Time travel, with Lawrence Krauss

Hello and welcome to Instant Genius, the Bite Size Masterclass Impogcast form. I'm Thomas Ling, digital editor at BBC Science Focus magazine. From H.E. Wells's The Time Machine to Doctor Who and Back to the Future, time travel has become a beloved staple of science fiction, but will humans one day actually be able to travel through time? According to physics, possibly. To explain this today, I'm joined by Lawrence Kraus, theoretical physicist and author of new book The Known Unknowns, The Unsolved Mysteries of the Cosmos. He delves into the strangest theories of time travel, time tourism and also what most time travel movies get wrong. Hello Lawrence, welcome to the show. It's great to be with you Thomas, at least virtually anyway. Fantastic. I'm going to start off with the big question, which is, is time travel possible? Let me give you the big answer. We don't know. That's what makes it exciting. As I mentioned in the book, I've been in the first sense of the new book. I don't know is probably the most important three words in science because it's an invitation to try and discover. The amazing thing about time travel is that as far as we know, it's not impossible. I think that's probably one of the most exciting things about it is that the laws of physics at this point don't preclude it. In fact, they almost beg for it to be possible in many ways because one of the great developments of 20th century physics was the connection and the unification of space and time. Space and time are really different manifestations of the same thing. One person's space can be another person's time in relativity. But there's a big difference. I can go from here to London and back. I can go, I do a round trip in space, but I've never, at least that I know, I've done a round trip in time. Time just seems inexorably to move in one direction. That is a frustrating thing and something that's clearly differentiates between the two, but in principle, nature shouldn't really be able to distinguish between the two. You could imagine that there should be a possibility of going on a round trip in time. Indeed, general relativity, Einstein's theory of general relativity, allows for that in principle. In fact, it says, you put together the right configuration of energy and momentum and you can create any sort of geometry of space and time that you want. So mathematically, I can write down the kind of energy and momentum I would need to have a space time that would allow me to do what we call in science, a closed time like curve, which is simply going back in time and where you're ending up where you started. So mathematically, I can write that down, but the big question is, is it possible to physically create that kind of energy and momentum? And that's the answer we don't know. And because we don't know, it causes us to, well, allows us huge latitude for speculation and fun in both physics and fiction. But do we know how much energy we might need for travel? Well, it's not the kind of how much energy, it's not quantity, it's quality. Let me put it that way. It's we need a certain type of energy and that's very strange. The type of energy we generally need often involves something called negative energy. I mean, the prototypical time machine is a wormhole. It's the best example of a time machine. And I can walk you through it if you want. No, it would be fantastic. So wormhole is a shortcut through space. You can imagine space being curved around and instead of going through space to get from one place to another, you sort of create a tunnel between what would otherwise be two distant points, but tunnel is not very long because space is curved. And that's kind of a wormhole. That's a wormhole now. Mathematically, that's possible to create general relativity. A wormhole would be a time machine. And to understand that, there's only one bit of relativity that I have to remind you of. Because that is, if you're going very fast through space, if I see you going very fast relative to me in a spaceship, your clocks will appear to have slowed down. And that's true. We can measure that. We measure that all the time on Earth. And then it's not science fiction. It's true. Now, imagine a wormhole with one end anchored to where you are. And the other end of the wormhole in space, but that other end of the wormhole moving around, very fast. Well, then because it's moving very fast, if you were sort of standing at the end of that wormhole, your clock will be traveling slowly. So say that wormhole does a big circle, say five light years around, and it's going near the speed of light. So it takes five years for it to do it. But if you're standing at that end of the wormhole, your clock is traveling slowly, and that whole trip might just be a week. So an observer at that other end of the wormhole is now five years minus a week behind you in time. So if you were able to go through the wormhole, you'd come out five years minus a week earlier. And then if you were at a rocket ship, you could zoom back to Earth and arrive back at Earth before you left. And so that's a wonderful time machine. The problem is, and it was Kip Thorne, I think, who first pointed this out, that in order to create a wormhole, you have to have a very special type of energy. Because the mouths of a wormhole in order to be configured as they are, they will generally collapse to form black holes. Another end of the wormhole will be a black hole, and you can't get out of a black hole. And in fact, you can prove that if normal energy is the only thing you have, the ends of the wormhole will collapse to be black holes. In a time shorter than it takes to traverse the wormhole. So there are no traversable wormholes, and you think, "Okay, well, that's that problem is solved. You can't have a wormhole time machine." But if you fill up the ends of the wormhole with a very special type of energy, say negative energy, which is gravitationally repulsive, then you can hold the wormhole mouth open. And you can have a time machine. So if we could produce negative energy configurations, we'd be able to produce, well, we'd be able to produce the energy necessary to hold a wormhole open. Whether you could have a wormhole itself, it's in create-win-and-space is a different issue. But then, of course, the question is, can you create negative energy configurations? And that's where we come to that remarkable three