[FredZarguna]

There will never be FTL travel. We already know this, because none of our descendants have ever visited us from the future.

[Ransomed]

What if they didn’t want us to know they were from the future? Seems like anyone that was capable of to-the-past time travel in the first place would also be slick enough to disguise themselves if they wanted.

Freegards

[Talisker]

There will never be FTL travel. We already know this, because none of our descendants have ever visited us from the future.

Not quite... Einstein's theory didn't address FTL - just time dilation in the speeds approaching light, and not even the actual speed of light. About what happens faster than that, he said nothing.

Although we already know that the speed of gravity is 20,000,000,000 light speed.

The Speed of Gravity - What the Experiments Say, by Tom Van Flandern, Meta Research [as published in Physics Letters A 250:1-11 (1998)]