Instant Genius - Time travel, with Lawrence Krauss

Episode Date: May 18, 2023

From HG Well’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 ever actually be able to jump through time? According to physics, quite possibly. To explain this today, we’re joined by Lawrence Krauss, theoretical physicist and author of the 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. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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I'm Thomas Ling, digital editor at BBC Science Focus magazine. From HG Wells' 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 Krauss, 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. So 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. And that's what makes it exciting. As I mentioned in the new book and the first sentence 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. And 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

of done a round trip and time. And time just seems inexorably to move in one direction. And 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. So you could imagine that there should be a possibility of going on a round trip and time. And 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 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 time travel? Well, it's not so, it's not how much energy. It's not, it's not quantity, it's quality. Let me put it that way.

It's, it's, it's, we need a certain type of energy and that's very strange. The type of energy we generally need often involve 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. That would be fantastic. So a 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, is that 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.

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 would 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, 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. Either 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, 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 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 mouths open and you could 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'd create one in 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 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, Beth, it could be done too. But we don't know for sure. So that at least allows for that possibility. Yeah, 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 forward for my 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 they're time travelers. 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 know, you can't disprove that. But there are much more severe problems. And they're the paradoxes that make time travel fascinating in science fiction. most famous paradox is the grandfather paradox, well, 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 absurd 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 could 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 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 know, you go back in time to kill Hitler, but you trip. or something. I mean, and so a closed 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

try 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. What do you make of sort of these, 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 those other universes or 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 know, you could affect the future in that universe and a universe in which you could be in because you would have could travel through the wormhole. So it just pushes the problem away 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 we're in a matrix and other science fiction.

And, you know, we're somehow in a computer game, vast computer game of a super intelligent 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 I don't think a wormhole 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 of the quantum mechanics is as many worlds interpretation, which is a fine interpretation of quantum mechanics. It's a clude. It says, well, every time you make an observation, you choose one universe

and the universe branches and there are many possibilities and there are many other. And it's just a classical way of picturing what's going on at quantum mechanics. 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 and 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'm looking at you now and, you know, and you're smiling, sort of, I could have, in a different branch of the quantum mechanical wave function, you could have been frowning. And so, or maybe in a different branch to quantum mechanical wave function,

I could be interviewing you, you know, 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 Kluge picture, you can't go from one branch to another, even though, again, there are some great science fiction episodes that involve that. When you make an observation, your reality is that reality,

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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 in science fiction, and as far as I know of. But the point is, 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 at relatively a constant rate, but it's moving at 30 kilometers per second around the sun. 30 kilometers per second.

The 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, we moved away 230 kilometers, more than the distance in 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 on its orbit around the sun, and I'd find myself in a very inhospitable location somewhere in empty space.

And so in order for a time machine to really work, you'd have to have any work in the sense that you could return to the same place in space. You'd either have to start out at rest, 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 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 HG worlds and it's really a shame to think that he couldn't do what he did even if he had a time machine I was thinking more back to the future and if that film was more accurate then it would be probably quite a short film that if Martin fly is just beams out into space. Oh, but the difference is they have a DeLorean, so they can travel.

Travel, and DeLorean, you know, moves around the earth. It's tied to the earth. So in any case, yeah, but that's probably not the only problem with it back to the future. What is the biggest problem for you with Back to the Future? Well, you know, actually, back to the future does have one thing that is maybe not a problem, but kind of realistic. I've argued that one of the proofs, you know, Hawking's proof that time travel is impossible is, you know, the fact that they would all go back and be inundated by tourists and the president

is one. But I think there's a much more remarkable proof. And that is, the more remarkable proof 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 week or two, I would be the richest person in the world because I would know what the stock market is going to do more with absolute accuracy. And if I just got a 5% return each day on my money in a month or two, 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, some people may say maybe it's because Elon Musk has a time machine, but I doubt it.

