A Physicist Explains Why Parallel Universes May Exist

This is Fresh Air, I'm Tiri Gross. Our interview today is about discoveries in physics and cosmology, but it may sound more like an episode of this. You're traveling through another dimension. A dimension not only of sight and sound but of light, a journey into a wondrous land whose boundaries are that of imagination, that's the sign posed up ahead your next stop, the Twilight Zone. If the Twilight Zone expanded your notions of reality, wait till you hear what Brian Green has to say. His new book is about parallel worlds. The theory that our universe might be one of several universes. Our universe may be just part of a multiverse. Although the idea of hidden realities may sound like science fiction, it comes out of very advanced mathematics. Brian Green is a professor of physics and mathematics at Columbia University. He's conducted important research on string theory and wrote a bestselling book explaining string theory called the elegant universe. It was the basis of a PBS series and a finalist for the Pulitzer Prize. Green's book The Fabric of the Cosmos was also a bestseller. Brian Green, welcome back to Fresh Air. So does everyone compare the concept of multiple universes to an episode of the Twilight Zone? Not everyone, but it certainly is the case that the science we're talking about is touching upon things that science fiction has explored, has made use of great many different environments. So the kinds of science we're talking about is in some way bordering a long science fiction. And that's what makes it sound both possible and absolutely impossible to me at the same time, possible because I've heard that story in fiction and impossible because that's fiction. But the wonderful thing about fiction is if you look at some of the ideas that have come out of science fiction, oftentimes they are just ahead of their time. It's fiction in the time that the piece is written, but some science fiction become science fact. So I'm having a lot of trouble wrapping my mind around the concept of parallel universes, because it seems like a contradiction in terms. As you point out in the book, I mean, I thought universe meant everything. I thought the universe was infinite and that infinite was beyond any amount that you could possibly imagine. Everything, everything was included in infinite. So how can there be more than one of those? Well, to some extent it is a question of language. If you define the universe as truly being absolutely everything, then you're right to talk about more than one universe would be a contradiction in terms. The reason why we have introduced a new term called the multiverse, which basically means multiple universes, is because as we have studied physics ever more deeply, we have found that what we have long thought to be everything is only a small part of a grander whole, only one piece of a much wider cosmos. And to really kind of communicate that idea, we've introduced this new terminology that our universe is just one of many universes, populating possibly a grander multiverse. And when you talk about many universes, there's lots of different theories about what those universes might look like, right? Yes, the wonderful thing about the subject is that there's not one monolithic notion of what a multiverse would be, as we have studied a whole variety of different areas of physics from relativity, quantum mechanics, cosmology, unified physics. It seems to be the case that whenever we follow the mathematics of these deep theories sufficiently far, we bump into one or another variety of parallel universe, idea. And to me, what makes it so compelling is it's not that we physicists are sitting at our desk saying, what kind of crazy idea can we introduce into science now? It's not like that at all. What we're doing is sitting at our desk trying to do what we always do, which is trying to understand the universe, come up with theories that can describe our observations, our data. And when we follow those theories far enough, we come across some version that our universe is one of many. Okay, so in one model of parallel universes, of a multiverse, in one model, I am interviewing you right now in another universe. Absolutely. What's that model? And how's our energy going there? Well, I hope it's going well. But there are a couple of variations on the multiverse theme, which would be compatible with that idea. The simplest is basically the idea that you began with that our universe may be infinitely big, that is space may go on forever. And if that's the case, it turns out that you can establish using pretty basic mathematics that there's only a finite number of different ways that matter can arrange itself. So if you have an infinite expanse with only a finite number of different possibilities, the possibilities have to repeat. I mean, if you think about having a deck of cards, when you shuffle that deck, there are just so many different orderings that can happen. So if you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe, and only a finite number of different complexions of matter, the way in which matter arranged itself has to repeat. So our collection of matter right now, with you interviewing me, that is repeating itself out there in the cosmos. But isn't that a kind of Earth-Ocentric notion of infinity that like space is endless, but the ways that matter can organize itself is finite. I mean, if space is endless, isn't there matter beyond our comprehension? Aren't there particles or