JAD ABUMRAD: Hello, I'm Jad Abumrad.

ROBERT KRULWICH: And I'm Robert Krulwich.

JAD: And this is Radiolab, the podcast.

ROBERT: Which means it's not Radiolab the real one.

JAD: No, this is a shorty.

ROBERT: In this podcast we're going to take you to New York city, to the 92nd Street Y where I do a series called Giants of Science. And on one night something very cool happened.

JAD: Was I in the audience?

ROBERT: I thought you were. I thought I saw you there in row three.

JAD: I can't remember about it. Anyhow, tell them.

ROBERT: The guest is—well, I don't have to. Let's just go to the hall, and Robert on tape will take over from here.

ROBERT: The man you are about to meet is. He's a professor at Columbia University of Mathematics. He's also a professor of physics at Columbia University. He's also the director of the Institute for Strings Cosmology and Astroparticle Physics at Columbia University. He's also a best selling author. He's also done work in what's called mirror symmetry, which means I'm not sure what exactly it involves, however, relating two different colabi yau manifolds, relating the conifold to one of the orbifolds, which I'm sure you know all about. He's a Rhodes scholar, he's the author of the Elegant Universe. So basically he's not a dumb person, not at all.

ROBERT: But he does, however, believe the most peculiar things. So if you ask him, and I will, he believes that if you travel from this room, from this auditorium on 92nd Street and Lexington Avenue in pretty much any direction at all. But if you have to go really, really, really far, light years upon light years upon light years out, you will, if you travel far enough, eventually find a galaxy. But that looks, and I mean exactly like our Milky Way. And in that galaxy you will find a planet that is an absolute copy, a dead ringer for our planet Earth. And on that planet you will find a city that is uncannily, down to the hairs on the city rats exactly like this city, New York City. And in that city there will be another 92nd Street YWYMHA with a building that has another auditorium in it that is exactly like this auditorium here.

ROBERT: And if you think that's an unlikely coincidence, I'm just getting started because I think Brian Greene will also tell you that in that distant, distant, distant auditorium on the other side of the cosmos backstage, waiting to come on stage, there is another professor who also teaches both mathematics and physics at a school that just happens to also be called Columbia University, who's also the director for Institute for Strings, Cosmology and Astroparticle Physics, who also just happens to be named Brian Greene with an E on the N like this one, the one out here who happens to have an identical set of also two sisters and the same mother and the same father with the same memories down to what he ate this morning for breakfast, which would not be an egg, because the faraway Brian Greene, like the nearby Brian Greene, are both of them vegans. And we're not favoring vegans tonight, we're just mentioning vegans.

ROBERT: And in these two rooms, these two identical professors with their identical passengers are going to be talking to. Yes, an identical audience watching, which is an exact copy of every one of you sitting here in this room right now with all your histories and all your memories, your exact copies are sitting in that other room on the other side of the universe, including you. You in particular, and you. And if you think no self respecting Columbia University professor who would actually, I don't mean this poetically, but actually believe that there's a copy of us on the other side to make it even crazier, Brian Greene believes there's not only one doppelganger room with a doppelganger Brian Greene and a doppelganger audience. He believes there are countless doubles of us in every direction out there in the great beyond.

ROBERT: And what's more, there are rooms exactly like this one with just you missing or just me missing, or the color of my shirt slightly different. In other words, in a big enough cosmos, in an infinite cosmos, Brian Greene thinks that anything you could imagine, almost anything that can be out there, will be out there, really out there. To which I say, oh, come on. Does he really believe this? And if so, why? So let's ask. And let's welcome the local homegrown version of a man I like to call Brian Greene. So far, so good. Did I get it right?

BRIAN GREENE: More or less. There's an assumption you left out, but we can get to that.

ROBERT: Okay, well, I want to talk about two words, infinite and universe. Let's do infinite. First. We're going to start infinite by discussing what you have in a manuscript you're working on. Now, you've described it as the Imelda Marcos wardrobe problem. So we have a woman here who does not like being seen in the same outfit twice. So tell us how she solved it.

BRIAN GREENE: That's an interesting way to get into the subject. I didn't anticipate that. That is where we would begin. But that's good. The issue that you're referring to is the following. If you have a finite number of distinct ways, in this particular case of dressing, and you have, say, an infinite number of days and evenings on which you have to stand, step out, then it's pretty clear that you're going to have to repeat an outfit once, twice, in fact, innumerable times, because you only, say, had 10 different outfits. Or in the case of Imelda, the example there, I Forget how many.

ROBERT: 500 dresses and 1000 pairs of designer shoes.

BRIAN GREENE: Right, There you go. So you can envision the number of distinct combinations of dresses and shoes. Very easy to figure it out. Just 500 times a thousand. And then if there are more than that many days than that individual that Imelda is going to have to repeat is going to have to repeat a given outfit if there's, say, infinitely many days and only finite number of outfits. Simple idea.

