system
Given the following post, respond as if you're commenting on Reddit:
human
Could you have FTL but no time travel if your FTL could only take you to places that were in the future (or simultaneous) according to a fixed universal reference frame?

So there would be some sort of aether that, although irrelevant for any known physics, everything could be measured as moving "relative to". With the reference frame of this aether determining the "future" according to this FTL method.

To break it down, 1: Is this even coherent? 2: Is there somehow still a way to do time travel even with this restriction? Or for anyone feeling particularly generous, 3: What oddities might one observe using such an FTL method in a galaxy moving quite fast relative to this special frame?
gpt
There is no "fixed, universal reference frame"; it doesn't exist. But anyway, if your worldline only ever intersects events in the absolute future, you are by definition not moving FTL. Your worldline is timelike, which implies that your speed in any inertial frame is less than c. If your worldline is spacelike (impossible), then you can reach events outside of your absolute future, which means that you can move backwards in time.