The idea behind a warp drive is that it utilises expansion of space, which is not subject to speed of light restrictions. The universe has already expanded faster than light without breaking causality.
The causality violation examples I've seen in relativity all involved going back to where you started. No return trips, which includes the cosmic horizon, no causality violation.
One question I've got though: does relativity (1) remove the need for universal frame of reference, or (2) preclude the possibility of a universal frame of reference?
Because if it's the former, then (I think) FTL doesn't need to also be a time machine?
> No return trips, which includes the cosmic horizon, no causality violation.
The drawback is that if a warp drive going from A to B leaves expanded space in its wake, a return trip will take longer than light would take to make a round trip. At the same time, an observer on B looking in the direction of the ship will see a shift to blue until the ship arrives, and, then, would see a marked shift to red when observing A.
