Closed timelike curve
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From the point of view of general relativity, a closed timelike curve (CTC) is a worldline of a material particle in spacetime that is closed. If CTCs exist, their existence would seem to imply at least the theoretical possibility of making a time machine.
If a second time parameter is introduced to discuss a CTC, then the question "Is it possible to travel backwards in time?" becomes meaningful, since at least two different time parameters are used. A simpler question is "Under what circumstances can a CTC exist?"