ways of organizing particles beyond our comprehension? You are right. There are potential loopholes to this way of thinking. We are definitely imagining that the physics that we have understood here on Earth does apply everywhere throughout the cosmos. And the reason we believe that is, when we look out as far as we can in the cosmos, and that's pretty far, we can look at billions and billions of light years from Earth very, very far away, everything we see seems to be describable using the laws of physics that we humans have been able to develop. And what we're basically saying is, let's therefore extrapolate and assume that those laws really do work everywhere and really every when. And if that's the case and the reasoning that yield this idea that matter must repeat is something that's a consequence of that mathematics. On the other hand, isn't there another theory of the multiverse that says in other universes, there are different laws governing matter and space and time and gravity and there might be principles we don't even know about. Yes, there's another variation on the multiverse thing that comes from our thinking about cosmology, the origin and the evolution of the universe that does yield a somewhat more robust version of the multiverse and the one that we're describing. And it's coming from something called inflationary cosmology. Now, most people have heard of the Big Bang theory, this idea of how our universe began in a very, very small nugget that about 13.7 billion years ago erupted with space and time being flung outwards, matter and energy coalescing into stars and galaxies over the course of billions of years of cosmic evolution. But the thing that we don't often stress enough about Big Bang cosmology is the Big Bang leaves out the bang. The Big Bang theory does not tell us anything about what actually happened at time zero itself. It doesn't tell us what caused that explosion, that outward push to happen. And this new theory, inflationary cosmology, is what fills in that detail. It tells us that there was a configuration of energy in the early, early moment of the universe that could give rise to something that sounds very strange, something called repulsive gravity. We're all used to the gravity's attractive, you let go of something it falls to the earth, earth pulls things toward it. But there's a kind of gravity that does the reverse, repulsive gravity pushes outwards. And we believe that in the early, early universe, repulsive gravity was an operation and that repulsive push is what drove everything apart. Now, the reason I bring them us all up is when we studied that repulsive push in detail, we find that it's not a one-time event, the kind of Big Bang outward push. There could be many big bangs, many outward pushes at various and far-flung places throughout this wider cosmos, giving rise to different universes. It's like a cosmic bubble bath of universes. Our universe is one bubble in this big cosmic bubble bath. So there were lots and lots of big bangs, one bang led to another bang, to another bang, and each of those bangs created a different universe? Yes, exactly. And to get back to your original question, when you study those universes in a little bit more mathematical detail, you do find that their features can differ widely. They don't have to have the same kind of particles. We know about electrons and quarks, protons and neutrons. Those are the kinds of particles we are familiar with and those other big bang universes, they don't have to have those particles. They don't have to have the forces that we know about. The electromagnetic and the nuclear forces, for instance, they may not be an operation in those universes. Other forces, instead, may take their place. So in this repeating big bang theory, I am not interviewing you in another universe. Well, it doesn't rule out that there could be other bubble universes in which the laws are very similar, maybe identical to ours. So we could, in fact, be having this conversation out there, but it allows for a wider variety of possibilities because the laws can, in principle, be different. Now, you said something that really baffles me you said when we study those universes in mathematical detail what do you mean by that I mean we don't even know those universes exist so when you say when we study them in mathematical detail what are you talking about well that is a confusing idea I think for for people who don't actually engage in the kind of research that I'm talking about because what we do is we sit down with equations equations that describe space in time equations that describe how matter can move through space in time and using those mathematical equations we can get a sense of what it would be like to be in one of those other universes even if we can't actually visit or see or interact with that universe in any real sense that's the power of mathematics and I have to say underlying everything that we're talking about in fact underlying everything I do with my entire life pretty much is a firm belief that mathematics is a short-footed guide to how reality works if that's wrong then all bets are off earlier in our conversation about the multiverse the idea that there are parallel universes are universes not the only universe you were describing how in one model of the multiverse everything that we are doing now is happening in another universe so the interview that we're doing now is happening in another universe what is the theory that backs up that model of the multiverse well there are two I mean one is I wouldn't quite call it a theory per se but it's the notion that space may go on for infinity and that fits within the general theory of relativity which is Einstein's theory of gravity which is the force most relevant on the largest scales of the cosmos and according to that theory the universe could be it doesn't have to be but it could be infinitely big it could also be that if you head out into space you might sort of circumnavigate the cosmos and return to your starting point much like what would happen if you walked on the surface of the earth in one fixed direction you'd come back you wouldn't keep on going forever in one direction if that's the case then space wouldn't be infinite and the ideas that we're talking about wouldn't be true there is another way that you can come to the conclusion that variations of this conversation are having a realization out there in the cosmos and that comes from a theory called quantum mechanics a completely different set of laws that are not as relevant at first sight to the largest things in the universe they're relevant to the smallest things in the universe yeah the quantum mechanics is the study of all those subatomic particles that make up matter and this is where I guess the laws of probability come in that's the key idea the sharp break from the older classical physics that arose when we learned about quantum mechanics is that in Newton's day his way of thinking about the universe was you tell me how things are right now and I will tell you with absolute precision how they will be in a minute or five minutes or an hour from now it was absolute definitive predictions quantum mechanics came along in the early part of the 20th century and said actually that idea is only approximate that idea is not fully correct when you study the universe with greater precision you learn that you can't make those kind of definitive statements the best you can do according to quantum mechanics is predict the likelihood the probability of one outcome or another so if you're studying say the motion of a little particle like an electron the math of quantum mechanics might say there's a 50% chance that the electron is over here and a 50% chance that it is over there and that is the best you can do in terms of delineating where that particle will be quantum mechanics says there's this inevitable portion of the world that described in terms of probabilities and how does that connect to the idea of multiple universes well here's the puzzle the the idea that the world is governed by probabilities is strange enough when you actually do the experiment to say figure out where that electron is you always find it in one location or another and indeed if the math said there was a 50% chance it was one place and 50% at another if you do that experiment a hundred times and pretty much 50 times you find it in one location and 50 and the other so the math is borne out by the experiments the little dark secret that doesn't get perhaps as much play as it should when we study the math the max of quantum mechanics we still do not understand how to go from the fuzzy probabilistic description that the particle might be here and it might be there we don't know how to go from that description to the single definite absolute reality that we see when we do the measurement we never find the particle partly here and partly there we always find it definitely here or definitely there how do we go from the probabilities to the definite outcome people have struggled with that they continue to struggle with that I've struggled with this problem back in the late 1950s a fellow named whoever it suggested a radical way to deal with this problem he said the idea that only one outcome happens in a given experiment that's just not right he said follow the math of quantum mechanics take it very seriously and it is telling us that there are two possible outcomes the particle can be here or there therefore what happens is there are two universes and one universe the particle is here any other universe it's over there and there's a copy of you in each universe measuring that particle and thinking incorrectly that that particles unique location is the only reality but in fact there are two of you thinking that there are two realities two parallel universes now she'd say when you frame this in terms of the position of electron it might sound kind of curious but it also might seem well not that relevant to everyday life who really cares about where one electron is here or there but when you take a counter the fact that everything you think everything you do everything you experience amounts to particles moving around inside your body moving around inside your brain every aspect of reality has to do with how particles move what we're learning from quantum physics if you take the math seriously is that every possible reality consistent with the laws actually happens in its own separate universe no are you convinced by that no I'm not convinced by that not yet I find it the most attractive way of dealing with this puzzle in quantum mechanics going from the fuzzy probabilities to the definite outcomes but when I study this theory in detail mathematically I find various holes in the mathematics holes as perhaps too strong a word I find various points in the mathematics and I'm not yet convinced that this is the right way of dealing with the issue there are other physicists in the world who if you were talking to them they would say I'm absolutely positively