6
$\begingroup$

I was reading a Wikipedia article on speed of gravity, https://en.wikipedia.org/wiki/Speed_of_gravity .

The following quote is taken from the mentioned article.

In the relativistic sense, the "speed of gravity" refers to the speed of a gravitational wave, which, as predicted by general relativity and confirmed by observation of the GW170817 neutron star merger, is the same speed[1] as the speed of light (c).

The speed of gravitational waves in the general theory of relativity is equal to the speed of light in a vacuum, c.[2] Within the theory of special relativity, the constant c is not only about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[3] This makes it the only speed which does not depend either on the motion of an observer or a source of light and / or gravity. Thus, the speed of "light" is also the speed of gravitational waves, and further the speed of any massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light (hence carrier of electromagnetic force), and the hypothetical gravitons (which are the presumptive field particles associated with gravity; however, an understanding of the graviton, if any exist, requires an as-yet unavailable theory of quantum gravity).

...

The detection of GW170817 in 2017, the finalé of a neutron star inspiral observed through both gravitational waves and gamma rays, currently provides by far the best limit on the difference between the speed of light and that of gravity. Photons were detected 1.7 seconds after peak gravitational wave emission; assuming a delay of zero to 10 seconds, the difference between the speeds of gravitational and electromagnetic waves, vGW − vEM, is constrained to between −3×10−15 and +7×10−16 times the speed of light.[29]

Is it correct to say that experimental verification that the gravity propagates at the speed of light came around 2017 once LIGO and VIRGO detectors observed the gravitational wave?

I assume that the curvature of space-time and gravitational wave are the same thing. I take space-time curvature as something static and gravitational wave as a dynamic and travelling curvature of space-time. Please correct me if I'm wrong.

Improve this question
$\endgroup$

2 Answers 2

Reset to default
6
$\begingroup$

General relativity tells us that gravitational waves move at the speed of light, and there is lots of experimental evidence to support general relativity. So before 2017 there was already a lot of indirect evidence that gravitational waves travelled at the speed of light. So much so that no physicist I know thought otherwise.

But what changed with GW170817 was that we observed the same event both optically and with a gravitational wave detector. This was the first direct experimental evidence that gravitational waves move at the speed of light. Or at least approximately the speed of light since there was a delay before the optical detection.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Thank you! Could you please confirm the following part? I assume that the curvature of space-time and gravitational wave are the same thing. I take space-time curvature as something static and gravitational wave as dynamic and travelling curvature of space-time. $\endgroup$
    – PG1995
    Commented Dec 19, 2020 at 11:28
  • 2
    $\begingroup$ @PG1995 yes the curvature of space-time and gravitational wave are the same thing. Compare this with an electric field: we can have a stationary electric field, as around a stationary charge, and we can have travelling electric fields i.e. EM waves. $\endgroup$
    – John Rennie
    Commented Dec 19, 2020 at 11:32
0
$\begingroup$

The question and the answers share a common legend: according to the theory of General Relativity, gravitational waves travel at c, and the question is how and to what degree this can be empirically proven. But in fact, GR only has a proof/derivation of the speed for a special case of gravitational waves. This special case is the weak (small perturbation) gravitational waves propagating on the background of flat spacetime. For this type, the Einstein equations can be linearized to yield the D'Alembert operator wave equation already known elsewhere. Its solution is this type of gravitational wave, and it indeed follows that it travels with c. So for these it is really just a matter of measuring it experimentally.

But for the general, other cases of gravitational waves, we do not have a similar proof, even theoretically; for strong gravitational waves propagating in flat spacetime, and for any gravitational waves propagating on the background of a significantly curved spacetime. So, in fact, for these, it is an open question what their propagation speed is. We can suppose that they also travel at c, but this is only a hypothesis with no rigorous basis. When we have a thorough theoretical and empirical knowledge of these in the future, it may turn out that they do not propagate at c. It may be smaller, but there is nothing to prevent it from being larger than c. There are some specific approximate calculations for gravitational waves propagating in a particular curved spacetime. Some of these yield c, but others do not, but establish a wider range around c by theoretical calculation. There may many more surprises to come about the speed of gravitational waves, and it saddens me to see how much is widely spoken and written about as a self-evident truth about something that is only an unproven hypothesis.

Speed of Gravity

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.