[Steuard]

Here's a link to a good discussion of some related issues by cosmologist Sean Carroll:

http://www.preposterousuniverse.com/blog/2011/09/24/can-neut...

(It's in the context of that mistaken "neutrinos move faster than light" claim from a few years back, but the arguments are general. Bob Geroch's article mentioned near the end is pretty much the gold standard for dealing with these issues as far as I know.)

I really need to sit down at some point and figure out how Alcubierre "warp" spacetimes would relate to causality. It may be a moot point: I've heard people claim that one feature of Alcubierre's solutions in General Relativity is that they are "eternal", in the sense that there's no obvious way to "turn on" a warp bubble around some given region or to "turn off" the warp once you reach some destination. If that's true, it wouldn't be useful for sending information, which might be all that's needed to preserve causality.

As for wormholes or similarly strange spacetime topologies, those really do open a can of worms for causality. There's some interesting physics/philosophy involved, and I'm not sure that we've got any solid answers. (I have my own strong suspicions about what the answer looks like, but I won't try to explain them here.) But that may also be moot: every known wormhole solution in relativity requires matter with properties entirely different than anything ever observed in the universe, so we won't be testing it out any time soon.

[pdonis]

The causality question was addressed years ago in this paper:

http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev...

TL/DR: Yes, the Alcubierre drive does violate causality.

[Steuard]

Thanks! I knew someone must have analyzed this before.

[mrfusion]

Thanks! Could you explain a bit more about how Alcubierre can't be used to send information?

Wouldn't a spaceship traveling FTL be able to carry information?

[Steuard]

You know what? I take that back. At some point, I'd had the impression that Alcubierre's solution required a fixed velocity (so you couldn't start or stop the spaceship), but evidently that's not the case. It gets to be problematic after all! :)

[ethbro]

I believe his point was "only if they can stop traveling at FTL".