The Multiverse: What's Real? | ENCORE Episode 1801 | Closer To Truth

The multiverse gives me chills. What could be more startling or arresting than many universes? Multiple universes, innumerable universes, perhaps an infinite number of universes. Our own one single universe alone is astonishingly vast. Even the small part we see has two trillion galaxies. And many galaxies have a hundred billion or more stars and even more planets. Now leap to the unimaginable multiverse. I am exhilarated. I am frightened. If all physical reality can be captured by one question, it is this. Is the multiverse real? But among cosmologists, the multiverse is becoming conventional wisdom. That's why I always want the latest thinking, the latest challenges. I'm always asking, probing, is the multiverse real? I'm Robert Lawrence Kuhn and Closer to Truth is my journey to find out. The ineffable vision of the multiverse and its irresistible enticement exemplifies the Foundations Questions Institute, FQXI, which dares to probe deep reality at the foundations of physics and the frontiers of cosmology. In every FQXI conference held every two years, no matter what the theme, a sub-theme is always the multiverse. I've arrived in Banff, high in the Canadian Rockies, wondering, what's the latest? In search of multiple universes, I set about to meet its staunch advocates and constructive critics. I begin with FQXI founder and scientific director, MIT cosmologist, Max Tegmark. Max, we've been talking about the multiverse for just about a decade. Where are we now? What are some of the latest developments, not just in observation, but also in your own thinking? I think it's been a pretty wild ride for the multiverse in recent years here. Of course, multiverses, parallel universes, are not theories. They're predictions of certain theories. So the more seriously we can test these theories, and the more seriously we're going to take their predictions, the multiverses. And for the level one and level two multiverses, which are basically our space way farther away than we can see, perhaps very diverse spaces. Those are predictions of inflation, and inflation is looking very good. The case has been strengthened a lot in the last few years by just these amazing precision measurements of the baby pictures of our universe. And then for the third level of the multiverse, the quantum multiverse, I think things are also looking very good. We've been testing the crazy weirdness of quantum mechanics on ever larger scales. And there is still absolutely no shred of evidence of quantum mechanics breaking down. If we can keep extending this and ultimately show that quantum mechanics can even be applied to us, and we can have you in two places at the same time, that kind of clinches it. Then there's just no way of avoiding that multiverse. And then there's quantum computers, which may very well accomplish macroscopic superposition long before you can do it with living things. Do those require multiple worlds? The way I think about it, the whole reason they work is because they're the ultimate parallel computers that tap into the parallel computing powers of all these other worlds. So if they can get the answer that would take longer than the age of our universe to do in our universe in one second, it becomes kind of hard to deny that some of that other reality is really there. Okay, I'm looking for these logical by force necessities. What's the confidence level that once inflation works in our universe, it has to branch off for level two universes that are universes that are outside our communication of space-time? What inflation generically does is it never stops. It just keeps making these vast regions of space that the inhabitants will call universes. But it does not guarantee any kind of diversity. It might just be minting vast numbers of kind of identical ones where the laws of physics seem the same, even if different things happen. To get to level two and have more diversity and maybe different amounts of dark energy or different numbers of quarks, whatever, you need, in addition to inflation, to have the fundamental theory of physics, be it string theory, loop quantum gravity, or whatever else, to have more than one solution. Because inflation is this very creative force which transforms potentiality into reality. So it'll not just make lots of space, but it will create parts of space with each of these solutions. And so far, all the quantum gravity theories that we have on the table pretty much have multiple solutions. So unless we come up with something very, very different, inflation would actually create this wonderfully diverse multiverse. So let me try to summarize your grand vision of reality. You have the multiverse within our normal space-time. But then you have another kind of level three, which is the quantum multiverse, which you have the quantum branching. So you have this vast number of universes within our current space-time, all of which nested within the quantum multiverse. And all of that with which is just one of the mathematical models of your level four. Did I get that right? Exactly. So I think of it as Russian dolls, one inside the other inside the other, except that inside the biggest Russian doll, the mathematical level four multiverse, there's many, many, many Russian dolls. All right. You know, one of them is this quantum level three multiverse. Inside of there, you have lots and lots of dolls that are different level two multiverses with different apparent laws of nature. And inside of each one of those, you have a vast number of level one parallel universes where the laws of physics are all the same, but we learn different things in history class. What a way for reality to be. Well, isn't it wonderful to have a really grand reality? Max's