If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:8:58

Video transcript

I'm boosting Rio I'm a philosophy professor at MIT and today I want to tell you about the grandfather paradox which is a famous paradox of time travel imagine Bruno Bruno has a grudge against his grandfather and rightly so his grandfather was a horrible man amongst many other things he perpetrated this fraud that left millions of people in poverty so there's nothing Bruno would want more than to kill grandfather unfortunately grandfather's already dead grandfather died many years before Bruno was born but that needn't stop Bruno because Bruno has constructed a time machine and intends to travel to the past to kill grandfather when he was still a young man in fact he wants to travel back enough in time that he can kill grandfather before grandfather had any children so picture Bruno he's traveled back in time he's brought a sniper's rifle with him and he's trained himself to use it he's been tracking grandfather's movements it knows exactly where he goes for his morning walk he's found a perfect spot in a nearby building and he has a clear shot grandfather is walking he stops to tie his shoelaces Bruno caresses the trigger and what happens we know that Bruno will not succeed in killing grandfather why well I already told you the grandfather is runo's grandfather and from that it follows that grandfather must have lived long enough to have children but on this story Bruno has traveled to a time before the time when grandfather had children so if Bruno were to succeed grandfather would never have any children which would contradict what I already told you about maybe you think that the story could be interpreted differently maybe you think that we can imagine a situation in which Bruno kills grandfather as a result the fraud never takes place so Bruno travels back to the present and finds that everyone is much happier I claim that that doesn't make any sense but I think it's easy to think otherwise because there are lots of movies that tell stories which are kind of like that one one good example of this is one of my favorite films Back to the Future now I don't know if you've seen Back to the Future but it's very easy to give you a quick summary so according to the film there's this guy Marty McFly who has a father George and nobody respects George but Marty gains access to a time machine which is especially modified DeLorean he gets into the DeLorean reaches a certain speed there's a flash of bright light and that allows him to travel to the past to a time when his father was a young man he interacts with his father he changes his father's character and when Marty comes back to the future he finds his father a changed man he finds that now everybody respects his father and his father is very very happy my colleague Brad scow once taught me a wonderful way of seeing just why this story is inconsistent because look at the beginning of the film were told that at a certain time 1985 George is respected by nobody and at the end of the film were told fed at that very time in 1985 everyone respects George but look it doesn't matter whether we're told one thing at the beginning of the film in a different thing at the end of the film it's still inconsistent one way to see this is to imagine that you're sitting on a the witness takes the stand and at the beginning of her testimony she says that at the crucial time she was by cigarettes of the store and towards the end of her testimony she says that at that very same time she was instead at home watching TV you can tell right there that not everything in the witness's story is true and that's because her story is inconsistent and it doesn't help fit part of the story was told at the beginning of her testimony in the other part was told at the end of her testimony the story is still inconsistent I think the same thing happens with Back to the Future just because we're told one thing at the beginning of the story in a different thing at the end of the story that doesn't mean that the story is consistent there is a different way of interpreting back to the future where it is a consistent story it's just that I'm this different interpretation it's not primarily a time travel story it's really a story about travel between universes and this interpretation what happens is that when Marty McFly boards the DeLorean what happens is not that he goes to the path of his own universe what happens is that he travels to a different universe in this different universe he meets copies of his parents interacts with them and then he goes to the future of that universe and finds the older copies of his parents to be very happy and respected by all notice however that that's not the interpretation that is suggested by the movie yes on that interpretation the movie is a tragedy think of Marty's poor original father stuck in our universe it's not only the case that nobody respects him he was also abandoned by his son Marty in summary I think that insofar as we interpret Back to the Future as an ordinary bonafide time travel story we have to conceive it even though it's a great movie it's logically inconsistent similarly when we interpret the story of Bruno and his grandfather as an ordinary bonafide time travel story we cannot say consistently that grandfather is his grandfather and the Bruno succeeds in killing him at a time before he have children this leaves us with a very tricky question we know that Bruno will not succeed in killing grandfather but what stops him here's some different options for answering this question one thing you could say is what stops him is that time travel is impossible so he could have never been in that position another thing you could say is that the universe contains a special anti paradox force and that if Bruno was about to succeed in killing grandfather somehow this force would come down and intervened I think there is no need to go in for either of these options if you ask me what stops Bruno from killing grandfather I say I don't know because you haven't finished telling the story but probably what stops in' is something quite mundane in some ways of developing the story it'll turn out that Bruno was pointing the rifle but lost his nerve at the very last minute and other ways of spelling out the story he got distracted maybe there was a barking dog in the background and other ways of telling the story he shoots but misses we know that there is some inconsistent time-travel stories but that doesn't mean that every time travel story is inconsistent there are perfectly consistent ways of telling the story Bruno and his grandfather and on some of these there are perfectly mundane reasons for explaining why Bruno failed it could be the time travel is technologically impossible so it could be that it will remain forever beyond our reach but that doesn't mean the time travel itself logically inconsistent in fact I think the concept of time travel is perfectly consistent