Imagine this as a counterexample. A wormhole could connect two distant points as being the same time in some specific frame. Go through either end at t=0, come

Imagine this as a counterexample. A wormhole could connect two distant points as being the same time in some specific frame. Go through either end at t=0, come out the other at t=0. Go through at t=1, come out at t=1. If that's the only FTL travel there is, then there are only two options to get back to your original position. The short way through the wormhole, but we've established that causality is maintained in that direction. Or the long way, but it'll take long enough that in any frame you get back after you leave. Either way, you can't get back before you left and can't affect things before you left. You get into trouble if there are two wormholes like this that connect points in different reference frames. You could go through the first wormhole at (t=0, x=A) to (t=0, x=B), then the next wormhole at (t'=1, x=B) to (t'=1, x=A), where I'm using t' as the time in this second wormhole's frame. Even if (t=0, x=B) is before (t'=1, x=B) in all frames, there could be some t' frames where (t'=1, x=A) is before (t=0, x=A), letting you return before you left. But at least there's some scenario (only one wormhole exists) that doesn't have tricky causal implications. And there are other setups where even going back in time isn't paradoxical. Take this viewpoint: You just can't kill your father when you go back. "It didn't happen" is the same as "It won't happen," so it's a non issue. If you go back and try to kill him, then instead of you succeeding, whatever happened will happen (which we already know wasn't your father dying). Mathematically, it's a loop. You can't continue forwards around a loop to get somewhere that isn't behind you. As a tautology, the only loop-containing solutions to the physical equations would be the ones containing self-consistent loops.