ROBERT: So just to make it simpler still or clearer still, let's do it as a coin toss. The odds against me getting, say, a thousand heads in a row, if it were infinitesimal, but if I kept tossing and tossing, and if I toss infinitely, then, of course I'm going to hit.

BRIAN GREENE: That's right. You do a thousand coin tosses over and over and over again, and look at all of the outcomes. Then even the most strange kind of outcomes will appear, such as all heads. And in fact, if you flip those coins enough times, say, infinitely many times, then that strange outcome will actually appear. Rarely, but infinitely many times.

ROBERT: So now let's just use that logic and apply it to just a different kind of system. Imagine a cosmos which has infinite space. Now, the key thing here, I think, is that the stuff in the cosmos you believe has some kind of finite set of rules. So let's take her. This woman. Are you in the pinkish kind of.

BRIAN GREENE: Yeah.

ROBERT: Red. Red. Okay. So she is a compilation of atoms which follow certain rules. Her memories are, I assume, you think, also atoms.

BRIAN GREENE: I do not. Everybody agrees that every aspect of your personality and your conscious being is reducible to a configuration of the atoms that make up your brain. But I certainly believe that.

ROBERT: And that's dreams, memories.

BRIAN GREENE: That's right. There's nothing else. All you are is a bag of particles acting out the laws of physics. That, to me, is pretty clear, however distasteful, say you.

ROBERT: The kiss of God. We just don't include in this arrangement.

BRIAN GREENE: You can if you'd like. I just don't see any particular need for it. So for this discussion.

ROBERT: Oh, come on. We're going to applaud. So if she is just a large number of atoms in a particular array, but let's say she can. We can travel infinitely and look at all of. And we're talking here what I guess about particles or about forces, or about.

BRIAN GREENE: You can reduce everything basically to particles. That's a fine way of phrasing the whole conversation. And the idea that you're getting at is indeed the fact that if the universe is infinitely big, the rough idea that we can make precise in a moment. If it's infinitely big, then the number of distinct configurations of particles, like the number of distinct possible coin tosses, actually is finite. And if the universe is infinitely big, then just as your coin tosses have to repeat, the configurations of the particles will have to repeat. So you go sufficiently far out there and the configuration of particles will repeat and will indeed make a configuration of particles that looks just like this room.

ROBERT: Looks on the outside, or is. So she had a dream last night about a black cat walking on a fire escape. She is also deeply in love with the man next to her, and they share an enormous number of experiences together. And she's wearing the pink thing, the red thing. So we have a lot of atoms there doing very different things, if indeed they're doing these things at all. You say that that combination, the intimate inside and outside combination, will be found elsewhere again and again and again.

BRIAN GREENE: Yes, I'm actually saying both. I'm saying that the outside will be found with a distinct inside, many places out there, but the outside with the same inside will also be found out there, because again, it's just another configuration of the particles. And since there are only a finite number of distinct possibilities, it's a huge finite number. It's a gargantuan finite number, but it's a finite number. And if you go sufficiently far out, there's just so many distinct configurations that there can be if they're finitely many. So if you go sufficiently far out, you have to repeat.

ROBERT: But won't you run into a quatrillion people almost like her?

BRIAN GREENE: Oh, absolutely. It's much easier to reproduce this woman in the first row with one particle in a different place or two particles in different. Those are much easier because there's less constraint if you want to reproduce this individual completely. Every single particle has to be in the right place. That just means you have to go a little further.

ROBERT: Now you. I know we're speaking here, and this makes perfectly logical sense. It just makes absolutely no practical Sense, obviously. But I'm also wondering whether it makes sense to you as a person. Do you seriously think. Do you literally think that there are two people exactly like us with our dreams, our thoughts in some other stage right now, or whatever you'd call now over and over again right now, all over the universe? Or you just say that.

BRIAN GREENE: Well, there's a key element that we have been assuming and we've been assuming that the universe is infinitely big, and we don't know that the universe is infinitely big. It could be that you go on for some distance and you walk and you walk and you wind up coming back from that direction. Sort of like the surface of the Earth. It has a certain size. It's not infinitely big. It doesn't have an edge. It can't fall off. But nevertheless, when you keep on going, you return to your starting point. It could be that the universe has that kind of a shape. The data seems not to support that at all.

BRIAN GREENE: At the moment, the best astronomical data does not support that. It supports a universe that is flat as opposed to curved. And if that's the case, it's still possible that the universe could be sort of like the surface of the Earth in the sense that it might be like the surface of a video game screen. If anybody who plays Pac man or any of the other video, you know that, you know, when Ms. Pac man falls off the right side of the screen, you know, she doesn't disappear. She comes back on the left side of the screen. That's another possible shape for the universe. It has a name. It's called a torus. And our universe might have that particular shape. Again, it's a finite shape, but you can't fall off. We don't believe you can fall out of the universe. So that would be a way to have a finite universe where you don't fall off. However, the other possibility is simply that it goes on forever and we don't know which is right. The simplest one, mathematically, is that it simply goes on forever. And if that's the shape of the universe, then yes, I fully believe that we are out there in some notion of now having this conversation not once, not twice, infinitely many times.