convinced there is no other way that this problem can be solved this is the right answer I have not gotten to that point yet I find this a wonderfully compelling idea but I'm not a full convert so you know we've been talking about this idea that we live in one of several universes there are other universes out there that we can't detect but the math that very very very high level math is showing that there really might be other universes when you write about this when you do research on it do any of your fellow physicists or mathematicians think that you've flipped that you've kind of gone over to the other side or is this idea of a multiverse pretty commonly accepted in your field I say it's highly controversial but there are a lot of people on both sides of the aisle it's not as though it's a fringe theoretical study with just a small number of people thinking about the possibility that we might be part of a multiverse there are many researchers many top researchers who are taking the idea seriously I mean the reason why the people who are not fond of this idea are critical of it is quite sensible we're used to science giving explanations of a different sort than a multiverse can give we're used to sitting down looking at data and coming up with theories doing our calculations and showing that our mathematical calculations yield the answer that agrees with what the experimenter or the observational astronomer has found that's the way science has progressed for a very long time if we are imagining we're part of a multiverse we're changing in some sense the way in which our theory and our observations affect one another after all we can't see those other universe we can't touch them we can't visit them and that is uncomfortable to many physicists and scientists are used to the more traditional way of doing science but if it is true if you and others are doing research now the will show we live in a multiverse not just in a universe then you're on the verge of a scientific revolution this would be of course a huge revolution it's a revolution in a way that would complete a meta revolution that's been in the making for five centuries me a long time ago we all know that we thought that the earth was the center of everything then Copernicus comes along and we learn that no the earth is going around the Sun and then later we learn the Sun is one of many stars in our galaxy one of hundreds of billions of stars in our galaxy then we learned that our galaxy is not the only galaxy there are hundreds of billions of galaxies out there if you take that Copernican revolution further it would suggest that what we have long thought to be the universe might also just be one of many universes in a grand or Cosmos is there a multiverse theory that you find most convincing it's a great question and I think it really does speak to what makes the whole subject of interest to me which is as I was saying before you almost can't avoid having some version of multiverse arise in your studies if you push deeply enough in the mathematical descriptions of the physical universe. And that, to me, is really the hallmark of what makes this an interesting subject. I mean, the stakes are very, very high because there are many of us thinking about one version of parallel universe theory or another. If it's all a lot of nonsense, then there's a lot of wasted effort going into this far out idea. But if this idea is correct, this is a fantastic upheaval and our understanding. Which of the ones is most likely to be, say, tested in the next few years, which is the only way that I'll be convinced of any of these, if they really can have some sort of experimental support behind them, is a version of the multiverse that comes from string theory. And that's your specialty. That's the theory that I've been working on for now 25 years, yes. And that's a theory that is attempting to realize Einstein's dream of a unified theory of physics. That is, in essence, one master equation that might be able to describe the big, the small, and everything in between. And as we've studied this theory, we have run into the idea that everything that we know about-- again, everything that we have long thought to be the universe-- might actually be taking place on a membrane. And the image that I like to have in mind is, imagine that our universe is like one slice of bread in a much grand or cosmic loaf, with the other slices of bread being other universes. It's called the brain multiverse. And again, it comes directly from the mathematics of this attempt to realize the unified theory that Einstein sought but never found. OK, let's backtrack just a little bit. So the unified theory that Einstein saw it and never found-- that's a theory that would explain both subatomic particles, but also explain the loss of gravity and speed and light in the cosmos and make the large coincide with the small. That's exactly right. What we have found is that in the 20th century, there are two major developments in physics. One, as you're mentioning, general theory of relativity, Einstein's theory of gravity. It does a fantastic job for big things, stars and galaxies, and so forth. The other development we're talking about, quantum mechanics. And it does a fantastic job at the other end of the spectrum for small things-- molecules, atoms, and subatomic particles. The big problem for 70 years is that each of these theories does fantastically well in its own realm. But whenever these theories confront one another, they are ferocious antagonists. The