and you go to vietnam, and it's becoming like unclear, so how do we measure success? it devolved into body count. and i am clear in my own mind that body count is a very bad measure of success. first of all, it's immoral. the warrior's job is to stop the other side from using violence, and when that other side stops doing it, then you're done. and the job is not to kill the other side, you sometimes have to kill the people on the other side to disswied them from doing what they're doing, that's the bad part of it, but the object i should not be -- objective should not be killing people, it's just inhumane. >> booktv.org. >> and now, brian greene talks about the possibility of multiple universes and explains how this would change our

understanding of reality. >> well, good evening. it's really a special pleasure and honor for me to welcome brian greeb to our -- greene to our fair city. and before we start talking about other universes, why don't we start talking about you. i know a lot of people would like to know some personal details about you. i understand you're a vegan. [laughter] >> yes. in this universe, i am. that's true. >> you stole my next question. >> i'm sorry. >> your doppelganger is a mediator. >> according to our understanding, that's quite possible. >> i was on an airplane just a few days ago, actually coming from london, and a woman next to me, i ordered vegetarian, she said, would you be offended if i ate meat? i don't care what you eat. anyway, i see you're offended by your doppelganger. >> but he doesn't sit next to me

on airplanes, so it all works out. >> if it's the kind i'm thinking, you both appear in an airplane. >> that's possible too. >> so tell me something else. i understand you have a number called, what is it -- >> [inaudible] >> yes. can you explain about that? >> um, yeah. you know, there's this idea of how many degrees of separation you are from famous people. so the original one was how far away a given actor was from kevin bacon. and then mathematicians wanted to compete and have their own version of kevin bacon which is paul err attorney who collaborated with many, many mathematicians. so the question is how far are you away from having written a paper with air coach. and then people say, let's put it all together and see how far a person is away from kevin bacon and air coach. and there are aren't too many

close to both, but there are a handful of them. >> how many are you? >> well, i used to be the world leader. >> what's your number? >> number five. >> but i've been overtaken. >> you're five and what? >> two from erdos and three from bacon or something like that. but i think, like, gwen earth paltrow has taken over the lead. she wrote a paper with -- no, i'm not sure. but there are definitely people who are taken over. >> i see. but in another universe, you have number one. >> that's always going to be case, yes. >> so what is this -- let me ask you, we all think that there is one universe. how could there be more? >> yeah. well, that is the essential question to start with. because, you know, a long time ago, you know, two years ago -- [laughter] um, the word universe meant everything, the totality, every star, every galaxy, the whole

she bang. so what sense could there possibly be in having more than one everything? and what we have found in research that actually dates back a number of decades but most vigorously relatively recently is that our mathematical investigations, our suggestion that what we have thought to be everything may actually be a tiny part of a much grander cosmos. and that grander cosmos can contain other realms that seem to rightly be called universe just as our realm has been called universe which means that you have many universes, multiple universes which we call them multiverse. >> sounds like a brand of cereal to me. [laughter] multicereal. >> you have a food thing going on here, don't you? is. [laughter] >> so tell me, i, i understand that physicses is a science, experimental science. >> yes. >> so where does this come in?

sounds more like a religion to me. i mean, there's this universe and another universe, i mean, how do we learn about these other universes? >> yes. so how can you gain confidence in an idea that speaks of realms that we can't see, that we can't touch, we can't visit, we can't observe directly? so let me give you the answer in two parts. one is in some versions of the multiverse, and i should emphasize there's not one proposal for how there might be one universe, in some there can be subtle questions between the universes that might allow us to have some experimental window onto them. but hold that to the side for the moment. let's think about the ones where you couldn't visit them. well, why do we think about these things? well, we have a belief found fod upon really hundreds of years of experience that mass can provide a gateway to reality. it can provide a window onto a reality that at the moment the math is being done, we can't

actually see or observe that reality. i mean, instein is the greatest -- einstein is the greatest example, right? he wrote down the theory of relativity. others looked at those equations and said the universe was expanding. einstein himself said, no, i don't believe that, but 12 years later the math was confirmed by observations. other examples are black holes. einstein didn't believe it, observationings now show that there are black holes. so we're following in that tradition. we are doing mathematical equations, following them, and as we can discuss in some specific cases they are leading us root by root to the possibility that ours is only one universe. does that mean the math is right? we don't know. it has to be confirmed, ultimately, through some kind of observation or experiment. but the possibility that the