grand reality of nested multiverses, the Russian dolls, defies the imagination. I meant my spontaneous reaction. What a way for reality to be. Frankly, I can't decide whether such an astoundingly immense reality counts for or against its actual existence. But ever more precise observations support the predictions of certain theories, especially cosmic inflation, the impact of which seems to lead inexorably to a multiverse. But how can we ever know for sure why are even simple multiverses, so to speak, still controversial? I asked Albanian-American cosmologist Laura Mersini-Houghton, who has her own theory of space-time and universal origins. Some of the controversies where the multiverse is concerned is the concern that we may never be able to collect evidence for its existence. And that kind of conclusion is based on general relativity that says no signal can propagate faster than the speed of light. And the only way for us to have direct evidence for an event is by collecting light rays from from that event. So in that sense, if we can't go beyond the horizon of our universe, then it will be hard to know what what lies beyond. However, there are many indirect ways of collecting data, and some of those are recently known as the anomalies in the cosmic microwave background in the sky. Those effects are heavily based in quantum mechanics. And one of the cherished principles of quantum mechanics is something known as the unitarity principle that simply puts as information about the system can never be lost. So in that sense, although our universe might be a small domain in a much vaster and complex cosmos, the multiverse, we are entangled, quantum entangled, with all the other domains, universes, and whatever else lies beyond our universe. And we can collect data about that entanglement and see it in our sky. So you're saying that everybody is entangled in this enormous multiverse because it all originated from the same kernel, so to speak. There are a lot of discussions and favorite theories and ideas on how the universe and the multiverse originated, but the two are intimately related to one another. And that's a question that goes back to antiquity. In the 20th century, Hugh Everett, a student of John Wheeler, who was studying the implications of quantum mechanics, reached the conclusion that quantum mechanics does not give you just one universe, but it gives a family of wave functions of the universes. But those are very different kinds of a multiverse than an inflationary. The Everett universe is a quantum mechanic generated, which is in a totally different abstract space, shall we say, whereas the inflationary universes are within our kind of space. Well, in reality, nobody knows in which space-time these universes exist. They could be embedded in the same space-time. Yeah, of course. One could be nested in the other. Or they could belong to totally separate space-time. So there are many variations. Okay, so I can speak about my favorite theory, which is the theory on how our universe was a wave packet before it went through the Big Bang. So we didn't even have a space-time associated with it. And then later on, of course, containing enough energy, it went through a Big Bang, through cosmic inflation, it grew up, space-time became big as well, and we end up with the classical universe, the big classical universe we see today. Multiple universes leaving traces in our universe? Different space-times? Our universe a wave packet before the Big Bang? Could reality really be like that? Do multiple mechanisms for generating multiple universes, all competing, help or hinder their real existence? Again, I can't decide. But even though the multiverse marches towards conventional wisdom, criticisms intrude. I meet cosmologist and unconventional thinker, Paul Davies. We have to distinguish the two different types of multiverse. One is that there are many universes, many Big Bangs scattered throughout space and time, the cosmological multiverse, and the quantum multiverse, which is that in quantum physics there's uncertainty, and all the possible outcomes of a quantum system, uncertain as they are, simply represent alternative realities, each of which is equally real, and each comes with its own observers who think they're unique. And so, in polite company now, if you don't believe this multiverse view, you have to be very careful. And so, there's still a handful of people here who I think have alternative views, and I think I'm one of them. I often say two cheers for the multiverse, because I think, you know, it's a nice try, but I think it has its own problems. I can tell you part of the reason why it's become so fashionable, and that is that if you're just dealing with an atom or an electron or something, you can always appeal to an external environment that somehow, by some magic we don't yet understand, turns the many different possibilities, the potential realities that are present at the quantum level into a single actuality. There's lots of physics that could go on in that transition from the micro to the macro. But many of the people here are cosmologists, and they like to apply quantum physics to the universe as a whole. And then you're a bit stuck, because there is no environment, there's nothing outside that can take this sort of shadowy superposition of many possible universes and say, this one is actually projected out and is the reality. And so, those people are sort of stuck with having to have that type of multiverse interpretation of quantum physics. So, if we have these two categories of generating multiverse, eternal inflation that we see macroscopically and this microscopic many worlds universe, you really can have a nesting of one in the other and the multiplicity of infinities. You know, I just stopped to ask