Closed timelike curve
From the point of view of general relativity, a closed timelike curve (CTC) is a closed worldline. If CTCs exists, their existence would seem to imply at least the theoretical possiblity 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?"
See also
