# Has the speed of gravity been measured experimentally? How? [duplicate]

In Newtonian physics, changes in gravity propagate instantly. In general and special relativity, gravity propagates at the speed of light, $c$. From reading answers to questions about gravity on this site, the consensus seems to be that the real world fits the relativity answer, $c$.

Numerous experiments have measured the speed of light in various media with increasing precision. Have there been any experimental measurements of the speed of gravity with generally accepted results?

If so, how did they work? Or if not, how might such a measurement be performed and why hasn't it been done yet?

## marked as duplicate by John Rennie, Kyle Kanos, ACuriousMind♦, HDE 226868, Qmechanic♦Aug 19 '15 at 19:00

• – Qmechanic Aug 18 '15 at 19:42
• People are actually eagerly looking for that signal you are asking about in LIGO (en.wikipedia.org/wiki/LIGO). That wave is called gravity wave! – Physics Moron Aug 18 '15 at 19:44
• @pinu: LIGO is looking for gravitational waves, not gravity waves; there is a difference between the two. – Kyle Kanos Aug 18 '15 at 20:33
• @KyleKanos really sorry, then I don't understand it properly. Any quick comments or references on this? – Physics Moron Aug 18 '15 at 20:38
• @pinu Gravity wave just refers to waves in a fluid under the influence of gravity, gravitational waves are the thing you're thinking of. – Jahan Claes Aug 18 '15 at 22:38

I shall first appeal to one of the key points of relativity:

If you are 22 years old, like me, I can only have affected things up to 22 light years away from me. Things beyond that are untouched by my existence in every sense.

If gravity were faster than light (by say a factor of 2) I could then have affected things up to 44 light years away!

This is neither a proof nor an answer to your question, it just shows that you can't have things faster than light if you choose faster than light to be your top speed.

Yes, finding this question: What is the status of existing measurements of the speed of gravity? which is in the related questions section we see:

The speed of gravity has also been calculated from observations of the orbital decay rate of binary pulsars and found to be consistent to the speed of light within 1% ( same link).

I shall not rip from Ben's answer though, nor find sources when he already has:

https://physics.stackexchange.com/a/63293/55341

When you talk about the speed of gravity, you are talking about the speed of gravitational waves. If something jerks to one side or changes shape in a way that the change in gravity could be detected at a distance, the progress of that change through space is nothing other than a gravitational wave.

They have just recently been detected by LIGO, but I don't think that was enough for a direct measurement of the speed. It would be possible to measure the speed in a straightforward way if a gravitational pulse were observed coming from something that could be detected by other means at a known distance. I don't know if a supernova would be expected to produce detectable gravitational waves, for instance.

So, direct measurement has not happened, but probably will some day. Others can do a better job than me of talking about indirect measurements.