# The Speed of Gravity

The thing that confuses me about the idea of gravitons, is how would gravity bend light? How would the gravitons “catch up”?

Gravitons, assuming they exist, don’t bend light, they bend space. Since light follows space, if space is bent, the light will bend to follow it.

if gravity does propagate instantly, does that mean that if we could somehow modulate the mass of something sufficiently, which would modulate the gravity, could we transmit information instantaneously? I thought that was supposed to be impossible?

QED: Ahh, I see. So would it be reasonable to think of gravitons as streaming out constantly from a massive obect, bending space wherever they are, and gravity lessening the further out you go as the gravitons spread out? Or is this a bad way to think of it?

I have read in other threads that gravitons is a kind of iffy word. If gravity was made up of gravitons then wouldn’t gravitons have a mass and then be affected by whatever mass was producing them.

For example. If a black hole pulled in objects with it’s gravitons how did the gravitons get out of the black hole?

quote-
How do we know gravity travels exactly at c? Answer: We don’t, but it seems consistent with other things we know.

Oh, and it discusses Tom van Flandern, who is constantly attempting to increase the speed of gravity; good luck to him. and perhaps one day he will suceed.
But I wouldn’t put money on it.

succeed.

I’m surprised no one brought up the SD staff report:

http://www.straightdope.com/mailbag/mgravityspeed.html

Gravity propagates at the speed of light and yet if the gravitational vector didn’t point to the present versus the retarded position of a mass then stable orbits wouldn’t be possible.

A similar situation exists in electromagnetics. If the electric field vector didn’t point to the present position of a moving charge there could be no electromagnetic radiation. It turns out that in EM the retardation is corrected to first order and the E field does point to the particle’s present position. There is a very unintuitive way of explaining this via potentials, but it is much easier to see using special relativity. I’ll see if I can find a link that does the job.

In any case, the same kind of thing happens in General Relativity except that the retardation is corrected not only for constant velocity but also for constant acceleration. So the gravitational field vector also points to the mass’s present position, and stable orbits are therefore possible.

This effect can lead some people to think c has been exceeded, but this isn’t the case as the information that appears to be instantaneously transmitted actually is generated at an earlier time and it is only, in effect, extrapolated to the present position.

You beat me to it!

if gravity is a wave of somekind, how does it attract rather then push mass a to mass b? does a magnet work in kind of the same way, by waves?

don’t mean to double post. but found this and wondered if it was bull or based on fact. seems kind of amazing

I screwd up again here is the site

http://www.autodynamics.org/new03/pico_gravitons.html

This is one of the very prolific theories of gravity based around the ‘push’ of imaginary, invisible particles and the ‘shadow effect’ of physical bodies;

this kind of theory is vey old, and dates back to medieval times, when the entities doing the pushing were thought to be ‘angels’.

I prefer to think of these theories under the heading
“Gravity doesn’t suck, it blows”.

SF worldbuilding at
http://www.orionsarm.com/main.html

To simplify things a bit (hopefully not too much!):

Gravitational attraction is a result of curvature of space-time. If you move one of the masses, it causes ripples in space-time that move at the speed of light (at least, if General Relativity is correct). These ripples are caused gravitational radiation.

Gravitational radiation has never been directly detected, in spite of many attempts. Probably existing detectors aren’t sensitive enough for the weak radiation that’s out there. But the effects of this radiation can be seen in binary star systems, where the stars are moving closer together due to the loss of energy by gravitational radiation.

To see that the masses move together when they lose energy, imagine trying to pull them apart. You have to add energy to get them farther from each other. So, losing energy will result in them moving closer together.

Gravitons, if they existed, would be the quanta of this radiation, in the same way that photons are the quanta of light. Up till now, there is no usable quantum theory of gravity, so any discussion of how gravity can be described in terms of gravitons is futile. Gravitons are not predicted by General Relativity.

FriendRob I’m probably reading your post wrong, but you’re not saying gravitational attraction is caused by gravitational radiation, are you?

Gravitational radiation has no longitudinal effect at all; it’s strictly a transverse phenomenon. If the radiation passed through a mass in the x direction it would alternately stretch and compress it in the y and z directions, with no effect whatsoever in the x direction.

The original question doesn’t make sense to me. Mass cannot simply appear where it did not exist before. Our Universe is partly defined by the distribution of mass within it. Creating or removing mass in a mind experiment immediately voids it.

I just watched a show called The Elegant Universe and in it they had a mind experiment where our Sun just instantly ceases to exist - but this cannot happen in our Universe so the experiment made no sense.

Well, a star can go supernova, and convert much of its mass to energy; the enery and expelled mass quickly exit the scene, causing a distinct change in the local gravity.

But even energy produces gravity, doesn’t it?

## How about a neutron star disappearing through a wormhole then? Too hypothetical?

perhaps the speed of gravity is irrelevant in the real world after all.

Robo99

Sun here -------------------- specular (anti) matter Sun here

------------>

WHAM!!!

No matter, just energy; matter is gone, gravity is gone.