The grandfather paradox is a proposed 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 `` Future Times Three '' .
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 '' ) .