math is revealing this new picture of reality is sufficiently compelling that many physicists, including me, are taking it seriously and investigating it vigorously. >> but i think the operational word here was can. because mathematics is not physics. >> exactly. >> and so sometimes the mathematics works, and sometimes it doesn't. you don't have to go very far, but you could say that end by cycles were invented by a grief mathematician, and tolmy argued -- doesn't describe reality. and you can go to later on, for example, -- >> before you leave that example -- >> sure. >> because i think that is a great example where you had some individuals that were looking at the motion of the earth and the

motion of the planets and coming to certain conclusions that we now know to be erroneous about how things were working. there were other physicists, mathematicians who looked at that math and said this is so complicated, this is so convoluted, and if we look at the math this way, it all simplifies. but the conclusion is that the earth is not the center. so we were propelled by mathematical investigation to imagine the earth is not the center. and then others using similar kinds of reasoning noted that the sun is actually not the center either. and then similar mathematical reasoning showed us our galaxy is not the center, it's one of many, many galaxies. we've gone through a sequence of cosmic demotion. we may be on the threshold of the next demotion by following exactly the same pattern. earth is not the center, sun is not the center, galaxy is not the center, our universe may not be the center. it may be one of many universes following exactly the same

pattern. >> i think the key is that mathematics is always simpler, in a sense -- >> that's certainly what we have found. >> but when you do very complicated mathematics and you trust your equations, often these equations are cumbersome. >> i wouldn't say so. i mean, i can understand where you might come to the cop collusion because -- conclusion because, you know, some of the multiverse ideas come from string theory which seems like a complicated subject when you hear about its features, but when you look at the starting point, it's actually pretty simple. >> how many strength theories are there? >> there's one now. there was a time when we thought there were a handful of distinct string theories, but wonderfully in the last decade we've realized what we thought were different theories are all the same, just express inside a slightly different language. so everything has been simplifying. you know, if you take, a good example, darwinian evolution. the principles of evolution are pretty straightforward, right? nevertheless, those principles

can yield the rich variety of life that we see on earth. the outcome can be complicated even though the starting point is simple. that is the way i would characterize our thinking about certain modern physical theories. the outcome, string theory if we get into it, extra dimensions, vibrating strings, it seems complicated, but this' like the richness of -- but that's like the richness of life. the starting point of evolution pretty straightforward. >> i see. so tell me, what are some of these theories that lead to the multiverse? in your book you describe several of them. i couldn't find one with the anti-universe, that's my favorite, actually, where your anti person is, you know, positive terror's an anti -- >> star trek there. >> so do you favor that route to the multiverse? >> well, there are many ways to the multiverse. maybe a good place to start would be what i consider the

simplest route of all which is to imagine the possibility that space goes on infinitely far, right? if you were to get into a rocket ship and head out into the cosmos, would you at some point hit a brick wall? no, most of us don't think that's the case. would you circle back to your starting point like what would happen on earth's is surface if you took a still journey? that's possible. or would you simply keep on going forever? let's take that third possibility seriously. if we do, there's a startling conclusion, and it's simply this: in any finite region of space, matter can only arrange itself infinitely -- in finitely -- >> large number but finite. similar if i shuffle the deck the order the cards differ, there are many different ores but still -- orders but still finitely. so if i shuffle the deck enough times, the order of the cards has to repeat. similarly, in in infinite space

the configuration of particles had to repeat too. you and i were just a configuration of particles, everybody in this room is just a configuration of the particles as is the earth and the sun and so forth. if configuration repeats some place out there in the cosmos, it means all we know is repeating. and that's a very straightforward. mathematical conclusion. >> right. but you're leaving out an important thing. the measure of that is zero when you go to infinity. >> it doesn't matter. >> no, the probability of us speaking in another universe is -- do you want to go there? >> oh, absolutely. [laughter] in fact, i don't need to frame it that way, let me do it in a concrete setting. if i had that deck of cards and i shuffled it over and over again, do you agree that sooner or later the order of the cards -- >> the deck is too large. t a really big deck. it's a really big deck.

