Restricted Directions for an Alcubierre Drive?
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?