It does not. The concrete evidence of this is relatively recent; theoretically nobody has expected that it would for a long time, but it has been theory.

But yes, we've coordinated detections between LIGO and radar astronomy now, and there's no reason to believe gravity travels FTL. As always, there are error bars, but they're tight enough not to get excited about. That is, who cares if gravity travels at 1.00000000000001 the speed of light? (The theoreticians would go absolutely ballistic, but pragmatically that would mean nothing.)

Huh? What about gravity waves allows... backwards time travel?

If you can send information FTL it creates time travel paradoxes in GR. Gravity waves are a propagation of information, so if they moved at 1.0000001 times the speed of light it would cause… the theoreticians to go ballistic, because it would either open the door for time travel issues, or something about GR is wrong.

At least, that’s my lay understanding of it.

So, nothing specific to gravity waves allows for backwards time travel, so much as anything that carries information traveling FTL would cause backwards time travel.

There’s zero experimental evidence to suggest this.

I actually went down a fairly deep rabbit hole to try and get to why GR proponents believe > C travel allows for “breaking causality”.

It basically comes down to a box plot mechanism they have breaking down when FTL is used, and is absurd.

I’ve yet to see any evidence that FTL would break causality.

Anything that travels faster than light is a violation of causality and would be perceived to move backward in time.

Only in our current understanding of relativity, which assumes that it is not possible to exceed the speed of light.

You can’t just break one postulate and assume the rest of the consequences are still ironclad.

> which assumes that it is not possible to exceed the speed of light.

That's... No, that's not what General Relativity says at all. The speed of light as universal speed limit is not a postulate, it is a result.

The reason is because going faster than light would break causality, as I mentioned, meaning time travel, and therefore meaning all kinds of paradoxes. That is why it is generally said that FTL travel is impossible; because the consequences of it being possible are absurd.