# Speed of Gravity the Same in All Reference Frames?

1. Jul 25, 2011

### jaketodd

Last edited by a moderator: Apr 26, 2017
2. Jul 25, 2011

### bcrowell

Staff Emeritus
Yes. The c in relativity shouldn't really be interpreted as having anything to do with light. It's a universal speed that is the same in all frames of reference. Massless things, including gravity waves and light waves, all move at this speed.

3. Jul 25, 2011

### jaketodd

Then how do you explain gravitational waves? They are measured in a change of amplitude of the gravity, not frequency. Frequency is the only way light can change from a moving source. And, a change of amplitude can only mean that the peaks and valleys from the gravity are overlapping, which can only mean that the gravity speed coming from a moving source is not reference frame independent because the actual peaks and valleys are overlapping and increasing in amplitude. Isn't this correct? If not, please explain.
BUT...
Ok, a change here: I found on Wikipedia that the amplitude "is not the quantity which would be analogous to what is usually called the amplitude of an electromagnetic wave"

So maybe I am making an invalid comparison? HOWEVER...

It also says on Wikipedia: "Speed: This is the speed at which a point on the wave (for example, a point of maximum stretch or squeeze) travels. For gravitational waves with small amplitudes, this is equal to the speed of light, c."

http://en.wikipedia.org/wiki/Gravitational_wave" [Broken]

So what happens to the speed when the amplitudes are not small? And what is "small" defined as? And if the amplitude of a gravitational wave is not like light, then what is it?

Jake

Last edited by a moderator: May 5, 2017
4. Jul 25, 2011

### Naty1

Try here:
http://en.wikipedia.org/wiki/Gravitational_waves

and note the dynamic illustrations and accompanying explanations,,

When we talk about the speed of light and gravitational waves we are using GROUP velocity:

http://en.wikipedia.org/wiki/Group_velocity

again, see the dynamic illustration....this is the speed that can be ussed for information communication....."c" for electromagnetic waves.

This is not PHASE velocity which can sometimes exceed the speed of light...but cannot be used to communicate...that is, cannot be used to transmit information.

5. Jul 25, 2011

### pervect

Staff Emeritus
I'm not quite sure what you're trying to say here. Let's start with the fact that light waves exist and move at the speed of light.

Clearly, it's possible for waves to move at the speed of light, thus if light waves can do it, gravity waves can also do it, it's not impossible in principle for a wave to move at the speed of light, we have an example, light itself.

Describing light waves as "measured by a change in the amplitude of electrostatic attraction",which is intended to be analogous to your description of gravitaional wave, is a highly suspect description of an electromagnetic wave. I suspect that's where you're problem is, in your conceptualization of gravitational waves.

For instance, maybe you're thinking that a gravitational wave is a longitudinal wave, it's not. It's a transverse wave, just like light is. Light has transverse and oscillating electric fields. Gravity waves have transverse and oscillating tidal forces.

6. Jul 25, 2011

### bcrowell

Staff Emeritus
Weak gravitational waves can be described as small perturbations on the Minkowski metric (+1,-1,-1,-1). Small means small compared to +1 and -1.

High-amplitude gravitational waves need not propagate at c. For example, GR predicts that a gravitational-wave pulse propagating on a background of curved spacetime develops a trailing edge that propagates at less than c.[MTW, p. 957] This effect is weak when the amplitude is small or the wavelength is short compared to the scale of the background curvature.
MTW - Misner, Thorne, and Wheeler, Gravitation

7. Jul 25, 2011

### atyy

I wonder if that's similar to or different from the "slow light" mentioned in http://arxiv.org/abs/0806.0464 ?

8. Jul 25, 2011

### bcrowell

Staff Emeritus
I don't think it is. The MTW thing is well understood and noncontroversial, as far as I can tell. The thing about charged particles and the equivalence principle is something that people have debated for 50 years without reaching a consensus.