The Physics of Time Travel

TIME travel has traditionally been the domain of science fiction, not physics. Fortunately, however, at least within Einstein's theories of relativity, discussions of time travel are open to physicists as well. Special relativity unifies the concepts of time and space. General relativity goes beyond unification and allows time and space to warp together in the presence of matter. General relativity even permits sufficient warping to allow "closed time-like curves". These seemingly perverse trajectories describe paths through space-time that always move forward in local time (i.e. an observer's watch always runs forward), but eventually end up back where and when they started. A space-time that contains closed time-like curves, localized in one region, can be said to have a "time machine". Closed time-like curves appear in explicit analytical solutions to the Einstein equation of general relativity. Previously such solutions were deemed "unphysical", simply because they contained closed time-like curves. Nevertheless, since these solutions obey the field equations, they should not be rejected out of hand. Indeed, interest in closed time-like curves has increased in the past decade. The reasons for this are varied, ranging from the practical (if time machines can be built, they would have a lot of potential uses), to the theoretical (perhaps quantum gravity can say something about the existence of closed time-like curves, or vice versa) to the philosophical (do the laws of physics allow or prohibit closed time-like curves?).