Do you believe there are species that have mastered FTL travel?

[Silent3]

35. Going from Point A to Point B faster than light leads to time travel paradoxes, no matter how...

... you accomplish it, be it "folding space", "warp drive", worm holes, or any other trick you can imagine. People get all hung up on the gross physical aspect of trying to go faster than light, how mass increases, how more and more energy is needed until it becomes infinite at the speed of light, etc.

But that's only half of the problem.

if a physical object, or even a massless information-bearing signal, starts at Point A and arrives at Point B faster than light in a vacuum can travel from A to B, then the arrival at B will be observed to occur before the departure from A in many frames of reference. This is not just a matter of when these events are seen to have happened, but a true reverse chronology that remains after light-travel time is taken into effect. Our normal concept of cause and effect gets turned on its head, because an effect can precede its own cause.

If one backward cause and effect pair were an isolated occurrence, no paradoxes would arise. But if you follow the principle of relativity, then the rules for FTL travel or FTL signal-sending must be the same for everyone in any frame of reference, there are no privileged frames of reference, and it becomes easy to construct "shoot your grandfather" types of paradoxes with interacting FTL events.

Suppose we have a Mars colony, FTL communication between Mars and Earth, and FTL communication between fast, but slower-than-light spaceships travel back and forth from Mars to Earth.

Alice on Mars calls Bob on Earth, saying "Hey, how's it going?". The relative speed of Mars and Earth in their orbits is slow enough compared to the speed of light that, for the purposes of an FTL voice call, the two planets can be considered to share the same frame of reference, and can share a common clock. Let's say Alice places her call at 1:00:00.0 UT, and the signaling system is so fast the Bob gets the call at 1:00:00.1 UT, almost instantaneously, and much faster than the 30 minutes that light-speed communication would have required.

Bob gets this call, and, hearing the slightly dejected sound in Alice's voice, realizes that he forgot to call Alice earlier himself to wish her a happy birthday.

In the frame of reference of a spaceship near Earth, traveling in a direction away from Mars at some substantial fraction of the speed of light, it would appear that Bob receives Alice's call anywhere up to 30 minutes before she makes the call. If Bob hangs up on Alice, picks up his phone again, arranges to have his phone call routed to the near-Earth ship instead of directly to Mars, the near-Earth ship routes the call to a near-Mars ship using FTL technology, and the near Mars ship routes that call to Alice, Bob can now say, "Happy birthday, Alice!" and Alice can receive Bob's call before she makes that dejected-sounding phone call of her own. (Yes, it does work this way. Don't make me have to pull out the Minkowski spacetime diagrams!)

Of course, if this happens, Alice will never make that phone call, and then Bob would never have been reminded of her birthday, so he won't make his backward-in-time call, but then Alice will think Bob forgot her birthday and will call him herself, and then Bob will make the backward-in-time call, but then...

While I won't say that FTL is impossible, if it is possible, then there are consequences that go beyond satisfying the immediate goal of wanting to move or send signals FTL. At least one of the following would have to be true:

1) At a philosophical level, at least, Einstein was wrong. There are indeed privileged frames of reference, and there is (at least locally) absolute time. The principle of relativity would still apply to known physical phenomena, but whatever new physics comes along to allow FTL would reveal a privileged frame of reference and a special reference frame of time such that all causes precede all effects within that frame.

2) The universe is OK with these kinds of paradoxes, and really weird, confusing shit happens when we start messing with FTL, more than we bargained beyond for in just wanting to make things go faster. Maybe something like the quantum multiverse idea, with different branches of the history of the universe existing in parallel which don't need to be reconciled with one another.

3) Some as-yet unknown force or principle blocks paradoxical events from happening. It's hard to not imagine that being a very weird kind of deus ex machina intervention, however, if you just graft FTL on top of anything like known physics.