# What is the speed of gravity?

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#### jeffreyH

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##### What is the speed of gravity?
« on: 21/03/2015 01:04:46 »
If the speed of light slows down in a gravitational field and the speed of gravity equals the speed of light then doesn't that imply that the speed of gravity also slows down in a gravitational field? So if the speed of gravity slows down would that not make it less effective in slowing down light?

#### PmbPhy

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##### Re: What is the speed of gravity?
« Reply #1 on: 21/03/2015 02:19:31 »
If the speed of light slows down in a gravitational field and the speed of gravity equals the speed of light then doesn't that imply that the speed of gravity also slows down in a gravitational field? So if the speed of gravity slows down would that not make it less effective in slowing down light?
No. The speed of light refers to speed at which light moves through space. However the speed of gravity is the speed at which changes in the metric, i.e. space. take place. Therefore the same reasoning which shows that light slows down in a gravitational field cannot be used to show that the speed at which changes in the metric, and thus space. take place.

#### jeffreyH

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##### Re: What is the speed of gravity?
« Reply #2 on: 21/03/2015 13:21:25 »
If the speed of light slows down in a gravitational field and the speed of gravity equals the speed of light then doesn't that imply that the speed of gravity also slows down in a gravitational field? So if the speed of gravity slows down would that not make it less effective in slowing down light?
No. The speed of light refers to speed at which light moves through space. However the speed of gravity is the speed at which changes in the metric, i.e. space. take place. Therefore the same reasoning which shows that light slows down in a gravitational field cannot be used to show that the speed at which changes in the metric, and thus space. take place.

That is VERY interesting.

#### PhysBang

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##### Re: What is the speed of gravity?
« Reply #3 on: 21/03/2015 15:41:17 »
No. The speed of light refers to speed at which light moves through space. However the speed of gravity is the speed at which changes in the metric, i.e. space. take place. Therefore the same reasoning which shows that light slows down in a gravitational field cannot be used to show that the speed at which changes in the metric, and thus space. take place.
This doesn't sound correct, since the speed of light is, essentially, the speed of causation and all changes to the speed of light can be represented as time dilation.

The standard answer to this question, from General Relativity sources, is that gravity propagates at the speed of light.

#### jeffreyH

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##### Re: What is the speed of gravity?
« Reply #4 on: 21/03/2015 17:08:39 »
No. The speed of light refers to speed at which light moves through space. However the speed of gravity is the speed at which changes in the metric, i.e. space. take place. Therefore the same reasoning which shows that light slows down in a gravitational field cannot be used to show that the speed at which changes in the metric, and thus space. take place.
This doesn't sound correct, since the speed of light is, essentially, the speed of causation and all changes to the speed of light can be represented as time dilation.

The standard answer to this question, from General Relativity sources, is that gravity propagates at the speed of light.

http://en.wikipedia.org/wiki/Speed_of_gravity
[For static fields, such as the electrostatic field connected with electric charge, or the gravitational field connected to a massive object, the field extends to infinity, and does not propagate. Motion of an observer does not cause the direction of such a field to change, and by symmetrical considerations, changing the observer frame so that the charge appears to be moving at a constant rate, also does not cause the direction of its field to change, but requires that it continue to "point" in the direction of the charge, at all distances from the charge.

The consequence of this is that static fields (either electric or gravitational) always point directly to the actual position of the bodies that they are connected to, without any delay that is due to any "signal" traveling (or propagating) from the charge, over a distance to an observer. This remains true if the charged bodies and their observers are made to "move" (or not), by simply changing reference frames. This fact sometimes causes confusion about the "speed" of such static fields, which sometimes appear to change infinitely quickly when the changes in the field are mere artifacts of the motion of the observer, or of observation.

In such cases, nothing actually changes infinitely quickly, save the point of view of an observer of the field. For example, when an observer begins to move with respect to a static field that already extends over light years, it appears as though "immediately" the entire field, along with its source, has begun moving at the speed of the observer. This, of course, includes the extended parts of the field. However, this "change" in the apparent behavior of the field source, along with its distant field, does not represent any sort of propagation that is faster than light.]

