The grandfather paradox is an unexplainable situation regarding time travel.
It is a proposed physical paradox of time travel first described by the science fiction writer Nathaniel Schachner in his short story Ancestral Voices, and by René Barjavel in his 1943 book Le Voyageur Imprudent (Future Times Three).
The paradox asks what happens if you were to travel back in time and killed your grandfather. Your grandfather would be dead, meaning you would never have been born, which means you could not have been alive to go back in time and kill your grandfather, but that means your grandfather would be alive and you would be born and so on with seemingly no answer.
Since, as far as we know, time travel of this kind is not possible, the paradox is not one we will face in practice.
- Schachner, Nathaniel (December 1933), "Ancestral Voices", Astounding Stories, Street & Smith Publications, vol. 7 (4); actually, the story refers to an ancestor of the time traveller not his grandfather.
- Barjavel, René (1943). Le voyageur imprudent ("The imprudent traveller"). actually, the book refers to an ancestor of the time traveller, not his grandfather.