Trying to understand how FTL would violate causality....

[Nugatory]

Is there a theoretical reason to assume the same propagation velocity of electromagnetic- and gravitational waves?

If you assume that these speeds are invariant, yes - there can be only one invariant speed, so they have to be the same. And there are fairly convincing theoretical reasons to expect both speeds to be invariant.

 

[timmdeeg]

If you assume that these speeds are invariant, yes- there can be only one invariant speed, so they have to be the same.

Yes, in this case.
The similarity is that in both cases oscillating charges and oscillating masses as well are emitting waves which propagate through empty space and can be detected elsewhere. So, naively one could suspect here a connection on a deeper level with the effect that both are either invariant (necessarily the same invariant speed then) or not. I'm not sure if this makes sense.

 

[Ibix]

If you assume that these speeds are invariant, yes - there can be only one invariant speed, so they have to be the same. And there are fairly convincing theoretical reasons to expect both speeds to be invariant.

I understand that the speed of gravitational waves drops out of the Einstein field equations, so more or less has to be the invariant speed unless we're chucking GR out of the window. But I thought it was possible to construct massive photons in a coherent way - Proca's work, further developed by Yukawa, I gather (e.g. https://galileospendulum.org/2013/07/26/what-if-photons-actually-have-mass/).

 

[Nugatory]

I understand that the speed of gravitational waves drops out of the Einstein field equations, so more or less has to be the invariant speed unless we're chucking GR out of the window. But I thought it was possible to construct massive photons in a coherent way - Proca's work, further developed by Yukawa, I gather (e.g. https://galileospendulum.org/2013/07/26/what-if-photons-actually-have-mass/).

That's what the hedge about "fairly convincing" was for.

 

[PAllen]

Is there a theoretical reason to assume the same propagation velocity of electromagnetic- and gravitational waves?

In classical GR there is a theoretical reason for GW to travel at the invariant speed. This is independent of whether you include EM in the matter energy sector of your GR model.

 

[timmdeeg]

In classical,GR there is a theoretical reason for GW to travel at the invariant speed. This is independent of whether you include EM in the matter energy sector of your GR model.

Okay, thanks.