The multiverse

[Music] [Music]

When I was a child, I remember being told, um, that in the distant past, a very learned person might hope to understand everything that was understood. Um, whereas now, because of specialization, because so much is known, that's impossible. That one person can only understand a very small fraction of what's known. And I really didn't believe this. I didn't want to believe this. And I envied the, the ancient scholars who might have aspired to knowing everything that was known at the time. And what I meant by knowing everything that was known or understanding everything that was understood is not that they knew in detail everything that happened, that they had lists of things which they remembered. That's, that's very far from what I meant. I meant that they understood all the explanations that were known. And I believe that, um, we are not heading away from an era in which one might understand all the explanations that are known, but towards it, because we are continually unifying and broadening and deepening, um, our explanations of the [Music] world.

The universe that we see around us is real, but it's only one small facet of reality. The whole of reality consists of many such universes, and they're all equally real. And we call the whole set the [Music] Multiverse.

I studied, uh, physics at Cambridge, and then, and after that, I was always on the borderline between different subjects. I studied, um, quantum field theory in an astrophysics department. And, um, then I studied, um, I, I, I came to the Multiverse theory, um, in, in the relativity center in the University of Texas at Austin. And, and then I was in the mathematical institute in Oxford. All my research has always been on the borderline between two subjects. I suppose the reason for that is that I'm interested in unifications and in broadening and deepening understanding of things. [Music]

Well, this is my office. I've never actually been here before. Why not? Uh, 'cause I work at home. Uh, so I've never used my office. [Music]

The Multiverse theory comes about as an explanation of the predictions of our best theory of physics, which is quantum mechanics. Quantum mechanics makes very accurate predictions, the most accurate predictions that any theory of physics has ever made. But if you want to explain why these predictions are so, how these physical events come about, there's no alternative but to postulate that what we see around us is not the whole of reality, that reality is much more varied and has a great multiplicity. This is what we call multiple universes.

But why would we need multiple universes? Suppose you take one of the classic, um, experiments of quantum mechanics, um, for instance, the prediction of the interference of light. Now, I can show you interference on a simple laser, which we can... What is it that forces us to believe in parallel universes? I'll show you the experiment. This is an experiment, a very old one, that's been, was first done hundreds of years ago, long before quantum theory was even thought of. It just involves shining light through holes. This is just a piece of cardboard with little pinprick holes in it. And there's one of them, just a single hole. This is a laser pointer that we use in, uh, giving lectures. I shine the laser through the hole, and the, there's, uh, we can see a little spot on the screen over there. And what would you expect to see? You'd expect to see just, if there's one hole, you'd expect to see one spot. But that's already not true. You see quite a complicated pattern. It's called an interference pattern. I'll tell you why. Uh, suppose we, what would happen if we made a second hole? Here's another place on the screen where there are two holes. We get a completely different pattern. And through three holes is a different pattern again. That's the pattern we get from four holes. This experiment has been known for, for, uh, many decades before quantum theory was even thought of.

But the key to quantum theory is what happens when we perform this experiment with very dim light. Suppose we take the laser, um, and we put in front of it a dark filter, very dark filter, so that, so that very few photons actually get through, maybe only one per second. And then, um, we can't see that anymore, but, uh, we have instruments that can see it. So we put the instruments in the, in where we know the light beam is. And when the, the, uh, detector is in, um, place where photons are arriving, it will start clicking. Go click, click, click. If we move it over slightly to a dark place in the pattern, it'll stop clicking. So we can, that replaces our, our eyes. Um, now, um, the surprise is that, um, even with this one photon at a time coming through the, the holes, the pattern is exactly the same as it was when we were shining the bright laser light through.

Well, the question is, how do we explain the fact that we see the same pattern, that we detect the same pattern on the screen when there is only one photon at a time going through here, as we did when the bright laser light was shining on it? You see, this one photon, the one we detect, must have gone through one of those two holes. And, but, but the place where it lands on the screen depends on whether there is one hole, two holes, or indeed three holes. However many holes there are, each number of holes makes a completely different pattern on the screen. So even when there's only one photon at a time coming through, the place where it lands depends on what other holes there are. And what we must conclude from this is that something is coming through those other holes and shoving our photon aside. And that, that process is interference. What is this something? Well, we can do further experiments to find out what that something is. The, uh, conclusion that we draw is that this thing behaves like light in every way we can detect experimentally, except one. And that is, we can't detect it. Always not there. It is there because it pushes aside the light that we can see. And it behaves like light. And, um, the only reasonable conclusion is that it is light of an invisible kind.

