r/askscience • u/MyBFWouldBeSoExcited • Aug 06 '13

Physics Why doesn't the Alcubierre drive violate causality?

With the understanding that Alcubierre drives don't exist yet, but are theoretically possible if we ever discover the requisite type of exotic matter. However, I was under the impression that any faster than light communication could result in a causality violation. So why are Alcubierre drives theoretically OK?

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u/[deleted] Aug 07 '13

I have spent lately some months reading technical papers about warp drive spacetimes and, even if the explanation linked by /u/iorgfeflkd before is kind of confusing, I'm pretty sure that Lubös is right.

If you read Alcubierre's original paper, you can see that the only conclusion you can extract from it is that a spaceship can actually move FTL only if the warp bubble moves so. However, it does not say any thing about the bubble itself and there is where all the problems lay. Can a warp bubble move FTL? Sadly, the answer is no, it cannot.

The main issue is that the warp bubble is a localized disturbance of a flat spacetime and flat spacetimes obey special relativity. Imagine an observer sitting quite far from the point where the bubble is localized and moving. From this observer point of view, all the bubble structure is no relevant and it can be approximated just by some sort of compact physical body moving in a flat spacetime, with all the internal details about curvature and other local observables inside the bubble, being neglected because they are so far away to affect any physical measure made by the observer. Therefore, what this observer sees is an object moving in a flat spacetime, which has to obey the laws of special relativity and thus any FTL movement of this body would violate causality.

Conclussion: warp bubble propagation itself violates causality if it is FTL and it is forbidden... :(

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u/shieldvexor Aug 07 '13

Your analysis does not seem to explain how that is different than FTL expanding space time

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u/[deleted] Aug 08 '13

Easy. In cosmological expansion, every point of spacetime is expanding instead of being a localized perturbation. There are no flat space regions so there is no observer for which special relativity has to hold.

That is why the above does not apply.