Paradox Compilation

The Grandfather Paradox

The time traveler goes back in time and kills his grandfather before his grandfather meets his grandmother. As a result, the time traveler is never born. But, if he was never born, then he is unable to travel through time and kill his grandfather, which means the traveler would then be born after all, and so on.

This an example of an infinite regression where the story goes on and on in a loop.

There are many variations of this paradox- Hitler paradox or Hitler's murder paradox, a fairly frequent trope in science fiction, in which the protagonist travels back in time to murder before he can instigate. Rather than necessarily physically preventing time travel, the action removes any reason for the travel, along with any knowledge that the reason ever existed, thus removing any point in traveling in time in the first place. Additionally, the consequences of Hitler's existence are so monumental and all-encompassing that for anyone born in the decades after World War II, it is likely that the grandfather paradox would directly apply in some way.

The Novikov self-consistency principle expresses one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible time lines are those entirely self-consistent-so anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency.