If you traveled back and stopped your own grandfather's birth, could you exist to do it?
Imagine a time machine lets you travel to the past, and you decide to stop your grandfather before he ever meets your grandmother. If you succeed, one of your parents is never born — and neither are you. But if you were never born, you couldn't have traveled back to intervene in the first place. Does this mean backward time travel is logically impossible, or is something else going on?
Reveal the answer
Philosopher David Lewis argued in 'The Paradoxes of Time Travel' (1976) that the traveler can try to stop his grandfather but will always fail for some mundane reason (a jammed gun, a distraction) — the past is fixed, not logically contradictory; 'can' just means different things relative to different sets of facts. The scenario is older still: it appears in Rene Barjavel's 1943 novel 'Le Voyageur Imprudent', often credited with naming the paradox.