that's easy. i'm talking about the universe, not a deck of cards. >> no, you're discounting the power of infinity. infinite space, this is the supposition. now, you can challenge that, but let's not. if you take onboard this idea that space goes on infinitely far -- >> no. >> -- then you've got a lot of room for this to happen. [laughter] and that's the point. >> i have a problem with space going infinitely far. >> that's a good place to try to poke a hole in it. >> in physics, the way i understand physics, these three dimensions in which we live in the force of time which einstein taught us is related to the other three, was created in the big bang. so i think if you think of the physicists, and check me if i'm wrong, space here was created in the big bang. we're not expanding into another space, we're creating space as we're going out. as the galaxies are expanding

with 13.7 billion years and so on. we are creating free space. so where is the other universe when -- as a mathematician, okay, this dimension goes on forever, call it x and this y and this z. >> yes. >> and i think in one of your universes, you've got up with here and one here and one here, and there are infinitely many of them, and that's okay, but does it really exist on a physical point of view when space and time were create inside the big bang? -- created in the big bang? >> so i do need to correct you a little bit with all due respect. >> sure. that's what i'm here for. [laughter] >> so there is an incorrect image that many people have in mind which is this: when we think about the big bang, typically we imagine that further and further back in time the entire cosmos was smaller and smaller and smaller, and way back toward the beginning the universe we sort of intuitively think of as very, very small and run that film forward and as you're saying space is created

from that big bang, so how could it be infinitely big if it was very small in the past? and if that was the right picture, you would be right. but that's the not the picture that's compatible with an infinite universe. as you head farther back in time, the universe is still infinitely big. if you go back in this heym and -- time, half of infinity is still infinity. one-third of infinity is -- >> what does it mean here? >> the traditional one. >> the universe is inmy snit. >> that it's infinitely big, yes. >> so what's the -- >> ah, that's the observable universe. >> so the universe goes beyond -- >> that's the key point. >> what is the big bang? >> the big bang is an event that gave rise to our realm, but if universe is infinitely big, then our part, the part we have access to is only a piece of the entirety. >> and the others are expanding

as well? >> exactly. so you need to make a distinction between the observable universe and the entirety. the observable universe is just the part we can see, and you're right, we can't see back than roughly 13.7 billion light years because that's the amount of distance light can travel from the beginning. almost nobody belief that is the universe ends at that point. most everyone believes it goes on at least a far distance beyond that, and the supposition is that it goes on infinitely far. >> brian, you're a magician. you pulled infinity out of the hat. [laughter] what does infinity have to do -- everything we learned about physics is finite. what does infinity mean? the integers or the continuum or the space of functions? i mean, to invoke infinity, you have to give me something. >> yes. and the most straightforward

analysis is the real line extended in exactly the way you know about from when you took. matt:ices at the young age. let me ask you this. if universe is not infinitely big, what happens when you travel out? >> i interviewed stephen weinberg a few months ago, and i asked him, the big bang is believed to be a quantum fluctuation, that's what created our universe. what was it a quantum fluctuation in? what was the medium in which we respond, if you are? if you will? and he said, that we don't know. we can't go there. >> oh, no. >> but you're telling me something else. you're telling me that there's an infin tuesday of space. mathematically, i agree with you. the real line exists, but it exist la on platonically -- >> no, i'm asking a concrete question. if you build a spaceship, and you go out and keep on going, what happens? >> well, if i take physics the

way physicses has been done, here's the big bang, it started here. but it has no, there's no -- location has no meaning. you can't define that point as being located in space because space doesn't exist before the big bang. i don't know about other universes. so if you start here, this space was create with the the big bang -- >> if you go into a rocketship and keep on going, what happens? [laughter] >> you know this, you can't. >> what are the possibilities? do you hit an end? do you cycle back to your starting point? >> you aim a telescope in this direction at night, and you aim a telescope in that direction at night, the two parts -- the farthest galaxies you can see, they're receding at a speed faster than light because of the expansion of the universe. you don't even need acceleration. so that part doesn't talk to this part. how are you ever going to get to one part from another with a spaceship that travels less than

the speed of light? >> so if you get in that ship, what will happen? >> where i don't know what will happen. i'd be lost in space. [laughter] >> so it's a mathematical question which in math language would be what's the overall topology. >> that's where i disagree with you. >> i think it exists in a mathematician's mind as a platonic orb there -- or thingst have nothing to do with the real world. mathematics is not fedsics. >> good. >> a lot of mathematics here doesn't do anything -- i'll give you an example. >> yes. >> we talked about those ep icycles. i'll tick your glass -- take your glass and give you -- >> you take it. i'll just drink out of this. [laughter] >> so we've got one of the

