The physics of time travel
Jonathan Z. Simon · Physics World · 1994
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”.