myself, how could reality be like that? Right. And now, it's easy to bandy this word infinity around. You actually have to be very careful when you're talking about infinite categories. There's a mathematical problem called the measure problem, that if you're going to compare one infinity with the other, you better have some rigorous definition of what you're talking about. A lot of this multiplication of universes and things, it doesn't actually come with any very well-defined measure. Paul gives two cheers for the multiverse, damning by faint praise. He brings up the measure problem. What does one do with infinities overlapping and intertwining infinities? I'm fine with infinities as mathematical objects, but I hesitate as physical realities. If physical reality is indeed infinite, then anything that can happen must happen, and it must happen an infinite number of times. So how can you measure anything? It gets crazy. I ask a theoretical physicist who critiques the multiverse and has sometimes unorthodox views, Andreas Albrecht. I'm very concerned about the way people are using the tools of physics to study the multiverse. Probably my biggest problem is actually probability. Every time we use probability, it's actually linked to quantum uncertainty, and it's linked via the chaotic processes that form, that the particles form in our body, in the world around us. So basically when you get to the multiverse, many important questions become questions without quantum probability answers. So the essential point is that when you have a multiverse, there's many copies of you, there's many copies of me. If I'm going to do an experiment and I want to know the outcome, which copy do I look at for the outcome? And you need to know the probability of which one. And so what I say is, you can't answer that question. If you don't know which copy you are, you can't assign a probability to it. Well, two of the ways to generate an infinite number of universes, or a very large number of universes, are one through eternal chaotic inflation, and another very different one is the multi-world theory. Very important, actually, yes. And so the kind of multiverse you get from the quantum branching in the every-worlds or whatever language you want to use, that's, in my view, legitimate, because that branching, the quantum mechanics is assigning probabilities as you go. Right. So are you a many-worlder? I am. But it is part of my intuition. I do think like a Neveridian, so it does slip in. Okay, but how about the chaotic eternal inflation? So that's where you run into this trouble, where the eternal inflation generates many copies of you, infinitely many, and you don't have any way of assigning probabilities. So I've been very harshly critical of the multiverse with all the measure problems, but certain questions just don't have answers. And maybe that's okay, and it's actually convinced me that maybe I should be a little bit more open-minded about the multiverse, to the extent where maybe there's enough symmetry. If there's enough symmetry where it doesn't matter which copy you are, then you should be fine. So integrating it all together, what's your probability, not the quantum probability, but the macroscopic normal probability, that there is indeed a multiverse? Okay, I guess I'd have to say 10%, and that's like a million times higher than last time we talked. So, it's quite a change for me. I like scientists who can change their minds. For Andreas, getting to 10% confidence in the multiverse is a big deal. I can't wait to tell Alan Guth of Andreas' progress. It was Alan's theory of cosmic inflation that provided the scientific mechanisms for generating multiple universes. But does Alan also worry about the measure problem? The basic issue here is that most versions of inflation have the property that once inflation starts, it never stops. And that's what's called eternal inflation. And that leads to a very difficult problem of defining probabilities. Anything that is allowed by the laws of physics will happen ultimately an infinite number of times. So to say that something is more common than something else, you're comparing infinities, and there's no unique way of doing that. So that has led many of us to be worried about this question of probabilities. We've been defining, exploring, I should say, different recipes for how to regularize the infinities to give rules for how to compare one infinity with another. And in order to talk about such things, we really do need to talk about not solely what happens physically, but we need to talk about what we expect observers to say, because that's ultimately what we're trying to predict, what we would say. So the observer enters by basically picking out parts of this multiverse, which are the parts where observers live, and we have to condition our probabilities on those observers. We have to ask, what's the most probable thing for an observer to say? And the one beauty of the multiverse here is that the set is infinite to start with. So no matter how much you narrow it, it's still infinite. So I think the least controversial thing is that if I want to make predictions about what I will see in the future, or what I will find as a result if I do some kind of experiment, I can look at observers who are all exactly like me. And I think that's the simplest way to describe how, when we pick out the observers in a multiverse, to use to make predictions. One still has this measure problem of how to count them, because there's still an infinite number of them. This does not solve the measure problem, but it is, I think, the right answer to the question of how do we pick out the observers that are relevant. To me, the most important measure problem in a multiverse is whether there is any measurement we can make