 When Doc Brown went back in time and helped the 1955 version of himself set up his weather experiment, was that a paradox? Alright, if we're doing a show about time travel, I've got to be ready for any sort of Connecticut Yankee and King Arthur's court nonsense. I still don't feel fully dressed, though. Hang on a second. Ah, there we go. Perfect. Time travel has been a pretty popular idea in fiction for a long time. I'm going to talk a little bit about how time travel works in fiction, how it might work in reality, and how it probably doesn't work. I say probably because we don't have a grand unified theory of physics yet, and so there's still some room to be surprised. But for the most part, writers of time travel fiction don't really care how it works or what relationship it has to physics as we know it. And so sometimes they write things that don't make a lot of sense. First, time travel to the future. This is certainly the oldest version of time travel that we find in fiction, with prophets and seers claiming to be able to see into the future as far back as 2000 BC. It's also relatively easy to imagine because every time you climb into bed, you time travel about eight hours into the future. It's also relatively easy to time travel into the future. And yes, I do mean relatively. According to Einstein's theory of relativity, an object that is undergoing some sort of acceleration experiences time slower than an object that is sitting still or moving at a constant velocity. So technically, you're experiencing the universe slightly faster than you would otherwise because you're within the Earth's gravitational field. You're time traveling right now. Of course, this effect is very small at the sort of accelerations that human bodies can withstand. For example, if you wanted to see a little bit more of the future, you could crawl into a giant centrifuge like the Gravitron at a state fair, and live your entire life at three times normal Earth gravity. And at the end of it, you would get to see an extra millionth of a second before you died. But if you were somehow able to withstand the forces at the event horizon of a black hole, you could watch the entire lifespan of the universe in the blink of an eye. This quirk of relativity is the source of the so-called twin's paradox, where one twin gets on a rapidly accelerating spaceship and the other twin stays on Earth, and when they meet up again, the Earth twin is actually older. It's not actually a paradox. We just call it a paradox because it seems weird to us. We don't usually travel at relativistic speeds. Now, time travel to the past is a little bit more complicated, and this is where writers of fiction tend to go a little bit crazy. So let's say that you have access to a time machine, and you think to yourself, hey, I want to see Josh while he's recording this. So you time travel to where I am now. If it's actually the past of your world, you would be here with me right now. It's not like there is ever a version of me recording this where you weren't here in the room with me. You've always been here. The same goes for if you traveled to 1963 or 60 million years ago. If it's actually the past, it has always existed with time travelers in it, and as such, they've never really been able to change how things turned out. They're back there with John F. Kennedy and Tyrannosaurs, and they've always been there. Our history already contains all of the time travelers that have managed to make it back. Now, there are some cute ways to get around this, like the many worlds interpretation of quantum mechanics. Basically, when very small things interact, if you add up all the possible ways they might have interacted, that summation is how they actually end up. The many worlds interpretation of quantum mechanics suggests that this isn't just an interesting coincidence. That there are actually an insane number of parallel universes out there where every possible interaction between particles is played out. And because everything is particles, this suggests that there are parallel universes where every possible situation plays out. So it's possible that you might time travel to some other version of the past, and when you arrived, your presence there would change which of those universes you're in. There would still be the universe that you actually came from, where you would just vanish as soon as you stepped into the time machine, never to be seen again. So if you pull the Marnie McFly, you could not in fact screw up your own past and slowly fade from existence instead of playing Earth Angel. You might prevent that other universe's version of you from ever having existed, but you'd be just fine. There are some theorized methods of time travel to the past that are consistent with current models of physics, including wormholes and cosmic strings, but no matter the method, there's one important thing to remember. Free will means a lot of different things, but it doesn't mean freedom from the laws of physics and logic. Like, you can't will yourself to stop obeying the law of gravity and fly off into space. That doesn't mean that you don't have some version of free will. It just means that you have to choose from a set of actions that are consistent with the rules of your universe. And the most basic rule of our universe, the thing that logic and science are based on, is self-consistency. Paradoxes just can't happen. There are things that violate our common sense and intuition, but that's more a failing of our ability to understand them than self-contradiction. With that in mind, there are only two ways that time travel can work. Either it's like Bill and Ted's Excellent Adventure, where you can have a conversation with your past self and it's always the same conversation. Or it's like the new J.J. Abrams Star Trek movie, where you can blow things up and screw with events willy-nilly and not have to worry about the existing canon. But not with black holes, that's just dumb. What's your favorite time travel movie? Does it obey the rules of self-consistency or is it just fun fiction? Does it matter? Leave comments, let me know what you think. Thank you for watching. Don't forget to blah blah subscribe, blah share, and I'll see you next week.