[chris (OP)]

I have heard it stated many times that the velocity at which gravity propagates is the speed of light.

What is the rationale and evidence for this argument?

[h3mp]

Surely if gravity is just curved space-time, then it doesn't propagate at all and has no speed :-s

[chris (OP)]

The theoretical force carrier of gravity is the "graviton" particle. These are purported to propagate at the speed of light, but why?

[jeffreyH]

The theoretical force carrier of gravity is the "graviton" particle. These are purported to propagate at the speed of light, but why?

That is a difficult question. I hope someone can give a reason.

[alancalverd]

IIRC there was a recent-ish experiment involving gravitational lensing which suggested that the speed of gravity was nonzero and not greater than c. I think it was at the upper end of this range.

[CPT ArkAngel]

Hi Chris,

Anything having a proper mass (or rest mass) must travel at a speed lower than the speed of light, because it would need an infinite amount of energy to attain the speed of light.

Gravitational Waves and gravitons, on the other hand, are unlikely to possess a proper mass because it would generate a possible infinite number of gravitons and gravitational waves. In the case of a quantum theory of gravity, this could be solved by a limit. In any case, you need a particle without a proper mass at the end of the line. In General Relativity, gravity has the speed of light and there were already many observational proofs of it.

The gravitational waves having been detected by LIGO have exchanged energy with it in order to be observed. This means that gravitational waves have some kind of inertial mass, but no proper mass. Thus, it can interact and it can travel at the speed of light. This is the same for photons, but they are different. (Maybe not that much...)

If you put photons in a box, the proper mass of the box is supposed to increase by the same amount of photon energy as E=MC2. Maybe it is not true at small scale and there is a direction to it.

For gravitational waves, I have no idea if it is the case. But curvature of spacetime, like a gravitational wave, in my opinion; should add proper mass to a box in a similar way as a photon.

[chris (OP)]

Thank you CPT ArkAngel; i understood some of that. But I don't think you've really answered my question, which was asking for the evidence that gravity propagates at the speed of light, and why...?

[guest39538]

Thank you CPT ArkAngel; i understood some of that. But I don't think you've really answered my question, which was asking for the evidence that gravity propagates at the speed of light, and why...?

Hey Chris, I think the ''speed of gravity'' is more related to the size of mass and terminal velocity being the ''speed of of gravity'' relative to the attracting mass.

[Space Flow]

There is no evidence yet of Gravity having any speed. On the other hand there is now even more evidence to support the general relativity view of Gravity rather than the particle physics view. The Standard Model is complete. There is no room for a fictitious graviton, and GR does not need one.
Having said all that lets just address the question as a single body Universe problem.
Take a Universe that is just Spacetime like your rubber sheet analogy. Now in this matter-less totally flat Universe, add one particle of matter. Does the almost infinite edge of this otherwise empty spacetime, not imediatelly feel the presence of this particle? The equations don't say anything about information transmission time. All they say is that the mere presence of this particle is felt right to infinity at a level dictated by the inverse square law.
Ask yourself how it could possibly be different.
The speed has to be infinite or nothing makes sense.
Similarly if you then somehow made this particle disappear. Would any part of the entire Universe show a deformity for a particle that doesn't exist? If the information only travelled at the speed of light, you would have observable evidence of a gravitational effect with nothing to cause it.
Yes I know you can't create or destroy Mass/energy but this is a thought experiment.
Gravity waves may propagate, and the speed that they propagate at would be related to some property of the spacetime medium that they propagate through.
But Gravity does not propagate.
That is the wrong way to think about it.
Gravity either is and if it is, it is to the end of the Universe, or it isn't and if it isn't, it isn't everywhere at once.
If conservation laws allowed either the creation or destruction of matter/Energy, then you would find that gravity was instantaneous.
As it is, gravity just is and if a mass changes position the Universe knows about it immediately. Otherwise you have delayed gravitational effects from where matter isn't, and next thing you know someone will scream "Dark Matter".

[guest39538]

There is no evidence yet of Gravity having any speed. On the other hand there is now even more evidence to support the general relativity view of Gravity rather than the particle physics view. The Standard Model is complete. There is no room for a fictitious graviton, and GR does not need one.
Having said all that lets just address the question as a single body Universe problem.
Take a Universe that is just Spacetime like your rubber sheet analogy. Now in this matter-less totally flat Universe, add one particle of matter. Does the almost infinite edge of this otherwise empty spacetime, not imediatelly feel the presence of this particle? The equations don't say anything about information transmission time. All they say is that the mere presence of this particle is felt right to infinity at a level dictated by the inverse square law.
Ask yourself how it could possibly be different.
The speed has to be infinite or nothing makes sense.
Similarly if you then somehow made this particle disappear. Would any part of the entire Universe show a deformity for a particle that doesn't exist? If the information only travelled at the speed of light, you would have observable evidence of a gravitational effect with nothing to cause it.
Yes I know you can't create or destroy Mass/energy but this is a thought experiment.
Gravity waves may propagate, and the speed that they propagate at would be related to some property of the spacetime medium that they propagate through.
But Gravity does not propagate.
That is the wrong way to think about it.
Gravity either is and if it is, it is to the end of the Universe, or it isn't and if it isn't, it isn't everywhere at once.
If conservation laws allowed either the creation or destruction of matter/Energy, then you would find that gravity was instantaneous.
As it is, gravity just is and if a mass changes position the Universe knows about it immediately. Otherwise you have delayed gravitational effects from where matter isn't, and next thing you know someone will scream "Dark Matter".