words. Those are remarkable three words. We don't know. There are lots of arguments that suggest it's extremely difficult to do in the laboratory. But there's no proof that I know of that shows that it's impossible. I'm betting that it can't be done. And I know my late friend Stephen Hawking bet that could be done too, but we don't know for sure. So that at least allows for that possibility. It's really interesting what you're saying about Stephen Hawking. Obviously, he said that time travel is going to be impossible because the present day will be filled with tourists from the future. What do you make of that? He wrote the book, the physics of Star Trek, because he was a Star Trek fan and appeared on it. And he talked about that in the book. And I countered him by saying that they all went back to the 1960s and no one noticed so that his argument was, I mean, if you were going to come back in time, why would you want to come back in time now? The 60s were much more fun. And you weren't around. But no one would have noticed if their time travel. But to be less facetious, that is one of the arguments. There are many arguments for why time travel is impossible in a rational universe. One is just that simple paradox that we don't see time travelers and why wouldn't we? And of course, people will argue, well, person X in history was really a time traveler. And you can't just prove that. But there are much more severe problems. And they're the paradoxes that make time travel fascinating and science fiction. Most famous paradoxes, the grandmother paradox, I usually call it, which is, let's say I could make a time machine and I could go back in time. And for some, up, certain reason, kill my grandmother before my mother was born. Well then my mother wouldn't be born. But if my mother wasn't born, then I wouldn't be born. And then if I wasn't born, how can I go back in time and kill my grandmother in the first place? And so it's all of these major issues that if you go back in time, you change the future. And that is, of course, the subject of much speculation and fascination and, of course, the plot of many, many science fiction stories, not just back to the future, but some of my favorite episodes of Star Trek and others. And that is a problem. And there are possible solutions, one of which is that if you go back in time, you're always doomed to repeat exactly what happened before. You go back in time to kill Hitler, but you trip or something. Until a cold, close time like curve will really repeat exactly the same things over and over again. Which kind of make time travel a lot less fun, don't you think? I mean, most people want to go back in time to either correct the errors of their youth or relive them, depending upon their mood. And if you can't change time, then maybe it makes it less interesting. But that would certainly get rid of that one way to get rid of the paradox. There are other possibilities too. Yeah, I think one way that some sort of sci-fi writers have tried to get around that problem is by using wormholes to go back in time to another universe. So the whole grandfather paradox wouldn't be a problem. So do you make all these plots and theories? Well, I mean, it's fun in novels, but in science, I don't think that really flies. Because first of all, you have to have another universe. And generally, at least in modern physics, I talk about in the book, while there may be other universes, we can't access them. They're causally disconnected from us. So there's nothing we can do in our universe that impacts on that. Those other universes are vice versa. And frankly, if a wormhole were to connect those universes, then you could cause the impact on one another. And then it wouldn't solve the problem. Because you could affect the future of that universe and the universe in which you could be in because you would, it could travel through the wormhole. So it just pushes the problem. A way a little bit. It's like it's the same as people who and I think I talk about this in the book people who imagine that somehow wearing a matrix Another science fiction and you know, we're somehow in a computer game vast computer game of a super Intelligence civilization and they say that's you know, that's the solution for how we came about and then but of course It begs the question well is that super intelligent civilization a matrix and another more super intelligent Is it turtles all the way down ultimately you have the same so you can push these problems around but but I don't think Or warm old to another universe is a solution and some people have made it even fancier make it sound fancier because One of the I argue misplaced ideas associated with quantum mechanics is this many worlds Interpretation which is a a fine interpretation of quantum mechanics It's a clue. It says well every time you make an observation You choose one universe and the universe branches and there many possibilities and there many other and it's just a classical way of picturing what's going on a quantum cancer It's as I argue in the book classical interpretations of quantum mechanics are probably misplaced because the real world is quantum Mechanical so why you try to explain it in terms of this classical illusion is one thing But some people have argued well one solution is that you go from one branch of the wave function to another You know when I when I'm looking at you now and you know and you're smiling sort of I could have and A different branch of the quantum mechanical wave function you could have been frowning and so and so or maybe and a different branch The quantum mechanic wave function. I could be interviewing you You know, it's that's what people like to think and then the argument is well Maybe you know a time machine would take you from one branch to another and then and then that's no problem because that universe is different Anyway, but again the whole point of quantum mechanics is even in that classical Cluj picture You can't go from one branch to another even though again There's some great science fiction episodes that involve that when you make an observation your reality is that reality And you don't have any choice to leap into another reality as not as much as it sounds like fun a point you make in the book I love is saying that a time machine would also need to be a space machine as well. Can you unpack why that is? Yeah, sure It's something that's never talked