Grandfather paradox

The grandfather paradox is a paradox of time travel. Suppose you traveled back in time and killed your biological grandfather before he met your grandmother. Then you would never have been conceived, so you could not have traveled back in time after all. Now did you travel back or not?

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced.

For instance, there could be "parallel universes" (see Everett many-worlds interpretation) and when you travel back in time and kill your grandfather, you do so in a parallel universe in which you will never be conceived as a result. However, your existence is not erased from your original universe. Alfred Bester's short story "The Man Who Murdered Mohammed" uses this premise.

Another resolution holds that, if one was to travel back in time, the laws of nature would simply forbid them from doing anything that could later result in their time travel not occurring. This theory might lead to concerns about the existence of free will (in this model, free will may be an illusion). This theory also assumes that causality must be constant: i.e. that nothing can occur in the absence of cause, whereas some theories hold that an event may remain constant even if its initial cause was subsequently eliminated. It is also possible that the time travellers intended action might be completed, but never successfully enough to result in cancellation--See Novikov self-consistency principle.

The grandfather paradox is rather overused though. For those people rather bored with killing their grandfather, there is an alternative which we'll dub 'The Oedipus paradox. The essential physics is the same, but the story is different. In the Oedipus paradox the time traveller returns in time and 'loves his mother' (to quote Tom Lehrer). The time traveller then has the potential of never himself being conceived (as his mother is already pregnant at the time of his conception). To avoid the getout of the time traveller being his own father, which would be possible genetically, though statistically unlikely, this would have to take place several months prior to his own conception.

One could argue though, that Novikov could prevent this paradox from occurring - as many time travellers would not be able to follow the lead of Oedipus.