math completely falls apart. They might say, when would they ever confront each other? One for the big, the other for the small. But there are realms in the cosmos, such as at the center of a black hole, where an entire star has been crushed to a very small size. A star is big and heavy. You need a theory of gravity. It's been crushed to a fantastically small size. You need quantum mechanics. And that domain, you need both of these theories. And when you bring them both to bear, everything falls apart. String theory is an attempt to fix that. How? Well, we have found rather surprisingly that a seemingly modest change to our picture of how the world is constructed allows us to sidestep the problem. In the older days, the older theories of physics, we envisioned that when you spoke about molecules and atoms and subatomic particles, when you got down to the particles, the electrons and the corks inside the nucleus of atoms, we envisioned that those particles were little tiny dots that had no structure, no size. They were really infinitesimal. If we change that idea and envision that these particles are actually not little tiny dots, but little tiny loops, little loops of string, little piece of strings that can vibrate at different frequencies, that change from a dot to a string is able to cure the mathematical inconsistencies between general relativity and quantum mechanics, at least on paper. We haven't tested. We have not been able to test these ideas yet. That's the big issue. But at least on paper, that modest change from a dot to a loop cures the problem. How does it cure the problem? That's a very interesting and difficult question. I figured it would be difficult. I'm absolutely willing to give it a shot. So first, let me just give you what the problem is in a touch more detail. Einstein's vision of space was that it was malleable, it was flexible, sort of like a trampoline. That's a metaphor that we physicist typically use. So the reason, for instance, that the Earth goes around the Sun is that the Sun sort of like sits like a bowling ball on the trampoline of space. And because it creates an indentation in space, as the Earth moves, it's nudged around by the curved surface of the warped space that the Sun creates. That's the way gravity, according to Einstein, works. Warps in curves in space. Now go to quantum mechanics. Quantum mechanics on the very small scales says there's something called the uncertainty principle at work. And the uncertainty principle basically says that you can't ever fully know both the positions saying the speeds of all the particles on microscopic realm. So there's a certain amount of chaos, a certain amount of tumultuous, frenetic behavior happening in the microscopic realm because you can't ever fully nail down what's going on. The frenetic behavior of quantum mechanics is very much at odds with the nice gently curving space picture that Einstein had for general relativity. And in fact, if you follow quantum mechanics and examine a little patch of space, magnify it with a fantastic magnifying glass. It says that if you magnify space to fantastically small scales, way down on small scale space is not gently curving as Einstein had in mind. It looks more like the surface of a violently boiling pot of water, a completely different image of space and one that makes Einstein's mathematics fall apart. That's the problem, the wild jitters of space on microscopic scales. Now how does string theory fix that? It basically spreads things out. When you go from a point particle to a string, you're spreading it out. You're spreading that point particle out into a loop. And when you spread anything out, you dilute it. Similarly, as you spread out the particle to a string, you spread out space. And the wild undulations of space that were the cause of the problem they get spread out, they're still there, but when they spread, they dilute. They dilute to a level that allows the math of general relativity and quantum mechanics to harmoniously coincide. OK, so I'm really working hard to absorb all of this. Me too. And now I want to take it a step further and have you connect everything that you've been saying about string theory and a unified theory and relativity and apply that to the multiverse, to the vision of the multiverse that arises out of your understanding of string theory. And if anybody understands string theory, it's you. Well, there are a couple of multiverses that come out of our study of string theory. One is something that emerged about 10 years ago or so, which was a realization that within string theory, the strings that we're talking about are not the only entities that the theory allows. It also allows extended objects that look like membranes, which are two-dimensional surfaces. There are also three-dimensional surfaces within string theory and so forth. And what this has opened up within string theory is the possibility that we might be living on one of those gigantic surfaces. And that gets back to the metaphor that I was mentioning before, where we are imagining our universe being a slice of bread in this big loaf. What that meant is our universe is living on one membrane and there can be other membranes floating out there in space. And that is what is known as the brain multiverse, that what we have long thought to be everything is actually confined to living on one of these giant surfaces and there are other giant surfaces out there. That idea actually may be testable in the next years at the Large Hadron Collider, this