I doubt it. But so I think, but you know, in back the future, the fact that they needed to use races for that, for the bully to become, you know, remember you got that, that sports. almanac and he became extremely credible. I think because he 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 paper and see what the stock market was doing the next day. And you could do it.

So maybe that's one of the accurate parts. 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. Do you have another favorite time travel story, a film or TV show that really demonstrates an overlook points about time travel that you just love? As 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 one that struck me as an episode where in what's called the next generation where

this data, who's this android, goes back in time and discovers himself, the 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 it at all because he did it hundreds of years ago. So it wouldn't as if, you know,

there wouldn't be this sudden change in the future where the past has changed because the whole timeline would change. But of course, that would have not made an interesting story. So if time travel were 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 we think is sensible. If the universe had a beginning, and people ask me, and I give the same answer that Stephen Hawking would have given, what happened before the beginning? And the answer is that may not be a good question.

Because if there was no time before the beginning, you can't even ask that question. But that again, raised the question. If there's no before, you know, physics works by cause and 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? No. No, no, I mean, I can't and you can't. The speed of light is a cosmic speed limit, and it's not as if it's just sort of, you know, it turns out that space and time conspire so that it's kind of like a cosmic catch-22. If you're going 99% of the speed of light,

and you step on the gas in your DeLorean or whatever, or your Tesla, and so you think you're going to 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 actually gain in mass. We measure that all the time when our elementary particles. In the Large Hadron Collider in Geneva, we're constantly dumping energy into those protons that are going around.

And they're going at 99.999% the speed of light. And you think, okay, can we dump more energy and make it go 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 elementary 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, it makes sense, it was a buzz. Yeah, there's always a bot.

It turns out that one of the laws of quantum mechanics is it's kind of like Washington or corporate America, if you can't measure it, anything goes. And so quantum mechanics says literally 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 fast in light. 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 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? It looks like. The electron is a negative charge going backwards in time.

But that looks like a positive charge going forward in time. And so when we put that together, it turns out the theory predicts that for every negatively charged particle, there must be an particle we now call an antiparticle of equal and opposite mass, equal mass and opposite charge. And it was that way of thinking about directs, theory that first, the relativistic theory of quantum mechanics and electromagnetism that suggested that antiparticles must exist, although that wasn't Dirac's argument. It turns out to be one way

of thinking about it. And in fact, Dirac, it 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. They, looking at cosmic rays, discovered the antiparticle of an electron called the positron now. and Dirac 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 factor, your clocks are slowing down. So if you were going at the speed of light, your clock 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 clock's 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 eventorized in a black hole, which is why the Russians used to call them frozen stars. It's not a sexy name. And so I would actually never see you fall 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 the speed of light and work out the equation of space time,

then the time variable would be going backwards for you, for you compared to me. 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 discuss the book, and guess what the answer is? We don't know. I don't know. But we have ideas, and it looks like the future, if I had to make my best guess, it's that the future is miserable.

And maybe that's the short-term future as well, but the long-term future. As 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. secondly, no one will be around to check. But it looks like here's 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 dark energy that making the universe expands. 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 light. You have to parse that more carefully. You can't travel through space faster than the speed of light, but space can do whatever the hell it wants to do. And so space like a surfer in an undertow, being carried out to see, no matter how much fast they swim, they're receding. Galaxies are at rest, but if the space is expanding, if the galaxies are at rest in space, their clocks are ticking at the same rate at 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 us, so they'll

disappear. The light from them will never be able to reach us. So most of the, 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, while eventually our galaxy will merge with its nearby galaxies, and the stars will die out, maybe form a large black hole, but that, if Stephen Hawking's right, that black hole will eventually evaporate in an unbelievably long time. And what you'll end up is a universe that's cold, dark, and empty. And that's the future. Ending on a rather bleak note there, that was Lawrence Krause, a theoretical position. and author of new book The Known Unknowns, the Unsolved Mysteries of the Cosmos.

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