Class.Quant.Grav.22 (2005) 5187-5190, S. Carlip, Reply to Comment on Model-dependence of Shapiro time delay and the`speed of gravity/speed of light' controversy''
http://arxiv.org/abs/gr-qc/0510056

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This is a good answer, but may give the incorrect impression that nothing at all is known empirically about this, only theoretically. Gravitational waves have never been detected directly, but the loss of energy from the Hulse-Taylor binary pulsar has been checked to high precision against GR's predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity's description of gravitational waves. –  Ben Crowell May 18 '13 at 18:48

Your question was first asked by Laplace. The following is from the Wikipedia article on "The speed of gravity"

"Laplace

The first attempt to combine a finite gravitational speed with Newton's theory was made by Laplace in 1805. Based on Newton's force law he considered a model in which the gravitational field is defined as a radiation field or fluid. Changes in the motion of the attracting body are transmitted by some sort of waves.[4] Therefore, the movements of the celestial bodies should be modified in the order v/c, where v is the relative speed between the bodies and c is the speed of gravity. The effect of a finite speed of gravity goes to zero as c goes to infinity, but not as 1/c2 as it does in modern theories. This led Laplace to conclude that the speed of gravitational interactions is at least 7×106 times the speed of light. This velocity was used by many in the 19th century to criticize any model based on a finite speed of gravity, like electrical or mechanical explanations of gravitation.

From a modern point of view, Laplace's analysis is incorrect. Not knowing about Lorentz invariance of static fields, Laplace assumed that when an object like the Earth is moving around the Sun, the attraction of the Earth would not be toward the instantaneous position of the Sun, but toward where the Sun had been if its position was retarded using the relative velocity (this retardation actually does happen with the optical position of the Sun, and is called annual solar aberration). Putting the Sun immobile at the origin, when the Earth is moving in an orbit of radius R with velocity v presuming that the gravitational influence moves with velocity c, moves the Sun's true position ahead of its optical position, by an amount equal to vR/c, which is the travel time of gravity from the sun to the Earth times the relative velocity of the sun and the Earth. The pull of gravity (if it behaved like a wave, such as light) would then be always displaced in the direction of the Earth's velocity, so that the Earth would always be pulled toward the optical position of the Sun, rather than its actual position. This would cause a pull ahead of the Earth, which would cause the orbit of the Earth to spiral outward. Such an outspiral would be suppressed by an amount v/c compared to the force which keeps the Earth in orbit; and since the Earth's orbit is observed to be stable, Laplace's c must be very large. In fact, as is now known, it may be considered to be infinite, since as a static influence, it is instantaneous at distance, when seen by observers at constant transverse velocity.

In a field equation consistent with special relativity (i.e., a Lortentz invariant equation), the attraction between static charges is always toward the instantaneous position of the charge (in this case, the "gravitational charge" of the Sun), not the time-retarded position of the Sun. When an object is moving at a steady speed, the effect on the orbit is order v2/c2, and the effect preserves energy and angular momentum, so that, and orbits do not decay. The attraction toward an object moving with a steady velocity is towards its instantaneous position with no delay, for both gravity and electric charge."

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And they used to think that gravity was the longitudinal wave to go with the transverse wave of electromagnetism (i.e. as in the P and S waves of seismology). –  Carl Brannen Feb 22 '11 at 3:53

the fact that distortion travels 'as soon' as a mass is removed or not is not implied in any way by gravity being due to a distortion of spacetime. In fact distortions of spacetime are as limited to travel to the speed of light as any other physical influence.

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From Hacker News https://news.ycombinator.com/item?id=6253263

This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe.

But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move on if this strikes you as tiresome.

Consider the Earth, and you on it. You're not floating freely, so clearly something's going on. We call that "gravity." We can call it, in the most generic sense, an interaction: you and the Earth are interacting somehow, and that's what's keeping you from floating freely. We can then ask what the speed of that interaction is by putting it in these specific terms: How much time will elapse between your changing your position relative to the ground and your beginning to fall?

Yes, it's the Wile E. Coyote problem. Wile E. Coyote runs off a cliff, floats in mid-air long enough to hold up a sign that says "Help," then begins to fall.

Clearly that's an exaggeration. But just how much time does elapse, in real life, between stepping off a cliff and beginning to fall?

We can approach the problem naively by remembering that all propagating phenomena in the universe are limited by the speed of light. Given that fact, it makes sense to hypothesize that the time between the moment when Wile E. steps off the cliff and when he begins to fall will be equal to or more than the distance between him and the ground divided by the speed of light. It certainly can't be less, right?

We can then construct a set of very, very precise experiments with very fine tolerances — probably involving electromagnets and lasers or something — to test this hypothesis. And then we can find that we're totally goddamn wrong.