ROBERT: Well, aside from the deep creepiness. Well, let's talk about the creepiness of that. I mean, there is a notion that we all have that we are unique individuals and that there is a real version of us. In my case, you're looking at him.

BRIAN GREENE: So you just said that a million billion times over in the other universes.

ROBERT: Well, so where does that leave you?

BRIAN GREENE: Do you just feel like personal identity.

ROBERT: Are you copy number418 or ...

BRIAN GREENE: Personal identity in this context is very, very strange. There's literally no difference between me and the other guy out there. And that's okay with me. I don't really have a problem with that. But I do agree that it challenges your sense of what your personal identity is all about. You are the collection of your memories, which are ultimately just a collection of impressions in your brain, just a collection of particle configurations. And if that other individual has the same brain configuration, because the particles are in identical state as they are in my head, that person is as much me as I am me.

ROBERT: So doesn't that automatically make you want to quickly run off the stage, hide under a pillow, do something that the other guy could never find out, and then be different?

BRIAN GREENE: Well, I know that he'll have exactly the same thoughts. It won't help me very much.

ROBERT: But that's really creepy to be chased around by the infinite number of doppelgangers so that everything you will to do, they also will to do simultaneously.

BRIAN GREENE: And there are infinitely others that will to do something slightly different. So there's just a richness to the world, but its complexion is somewhat different than what you had in mind. I think most of us have in mind that if the universe is infinitely big, you keep on going, you'd see new landscapes. Keep on going, you'd see new stuff. New stuff, new stuff. That's our picture of what reality is in that kind of a universe, our universe. And I'm saying that the laws of physics don't support that notion. The laws of physics support, yes, it's a rich world out there, but the distinct landscapes, the distinct environments, the distinct configurations that you can see are limited.

ROBERT: What about how far away is the nearest one to me?

BRIAN GREENE: Well, that's a good question. You can actually calculate that. And as you might imagine, it's a pretty big number, because I'll give you.

ROBERT: A number from a guy named mark Tegmark at MIT. I have no idea what this means. What is 10 to the power? 10 to the 29th meters.

BRIAN GREENE: That's a big number. And you know, you know what it means to take.

ROBERT: It's not like, hi, it's not a.

BRIAN GREENE: Well, just to quickly say what that means. I think you're familiar with what 10 to the two means. It's 100, just a one with two zeros following it. In this particular case, you take 10 to the 29. That's a big number. One with 29 zeros. After it. And then you take 10 to that number, which is that many zeroes following 10 to the 29 zeroes following the one. And it's so big that we don't have a name for, I mean, called the crawl witch, whatever. I mean, it's a huge number. And it just is commensurate with the fact that even though we're saying that there are a finite number of ways that my particles, my electrons, my protons, my quarks, there's a finite number of distinct ways that they can be configured. It's a huge number. So you have to flip that coin many, many, many times before you reproduce that particular sequence, this particular configuration. But in an infinite universe, let me just stress, in an infinite universe, 10 to the 10 to the 29 is really small. I mean, infinity is a hard concept to wrap your mind around. I mean, you take half of infinity, you've got infinity. You take a third of infinity, you take a tenth of infinity, you've got infinity still. So this number, however big it may seem by human standards, is just a little tiny blip in this infinite cosmos.

ROBERT: Well, there was one other assumption that we made, and I just, maybe I should call attention to it. You said that all of the particles that make her up and all of the forces that join those particles are of finite group. Now, how do you know that? All you know is what you can see here, where we are. Why wouldn't you know? Who says?

BRIAN GREENE: Well, again, there are underlying assumptions that the laws of physics that we know about here are the laws of physics that govern the universe everywhere. That's again, a rather simple assumption that we don't know to be true. Certainly our observations of the universe suggest that the parts that we can see visually are governed by the same laws of physics that govern the phenomenon that we can see the world around us. That doesn't prove that if you go sufficiently far out, the laws might change. Simplest assumption is that they don't change. And that's certainly what I am assuming. And then the idea is, you know, perhaps it's slightly confusing focusing on one individual's particles. What we really mean is all of the particles in the observable universe, the part of the universe that can influence itself by virtue of exchanging particles and exchanging forces. This particular part of the universe has a certain large number of particles, and that large number of particles has a large number of possible arrangements, but a finite large number of possible arrangements. So in most of those arrangements, there won't be people at all, because the particles will be haphazardly arranged, and they won't have coalesced into structures that we call humans. In some of those universes, there'll be one lonely human, the only person that's formed out of the particles in that universe. And in a very tiny fraction, there'll be universes that look like ours.

ROBERT: Well, since you've slipped in this plural for universe, now, universe says let's. Because I don't know that I should just let you do that. Let's just do that. When you use the word. When I use the word universe, I use it in the common sense that it includes everything that we know of or that we can imagine. That seems to not be even remotely what you're doing.