Now, that doesn't take you all the way to concluding that there are many universes. But the thing is, quantum theory predicts all this. And in the equations of quantum theory, these invisible bits of light appear in the equations. And they are treated on, in exactly the same way as the light that we do see. What's more, all particles behave like this, not just light, but even the particles in the screen, the particles that you and I are made of. They all are affected, shoved aside by counterparts of themselves that behave exactly like those particles, but cannot be seen. So they are somewhere else. They are, um, real matter, real energy, real light that we cannot see. And so they are an entire parallel world of matter, energy, and light. That's why we call it a parallel universe. And there are many of them. In this experiment, we can see, however many holes we make, um, there is a different pattern, even when one photon is going through. There's a different pattern for six holes, for seven holes, for a hundred holes, for a thousand holes. And that means that there must be something traveling through every one of those thousand holes and making a difference to the photon that we see, making thousands of parallel uniform. And that means there must be at least thousands of parallel universes. Uh, in fact, the theory tells us there are a lot more than that. [Music] [Music]

At the time of the Big Bang, according to quantum mechanics, all the universes came into existence, but they were all very alike. Um, and then with the, uh, interactions between them, interference and other interactions, caused them to become different. Are you real? All the other universes are as real as the one you and I are in now. Suppose you're right. Suppose we have a lot of different and a lot of the same universes all existing together. What would it mean for me? I mean, am I a part of many universes? Um, you have to start thinking of yourself as not as an entity existing in one universe, but an entity existing in the Multiverse as a whole. And so there are other copies of you and of me in other universes. Some of them, some of these copies are completely identical to us. Now, in those cases, it's really just a matter of words to say whether they are other universes or whether they're the same universe. If they're completely identical, there are other universes which are very like this one, but differ only in the position of one atom somewhere. Now, those universes are interfering with ours, and they are producing interference effects, um, which we could detect in the laboratory if we wanted to. And there are others which are very different, where the interference is so small that we will never see them. But they form part of the explanation of the things that we do see. So you exist in multiple copies. I exist in multiple copies. And there are some universes in which I'm sitting here talking to you about something slightly different. And there are other universes where I've just got up and gone to have a cup of tea.

What makes me go into multiple copies in multiple universes? You already exist in multiple copies. Initially, they are all identical. And then, when a moment of choice happens, both a microscopic one and one made by you consciously, um, these identical copies become different from one another. So, suppose you're walking in Oxford and you have to choose to go left or right. In that case, for instance, it could be that in half the universes I go left, and in half I go right. And could you say that the universe splits up as soon as you go left or right? It's not a matter of splitting. This, this terminology of splitting is, uh, was the way that the many universes theory was originally introduced when they thought of there being a universe which then splits into two universes. But nowadays, it's better, we find it better to think of there being just a certain number of universes, perhaps an infinite number, already there, and then half of them do one thing and half do [Music] another.

How do I know if which universe I am, or how do I even know who I am, what I am, if there are multiple copies? For me, that is, uh, the same question that one of a pair of identical twins might say. If, if there are two identical twins, um, called Joe and Jack, and, um, Joe might ask, well, why am I Joe and him Jack? Well, the answer is, if you were Jack and he was Joe, you'd still be asking this question. The thing is, the fact of the matter is, there are two of you, and they are identical, or they, they were identical until they started to become different by having different things happen to them. And it's the same with you and your counterparts in other universes. They start off identical. When they're identical, it doesn't make any difference whether you call them one copy of you or several. But then when they start becoming different, it is worth calling them different versions of you, slightly different versions. Some of them in more distant parts of the Multiverse will be so different from you that it isn't worth calling them you anymore.

It sounds pretty bizarre, actually. We have to get used to the fact that new advances in science get further and further away from common sense. If you want science merely to predict the outcomes of experiments without telling you why, then you don't need to believe in multiple universes. In fact, you don't believe, need to believe in multiple planets. You don't need to believe that anything outside this room exists, if you only want to predict. But if you want to explain, then you must adopt the explanation which meets the facts. And in quantum mechanics, there is only one explanation that meets the facts, and that is the multiple universal explanation. We can see the behavior of things like photons being affected by things we can't see. Our only choice is to say it behaves as if it were affected by those, or to say it really is affected by those. And it only makes sense to say it's really affected, because something that doesn't exist surely can't affect things that do exist.