To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. Not after a very small interval of time, but absolutely instantaneously. As in zero time elapses between dropping and falling. This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.

I'm going to skip ahead a bit here, because I don't feel like explaining the entire theory of general relativity, and it won't be that useful in answering the question anyway. Suffice to say that no, no time elapses between dropping and falling, but at the same time no, no signal or interaction has to propagate upward from the ground to Wile E. in order to make him start falling. In fact, what's going on is that Wile E. is always falling, due to the curvature of spacetime created by the Earth. Whenever he's standing at the edge of the cliff, on the ground, the ground beneath his feet — paws? — is arresting his fall by, effectively, pushing up against him. The very instant that's removed, he starts falling. So in that sense, gravity has no speed. Because it doesn't actually propagate through space. One way to look at it is to say the gravitational field fills space, so wherever you are, you're already being affected by it all the time. Another way is to say that gravitational essentially is space, so it affects you simply by virtue of existing. The two are essentially equivalent English translations of the equations that actually describe the phenomenon.

But okay, that's half the problem. The gravity of a static body fills space, or is space, and as such can't be meaningfully said to have a speed. But what about the gravity of a changing body? Like you said, what if "suddenly a black hole appeared?" Well, the answer of course is that that never happens, ever. Gravitation doesn't suddenly anything; macroscopic things don't just appear out of nowhere, and teleportation is impossible. So we don't have to think about that … and in fact we couldn't get meaningful answers if we tried.

But things do move. The moon's moving relative to the surface of the Earth; we can tell, even apart from the fact that we can see it up there, because the moon is the major contributor to the tides, and the tides rise and fall. But what's the relationship between the moon's position in space and the tidal acceleration on the Earth? Are the two somehow always in perfect sync, or is there some lag? If so, how much, and in what direction? That's actually a much harder question to answer than you might think. There was a now-infamous paper some years ago by a fellow named Tom Van Flandern (recently passed, God rest his soul) that asserted that the change in gravitational acceleration in a dynamical system actually propagates many times faster than the speed of light — at least twenty billion times faster than the speed of light — but not instantaneously. This got a lot of attention at the time. If the propagation speed of changes in spacetime geometry were equal to the speed of light, that'd be fine. If it were literally instantaneous, that'd also be fine, more or less, though our theory would need some tweaking. But faster than c but still finite? That was really hard to explain.

It turned out not to be a problem though. Because Van Flandern just made a mistake in his paper. See, the relationship between motion and gravitation is not as straightforward as it might seem. In fact — and I'm glossing over this now, because the maths are damn complicated — whenever a gravitating object moves inertially, the gravitational acceleration vector at a point removed actually points at where the object actually is at a given instant, as opposed to where the object's light is seen to be coming from at that instant. So in that sense, we're back to gravitation being instantaneous again!

But is it really? No. Because you see, if the inertially moving object were to come to a stop instantaneously, the acceleration vector would continue to point toward its future position for a time, as if it were still moving inertially, even though the object is actually somewhere else. The sum of effects that serve to cancel out aberration when everything moves inertially would break down, and the acceleration field would point toward empty space for however long it takes for the change in geometry to propagate through space at the speed of light from the gravitating object to the point in question.

Except things don't stop moving instantaneously. Things accelerate, and acceleration requires energy, and when you factor that in, the equations balance out again. (If you feel up to the challenging of following a lot of advanced mathematics, here's the best paper I know on the subject.) - http://arxiv.org/pdf/gr-qc/9909087v2.pdf

So what does that mean? It means that the "speed of gravity" is the speed of light … technically. Changes in the geometry of spacetime actually propagate at the speed of light, but the apparent effects of gravitation end up being instantaneous in all real-world dynamical systems, because things don't start or stop moving or gain or lose mass instantaneously for no reason. Once you factor in everything you need to in order to model a real system behaving in a realistic manner, you find that all the aberrations you might expect because of a finite speed of light end up canceling out, so gravity acts like it's instantaneous, even though the underlying phenomenon is most definitely not. The universe is pretty damn cool, if you ask me.

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I feel like this question is being asked wrong and/or it is being interpreted wrong for what you're actually asking. It is understood that the propagation of anything cannot exceed 'c', but I don't think propagation is necessary to answer the question, or to create a valid thought experiment. First off, gravity is not fully understood by any mainstream science and a lot of the paradoxial problems inherent within our current accepted understanding tend to leave many scratching their heads. I'm no physicist, or scientist for that matter, but this has been on my mind for a very long time and I decided to throw it out here and allow you all to tear it to pieces or at least lead me in a better direction lol.

The question, how would the sudden disappearance of the sun affect gravitation, and would it follow 'c' or happen instantaneously?