It’s a classic science fiction trope: a time traveler journeys back in time and causes a change in history that has disastrous effects on the present or even threatens their very existence.

If these changes jeopardize their ability to travel back through time in the first place, then surely the traveler can’t make that change to time, right? But then they can go back in time again, so, can make those changes again … and so forth.

That’s the essence of a trap called the “grandfather paradox,” an idea that has been used to great effect in books, films, and TV shows—from Ray Bradbury’s short story A Sound of Thunder to Futurama to Back to the Future. And as much fun as this concept is in science fiction, it’s also something that actual physicists and philosophers are intensely thinking about.

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