big accelerator in Geneva, Switzerland. - How would you test that idea on this accelerator? - Well, if we are living on one of these giant membranes, then the following can happen. When you slam particles together, which is what happens at the Large Hadron Collider, protons are slammed against each other violently there, sped up near the speed of light and you have these head on collisions, some debris from those collisions can be ejected off of our slice of bread, off of our membrane and be ejected off into the grander cosmos within which our membrane floats. If that happens, that debris will take away some energy, which means there'll be less energy for our detectors here on our slice of bread to measure. So if we measure the amount of energy just before the protons collide and compare it with the amount of energy we record just after they collide, if there's a little less after and if it's less than just the right way, that would indicate that some had flown off indicating that this brain picture is correct. - Wow, okay, so it must be so both exciting and frustrating for you, you're theorizing that there could be other universes and it's conceivable, you could mathematically prove that but you wouldn't be able to get there to actually experience it with your own senses. - Yes, so you can ask yourself, is it really worth thinking about if it's something that is purely intellectual? It seems esoteric and perhaps even standing outside of what we'd want to call nuts and bolts science. And the reason why we are compelled to think about these ideas Well, one it's just wonderful. and interesting, but beyond that, there are certain problems, certain problems that we have struggled with for decades, which when you refrain them in the context of a multiverse theory, some of those problems simply evaporate, and that capacity to go from complete lack of understanding of certain problems to the problem simply going away when you think about them in this other setting is quite compelling. One of the things people love about space travel is that it gave us Teflon and Tang and all kinds of materials that are used now, it had practical applications, in addition to being like fantastic and mysterious and all of that. Are there practical, I think a lot of people would want to know, are there practical applications of the kind of really abstract research that you're doing? Really hard to answer, I certainly can't think of any because the stuff that we're talking about is so far removed from me every day, but what I think needs to be said is, if you were to have asked that very same question to the people that were developing quantum mechanics in the early part of the 20th century, people like Niels Bohr or even Einstein or Schrodinger, I think they would have said, I don't see any application of these ideas, we're talking about atoms and subatomic particles, that's just too far away from everyday life to really affect anything that we do in everyday life. But now, 80 years later, 90 years later, the equipment that we make use of, the fact that you have a cell phone, your personal computer, all manner of technology that has an integrated circuit relies upon quantum physics. Without quantum physics, there wouldn't be any of that stuff. Someone actually estimated that something like 35% of our gross national product comes from quantum physics, which is all just to say, you don't know where science will go, 80, 100 years, 500 years after fundamental discoveries are made. So this kind of science, if it's correct, and I really need to emphasize, this is cutting-edge stuff, we don't know that it's correct, but if it is, it could have a major impact on the way we live. What got you interested in investigating this kind of cosmological level of physics? The math itself, was it a desire to find answers to really profound questions? There are people who are driven from different angles, some are driven by the mathematics, and it just takes them wherever it does, and they just find it exciting to follow the equations. I do not like that. For me, it is the idea, I mean, I love the idea that what we're studying is something that is so big, so transcendent. We're trying to talk about not even the universe we're talking about, perhaps other universes, but all within a rational, logical framework that allows us to make some definitive statements. To me, that's enormously exciting to step outside the everyday and really look at the universe in these mathematical terms on its grandest of scales. What do you most hope to learn in your lifetime? I'd love to understand how our universe began, if indeed there is one universe, or if there are many universes that still like to know how this one got started, and associated with that is the most puzzling question to me of all, which is, what really is time? What is the nature of time? I mean, we all think about time, we all live within time, but we are still struggling to figure out what time actually is. And if our mathematics, if our theories could answer that, I think that would be a profound step forward. Well, Brian Green has been great to talk with you. Thanks. Brian Green's new book is called The Hidden Reality, parallel universes, and the deep laws of the cosmos. You can read a chapter on our website, freshair.npr.org. Green is a professor of mathematics and physics at Columbia University. Coming up, a rock historian Ed Ward talks about a Memphis record label that produced great soul records in the '60s, but remains almost unknown. This is freshair.