Tag Archives: time travel

Changing the Past: The Intention Paradox


(a) This is an adaptation of my opening talk for an exhibition by artist Sinead MacDonald this time last year in the Draoicht Art Centre. 

(b) This is about time travel. This is a warning in case you think that is fanciful, and fanciful things don’t interest you.


Many people want to change something in their past. Some of these things – for example, earthquakes – couldn’t be avoided, and so the desire is futile. But others seem tied to our decisions and choices. Let us say you have a time machine. If you could, wouldn’t you want to go back and, at least, try to change some of your decisions and choices — make the other decision, bring about the other outcome? I certainly would; my regrets are not too few to mention.

This seems to be the idea of many people (at least, in the US, and there’s little reason to think it varies in other cultures). Research by the Pew Institute asked the American public what futuristic invention would they most like. 9% of respondents said they would like a time machine. And, according to the show This American Life, most people would like a time machine because they could then go back and change things. For example, a lot of respondents said they would then go back and kill Hitler before he rose to power.

So, it is a common belief that, if you could travel in time, you could also change the past. And this is a trope in lots of time travel stories: Back to the Future, Looper, Primer (although, not Bill & Ted, or Interstellar, or Twelve Monkeys).

However, there is a problem commonly raised about changing the past. At least in some circumstances, you can run into The Grandfather Paradox.

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Edge of Tomorrow/Live Die Repeat: “Why Help the Time Traveller?”

[Please note: as with my other ‘fast posts’ I consider this a brief comment on a thought with only a small amount of technical detail. ]

Just saw Edge of Tomorrow. Fun film (was the only person in the screen; odd experience for a big action movie). I don’t think anything I’ll say here will spoil it (but be warned just in case). 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. I think that generally the ‘changing-the-past’ plotline of many time travel stories is not a terribly good one. 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, Edge of Tomorrow. I just had a thought after watching the film that there is a way to approach time travel which allows such changing-the-past. Or rather the appearance of changing-the-past. But it isn’t exactly changing the past. You don’t really go back in time. What you do is reverse physical processes. This solves a problem I have with such films.

Next page (page 2 below): Why Help the Time-Traveller?