about science fiction and I as far as I know of but the point is the earth You know, we this is the example of the fact that we feel like we're standing still you and I feel like we're standing still Because the earth is moving it relatively a constant rate, but it's moving at 30 kilometers per second around the sun 30 kilometers for again this sun by the way is moving at 200 kilometers per second around the galaxy and so you know every second Relative to where we were in the galaxy, you know moving it to you would move to way 230 kilometers The distance more than the distance seen London and Paris and if I was in a time machine and I Went back in time and I didn't go back in space when I came returned at the same point in the space the earth would now be very very far away And it's orbit around the sun and I'd find myself in a very inhospitable Location somewhere in empty space and so in order to in order for a time machine to really work You'd have to have it any work in the sense that you could return to the same place in space You'd either have to start out arrest and what I mean by rest depends upon your relative frame But if you were on the earth you'd have to start out on the earth and you'd have to make sure that somehow The machine was doing an orbit around the sun at the same rate at the as the earth was all the time that it was going back in time And that would be an even more complicated thing to build Which is a shame because one of my I certainly I'd have to say one of my favorite science fiction stories is the time machine of a few worlds And it's really ashamed to think that he couldn't do what he did even if he had a time machine I was thinking more about the future and if that film was more accurate than it would be Probably quite a short film that if my submit fly is just beams out into space Oh, but the differences they have a delorean so they can travel Travel in the lorry and lorry and lorry you know moves around the earth It's tied to the earth so in any case um, yeah, but that's probably not the only problem with it back to the future But what is the biggest problem for you with back to the future? Well, you know actually back the future does have one thing that is is maybe not a problem but kind of realistic I've argued that One of the proofs it's you know, Hawking's proof that time travel is impossible is Was you know the fact that they would all go back and be be inundated by tourists and the president is one But I think there's a much more remarkable proof and that is the more a Marko proof is that is that Elon Musk is one of the richest people in the world If I could go forward in time a day and keep doing that within a within a week or two I would be the richest person in the world because I would know what the stock market is going to do the more with Absor accuracy and if I just got a 5% return each day on my money in a month or two I you know, I could be certainly richer than most people in the world and the fact that there are so few billionaires or the fact I mean something that may say maybe it's because Elon Musk has a time machine but I doubt it I doubt it but so so I think but you know In in back the future the fact that They needed to use races for for that for the bully to become you know Remember you got that that sports almanac and he became extremely critical I think as you could bet on races That's one way but you wouldn't have to do anything so fancy you just have to just read the read the paper and and see what this dog market was doing the next day and and You could do it so so maybe that's one of the accurate parts but why but of course The question is why the doctor wouldn't have done that and become rich enough is a bigger I think a bigger problem So you have another favorite time travel story? You know film or TV show that really demonstrates an overlook point about time travel that you just love Yes, you know as I mentioned I wrote a book called The Physics of Star Trek And there are lots of Star Trek episodes involving time travel The the one that struck me as an episode where in the in what's called the next generation where this data Who's as android goes back in time and discovers himself this skull or his head and What's interesting about it is it demonstrates the problems with thinking about time travel because He goes back in time and of course And does something but it's only the instant he does something back in time that suddenly things change in the future But of course if you think about it That's not at all because he did it hundreds of years ago So it wouldn't it wouldn't as if you know there wouldn't be this sudden change in the future or the past has changed because the whole time I would change but of course that would have not made an interesting story So if time travel were the where possible all of these classical notions about how things work and that make the world sensible And make storytelling sensible would have to be changed But that's the great thing about science, you know as I talk about in the book Science forces us to change what what we think is sensible if the universe How to beginning and people ask me and I give the same answer that That's even how can we get what what happened you know before the beginning and and the answer is that may not be good question Because if there was no time before the beginning you can't even ask that question But that again raise the question if there's no before you know the physics works by causing effect and all I mean, so you have to change language and That's okay. It makes us uncomfortable But the universe doesn't exist to make us comfortable. So I think in a lot of TV shows and films you get a lot of people who travel through time by going faster than the speed of light Is theoretically that possible? Ah No No, no, I mean I can't and you can't and the speed of light is a A cosmic speed limit and it's not as if it's just sort of you know it turns out the space and time can fire So that it's it's kind of like a cosmic catch 22 Step up if you're going 99% of the speed of light and you step on the gas and you're delorean or whatever or your Tesla And so you think you're gonna go faster what happens is you just get heavier You get heavier you don't go faster because that's the way relativity works You get you gain in mass we do we measure that all the time when our elementary particles week in our in the lot in the Large-addron collider