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So, I've heard that while Alcubierre drives might present a semi-plausible way of getting around the light speed limit (if you can get ahold of negative energy), they do so at the expense of creating backwards time travel. Is this an inevitable consequence of using such a drive, or just a possible way of using it? If you plan on using an Alcubierre drive to travel faster-than-light to a distant location, and then travel faster-than-light back to your starting point, will the nature of your drive mean that, no matter what you do, you'll have gone back in time?

I'm running off the assumption that, if there's a way to use an Alcubierre drive without violating causality, then I can handwave in an explanation about how attempts to use such drives to go back in time create insane amounts of radiation which vaporize everything inside the warp bubble, creating a practical barrier to use of them for time travel.

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  • $\begingroup$ A Relatively Interesting Link Though as you're trying to make yourself end up 'back in time' it's not an actual answer in-and-of-itself. You will need to watch out for other people seeing you time travel by accident though. $\endgroup$
    – Joe Bloggs
    Oct 24, 2019 at 16:31
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    $\begingroup$ Nothing about Alcubierre drive says it would necessarily be able to actually go faster then light, on contrary, I read that it wouldn't be feasible. Nonetheless it might still be better then alternatives, because of quick getting to sublight speed without feeling any acceleration. $\endgroup$
    – Mithoron
    Oct 25, 2019 at 0:35
  • $\begingroup$ To quote Wikipedia quoting Alcubierre: "The [cosmic censorship] conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole." This may be the kind of thinking you're looking for. $\endgroup$
    – Kajelad
    Oct 25, 2019 at 1:13
  • $\begingroup$ The idea of faster-than-light travel allowing backward time travel is based worldline diagrams. However, superluminal paths aren't real worldlines. Real worldlines don't point backwards in time under lorentz transformations. Therefore, faster-than-light travel does not allow you to travel backward in time in any practical sense. See also youtube.com/watch?v=HUMGc8hEkpc $\endgroup$
    – cowlinator
    Oct 25, 2019 at 1:35
  • $\begingroup$ @Mithoron And it might also be better for sublight just because it gets around the tyranny of the rocket equation without creating a true reactionless drive. The ship can rocket around a star system at significant fractions of the speed of light without needing to be 99% fuel, and without carrying the kind of kinetic energy needed to kill a few billion dinosaurs. $\endgroup$
    – FlyingLemmingSoup
    Oct 25, 2019 at 1:46

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They just allow it.

Superluminal time travel requires two FTL jumps in two different reference frames in order to return to your place of origin before you departed.

The Alcubierre drive actually makes switching reference frames like this quite easy- all you need is a powerful subluminal rocket engine.

Here's how to do it:

  1. Fire up your Alcubierre drive to travel some substantial distance away from Earth, then turn it off.
  2. Fire up your sublight drive, and accelerate until you're moving away from Earth at a large fraction of the speed of light, then turn that off.
  3. Turn around, and fire up your Alcubierre drive again to return to Earth. You'll want to overshoot a bit, since when you drop out of warp, you'll be moving with the same velocity as you had when you turned the warp drive on.
  4. Fire up the sublight drive again to slow yourself to a halt.
  5. Done! If you've used the right speeds for both the sub- and superluminal steps, you'll now be in the past.

To prevent this, you can have your Alcubierre drive interact with some reference frame that all observers can agree on (e.g. the Cosmic Microwave Background), such that if it's activated while not stationary in that reference frame, it'll collapse into a black hole. That's your handwave.

Edit: Here's a video from PBS Space Time showing how a ship with an Alcubierre drive capable of a maximum of 2x lightspeed can take a trip deep into interstellar space and return to Earth long before it was even built.

The video doesn't go into much detail about how the change of reference frame is to be done, but to my understanding, burning a more conventional rocket engine should do the trick.

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    $\begingroup$ +1. I remember seeing a youtube video that demonstrated this; was going looking for it. $\endgroup$
    – Qami
    Oct 24, 2019 at 15:49
  • $\begingroup$ @Qami Indeed; PBS Space Time did a video on it a while back; I'll edit a link into my answer now. $\endgroup$
    – Someone Else 37
    Oct 24, 2019 at 16:18
  • $\begingroup$ Note that if your Alcubierre drives collapse into black holes if activated while not stationary in a certain reference frame, that'd turn them into very dangerous weapons of mass destruction, since very small black holes would very rapidly ablate their mass away via Hawking radiation. You'd basically be turning your ship into a 100% efficiency matter-annihilation bomb. $\endgroup$
    – nick012000
    Oct 25, 2019 at 2:51
  • $\begingroup$ you're also leaving relativity behind, in which case the old saying holds again: relativity, FTL, causality - pick any two. $\endgroup$
    – ths
    Oct 25, 2019 at 14:42

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