Hi space, can you confirm my understanding of something, I though gravity was the invisible force between two bodies caused by the two bodies being present, what affect would gravity have on a single particle in a void if there was nothing to attract it, would the speed remain 0 would the idea of gravity even exist?

[evan_au]

I have heard it stated many times that the velocity at which gravity propagates is the speed of light.

This is the common view (as assumed in Einstein's General Relativity), but you can't say that it is proven.

Quantum theory and General Relativity currently have irreconcilable differences, but researchers are hoping that  mediation may still prove successful.
 
If there is to be a reconciliation, it will probably be related to the hypothetical particle carrier of gravitation, the "Graviton".

If you look at the Wikipedia page on the graviton, you can see some of the open conflicts, eg:

• The information box states that the rest-mass of the graviton is zero. Since we know that gravitational waves can deliver momentum into a detector, this implies that the graviton must have momentum, and the only way a zero-mass particle can have momentum is if it travels at the speed of light (as per Einstein).
  
• From the recent detection of gravitational waves, researchers placed an upper bound on the rest mass of a graviton: < 10^-22 eV/c2. This is miniscule, but it doesn't prove that it is exactly zero. If the mass is ever-so-slightly greater than zero, then the graviton cannot travel at c (but it could be extremely close to c).
  
• "gravitational waves must propagate slower than c in a region with non-zero mass density if they are to be detectable". I assume that this is analogous to the fact that light travels at speeds < c in a volume containing matter. So the speed of light (and possibly gravitation) is only c in a true cosmic void.
  
• String theory has been shown to be compatible with General Relativity, and to have a particle like a graviton. But it, too cannot be reconciled with the Standard Model of particle physics at this time.
  
• More speculatively, slight differences in the properties of gravitons (like traveling slightly slower than c, or leaking into other dimensions) might explain some of the characteristics of dark matter
  

It's a brave pleb who would bet against Einstein, but quantum gravity is clearly something that is beyond the scope of Einstein's general relativity.

An Upcoming Experiment?: As of this month, we have demonstrated an ability to detect gravitational waves. By 2020 we should have enough operational detectors worldwide to be able to locate a source like colliding neutron stars, and point our telescopes in that direction. In this case, we should be able to compare the time of arrival of the gravitational waves with the arrival of the optical/radio signal. This will give us a direct comparison of the speed of gravitational waves and light. It would be a bonus if that mass of decaying neutrons sprayed into space gave us a simultaneous pulse in the world's neutrino detectors!

A similar analysis with Supernova SN1987A allowed us to place fairly tight limits on the speed of neutrinos compared to the speed of light (it was very close to c; now we know from neutrino oscillations that it is ever-so-slightly slightly less than c).

But curvature of spacetime, like a gravitational wave, in my opinion; should add proper mass to a box in a similar way as a photon.

We know how to confine light in a reflective box. I don't know of any way to confine a gravitational wave inside a box.

But Gravity does not propagate.

The fact that last September researchers detected a gravitational wave oscillations that seemed to come from merging black holes about a billion light-years away suggests that sudden changes in gravitational fields do propagate through space "to infinity", as predicted by Einstein.

Just like sudden changes in electric fields propagate through space "to infinity", as predicted by Maxwell.

I don't think it's any harder to believe in propagating gravitational fields any more than it is to believe in propagating radio waves.

[chris (OP)]

Thanks Evan; excellent answer; now I understand what we do and don't know much better. Chris

[evan_au]

Also see: http://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements

This suggests that measurements of a binary pulsar radiating gravitational waves is consistent with the speed of gravity being within 1% of the speed of light. Again, it is not proof that gravity travels at exactly the same speed as light.

[CPT ArkAngel]

There are two LIGO interferometers, one in Louisiana and the other in Washington state.

The gravitational waves were detected by both detectors. The delay measured between them is consistent with GW moving at the speed of light within the margin of error. The margin still includes the possibility of a lower speed.

Experimentally, General Relativity is a great success. It would be extremely surprising if gravitational waves had a different speed than C. Though GR is certainly wrong at some point, it is very unlikely in this case.

Concerning the box containing photons, just imagine a box large enough in a huge and relatively flat space...

