If you knew your grandfather was going to be an evil despot and you traveled back in time to kill him, what would happen?

One theory holds that if you kill him in the past, you won’t exist in the future — a paradox that bedevils time travel theorists and screen writers.

Seth Lloyd, a mechanical engineering professor and self-labeled quantum mechanic, holds with the second fundamental time travel theory that says no matter what you did, you wouldn’t be able to kill your grandfather in the past, and so you would always exist in the future. He says recent research with photons by his team at the Massachusetts Institute of Technology provides some evidence.

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