fathers of quantum mechanics in the '20s. he built this theory, everybody knows about the uncertainty principle and maybe about matrix mechanics, and then he went a step further. he thought he was going to go into something else, and he said here's a proon the -- and i need three ice cubes here and three -- anyway, so we've got a proton and a neutron. i'm going to use the mathematics of symmetry to explain why these two are so similar, and he calls it su ii which, of course, you know. and that assumption was wrong. that was taking mathematics that makes a lot of sense in your mind as a mathematician but has nothing to do with the real world in the sense of the proton and electron. they worked very similar because of a accident of nature. a lot of them is a lot heavier than the other is absolutely terms, but when compared to the

math of the two, then you think they're really very, very similar. and he went into symmetry. now, of course, we know later all kinds of thing were done, and the mathematics sort of came back. but at that moment what you have as mathematics is very powerful and absolutely useless. >> i rest my case. [laughter] there are but it's a case i agree with. what i would say is mathematics opens up the realm of possibilities. thank you very much. and what the art of physics is being able to sniff out which mathematics is relevant for reality and which mathematics isn't. now, experiments and observation are a key part of that story, and the one you just mentioned, ultimately, it was an observation experiment that kick tated -- dictated that math wasn't the right direction to go. so what we need to do and we spend our professional live

doing is trying to understand which body of mathematics is relevant and which isn't. now n this particular case that we're talking about, the argument makes the assumption that a certain body of mathematics, space can go on infinitely far, is relevant to reality. if that's right -- and it may not be -- but if it is, you come to this startling conclusion. if it's not, then you don't. and i think that's the mode of thinking about these multiverse proposals. many of them start with a certain mathematical framework, push the math as far as we can to the border of understanding, and then using that to look over thize season and see what's there. are we seeing reality, or are we seeing mathematical ideas? that's a question, ultimately, that has to be confirmed or disputed by observation. now, let me just give you an example where that mode could helps us here. so people have asked themselves, if space doesn't go on infinitely far.

could we, perhaps, observationally establish that? that would be a nice thing to do. if it doesn't go infinitely far and it does have the shape like the surface of the earth where it comes back on itself, then, as you know, there are structures in space that give off light, if universe has that shape light can also pass by us, circle around the universe and come back a second time or a third time. so if you can see multiple copies of a given object, that would be a nice piece of observational evidence showing that space is finite. no such evidence yet. could be big, so big that it hasn't had time to cycle around. but that's exactly what physics is about, you know, doing mathematical calculations, pushing to the limit and then trying to find observational tests. >> right. so tell us about some of these specific theories. let's start with the one i dislike the most. >> yes. [laughter] >> how about the many worlds?

>> oh, many worlds. >> i can't even say. >> many worlds is a somewhat different character of proposal for how we can be one of many universes, and you may note that in the book it's actually one of the later chapters because -- >> yeah, i was worried about that. >> why is it later? you're right, chronologically earliest, that's right. because i think in thinking about this subject marching through the developments chronologically doesn't give you the most pedagogically sensible way of thinking about where we are today because the many worlds of quantum mechanics stands outside the chronological martha ends up with string theory. but it is an interesting proposal, and that's why i have a chapter devoted to it. >> it's weird. >> you're right, it is weird. and in that chapter i basically come to conclusion that i don't think it works. >> whew. [laughter] >> but that doesn't mean it doesn't, and if you were talking to other people like david

deutsche from oxford or various other researchers, david wallace, they would say it absolutely does work. so i don't want to give the wrong consensus -- idea that there's a consensus. so the idea of quantum mechanics whereas newton said tell me how things are today, and i will predict how they will be tomorrow, the universe is like a giant clockwork, i'll use my mathematics to turn the crank forward and predict how things will be, and it established that thinking of things was very accurate when applied to everyday omits like glasses -- objects like glasses. newton can tell you what will happen. you do the observation, and it does happen. great. when people began to probe the microscopic realm, that whole structure began to fall apart. >> different universe there. >> completely different universe. completely different realm. let's not use the word universe in too many different ways

tonight. [laughter] and in some ways, why should the laws that work on everyday scales also work on tiny scales? and it turns out that they don't. the laws of quantum physics, and the new idea is that you can only predict the likelihood, the probability of one outcome or another. so if i'm not dealing with iraq or the moon, but an electron and i want to know where it is, the logs may show there's a 50% chance over here, a 50% chance over there -- >> or both. >> well, no, the 50% chance of each, and you can't do any better than that according to quantum physics. now, the weird thing is when you do enough observation of the electron, you always find it either here or there. you never find it sort of half here, half there, there's never some sort of melding of the two. so the puzzing has been for -- puzzle has been even though results are confirmed by doing

