Grandfather Paradox – Solution

Published on May 13, 2016

What if you went back in time and killed your own grandfather? Would you still be born? Or would you have thus killed yourself? Thanks to Google #sciencegoals for sponsoring this video!

Link to the previous MinutePhysics video: https://youtu.be/XayNKY944lY

If you could travel back in time, and you killed your grandfather, would you be killing your future self? What do physics, complexity theory, and computer science have to say about this famous murderous time-travel paradox?

Scott Aaronson Notes that discuss time travel and computation, computational complexity, closed timelike curves, and the grandfather paradox: http://www.scottaaronson.com/democrit…

Technical paper that discusses the same thing: http://arxiv.org/abs/quant-ph/0502072

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Grandfather Paradox

0:01As far as we know, time only moves in one direction – forward – but if you could
0:06loop back to visit the universe at an earlier point in time, a famous paradox arises. What
0:10if you killed your grandfather when he was a child? Then your father or mother wouldn’t
0:14have been born, so you wouldn’t have been born, so you wouldn’t have been able to go
0:18back in time to kill your grandfather in the first place. Paradox.
0:22The simplest resolution to the grandfather paradox is that when you go back in time,
0:26you’re actually not going back into your own history but to a copy, and everything you
0:29do there influences the new alternate future of that universe, not your own past. But that’s
0:34boring, because it just avoids the paradox.
0:37If what you do when you go back in time actually influences your own past, and the effects
0:41of your time travel do loop back to the present…future…past – no problem. Let’s just follow the paradoxical
0:48timeline through BEYOND its paradoxical conclusion. You go back in time, kill your grandfather,
0:53thus you aren’t born so you can’t go back in time, thus your grandfather isn’t killed,
0:58thus you are born, so you go back in time and kill your grandfather, and so on… I’m
1:02showing this as a linear series of events but really it’s two entangled histories happening
1:07in parallel. Is that even possible? Well, I don’t know about the time travel part, but
1:11subatomic particles regularly do multiple different things in parallel – it’s called
1:15quantum superposition and is responsible for the weirdness of the double slit experiment,
1:19many properties of atoms and molecules, fusion in the sun’s core, and so on.
1:23So if the universe were to exist in a superposition of two states – your grandfather is alive
1:28and your grandfather is dead – then the natural result is a superposition of two states:
1:31you’re born and able to go back in time to kill your grandfather, and you’re not born.
1:36And the natural result of these is a superposition of two states – your grandfather is dead
1:40and your grandfather is alive – and so, at least from a logical perspective, this
1:44looping timeline is entirely consistent and there’s no paradox. And a similar paradox-free
1:49solution can be obtained by viewing the problem as a steady-state solution to a Markov chain
1:53– but I won’t go into that here.
1:54Now of course nothing about these solutions to the grandfather paradox suggests that closed
1:58time loops are actually possible – in fact, some of the implications this kind of time
2:02loop have in the study of complexity theory seem to suggest that time loops – and thus
2:06time travel into the past – must be impossible. But the main point is sometimes we think a
2:11situation creates a paradox when it doesn’t, and really the only paradox is how our thinking
2:16can be twisted enough to dream up time-traveling murderous grandsons, but not twisted enough
2:20to think about twisting time.

Solution to the Grandfather Paradox In Under Three Minutes