# Speed of gravity

#### peter0302

Why not? If you calculated the orbit assuming instantaneous attraction, you'd get the Newtonian result. If you calculated the orbit using virtual gravitons travelling at the speed of light, you should see e a slight lag in the pull from the star due to the finite speed of the gravitons versus the momentum of the planet in the orthogonal direction, because the planet is tending to move away from the star ever so slightly. Granted, this is just a hueristic. It would be interesting to do the math and, ignoring the GR field equations, just use a Feynman "virtual graviton" model to see what the orbit looks like given the lag from the speed of gravity.

#### Phy6explorer

I don't even think that gravity travels. I mean it's like asking "Does vaccum teavel?"

#### peter0302

Well, you agree, don't you, that the electromagnetic force propagates at the speed of light, carired by the photon, right? So, same idea for gravity/gravitons.

#### Phy6explorer

That surely proves me wrong!!!

#### Nickelodeon

whatever property of space and time that makes disturbances of EM propagate at a finite speed is what makes disturbances of gravity propagate at the same finite speed.
'disturbances' of gravity is a different thing and I wouldn't dispute the idea that a disturbance would more than likely propagate at the speed light.

To clarify things (hopefully) with an analogy - there are two fishes swimming some distance apart in a river. One fish claps its fins and creates a 'disturbance' which is heared by the other fish. This disturbance propagates at a speed governed by the properties of the water.
The speed of the flow of the non physical river is what we are talking about when discussing the speed gravity.

#### DaveC426913

Gold Member
Why not? If you calculated the orbit assuming instantaneous attraction, you'd get the Newtonian result. If you calculated the orbit using virtual gravitons travelling at the speed of light, you should see e a slight lag in the pull from the star due to the finite speed of the gravitons
I get that that's what you're claiming, I'm just not convinced that that's what Mercury's precession is from. I might be wrong. I'd like an authority to weigh in on that.

#### rbj

To clarify things (hopefully) with an analogy - there are two fishes swimming some distance apart in a river. One fish claps its fins and creates a 'disturbance' which is heared by the other fish. This disturbance propagates at a speed governed by the properties of the water.
The speed of the flow of the non physical river is what we are talking about when discussing the speed gravity.
sounds like a "gravity aether" to me. i don't think it exists and i don't think that is what is meant by this Minkowsky space-time.

#### RandallB

Phy6explorer said:
I don't even think that gravity travels. I mean it's like asking "Does vaccum teavel?"
peter0302 said:
Well, you agree, don't you, that the electromagnetic force propagates at the speed of light, carired by the photon, right? So, same idea for gravity/gravitons.
That surely proves me wrong!!!
Nothing proven here.
All you have shown is the discrepancy and incompatibility between General Relativity Astrophysics vs. Quantum Mechanics Particle Physics.

GR expects some extra dimensional curvature to account for gravitational field strengths or affects that according to the theory we might never directly observe.
QM and the standard model expect no curvature or fields but physical particles “gravitons” to account for gravity.

So does gravity travel in particles or do fields change in strength as the masses move and change the shape GR curves?
Until one theory can replace the other theory in that other’s domain, the best you can say is “I don’t know but from within the domain of each theory I have a tool that works”.

#### redtree

Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.
In the case of a black hole, assume a change in the mass (perhaps from a large number of photons striking the black hole on the opposite side), what path do the "changes" in gravity follow traveling at c to escape the event horizon? How is that path different from the path a photon might travel (which is unable to escape the event horizon)?

#### Chaos' lil bro Order

Gravity doesn't travel at the speed of light, changes in gravity (in the form of waves) travel at the speed of light.
Is this proven Dave? That gravity travels in waves? I thought experiments like LIGO and LISA were meant to test this assumption. Is there more evidence for gravity being wavellike rather than point like?

#### Phy6explorer

But I still don't get! I read in a reliable website that even light cannot escape from a black hole and a black hole sucks stuff due to extremely strong gravitational field. And according to general theory gravity travels at c. So how does a black hole suck light???

#### Nickelodeon

And according to general theory gravity travels at c.
It's a pity we don't have Einstein here to ask if he really said that.

In answer to your question I think we have to assume that, whatever the cause of gravity, it has to be either travelling faster than light or that it modifies the mechanics of EM radiation to such an extent that it can no longer propagate.

#### RandallB

(And according to general theory gravity travels at c.)
It's a pity we don't have Einstein here to ask if he really said that.
It is an entirely different perspective to use gravity fields of GR vs. graviton particles to visualize how gravity works.
I think the problem comes in trying to use the same “sense of seeing” for both theories and that does not work on fields so easily.

I believe Einstein would say something to the effect that the fields are fixed potentials set by the GR masses establishing the extra dimensional GR curves. Thus, just as it is impossible to actually perform the hypothetical examples often put forward such as “What if the Sun ‘just disappeared’ what would the earth do?“. We can only expect to displace or move the sun at a speed of c. Further even if we suddenly move the sun at the speed of “c” we should not expect that rate of displacement to be translated to an instant new curve at distances away from the sun to realign those fixed fields to the new position of the sun. Any change in the gradient of curve that needs to propagate out to distant gravitational fields should be expected to take some time.