BRIAN GREENE: Well, we found that that notion of universe, however all encompassing it sounds, is a little too coarse to accurately describe modern thinking about how the physical universe actually works. And we find it more useful to have words that delineate the universe with finer characteristics than lumping it all together into everything that you can imagine, everything that's out there. So, for instance, we imagine the observable universe. That's the part of the universe that you can, in principle, see. With a sufficiently powerful telescope, with a sufficiently powerful receiver, you'd be able to exchange signals with those parts of the universe. We believe that there are parts of the universe that are just beyond our ability to see. Because there's not enough time since the beginning for light or any other influence to travel from that distant location to us. So that's a part of the universe that's kind of separate from us. It's not within the observable universe. So some people like to think of our observable universe as one contained universe. And those other regions as, in some sense, other universes. They're not that distinct in principle. You can get to them if you travel sufficiently far. They're just regions that are beyond our ability to exchange influence with today in.

ROBERT: A Christopher Columbus sort of way, you get on a boat, you ride to the horizon, you go over the horizon, you keep going, and you see something that no one has seen before that you know.

BRIAN GREENE: Yes, hugely. So, in fact, we use the same word, horizon. So just as you mentioned, Christopher Columbus can look to the horizon and knows that there's something beyond the horizon, but can't actually see it. We look out into the cosmos as far as we can, and we believe that there's stuff beyond what we can see beyond our cosmic horizon.

ROBERT: How much space can we see? What's the observable?

BRIAN GREENE: Roughly 42 billion light years in any given direction.

ROBERT: 42, that's the. That's the entire diameter. I mean, that's all the way across. You turn to the left, you look some number that way, you turn to the right.

BRIAN GREENE: Exactly right. So you look out this way, you can see about 42 billion light years. Again, a light years, about 6 trillion miles. So again, it's far, but it's a finite size.

ROBERT: What is your hunch if you do a Columbus, you go over the horizon and you travel. Oh, no, let's stay. If you're in the observable universe, but you move over 800,000 light years to the right. So can you now see stuff that people from here couldn't see because you're now over there?

BRIAN GREENE: Oh, absolutely.

ROBERT: You have any sense of whether the stuff you're going to see will be America, Big surprise. Or just more of the same?

BRIAN GREENE: It will. In that small motion that you described, say 42 billion plus light years, we believe that will be particles, entities that obey the same laws of physics, that particles and entities in our observable part behave according to the same rules, but they'll be configured somewhat differently. You won't see another America, if that's what you're asking out there, because it's so unlikely. You need to go far enough away that you sample more possible configurations than just one, the neighbor next door. That's as if you said to me, if I take a coin and I flip it once and I get a particular sequence. Heads, tails. Heads, heads, tails. If I flip it a second time, will I get exactly the same sequence? That's unlikely. It's possible, but unlikely. But if I flip that five coin sequence, if I do it a billion times, will I ever get that same sequence? Yes, that's very likely.

ROBERT: So if. Is there a place out there where conceivably what is predictable over here changes out there? Is there any neighborhoodness where the laws actually might.

BRIAN GREENE: Yeah, yes, absolutely. So in fact, when we study modern cosmology with greater intensity, we find that it's probably likely that if you examine the universe from say, a God's eye view. So using that capital U universe that you started with, if you can sort of see in some sense the whole thing, not just what we individual humans have access to, if you can see the whole thing, then you'd probably find, according to a theory called inflationary cosmology that we can discuss if you'd like. But according to this theory, the likely complexion of the universe is that there are a variety of bubbles, bubble universes. Ours is just one of this vast cosmic bubble bath of universes. And in the different bubbles, the laws of physics can appear to be somewhat different from the laws that we are familiar with.

ROBERT: When you say they can be different. So are you talking about all hydrogen but no oxygen, or are you talking about, oh, actually we don't have gravity here. We don't do gravity. We don't actually have a weak force.

BRIAN GREENE: It's more the former. But actually the distinction between those two examples is not as clear cut as you would envision. So, for instance, if you talk about not having the nuclear forces, which is what you were alluding to, it's possible that the nuclear forces can be in our universe as we measure them and in that other bubble universe, but in the other bubble universe, they might be so weak compared to their strength in our universe that they don't have much of an impact. The impact might be infinitesimally small compared to the impact in our world. So for all intents and purposes, there might not be a nuclear force in that universe Compared to the nuclear force in ours.

ROBERT: Well, now this is getting a little complicated. So we're here with a set of rules and a set of operating systems that we have somehow magnificently begun to figure out. And then you say there might be a neighbor where all of the principles that we, that we. That have organized life and existence here may not be operating quite the same way. But I guess I kind of thought that we here eventually attach to them there. So these bubbles you talk of, these.

