Glossary:Grandfather paradox

The grandfather paradox is a paradox of time travel, first conceived by the science fiction writer René Barjavel in his 1943 book "Le Voyageur Imprudent" ("The Imprudent Traveller"). The paradox, stated in the second person, is this: Suppose you traveled back in time and killed your biological grandfather before he met your grandmother. As a result, one of your parents (and by extension, you) would never have been conceived, so you could not have traveled back in time after all. In that case, your grandfather would still be alive and you would have been conceived, allowing you to travel back in time and kill your grandfather, and so on. According to this theory you would be stuck in an endless time-loop from which there would be no possible escape. You would, however, never know of this loop.

An equivalent paradox is known (in philosophy) as autoinfanticide — that is, going back in time and killing oneself as a baby.

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

Parallel universes resolution
There could be "an ensemble of parallel universes" such that 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.

In the Marvel Universe, any change made to the timeline results in an alternate timeline. Some characters know this and use it to their advantage (such as Vance Astro of the Guardians of the Galaxy, whose timeline shift allowed an alternate self to become Justice.)