Many years ago I read a haunting short story (I believe it was called As Never Was, by P. Schuyler Miller), about a curious possible aspect of time-travel. In the story, which I recall only vaguely, there was a museum that sheltered a celebrated artifact: a strange and marvelous knife that had been brought back from a time in the far future when civilization lay in ruins.
The twist was that it turned out that the shattered building in which the knife had been found was in fact the wreckage of the very museum in which the knife was being displayed. In other words, the knife’s existence consisted of a loop in time: a strange circular path in which the artifact itself had no apparent origin.
Physicists refer to this as a closed timelike curve — and as preposterous as it may sound, it is far from clear just what, if anything, would make this re-entrant life-history an impossibility. A reader recently sent along a link to an engaging little article that explores some of the implications of this strange idea; you can read it for yourself here.