[I’m pushing this 2014 post to the top because it is related to chapter 10, the time travel chapter, in my recent book ‘Philosophy of Time: A Contemporary Introduction. It concerns an issue I’d no room to post there, about why we would help a time traveller at all…]

In Edge of Tomorrow, a character played by Tom Cruise, computer-game-like, keeps going back to the same point in time every time he dies. And he dies a lot because he is in a battle against aliens. Generally, I’m not a fan of the ‘changing-the-past’ plotline of many time travel stories. It works best in Twilight Zone circumstances, where paranoia and Oedipal fate issues make it twisted and interesting. But the idea of, say, going back to save your now-dead husband just has a lot of problems. You go back and stop it happening. So does that mean that, once you stop it, it never happened? Then you don’t go back. So who stopped it happening? You didn’t — because you changed it so he survived and you didn’t go back.

This is often called the Grandfather Paradox, after a thought-experiment where someone goes back for a far less compassionate reason. They go back to kill their own grandfather. But in succeeding, they prevent their own birth. So, they don’t exist to go back. They don’t kill their grandfather. Their grandfather survives. They are born; they go back; they kill their grandfather; so they cease to exist.  So, they don’t exist to go back. They don’t kill their grandfather; their grandfather survives. They are born; they go back; they kill their grandfather. So they cease to exist.  So, they don’t exist to go back. They don’t kill their grandfather; their grandfather survives. They are born; they go back; they kill their grandfather; so they cease to exist.  So, they —

But the grandfather paradox doesn’t need all this killing. All you need is knowledge about the world, the will to go back to change the past, and then to succeed in doing so to get the same paradoxical situation.

If I knew that my housemate had eaten my mandarin oranges, then I travelled back to stop them, then stopped them, the same paradox results. Why did I go back if I succeeded? They didn’t eat them if I succeeded — thus wiping out my reason to go back. So I don’t go back (at least for that reason) and so my housemate eats my oranges, and …

Anyway, I talk about this elsewhere, calling it The Intention Paradox. For more, see this post Changing the Past: The Intention Paradox (it’s password-protected. I’m happy to provide it if you contact me).

Edge of Tomorrow highlights a different issue with time travel: When the protagonist goes back in time, where does everyone else go? And why the hell would they help him?

The answer I propose is an approach to time travel which allows both changing-the-past – or, at least, the appearance of it — and also explains why anyone else would help. Continue reading →