# Why do gravitational waves propagate at the speed of light?

KarminValso1724
If things such as quantum entanglement and the expansion of space can travel faster than light, then why can't gravitational waves, which are vibrations of spacetime? I thought that only matter cannot move through space faster than light. Also, has it been 100 percent proven that gravity waves travel at the speed of light?

Homework Helper
Gold Member
The speed of propagation of gravitational waves is derived in the process of solving a system of equations. In the solution, it turns out they propagate at the speed of light. So it's not a matter of 'could they travel faster?' It's simply that the theory predicts, via the equations, that that's the speed at which they will propagate. If they propagated at any other speed we'd need a new theory of gravity.

Mentor
If things such as quantum entanglement and the expansion of space can travel faster than light
Neither of those travel faster than light.... Because they aren't travelling at all.

Markus Hanke
If things such as quantum entanglement and the expansion of space can travel faster than light, then why can't gravitational waves, which are vibrations of spacetime?

In classical electromagnetism, it follows from Maxwell's equations that the propagation of electromagnetic radiation is governed by a certain wave equation which dictates that it does so at the speed of light. Likewise, the Einstein field equations imply that the propagation of gravitational waves is governed by a wave equation which dictates that this happens no faster than the speed of light; that is quite simply what the respective laws of physics demand. I suppose you can think about it as nature's way to ensure that there is no possibility of causality violations.

Giulio Prisco
If things such as quantum entanglement and the expansion of space can travel faster than light, then why can't gravitational waves, which are vibrations of spacetime? I thought that only matter cannot move through space faster than light. Also, has it been 100 percent proven that gravity waves travel at the speed of light?

Saying that entanglement and the expansion of space "travel faster than light" is misleading, because entanglement and the expansion of space can't be used to send faster than light (FTL) signals (that the expansion of space can't be used to send FTL signals is evident, that entanglement can't be used to send FTL signals is a proven result of quantum mechanics).

Gravitational waves are solutions of Einstein's field equations of general relativity, which predict gravitational waves traveling at the speed of light. The recent detection of gravitational waves from merging black holes (LIGO experiment) seems entirely consistent with general relativity.