BRIAN GREENE: Are a different kind of bubble bubbles where we don't attach. So before, when we were talking about the laws of physics here being the same as the laws of physics way out where we can see and then beyond the horizon. Think of that as taking place in one big bubble. When we get to this, that's the easy, ordinary. That's easy ordinary, but. And the thing I really want to stress is these ideas do not come because we don't have enough to think about. And the imagination runs wild when we study cosmological data. And again, I'd be happy to talk about that data, But I'm not sure how detailed we want to get when we study the cosmological data. It leads us almost inexorably to this picture where there are more than one bubble. And these bubble universes are not talking to one another, they are not touching each other. You can't reach the other bubble. You can't traverse the space in between in a meaningful way.

ROBERT: So does that mean that over here, let's say we live Here. So we have. I love this thing. So we are over here. So that's usually. Now, does something come out of this and do it again, or does this happen and then over there your guy happens?

BRIAN GREENE: It's a little hard.

ROBERT: Am I being technically accurate in these questions?

BRIAN GREENE: Let me just say another way of thinking about it that may help, and then we can sort of come back to that if you think that will clarify things. The way I like, I thought I.

ROBERT: Was startlingly clear myself.

BRIAN GREENE: Yeah. The way I like to think about it is this picture, this inflationary picture. It's as if the universe is a big block of Swiss cheese. And I'll just stop right there. Thank you. It's like a big block of Swiss cheese where the holes are the universes that we're talking about. So everything that we know about is one hole in the cosmic Swiss cheese. But there are many other holes in the cosmic Swiss cheese. And the point is, when we study this picture that emerges from studying the cosmological data, not from an overworked imagination, we find that the meaty part of the cheese. Sort of a bad vegan metaphor there, I guess. But, you know, the white part of the cheese expands so quickly that the bubbles, the holes in the cheese, are pushed apart at an enormously fast rate, faster than the speed of light. And that's what prevents one from even, in principle, going from one hole, one bubble, our universe, to another hole, another bubble. So our universe and the other universe is. Are all embedded in this environment, this cosmic Swiss cheese. But we can't traverse the cheesy part because it's growing so enormously quickly. But.

ROBERT: Oh, dear. See, now, let's go with your metaphor here. So we have this big Swiss cheese of a mega. Something of the place.

BRIAN GREENE: Multiverse is actually the word that. Yes.

ROBERT: So now I. We're inside. We're in a space. So this is your cheese hole now.

BRIAN GREENE: Yes.

ROBERT: Okay. So I'm living in there, you know, and my universe is getting bigger and bigger. So this Swiss cheese has a hole that is growing.

BRIAN GREENE: The hole is growing larger.

ROBERT: Hole is growing. And you're saying that the cheese.

BRIAN GREENE: But the right part of the cheese is also growing and it's growing faster than the rate of which that hole is growing. So there's bigger holes, but more cheese.

ROBERT: So if you're in your. This particular evening is brought to you by the American Dairy association making the popcorn. And we'll be doing it in Wisconsin next week. So the reason I can't talk to you if you're in your hole And I'm in. Mine is, I'm here growing, you're over there growing. But the space between us, we would be flying to either side of this stage. Yes. Faster than the speed of light, you say?

BRIAN GREENE: Yes, faster than the speed of light. Sometimes people say, well, how's that possible? Nothing goes faster than the speed of light. And the thing to remember is that when Einstein established that nothing goes faster than the speed of light, what he really established is that nothing can traverse space at a speed greater than the speed of light. But his equations full well allow space itself to grow at any speed bigger than the speed of light is perfectly fine.

ROBERT: Okay, I'm going to have to keep summing up because otherwise I'm going to get completely lost. So the universe then is a series of expanding empty spaces inside a web of some kind of. We'll call it the big cheese. And the webs are growing. What I don't still understand is where did these holes come from in Swiss cheese? I have no idea where those holes come from either. I consider this the dual hole problem myself. What could you address, please? The spatial holes. And we'll leave the dairy holes for another occasion.

BRIAN GREENE: Yeah. So in this version of cosmology, again, it's called inflationary cosmology, what happens is in the beginning, whatever that means, space is filled with something that we call a field. An inflaton field is the name of it. And a field is very much like an electromagnetic field. I think we're all pretty much familiar with that idea that there are electromagnetic waves going through this room. That's how you can use your cell phone to talk to somebody far away. The electromagnetic waves are suffusing space. Those electromagnetic waves are fields. Electromagnetic fields. This inflationary theory invokes a new field, not the electromagnetic field that fills all of space. And it has the amazing property that by virtue of filling space, it causes space to be filled with repulsive gravity. That's what this field does. Repulsive gravity pushes everything apart.

ROBERT: So this is gravity. Normal gravity brings things together. This is the ugly twin.

BRIAN GREENE: That's right.

ROBERT: Get out of my way.

