Q&A

# Restricted Directions for an Alcubierre Drive?

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It's my understanding that if an Alcubierre drive were to be constructed, it would be able to function as a time machine because there would be certain paths through you could take to get to a destination which would result in you arriving before you started. It is, however, also my understanding that this isn't a necessity of the concept, merely a way that it can be used.

Assume that an Alcubierre drive has been invented, but that there is some intrinsic property of the process that makes it impossible for a ship to arrive before or at the moment that it left. Turning the ship onto a course which enables it to arrive before its actual speed should theoretically allow it to (e.g. a ship that can travel 10x the speed of light arriving ten light-years away in less than a year) starts to destabilize the 'warp field', making it harder and harder to maintain; and turning the ship onto a course which takes it back in time instantly collapses the 'warp field', with no way to maintain it.

I still don't have an intuitive grasp on how FTL travel in this fashion generates time travel, so I don't know if this setup makes any physical sense. Assuming that it does, how would this restrict which directions a ship could travel in, and when?

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Yes. By restricting the directions it can travel in, you can create an Alcubierre drive that can't be used as a time machine to create paradoxes.

The simplest way to do this is to pick a single frame of reference for the warp drive. The Alcubierre drive must never go back in time /according to this reference frame/. There are no other restrictions. It's hard to explain further without turning this answer into a primer on special relativity and Minkowski spacetime.

Relativity, Causality, FTL ← Pick 2

"Relativity" is the idea that the laws of physics are the same for every reference frame. An Alcubierre drive with a preferred reference frame would therefore violate the abstract idea of "relativity". But Newtonian relativity, special relativity and general relativity would all be preserved.

In other words, you get to have 2 of the following 3 things.

• Relativity (background independence)
• FTL (faster-than-light travel)
• Causality (world without time travel)

Our universe has relativity and causality. If you want a causal world with FTL then you have to throw out relativity, but just for the Alcubierre drive itself. The rest of the universe would still work the way we're used to.

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