The "warp drive" they are discussing, and which was so hyped in 2010 or so, doesn't involve FTL, it involves contraction and expansion of spacetime as the ship moves in freefall. So no time travel.
[0] https://en.wikipedia.org/wiki/Alcubierre_drive [1] https://en.wikipedia.org/wiki/Chronology_protection_conjectu...
> GIVEN: That the universe of discourse permits both time travel and the changing of the past.
> THEN: A time machine will not be invented in that universe.
> For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past.
thus
> Niven's Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
It turns out that if you use any remotely sane numbers on the speed of an FTL craft, it is extremely difficult to travel in time. To get time travel, you have to accelerate on the other end to very significant fractions of the speed of light, which itself takes so much time that it's hard to recover it via the FTL drive again. You have to put in absurd numbers for the speed of the drive and your ability to accelerate (and your ability to carry energy for conventional acceleration!) before you can get time travel out of most FTL drives, because bending your light cone enough to put the past into your FTL drive's path takes a lot of energy.
The only fictional drives that have serious time travel possibilities are the ones in universes where accelerating to significant fractions of the speed of light is easy-peasy, and the FTL drive is very, very fast (some are instantaneous) and can be arbitrarily steered. It's actually not that long of a list; a lot of FTL drives can hypothetically be used for time travel but the engineering in the universe still doesn't practically permit it. Just adding a cosmic censor post facto would preserve the story just fine. Many of the universes that meet those criteria are already so fantastical that they do in fact have time travel, so... those work out too.
(There's also several video universes where it's nominally possible, but the video wildly contracts the documentation. Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, and with refueling being necessary often enough that it's hard to believe they could actually make it.)
I don't know about Wars, but Trek strongly implies that impulse drive reaches significant fractions of light speed with near-zero acceleration time; there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G, and there is little on-screen indication that refueling (as opposed to drive system maintenance, and replacing non-fuel consumables used onboard) is regularly needed.
The indication I refer to is that when you see a ship moving on screen from the perspective of another ship, it never suddenly blinks to "near lightspeed", because what that would look like is that in one frame the ship is there, and the next it isn't. That is never what happens. What happens is that the ship lumbers off, and slowly turns over the course of a couple of seconds, and can be seen to visibly accelerate in a way that the ship doesn't even cover its own length in less than five or six seconds. If you work the math it comes out on the order of 50-100G. Accelerating to near-lightspeed in a couple of seconds is several orders of magnitude larger than that.
Of course the ships don't blink around at near lightspeed; it wouldn't be photogenic. So all ships on all shows in all screens move within roughly a single order of magnitude of screen speed, no matter what the tech manuals say. Similarly, no matter what the claimed ranges of the weapons are, all engagements take place within a range where the two ships can clearly see each other, meaning no more than a handful of kilometers at most. No matter how far the ships are nominally apart from each other, a bolt of energy fired from one ship to another will take on the order of half a second to arrive. Almost no matter how zoomed in we are or are not, the energy bolts will subtend roughly the same amount of our eyes. If the ship is, say, firing on a planet, the bolts will perspective shrink maybe by a factor of two or three, if they shrink at all, rather than doing what they should do which is disappear entirely (except for perhaps one lit pixel if the bolt is sufficiently emissive). There's this one ST:TNG episode where the Enterprise uses phasers to quickly mine holes in the ground and when you start to see this sort of thing it's hard not to see the Enterprise as firing on a roughly 5-mile-diameter ball sitting right next to it, because if you follow the perspective of the "beam" that's what it implies about the size of the target.
Space isn't photogenic. Even today on earth, real combat takes place at ranges where you don't have a direct visual on the target a lot of the time. Any even vaguely realistic space combat will take place across ranges where you can't see a mile-sized ship with the naked eye. But that's not what they want to show. Consequently there's a massive, massive mismatch between the numeric claims of performance and what we actually see on the screen. So you might say something like "oh, the screen is maybe being symbolic", but then they routinely do things that imply what's on the screen is the real truth, such as in the one TNG movie where Data leaps between two ships. That's just one example, there's plenty of other situations like that.
https://www.hindawi.com/journals/isrn/2013/482734/
I suspect using GR to design a warp drive is like using a spoon to build a freeway. Without a theory of quantum gravity - which appears to mean not just a theory of quantised space time, but also of the non-locality and complex distributed relationships that haunt QM - you're probably not going to get very far.
That’s not time travel. You could get the same effect by just blocking the photons before they make it to B, and then traveling at conventional speed.
The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing.
The distance of travel requires time relative to both reference frames.
If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality.
Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates.
If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.
If you start being able to travel >c those relationships immediately get impossibly and paradoxically twisted.
See e.g.
http://www.theculture.org/rich/sharpblue/archives/000089.htm...
Congrats, you have just traveled “back and forth” through time. To people in both the Point A and Point B reference frames, you have “seen the future” and acted on it.
Now, the Legbreakers aren’t stupid. Once they know someone is screwing around with FTL they’ll just hold bets and payoffs until light can travel. Yes, but… how do they synchronize their clocks between A and B? And how can they distinguish between a lucky person and a time traveler? Etc etc, ad regressum.
"Now" and "Here" as fundamental concepts only make sense inside a single reference frame. If you break the idea that information has a speed limit, then both of those ideas shatter into tiny little pieces.
Similarly, if wormholes are real, and you construct one in a gravity well, what happens to gravity? Especially if both ends are in a gravity well?
I'm not sure what to think about gravity "reaching through". Gravity and the wormhole are both just curved spacetime. Maybe the larger body distorts the wormhole throat in a way that looks like "reaching through"? Certainly, you need to have some effect that causes energy conservation when you make a perpetual water wheel by suspending one mouth over another and running water through it...