BRIAN GREENE: That's right. And Einstein himself is the one who found this. This is an old idea of repulsive gravity. It goes back to 1917, even though it's not widely appreciated that gravity can be repulsive. And in this environment of this field filling space, gravity is repulsive, drives space apart. As space is driven apart, it turns out that there are regions where this field drops to zero value. It quantum fluctuates to zero value. And those are the holes. So we, our universe is simply a fluctuation in this field where it had a high value and it dropped to a low value, thankfully then giving rise to the universe that we are familiar with. And let me just quickly say, as it drops to zero, it gives up its energy to the familiar particles that make up you and me and the world. So all the particles that we know about in this version of cosmology come from this field, dropping down to zero, releasing its energy to particles that give rise to the stuff that we are familiar with. Now that field can drop to zero all over the place, giving rise to a bubble universe here, a bubble universe there, a bubble universe there. But in between, the field still has a finite value driving space to expand.

ROBERT: Well, I understood about 5% of that, but maybe 10. So let me say it back to you and tell me how close I am. So under at the. Underneath everything, there's this thing. Sometimes it gets hot, sometimes it gets cold, it's in flux anyway. And sometimes it spits out this space, get out of my way and creates space. And sometimes it just, I don't know, goes home and has a sandwich or something. So sometimes you get an explosion of nothing which then melts down into us. And sometimes you don't get very much at all. So you just. And there's this sort of thing underneath everything that's doing it.

BRIAN GREENE: As long as by thing underneath everything, you mean the region of the universe in which this field has a repulsive gravity effect and drives space to expand. So the cheesy parts of the Swiss cheese are the places where the field has a finite value. And the holes in the Swiss cheese are where the field has dropped to zero value.

ROBERT: Okay, so this leaves me then infinity. I get on the universe side. What we end up with then is very distinctly different neighborhoods. Each of these bubbles can have its own character. So some of them, you could have atoms that come together happily and form chains and then form us and then even give rise to intelligent things and other things. You just get like eh, sand or I don't know, it would be really boring for us.

BRIAN GREENE: Yeah, yeah, exactly.

ROBERT: So then the universe has got. Is this an infinite universe, this multiverse?

BRIAN GREENE: Well, that's a good question. So there are a number of kinds of infinities that now come into the story. Number one, you can ask, how many different bubbles are there? How many different holes in the cosmic Swiss cheese are there? And indeed this is a process that we believe to be eternal. And therefore there would actually be an infinite number of these little Bubbles with our bubble being just one. And now you might say, but wait a second. You started off this conversation talking about space that we have access to going on infinitely far. If our universe is a bubble, no matter how big, it doesn't go on infinitely far. It has some size to it. And in a way that I urge you not to try to describe in any detail here, because it's pretty tough. To someone on the outside, it looks like this bubble has finite size. To anybody on the inside, it looks like it has infinite size. So the conversation that we had 15 minutes ago about a universe that goes on forever applies here, even to each of those individual bubbles. So in each bubble, space goes on infinity. So there are an infinite number of copies of us having this conversation inside our. Inside a single one of these bubbles. And then you got infinite number of bubbles where anything else can be going on.

ROBERT: Could there be.

BRIAN GREENE: Again, there's a danger. There's a danger that people say, oh, my God, this is so far out. And nuts. This is.

ROBERT: It's pretty far out.

BRIAN GREENE: And this is a very minimal, a very minimal approach to describing observations that we have. Observations of the cosmic microwave background radiation, observations of the early universe by looking deeply into space can only be understood, largely speaking, in the context of the frameworks that we're discussing here.

ROBERT: You're saying to me that all these nutty ideas come from very, very serious premises.

BRIAN GREENE: Yes. They don't come from people saying, let's think up a nutty idea and examine its consequences. They come from people saying, let's do measurements of the universe and try to come up with theories that explain those measurements.

ROBERT: I'm not even going to ask you what measurements those were.

BRIAN GREENE: No, literally. That's not hard to describe. It's literally measurements of the temperature of radiation that we believe is left over from the early stages of our bubble universe. You measure the temperature over there, the temperature over there, and so forth. And you get certain numbers and you try to explain them with theoretical equations. And the framework that is able to best describe them winds up entailing all of the additional structure that we're talking about. It comes along with the theory. It's not something that you put in from the outside.

ROBERT: Well, I'm going to get as weird as I know how to get here based on what you've told me before. In addition, imagining now a universe in which there's a bubble, in which we have carbon and hydrogen and oxygen and all the forces that we're familiar with, and we have planets and mass and clouds of gas and you and me, over and over and over again, that universe. And then we have another universe not too far over in that direction in which very little happens, another one in which we get halfway there, another one which very odd things happen. What about this? One of these bubble universes produces super intelligent computer geeks, intelligent beings. And what they do is they do the Matrix. They come up with a supercomputer, fabulous supercomputer, that creates a universe so spectacularly realistic that the critters in that phony fake universe think they're alive. If you can make give me our universe and weird universes and weirder universes, can you eliminate the possibility of totally fake universes?

BRIAN GREENE: Not only can't you eliminate it, it's actually quite likely. I mean, you see in our universe, just using that as a benchmark, that we're getting closer and closer to being able to do exactly what you're talking about. Imagine technology 1,000 or 10,000 years or 100,000 years down the road. One presumes that if progress continues apace, that we will be able to create the kind of simulations, computer simulations, that to those bits that are being manipulated in that simulation appears to have a reality. In much the same way that you're familiar from the Matrix, you anticipate that that's likely to happen. In fact, you can go one step further just to make sure.