[Space Flow]

Quote from: Space Flow
But Gravity does not propagate.

The fact that last September researchers detected a gravitational wave oscillations that seemed to come from merging black holes about a billion light-years away suggests that sudden changes in gravitational fields do propagate through space "to infinity", as predicted by Einstein.

Just like sudden changes in electric fields propagate through space "to infinity", as predicted by Maxwell.

I don't think it's any harder to believe in propagating gravitational fields any more than it is to believe in propagating radio waves.

Although your answer was very good and extremely comprehensive as far as the propagation of Gravitational waves are concerned, my postulations were never about the propagation of Gravitational waves.
I have no argument with anything you said. The future will tell whether your highly hypothetical gravitons have anything to do with reality or not.
Anything that I stated was to address the question posed by Chris to start with and that as I understood it was not about the speed of Gravitational waves, but about the speed of Gravity.
I see no way to connect the two. One is a compression wave traveling as a compression wave through a medium and as such would have to obey rules that have to do with transmission of compression waves through the medium. This medium in other words is a physical presence with physical characteristics that influence such propagation. Even still EM waves are as far as I understand not dependent on a medium so we would be comparing apples with oranges if we assign gravitational waves the speed of light. Entirely different methods of propagation.
Gravity being the distortion of this spacetime itself is not a compression wave. It is the influence of Mass on the shape of all of spacetime, from the coordinates of the mass to infinity. It is, rather than it propagates.
If you somehow magically removed our sun from reality, all of spacetime would know immediately, as there is no known way to describe a curvature of the medium at any distance without the mass to cause it.
Although I generally love your very thorough treatment of any question, I believe you have done this question an injustice by making it about Gravity waves and Hypothetical gravitons which it never was by the tittle.
If that was the question I would not have offered an opinion.

[Space Flow]

Hi space, can you confirm my understanding of something, I though gravity was the invisible force between two bodies caused by the two bodies being present, what affect would gravity have on a single particle in a void if there was nothing to attract it, would the speed remain 0 would the idea of gravity even exist?

Even though I promised myself that I would stop responding to you, I will answer this misconception.
Gravity as is currently understood by GR is the distortion of spacetime by the presence of mass. Matter does not in any reality attract matter. If there are two particles in the universe than they will both be influenced by the distortion that each impose on spacetime. That is not the same as attracting each other. They are both under the influence of a distorted spacetime.
So having explained that part you should be able to see that gravity in its simplest terms can be described as a single particle and its effect on the so called fabric of the universe. That is the only interaction needed.
Mass and spacetime. Not mass and mass.

[CPT ArkAngel]

The detection of gravitational waves implies a gravity travelling at the speed of light.

GWs are produced by the transfer of energy from gravity to gravitational waves. It is the decrease in potential gravitational energy of two bodies spinning together while accelerating toward each other which produces gravitational waves. This is real energy so gravitational energy is real. If energy can travel faster than light then you can go backward in time and kill you grandfather before you were born...

If you want to break this law, you must be able to explain a few unknowns in physics, at least...

Concerning the term propagating (through space), it implies that space is some kind of material. Sure that space need a description, it is not so sure that what we perceive as space is a real medium. That is why I avoid the term 'propagate'. Because I don't know and it may be a misconception, or maybe not...

In the case of entanglement, there is no energy transfer. probably...

[alancalverd]

Propagate is a useful term because, for instance, it distinguishes between a particle travelling through space and a selfpropagating electromagnetic wave, neither of which requires any material. 

[CPT ArkAngel]

Do you imply that if you could make disappear all matter in the universe, Space would still exist?

Personally, I don't think so. I think this exactly where GR cannot fit with a quantum gravity theory.

[Space Flow]

Do you imply that if you could make disappear all matter in the universe, Space would still exist?

Personally, I don't think so. I think this exactly where GR cannot fit with a quantum gravity theory.

No I am not implying that space can exist with no matter, just as I can not imply that matter can exist with no space. It is the interaction between these two physical qualities (Matter and Space) that allow an animated Universe. I don't know of any other kind of Universe.
Breaking it down to one particle of matter is of course an oversimplification to show the Physics of the interaction.
Magically removing the sun does not imply that I believe in magic, or that this could be done. That again is just a thought experiment to transmit an idea.   
As far as GR not fitting with Quantum Gravity Hypothesis (not yet theory), is concerned. You can not possibly doubt the validity of GR when every advance we ever make again proves its predictive ability exactly right. So if some other view does not agree with GR than the logical thing to do is to assume that the problem is with the other view. Be it Quantum Gravity or any other view that does not agree with such a 100% success rate. 
That in my view is science and everything else is conjecture.

If we are to be bringing conjecture into this discussion than I feel obliged to point you to my view of gravity that doesn't actually clash with either GR or QM, and has nothing to do with weirdo ideas like Quantum Gravity, ridiculous vibrating strings, or any Multiverses that we can never prove.