an ec appreciate over and over -- experiment over and over again, how do you go from the fuzzy, hazy problem listic mathematics of quantum theory to the single, definite reality that we observe when we do an experiment? nobody has answered this question yet, shockingly. it's 2011. but the proposal that comes from the hugh everett in 1957 is this. he says, look, if math says there's a 50% chance the electron could be here or here, he says when you study the math diligently and really follow it through and apply it to the experimenter as well, the math seems to see that when you do the observation, you find the electron here, and be you find the electron here just in two different universes. in each universe there's a copy of you thinking incorrectly that this there's a single definite outcome, but from the bird's eye view, there are two of you thinking that.

the idea is that all of the possibilities allowed by the quantum laws are realized in one universe or another, and this grand collection of possibilities, that we call the quantum multiverse. that's the idea. >> but you believe it. >> no, i don't believe it. i don't believe it because i don't think that we've established yet in any of the an cease, and again this is controversial, some people think we have, i don't think we've established yet how this way of thinking about quantum mechanics actually describes our observation. that link, i don't think, has been established. >> i think we just don't understand quantum mechanics. >> no, but that's tantamount to exactly the same statement. to understand quantum mechanics is to say how does it link up with observation, and i don't think we've answered that yet. >> well, it just doesn't appeal to our understanding of the universe because we are live anything a space where things don't have the way they happen in the microworld. >> let me just add a small footnote to that.

>> mostly, except like things like that. sometimes you can see large object behaving quantum mechanically. >> and i wasn't going there. i just want to emphasize that what you're saying explains why quantum mechanics is counterintuitive. >> it's worse than that. [laughter] >> it's crazy. >> what have word you like. whatever word you like. >> einstein couldn't accept it. >> exactly right. but why is that? and there's two parts to that story. >> why? >> no, i wasn't actually answer ago question. [laughter] >> i want to answer it. >> can you know, there's a part of quantum mechanics that feels very uncomfortable because it's so at odds with experience. and that's the part that makes it hard to accept these crazy ideas. but if these crazy ideas have been fully worked out mathematically and the link to observation had been made, then we'd have to accept that our intuition has been built up from thousands of years of live anything a world this size, and there's no evolutionary

advantage to understanding the probablistic motion of an electron. when you're out on the savannah trying to get your next meal, it doesn't matter if you understand the probabilities of quantum physics, it matters if you understand where that animal's going to be in five seconds so you can jump it and eat it. and that's why our brains have developed to be newtonian. if i took this glass, okay, and i took the water out and i threw it, somebody could catch it. if i were to do the same thing with an electron they wouldn't be able to catch i want because they don't have the same intuition. how do you go from the problem listic mass to the probable r59? that has not been solved. >> brian, are you a gambling man? >> why do you ask? [laughter] >> we talked about your food habits -- have you been to a

casino? >> yes. >> so in a casino you have a roulette wheel or something like that, and it rolls around and falls on one number. there are 36 numbers and 0 and 00, but it chooses one number. >> with yes. >> request do you have a problem with that -- do you have a problem with that? >> do i have a problem with that? i don't think i said i did. >> why do you have a problem with the world based in probabilities? >> i do have a problem that is incomplete. >> then you're an einstein -- >> no. >> he said the theory's incomplete. >> for a different reason. >> einstein's problem had to do were do diswhrel, it had to do with a lot of things. >> exactly. >> even though he had the vision to actually understand, you know, something we call today entanglement and the epr paradox and so on. what i'm asking you is something at a lower level. you have no problem going to las

vegas -- well, maybe you do. but gambling, you have no con shep chul problem, as the giew on the prairie -- guy on the prairie hunting mast done, if another time you're chasing an animal, that's newtonian if a sense, right? what you're talking about? would you need to see a shrink if wolf went one way and the next time it -- >> if wolf looked like my mother or father, i might -- i'm not sure of the point you're making. >> okay, here's my point. you do an experiment and when you can observe it, the electron goes one way. it can be to the right, and in another universe, to the left. >> yes. >> but when you done, the electron goes both ways, right? we know that because we can think quantumly. >> if we're trained. >> with right. it's okay for us. it interferes with itself.