What might that speed of propagation be? Although it is not quite the same thing as gravitation particles moving in some form of plenum, it hard to see any explanation use anything other than something very close to “c”.
But the idea that changes in curve shapes (not dependent on particle movements like gravitons in GR) could accumulate from multiple masses to establish larger and larger ‘curves in space’ should not be that hard to visualize. Even curves so steep that real particles such as light would need speeds faster than “c” to climb over.

Visualizing changes in GR curves within the GR theory is simplified since it is not dependent on the propagation of “gravitons” just the propagation of the gravity field change information.

It might be true that QM in expecting gravitons in the Standard Model “Cannot Work” here, But QM does not predict Black Holes GR does. GR “does not work” in the microscopic domain of QM either, that’s why we use both theories in their appropriate place.

#### Hans de Vries

Gold Member
Yes, gravitational forces propagate at the speed of light according to all accepted theory and known experiments. If the hypothesized graviton (gravity force carrier) exists, it will travel at the speed of light like a photon.
Correct

You needn't even consider hypotheticals to understand this. Consider the planet Mercury. If you calculate the orbit using Newtonian mechanics (which assume gravitation is an instantaneous force) you will find that Mercury's true orbit is slightly different than what you calculate. The perihelion, the point at which the planet is closest to the Sun, actually moves over the course of time. This is explained by General Relativity due to the curvature of spacetime or, another way of looking at it, the fact that gravitational forces propagate at the speed of light.
But Mercury's orbit is a different effect caused by the curvature of space-time yes,
but not due to the speed of light.

The direction of the gravitational (or electric) force coming from a moving object is not
in the direction of where the object was, but where the moving object is if it continues
to move in a straight line during the time needed for the propagation.

For electric forces you can check this by using the Lienard Wiechert potentials which
are derived by assuming that V propagates with c. The gradient of V, the electric field,
points to a "Phantom position" which is the position where the source was plus the vector
"speed x duration".

The mathematical derivation is for instance given in chapter 2 of my book.

http://physics-quest.org/Book_Chapter_EM_LorentzContr.pdf

specifically sections 2.2, 2.8 and 2.10

Regards, Hans

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#### DaveC426913

Gold Member
But I still don't get! I read in a reliable website that even light cannot escape from a black hole and a black hole sucks stuff due to extremely strong gravitational field. And according to general theory gravity travels at c. So how does a black hole suck light???
The light does travel at c, even near a black hole. The gravity bends its path into a circular orbit, so that it never escapes - it doesn't actually stop light in its tracks.

#### redtree

The light does travel at c, even near a black hole. The gravity bends its path into a circular orbit, so that it never escapes - it doesn't actually stop light in its tracks.
A photon emitted exactly normal to the "surface" of a black hole, which direction will it orbit? How will its path be bent?

#### DaveC426913

Gold Member
A photon emitted exactly normal to the "surface" of a black hole, which direction will it orbit? How will its path be bent?

It won't. It will head straight out. But it will be almost infinitely red-shifted - its frequency will approach zero and its wavelength will approach infinity.

#### redtree

And will it escape the event horizon?

#### Phy6explorer

Are you asking whether the light will pass through the event horizon or go on without passing the event horizon? I mean, if the light passes through the event horizon, that's it, isn't it? The observer cannot see anymore of it? I don't think it will go on without passing through it ?

#### Nickelodeon

It won't. It will head straight out. But it will be almost infinitely red-shifted - its frequency will approach zero and its wavelength will approach infinity.
The frequency could probably stay the same - the wavelength approaching infinity would do the trick.

This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?

#### dst

The frequency could probably stay the same - the wavelength approaching infinity would do the trick.

This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?

The constant denoted c is as you say, THE universal constant. It places a limit on how fast anything capable of conveying information can go. Since gravity can convey information, it's limited as such.

#### DaveC426913

Gold Member
The constant denoted c is as you say, THE universal constant.
Yes, as dst eloquently points out, c is THE limit, not just on light.

#### Janus

Staff Emeritus
Gold Member
This is probably another post, but why is the universal constant of the speed of light such a sacred cow in GR and why should it determine the speed of gravity which is more than likely an entirely different beast?
To build on what has already been said:

The speed of light is considered a constant in GR because it is postulated as such in SR, and SR is just a limited subset of GR. Also, every observation to date has upheld that postulate.

Also, once you establish that a given speed is constant for all observers, it follows logically that this speed is also the ultimate speed limit for information.

#### redtree

Are you asking whether the light will pass through the event horizon or go on without passing the event horizon? I mean, if the light passes through the event horizon, that's it, isn't it? The observer cannot see anymore of it? I don't think it will go on without passing through it ?
I mean if a photon travels away from a black hole with a non-zero velocity, then at some point in time, how would the photon not cross the event horizon? Particularly since, as the photon traveled further from the black hole's center of gravity, the gravitational dilation (or curvature) of space-time would diminish.

#### DaveC426913

Gold Member
I mean if a photon travels away from a black hole with a non-zero velocity, then at some point in time, how would the photon not cross the event horizon? Particularly since, as the photon traveled further from the black hole's center of gravity, the gravitational dilation (or curvature) of space-time would diminish.
Yes, the photon will escape the black hole! Saying that light can't escape a black hole is a bit sloppy, so there's no paradox.