The Grandfather paradox

The grandfather paradox is a proposed paradox of time travel, first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (The Imprudent Traveller). The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.

An equivalent paradox is known (in philosophy) as autoinfanticide—that is, going back in time and killing oneself as a baby—though when the word was first coined in a paper by Paul Horwich he used the odd version autofanticide.

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, a number of possible ways of avoiding the paradox have been proposed, such as the idea that the timeline is fixed and unchangeable, or the idea that the time traveler will end up in a parallel timeline, while the timeline in which the traveler was born remains independent.

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