and Geneva We're constantly dumping energy into those protons that are going around and they're going at 99.9999 percent the speed of light and you think okay Could we dump more energy and make no faster? But what happens is they literally get heavier they get more mass Which is why we can then use them when we smash them together to try and create new parliamentary particles that are very heavy like the Higgs boson So space and time conspire to unfortunately not allow you to do that now having said that however and makes it It was about Yeah, there's always a button. It turns out That One of the laws of quantum mechanics is it's kind of like Washington or corporate America if you if you can't measure it anything goes and so Quantum mechanics says literally that that a particle is doing many different possible things at the same time when you're not measuring it Including things that may seem like they're classically forbidden so if you can't measure it over a very short period of time a particle Could go faster than light But what would that mean that would mean it would look Like it was going backwards in time because that's what happens if you're going faster and like okay So given that possibility It turns out that when you think about the quantum mechanics of space and time and particles doing their quantum things then These particles that are momentarily going faster in the speed of light look like their particles going backwards in time But what's an electron that looks like it's going backwards in time looks like an electron is a is a negative charge going backwards time But that looks like a positive charge going forward in time and so when you put that together it turns up the theory predicts That for every negatively charged particle there must be a particle we now call an antiparticle of equal and opposite mass A-ball equal mass and opposite charge and it was that way of thinking about directs The theory that first the relativistic theory of quantum mechanics and electromagnetism that suggested that antiparticles must exist, although that wasn't Direct's argument, it turns up to be one way of thinking about it. In fact, Direct just seems so absurd that he refused to believe it until in fact a year after he developed his relativistic theory of quantum mechanics in 1929 and 1930, I think it was looking at cosmoparase, discovered the antiparticle of an electron, called the positron now, and Direct said his equation was smarter than he was. I'm going to need you to explain one concept like I'm a five-year-old. Why is it that going faster than the speed of light would mean going backwards? Well, because look at it this way. In fact, this is kind of what happens as you go into a black hole. When you go faster and faster, in fact, your clocks are slowing down. So if you are going at the speed of light, your clocks would stop, right? They'd be stopped. In fact, it is true that for a photon, for particles of light, the entire history of the universe happens in an instant. It happens that there's no time for a photon because time has stopped. And in fact, if I were looking at you falling into a black hole, another place where time gets affected, as you're falling in, your clocks going slower and slower and slower, and it looks, therefore, I'll never see you fall in. It looks like you will freeze at the eventorize of a black hole, which is why the Russians used to call them frozen stars, not a sexy name. And so I would actually never see you falling into a black hole because you would appear to slow down. And so it's a natural extrapolation. If you think about things slowing down and stopping when you get to the speed of light, if you work things out, if you were traveling faster than speed of light and work out the equations of space time, then the time variable would be going backwards for you compared to me. If finally, it would be good to ask, if time travel was possible, what could be the latest that a person could travel to in the universe? Is there going to be an end of days for our universe at some point? Well, yeah, that's one of the questions I discussed in the book and guess what the answer is. We don't know. But we have ideas. And it looks like the future, if I had to make my best guess, it said the future is miserable. And now, maybe it's a short-term future as well, but the long-term future. I often say, I don't make predictions about the future less than two trillion years in the future. First of all, because it's easier and secondly, no one will be around to check. But it looks like the future, the far future is that our universe is expanding faster and faster as far as we can tell, due to this weird stuff called our energy that making the universe expand. And if that's the case, then distant galaxies will eventually be receding from us, believe it or not, faster than the speed of light. Now I just told you, you can't go faster than speed of light. You have to parse that more carefully. You can't travel through space faster than the speed of light. The space can do whatever the hell it wants to do. And so, space like a surfer in an undertow, it being carried out to see, no matter how much faster they swim, they're receding. Galaxies are at rest, but it's the space is expanding. If the galaxies are at rest in space, their clocks are ticking at the same rate as us, but the space between us is causing them to separate faster than light. So they're not moving in their own frame faster than light. But anyway, they're moving away from it. So they'll disappear. So most of the universe will disappear in a time of about two trillion years it turns out. And then all the stars are in our own galaxy. Well, eventually our galaxy will merge with its nearby galaxies. And the stars will die out, maybe form a large black hole, but if Stephen Hawking's right, that black hole will eventually evaporate in an unbelievably long time. And what you'll end up as a universe is cold, dark and empty. And that's the future. Finding on a rather bleak note there, that was Lawrence Kraus, theoretical physicist and author of new book The Known Unnones, the unsold mysteries of the cosmos. Thank you for listening to this episode of Instant Genius brought to you by the team behind BBC Science Focus magazine, which you can find on sale now in Supermarkets and news agents as well as your preferred app store. You can of course also find us online at sciencefocus.com. [Music]