ROBERT: So the people in this fictive universe would be creatures of fiction made up by a computer program, but the fiction would be so specific, so beautifully detailed, that these creatures, fictional creatures in someone's mind through a computer, would think they were alive, would think they fell in love, would think that they were having sandwiches, would think they wanted to be vegans. Sure.

BRIAN GREENE: But bear in mind that it has nothing specific to do with any of the work that we physicists and string theorists and cosmologists are working on. I'm going to take that as a given that the natural progression of technology can lead to that kind of an outcome. I have no particular expertise to say that that will happen, but a glance at the trajectory of technology suggests that that's a possibility. Now, the place where the ideas that we're talking about come in is the following. If you ask yourself the following question, Is it more likely that if I'm a being that sees a reality, is it more likely that's a real reality that came from the universe creating it as, say, one of these bubbles? Or is it more likely that I'm in a simulation? And all we're talking about here leads to a rather Interesting conclusion, because for the universe to create a reality that we're familiar with is a relatively unlikely course of events. Because the universe is creating all these bubbles. The particles are coming together in one way or another, randomly joined together according to the laws of physics. And only in some real tiny fraction will the configurations give rise to human beings, say, that see a world akin to the one that we see. That's rare. In one of those bubbles, though, if you do have these super intelligent beings, maybe they're not even super intelligent, just super expert at dealing with the technology of their time. If they can create simulated universes, it's much more direct route to a reality that we would consider akin to what we experience. It's not random. It's individuals actually creating the simulation. In fact, they can create simulation after simulation after simulation after simulation. So in that universe, there are many, many, many simulations, simulated worlds that beings in those worlds consider real. And therefore from that perspective, there are more simulated worlds that appear like the one that we see around us than there are real ones.

ROBERT: Oh, no. So the chances of us being in a naturally occurring real world might be smaller than the chances that we belong to the fifth grade project of a geeky kid in the planet Xantar. I don't know how you live with this stuff, and we've only just sat down and you've already told me that we are doppelgangered up the gazoo. We therefore seem to have no real identity. Free will is an open question, and we're probably a fake. Why didn't you want to become like a soccer player or something more normal? Actually, it's a serious question. Do you actually, assuming that you believe these things to be true, doesn't it get you down?

BRIAN GREENE: No, I think it's incredibly exciting. I mean, to me, the most wondrous thing about science and physics in particular, is the fact that through the power of thought and calculation and observation, you can be led to conclusions vastly at odds with what you would think based upon experience. I don't think there's anything more wondrous in that moment when you think the world is one way. And your equations, your math, your ideas, your theories begin to convince you that it is another way. And we've seen this played out in the history of science over and over again.

BRIAN GREENE: I mean, the gentleman developing quantum mechanics in the 20s and 30s, from Heisenberg to Bohr to Schrodinger and so forth, they began by thinking that the world was basically a Newtonian world, a clockwork universe as it was called, where Things evolve according to a very precise set of laws. You tell me how things are today and the clockwork universe tells you how they will be tomorrow in a generation. They realized that that picture, however compelling, was wrong. They realized that the world arises and develops according to probabilistic rules where the best you can do is say it's a 30% chance that things will be this way in an hour, 4% chance of that and 22% of that. That was such a paradigm shaking way of thinking about reality. It's not something that gets you down, it's something that pumps you up. BRIAN GREENE: And if we are now perhaps learning that the very nature of reality at least needs to be questioned, it doesn't in any way establish that we are in some simulation because it's more likely, perhaps, that we are in one. We could still be in a real universe. But to me, it's wonderful to get your sense of reality kicked, to get a kick in the head, and to at least be able to contemplate the real possibility that the laws of physics are suggesting that reality is not what you think it is.

ROBERT: Although if you're seeking to be pleased by what you see, if you want to feel joy, I'll give you a joyous image. If I shoot a cannonball into the air, Newton and all physicists can tell me if they knew enough exactly where the cannonball is going to go. It's fixed and it's understood, and the cannonball has no free will in this matter. If you know well, the image I like to think is if I, or if I took a dead bird and threw it in the air, it would fall predictably to a place on the ground. But if I took a live bird and threw it up in the air.

BRIAN GREENE: I could in principle predict where it will land.

ROBERT: Oh, but you'd have to know so much about.

BRIAN GREENE: I need to know so much. But that's a fine detail. That's the fine print you, I think, have in mind that the bird has some free will to go where it wants to go. And as far as I certainly can tell, there is no place for free will in our understanding of the laws of physics, however diminishing that may make some people feel. I think we need to perhaps face up to the real possibility that the notion of having free will is an illusion. It's a useful illusion. It makes life interesting, and it's one that you need to put to the side because you need to live as though you do have free will. But all there is is physics.