in our universe that can demonstrate the actual existence of other universes, even one other universe. Otherwise, as some cosmologists are now arguing, the multiverse is not science. I ask the co-scientific director of FQXI, cosmologist Anthony Aguirre. Are there things that we could actually go out and measure that could tell us that there is a multiverse? And the good news has been that there are. Bad news is that those things haven't shown up. Which doesn't mean the multiverse is not true, but it just is hard. What are some of those? That would really be interesting. Bubble universes can collide, and in our universe, if everything went right, would literally look like a kind of sphere, a bruise on the microwave background radiation that we can look for. So, it tells us kind of what the density and temperature of the universe looked like at one very early time, so we have this snapshot. But if this other bubble ran into ours, it would perturb that in some region, and that region would look like a kind of disk on the sky. When they look out, they'd see, well, over there, it's a little bit warmer than it ought to be. Is that generally accepted, that that type of imprint would be an indication of a separate universe? I think among people who understand the arguments, yeah, I don't think there's... Among the good people. Right. Among the people who agree with us, there's consensus. Okay. Yeah. That's good to know. But that's not really the main reason that people think about the multiverse. They came upon it as this side effect of inflation that we believe for other reasons. And I think it's interesting to see that the reasons for believing inflation have held up quite well. I did a little poll at this meeting, and I asked, what fraction of people who are cosmologists thought that inflation happened at 75% likelihood or more? So they weren't sure, but 75% confidence level. And it was about 80 or 90% of people. And then I asked, of those people who have studied inflation, what fraction of those people think that with 75% confidence, there was eternal inflation? And that was about a third, a third of the people who thought that inflation happened. So you combine all these numbers together, you get sort of 25% likelihood of eternal inflation. Moreover, I would claim that if you actually laid out the different possibilities, like, okay, if it wasn't eternal inflation, tell me what it was, you know, and tried to list all those possibilities, that none of those would have a higher probability than eternal inflation. Oh, for sure. So in that sense, I think we can say that the multiverse is the mainstream theory of cosmology. Which doesn't make it right, of course. Doesn't make it right, but it is, does mean we have to take it seriously. Anthony considers the multiverse as mainstream because eternal inflation is believed by a plurality of cosmologists in that no other theory has more adherence, even though the multiverse is not believed by a majority of cosmologists. I count myself among those who hope there is indeed a multiverse. I'd like my reality as grand as possible. But I still see the observational support as inferential, modest, and second order at best. Maybe it's because I'm not a professional cosmologist that my confidence does not rise to that of mainstream cosmologists. That's why I ground myself with those who are less committed, more cautious. I meet physicist Carlo Rovelli, who seeks an integrated theory of quantum gravity. I am skeptical about multiverse. I don't see neither the strong evidence for them, for multi-universe now, nor some compelling reason to go there, and especially it seems to me that we have more urgent problems to solve. I mean, we don't know quantum gravity here in our universe. We see thousands of black holes forming. What happened in the center is a problem of quantum gravity, which has nothing to do with multi-universe. Sure. But we see our own big bang, and something quantum has happened there. I don't see a direct use of the notion of multi-universe so far, and I don't see any nearby possibility of measuring effect of multi-universe. So if some courageous mind or soul want to explore it, it's fine, but I think it's a bit premature. Well, I don't think that should stop us. Multiple universes seem such a fundamental part of the structure of reality. If we're the only universe, what does that mean? Or if there's an infinite number of universes, what does that mean? Either way, I'm just, you know, just terrified. I believe that we are very far from understanding everything about nature. We are very far from the final theory of the world. Can I comprehend the multi-universe, grasp it all in a single frame of thought? I cannot. But occasionally, for a fleeting fraction of a second, I feel almost as if I can, and I shiver. Multiple universes are awe-inspiring and sublime, but are they real? What do cosmologists assert about multiple universes? One, almost everyone accepts that our universe extends beyond that which we see. Two, the most accepted theory by far is eternal inflation, which generates multiple universes by squeezing them off from our universe. But there are skeptics. Three, an increasing number of physicists accept the many-world interpretation of quantum mechanics. I'd be dumbfounded if all those branching worlds were real. But what do I know? The multiverse has two kinds of measurement problems. Theoretically, if the multiverse is infinite, how can we know anything? Because everything possible will happen infinite number of times. Practically, could any evidence be observed that could suggest other universes? Where am I on the multiverse? I say it again. What a way for reality to be. Closer to truth. For complete interviews and for further information, please visit closertotruth.com