And from the same source:

[The finite speed of gravitational interaction in general relativity does not lead to the sorts of problems with the aberration of gravity that Newton was originally concerned with, because there is no such aberration in static field effects. Because the acceleration of the Earth with regard to the Sun is small (meaning, to a good approximation, the two bodies can be regarded as traveling in straight lines past each other with unchanging velocity) the orbital results calculated by general relativity are the same as those of Newtonian gravity with instantaneous action at a distance, because they are modelled by the behavior of a static field with constant-velocity relative motion, and no aberration for the forces involved.[16] Although the calculations are considerably more complicated, one can show that a static field in general relativity does not suffer from aberration problems as seen by an unaccelerated observer (or a weakly accelerated observer, such as the Earth). Analogously, the "static term" in the electromagnetic Liénard–Wiechert potential theory of the fields from a moving charge, does not suffer from either aberration or positional-retardation. Only the term corresponding to acceleration and electromagnetic emission in the Liénard–Wiechert potential shows a direction toward the time-retarded position of the emitter.]

#### jeffreyH

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##### Re: What is the speed of gravity?
« Reply #5 on: 21/03/2015 17:58:37 »
aetzbar NEW THEORY! WRONG SECTION! Are there any moderators operating on the forum please?

#### JohnDuffield

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##### Re: What is the speed of gravity?
« Reply #6 on: 21/03/2015 18:11:26 »
If the speed of light slows down in a gravitational field and the speed of gravity equals the speed of light then doesn't that imply that the speed of gravity also slows down in a gravitational field?
IMHO the answer to this is yes, because gravity and electromagnetism are related via $$c_0={1\over\sqrt{\mu_0\varepsilon_0}}$$.
Note hoever that whilst the coordinate speed of light varies with gravitational potential, one can't be certain that the speed of gravitational waves is the same as the speed of electromagnetic waves, just as one can't claim that shear wave speed is the same as compression wave speed.

So if the speed of gravity slows down would that not make it less effective in slowing down light?
IMHO the answer is no, the speed of light depends on the state of space, and so does the speed of gravity.
« Last Edit: 21/03/2015 18:13:34 by JohnDuffield »

#### yor_on

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##### Re: What is the speed of gravity?
« Reply #7 on: 21/03/2015 18:17:00 »
Gravity is gravity. our (ahem) safety net for a universe. Its reach is presumed infinite, and it must be dynamically updating itself constantly as everything is in relative motion. Those 'updates' are what is defined to 'c', but if you think of it in terms of you, doing a sharp course correction in a 'flat space', you will find inertia acting on you. Depending on definitions you can think of that as a result of a Higgs field existing everywhere, or just 'gravity' existing everywhere. But I still think 'c' will be involved in the 'action and reaction'.
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#### jeffreyH

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##### Re: What is the speed of gravity?
« Reply #8 on: 21/03/2015 18:28:07 »
Gravity is gravity. our (ahem) safety net for a universe. Its reach is presumed infinite, and it must be dynamically updating itself constantly as everything is in relative motion. Those 'updates' are what is defined to 'c', but if you think of it in terms of you, doing a sharp course correction in a 'flat space', you will find inertia acting on you. Depending on definitions you can think of that as a result of a Higgs field existing everywhere, or just 'gravity' existing everywhere. But I still think 'c' will be involved in the 'action and reaction'.

Some interesting points. Especially when considering course corrections through the field.

#### PmbPhy

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##### Re: What is the speed of gravity?
« Reply #9 on: 22/03/2015 16:45:51 »
Quote from: PhysBang
The standard answer to this question, from General Relativity sources, is that gravity propagates at the speed of light.
I agree with that statement. If you thought that I implied otherwise then I don't see how. If so then please point it out for me? Thanks!

Quote from: jeffreyH
Where did you get that impression from? Please point out the source of your impression so I can read it for myself. Thanks.

Quote from: jeffreyH
In particular:

http://en.wikipedia.org/wiki/Speed_of_gravity
[For static fields, such as the electrostatic field connected with electric charge, or the gravitational field connected to a massive object, the field extends to infinity, and does not propagate.
That's for a static gravitational field, not a time varying one. Only when there is a change in the source will there be a gravitational wave generated.
« Last Edit: 22/03/2015 16:51:23 by PmbPhy »

#### PhysBang

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##### Re: What is the speed of gravity?
« Reply #10 on: 22/03/2015 19:27:18 »
Quote from: PhysBang
The standard answer to this question, from General Relativity sources, is that gravity propagates at the speed of light.
I agree with that statement. If you thought that I implied otherwise then I don't see how. If so then please point it out for me? Thanks!
Why don't you read the passage that I quoted?

#### jccc

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##### Re: What is the speed of gravity?
« Reply #11 on: 22/03/2015 19:51:11 »
what's the speed of em force?

what's the speed of electrostatic force?

what is/causes gravity?

then you will have the correct answer.

i explained everything, no one openly agreed yet.