We define time travel to mean departure from a certain place and time followed (from the traveler’s point of view) by arrival at the same place at an earlier time (from the sedentary observer’s point of view). Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the time-like ordering of events is not invariant. This violates our intuitive notions of cause-effect relation. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is more mysterious than we think" The answer is more interesting than you might think.
The B-movie image of the brave chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch
We define time travel to mean departure from a certain place and time followed (from the traveler’s point of view) by arrival at the same place at an earlier time (from the sedentary observer’s point of view). Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the time-like ordering of events is not invariant. This violates our intuitive notions of cause-effect relation. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is more mysterious than we think" The answer is more interesting than you might think.
The B-movie image of the brave chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch
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