ROBERT: Don't you There are a lot of people, Paul Steinhardt, David Gross, Martin Gardner, who have some real problems with this multiverse concept. And I'm just going to run a few of them by you. One of them is to conjure up a universe in which everything that could happen is happening. You know, so there are lots of Brians and Roberts, and there are lots of almost Brians and Roberts and there are lots of very almost and post almost and all this kind of thing. Doesn't that break one of the basic rules of science that you know, the keep it simple rule, Occam's razor. I mean if you can, or I'll just quote Martin Gardner. Surely the conjecture that there's just one, one universe and its creator is infinitely simpler and easier to believe than there are countless billions upon billions of worlds.

BRIAN GREENE: It does sound simpler in some ways. And I have to say I'm going to give you two answers to this issue. If it turns out that our future understanding eliminates this idea of parallel universes, multiverse theory and winds up showing, ah, the way to think about it is this. There's one universe, we can calculate everything. It's uniquely determined by the laws of physics. I think I'd be happier with that outcome. So don't get the misunderstanding that I have some emotional connection to this idea. It's where we're naturally led by a variety of interesting ideas and therefore needs to be taken seriously. But in terms of it being simpler to have one universe versus many, it really depends on how you define simpler, right? Is it simpler to have one number versus many or all numbers? In fact, if I give you a 22 digit number, just one 22 digit number, and I ask you to repeat it to me versus I simply say think of all numbers and just count them all for me. Which tasks do you think you will succeed at?

ROBERT: Oh come on. You make 22 digit. How about working this way?

BRIAN GREENE: No, no, no.

ROBERT: Give me a two digit number or give me a.

BRIAN GREENE: But that's the thing. Our universe is not characterized by a two digit number. Our universe is characterized by a great number of distinct features. The mass of the electron, the mass of the quark, the strength of gravity, the strength of the electromagnetic force, the gauge symmetry of the standard model, the equations of general relativity, and on and on it goes, a long list of ingredients that need to be just so for the universe as we know it to appear. You get the mass of the electron a little bit wrong. When you repeat it to me, the world is different. You get the strength of gravity a little bit Off. When you repeat it to me, the world is completely different. So there really is a tight analogy. There's a long number, if you will, that characterizes this data versus there being all universes versus there being all numbers. And in some sense, it's much easier from a complexity standpoint to describe all numbers than it is to describe this one particular 22 or 44 digit number. In fact, you can make this precise. There's a notion of algorithmic complexity, which is the amount of information necessary to, say, give to a computer to spit back a piece of data that you're interested in. Now, I can write a very short computer program that will spin, spit out 1, 2, 3, 4, 5, 6, and 1 line computer code. If I want to spit out some particular number, well, I may need to actually type in every single digit of that number for it to spit it back to me. That is greater complexity, not less complexity. And I'm only trying to emphasize the fact that even though you think of one universe as being more simple than many, that reasoning need not actually hold up under close scrutiny.

ROBERT: So just so I give you your due, you're saying that, and I'm not even going to ask you what the problems are, but serious people trying to figure out things that they can measure and things that they can see and things that they can examine when they look to the math, because I look behind the math, leads them to these, to this multiple universe concept directly or over and over and from many directions.

BRIAN GREENE: From many different directions, from classical physics, from quantum physics. There are a variety of different ways of having multiple universes. We've actually described two tonight. The idea that if you go sufficiently far out, the universe repeats as if there are many worlds out there much like ours. Or there can be these many bubbles where there are many universes out there that may be like ours or not. And these are naturally coming from simple ideas that we allow to run their course. I should emphasize this is not a certainty by any means. Physics, science, we all know, is a dynamic subject where new ideas come in and force us to reevaluate and to rethink things. And I would be thrilled if ultimately we do find that there's one unique universe and we can sit down and calculate everything about it. That would be thrilling. But it's also thrilling to imagine that that isn't how the world is really put together and that the data that we now have and the ideas that we're coming up with may be revealing the true underlying reality.

ROBERT: All right, so we're left with a bunch of choices. We are infinitely duplicatable. We live in cheese. In expanding wholes. We might be a reflection of a computer. We might be a pale reflection of mathematics. We might be actually made of real stuff in the here and now. Or not.*

ROBERT: That was professor Brian Greene in conversation with ...

JAD: You.

ROBERT: Me. [laughs]

JAD: This recording was generously provided by the 92nd Street Y, and they have more of their lecture series available at their website, www.92Y.org. You can also check out our website while you're online, www.Radiolab.org.

JAD: Radiolab is funded by the Alfred P. Sloan Foundation.

ROBERT: Or Al Sloan, as I like to call him.

JAD: Exactly. The Corporation for Public Broadcasting and the National Science Foundation. I'm Jad Abumrad.

ROBERT: And I'm Robert Krulwich.

JAD: Thanks for listening.

 

-30-

 

Copyright © 2024 New York Public Radio. All rights reserved. Visit our website terms of use at www.wnyc.org for further information.

 

New York Public Radio transcripts are created on a rush deadline, often by contractors. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of programming is the audio record.