In Harry Potter and the Prisoner of Azkaban, Harry is on the verge of being killed by tens of dementors. With just moments left of his breathing life, an unknown individual bursts onto the scene to cast away the dementors, just in time for Harry to be found and taken to the hospital, where he survives. The savior, it turns out, was Harry himself who had time-traveled back to save his own life. This story is metaphysically impossible. Here’s a formal explanation of why.
- In order to time-travel at time t+1, Harry must survive at t0.
- In order to survive at time t0, Harry must time-travel at t+1.
This is a viciously circular argument. The actualisation of premise 1 necessarily depends on premise 2, and the actualisation of premise 2 necessarily depends on premise 1. Here’s a logically equivalent paradox to make clearer the absurdity of Harry’s time-travel.
Time-Travel Box paradox
Imagine you have a time-travel device that you’ve never used. The device sits inside a box (the Time-Travel Box) that can only be opened by a special key. Once the Time-Travel Box is closed, it locks and can never be opened unless the key is used.
One day, by accident, you leave the key inside the Time-Travel Box and close it. The Time-Travel Box is now closed…
