Abstract
It is a curiosity of all warp drives in the Nat ´ario class that none allow a free-falling passenger to land smoothly on a moving destination, for example a space station or an exoplanet, as we demonstrate here. This has implications on the hypothetical practical usage of warp drives for future interstellar travel, but also raises more fundamental issues regarding the relationship between superluminal travel and time travel. We present a modification to the Natario warp drive, using a non-unit lapse function as in the ADM formalism, that removes this pathology. We also provide a thorough geometrical analysis into what exactly a warp drive needs to be able to “land”, or transition between rest frames. This is then used to give generic and explicit examples of a spacetime containing two warp drives, such that the geodesic of an observer travelling along one and then the other naturally forms a closed timelike curve. This provides a precise model for the connection between faster-than-light travel and time travel in general relativity. Alongside this, we give a detailed discussion of the weak energy condition in non-unit-lapse warp drive spacetimes.
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