The Grandfather
René Barjavel · 1943
The puzzle
You travel back in time and kill your own grandfather before he has children. Your parent is never born; you are never born; you cannot have travelled back to kill anyone.
Note
Different physics give different answers. Closed timelike curves under general relativity (Gödel 1949, Tipler 1974) are mathematically permitted, but the Novikov self-consistency principle insists that any time-travel scenario must be constrained so the past you see is the past that produced you — your gun jams, you miss, you find the wrong man. Many-worlds quantum mechanics resolves the puzzle by branching: you kill the grandfather of an alternate-you in a parallel timeline. Free-will accounts treat both as cheats, since neither lets the past actually change. The paradox keeps doing work because it tests, more cleanly than most thought experiments, what we think causation actually is. Barjavel’s 1943 novel Le Voyageur Imprudent gave it the name; pulp SF had been worrying the structure since the 1930s.