Topics Time

Question Not to be silly…but if I could build a time machine would it be possible for me to go back in time and stop myself from building the time machine?

Accepted:
March 28, 2009

Comments

Not a silly question at all, absolutely not! But the answer is no.

Suppose you built a time machine last year, 2008. Then it is true now that you built a time machine in 2008, and it always will be true that you built a time machine in 2008. Suppose now that, next year, you decide that it would be amusing to create a paradox by using your machine to go back in time and prevent yourself from building the machine in the first place. But it's still going to be true that you did in fact build it in 2008. Which means that, no matter how determined you might be, it will still be a fact that you didn't succeed in your plan of preventing this. Logic alone can show that something or other must have scuppered your plan: because success would indeed generate a paradox, whereby you both did and did not build the machine, which is a logical impossibility. Now, what logic won't show us is what scuppered your plan. Maybe you had a last-minute fit of conscience and just decided not to go through with it. Maybe you did make an attempt, but your 2008 self managed to overpower your intruding 2010 self. Or maybe something else intervened. If you had CCTV cameras set up around your laboratory last year, you might actually be able to find out what prevented your scheme. Now in 2009, you could go back and watch the images of your 2008 self as you were still tinkering with the machine, and maybe you'll spot your 2010 self, tiptoeing up out of the shadows, about to conk the other one on the head with a spanner... and then slipping on a banana peel, knocking themselves out instead, while the younger one, quite oblivious to what's going on behind them, successfully completes the machine. There will be some perfectly ordinary explanation for why you failed to prevent the building of the machine, and this will be a proper subject for historical research -- because, even if it was initiated by events in 2010, that failure already happened in 2008. But something will have thwarted your plan, because the fact is that you did build the machine.

My own thinking on matters like this has probably been most influenced by the late, great American philosopher, David Lewis, particularly his article 'The Paradoxes of Time Travel'. It first appeared in the American Philosophical Quarterly, 13 (1976) 418-46, and is reprinted in his own Philosophical Papers, vol. II. I recommend it.

Question Not to be silly…but if I could build a time machine would it be possible for me to go back in time and stop myself from building the time machine? Accepted:March 28, 2009

Question Not to be silly…but if I could build a time machine would it be possible for me to go back in time and stop myself from building the time machine?

Accepted:
March 28, 2009