Can I ask you a personal question? For somebody who is busy with logic and mathematics and, uh, quantum theory, it looks rather messy around here. Yes. Well, you have to remember, this is my place of work. Um, I, as you know, I don't go to my office, so this is where I work. I, I sit at my computer and write the book that I've been writing. And then, uh, when I'm working on other kinds of research, if I receive some papers, then my filing system is my floor, but I know where everything is. [Music] [Music]

Well, like immortality or perpetual motion, time travel is one of the enduring fantasies of the human race. But until recently, the view held by scientists has been that time travel is impossible, not because there is any law of physics forbidding it, but because if it were possible, it would create all sorts of paradoxes. Operator, stand by for time travel. You love time travel, don't you? Um, it's always fascinated me, um, ever since childhood. I wanted to travel in Doctor Who's time machine. You actually did more recently. Uh, I, I made a film in which I had the privilege of traveling in the actual TARDIS of Doctor Who to explain how, um, the many universes interpretation of quantum mechanics solves the problems, um, which in science fiction are usually thought to exist with, um, with time travel. The obstacles, the paradoxes, which are usually thought to prevent time travel, actually do not prevent it if we perform a proper quantum mechanical [Music] analysis.

Does physics give the possibility of time travel? We have to answer that as two questions. The first is, is it physically possible to build a time machine? That is, a path from the present time into the past. And, uh, the answer there is, it's still an open question. But as far as we know, there are physical processes which would allow the, um, the tearing of the fabric of spacetime in such a way that there would be a pathway into the past. This would involve something like a rotating black hole, some very violent event in spacetime. The other half of the question is, if we had such a path, would it be possible actually to travel on it? And what would then happen in regard to the paradoxes? For instance, what if we try to change the past? Would the past be as it is recorded, or would it get changed? If, for instance, I go into the past and prevent myself from ever entering the time machine, or from even building the time machine, then what would, um, what would happen? Would I prevent myself? If so, who was it that went back into the past to to stop me? Uh, and if I couldn't, what would stop me? Sir, I have a time quake approaching. Time quake approaching. Four. [Music] Three. A paradox. Louise, you've changed the past. I know. Down. While we can't change the past, it catches up with us. That is apparently a paradox. And the resolution of the paradox is that when one does things like that, when one goes into the past and changes things, um, the result of that is that one goes, one has actually gone into the past of a, of a different universe. Suppose we would have a time machine. Would you go with it? I don't think I would, because, um, when you travel in time, one thing we do know is that you can't get back to your original universe. So, at the very least, what would happen is that I would end up in a universe where there was another copy of me, the copy who hadn't set out in a time machine. And in that universe, there'd be two copies of me. And in my original universe, there'd be no copies of me. And that would mean that all my friends and and people I know, um, in this universe, they would see me entering the time machine and not come out. And from their point of view, I would be lost forever. There would be another group of the same people in another universe. They would see, have, uh, one copy of me, and they would see a second copy arising, coming out of the time machine. And from then on, they would be in the company of two copies of me. They might not consider that to be an improvement on [Music]

Is the Multiverse theory testable? The fact that we can't directly detect parallel universes is to some degree an accident of the way we're built. If you wanted to detect the motion of the Earth with your human senses, you wouldn't be able to do so either. The way we detect the motion of the Earth is, for instance, by taking a long pendulum, Foucault pendulum, and letting it hang for a while. And in the course of a few hours, we see the plane of its rotation changing. And this is because the plane actually stays the same, and the Earth is rotating underneath it. So this is an instrument which detects the rotation of the Earth. If, by chance, our senses had contained an instrument equivalent to this pendulum, we would be able to feel the rotation of the Earth. In a similar way, if our senses happened to work through quantum interference, then we would be able to feel directly the influence of parallel universes. So saying that you don't feel other universes is like the Inquisition saying to Galileo that they don't feel the Earth moving beneath their feet. Well, the reason why they don't feel the Earth moving beneath their feet is because Galileo's theory explains why they won't feel it moving. But if they do a different experiment with a Foucault pendulum or with telescopes, they will indirectly be able to tell that the Earth is moving.

The core of every scientific advance is a better explanation. Now, quantum mechanics and the many universes interpretation are our best explanation of the physical world at the fundamental level. In fact, I would say they are the only current explanation of the physical world at the fundamental level. There are no real rival explanations. We don't understand it fully yet, and I don't think one ever understands a theory fully until one has its successor. One day, there will be a successor to quantum theory, just as quantum theory itself is the successor to classical theories of physics. And then we will understand the full implications of quantum theory and where it went wrong. And I would expect, again, on the basis of what usually happens in science, we would expect that future theory to be even stranger than quantum theory. [Music] [Music]