typical experiment with one particle, right? you have no problem with that at all. when you open the box, you collapse the weight, so to speak, you today the coin, you roll the roulette wheel, and it goes one way or another. why is it -- and, by the way, the problem is not of mathematics. finish for a mathematician,hill birth's space -- >> how many people are familiar with hill bert spaces? [laughter] and i'm three. i think we're going a little far afield. let me just be a little bit clear here. my problem with quantum mechanics has nothing to do with the fact that it involves probabilities. so i'm happy with probabilities -- >> so then no more mini worlds? >> somehow we're talking at cross purposes. >> well, that's the alternative to the probabilities. >> no. no, no, no, absolutely not. >> okay. >> the people who believe in many worlds also believe in probabilities, they're just trying to make a link between the probable listic predictions

and the fact that you see a definite reality, and that has resisted solution for about 50 years. so if you were talking to a person who does believe that there are many universes in quantum mechanics, you would ultimately find that they're trying to explain the very same probabilities that middle east boar was trying to explain back in the old days. so it's not like einstein where einstein had in his mind that physics needed to make definite predictions. >> no, no. we've long since gone beyond that because observations do show that the probabilities work. we're trying to close the gap in the actual quantum -- but my suggestion is that we move on from this because this is simply one variation of the theme of multiverses. >> right. what's your favorite multiverse? >> you know, it depends the way in which you judge favorite. but i certainly have a leaning

towards those that have a chance of being experimentally tested in the shortest time frame which is one way of thinking about the subject. and from that there's a multiverse that comes from string theory which i find particularly exciting along these line which is called the brain universe, brane multiverse. and it comes from the following idea: so within string theory, and i think many people have at least heard of what string theory is. it's this idea that the elementary constituents of part billions, little tiny dots, the new idea of string theory is that within these particles is a tiny filament that vibrates in different patterns. the idea is deep in the heart of matter is little, tiny vibrating strings. as we've studied the math of this theory more and more, we've come upon the following very

interesting idea. there are not only these tiny filaments, there can also be what we call membranes, giant sheets, if you will, that can have two dimensions or even three check ins and so forth -- dimensions and so forth. and the math seems to suggest at least it's possible that all that we know about every star, every galaxy and so forth is living it life out on one of these membranes. let me just do a two-dimensional analogy. imagine a big slice of bread where every star and galaxy that we know about is on this slice of bread. that is our universe. now, this proposal suggests there could be other slices of bread, other membranes, other universes that if you will are all part of some cosmic loaf, to use the metaphor, with our universe being one slice of bread in this grand collection. now, to your question why do i find this particularly exciting? well, there's a chance that this proposal might be tested. how would that be? well, the collider slams protons

against protons at fantastically high speeds. and the math shows that in some of those collisions if they're moving fast enough, when the protons collide, they can create some debris that would get ejected off of our universe, off of our slice of bread. how would we know that? well, the debris would take away some energy with it. that means there'd be less energy left for our detectors to measure after the collision than before. there'd be some missing energy. people are looking for these missing energy signatures, and if energy's miss anything the way that the math suggested it should be, this would be interesting evidence that this brane picture is correct suggesting that there might be other universes out there. >> have you been depressed recently? >> why to you ask? >> >> because you know the lcv hasn't found anything. right now as a lot of people may have heard, the results are negative on that. and they're also negative on something else. >> let me just respond to that

too. you know, t very, very early. in fact, if they found anything at all, they wouldn't announce it because it'll take years of analysis before they do. you're making a great point. not depressed, i'd be thrilled if it wasn't because this is meant to be an experimental science. >> right. >> if we could rule out string theory, let me just be on the record very clearly about this, would i be depressed? i would jump for joy. because i'm not whetted to a particular theory. i'm whetted to working toward truth. i don't know what you think. i think you go around once. and if you go around once -- >> in this universe. >> in this universe. i don't want to spend my time working on a theory that's incorrect. so if string theory's wrong, i'd like to know today, or i'd like to know yesterday. so it's not a matter of having a certain emotional investment in one outcome or another, i have an emotional investment in contributing however minimally that may be to the ongoing human search for truth and finding that a given theory is wrong is

progressing because you can throw that one away, winnow down the probabilities. >> good. so you'll always be excited. >> that, to me s the nature of reality, nature of the group verse. >> 30th of march is when they started. of course, they stop for the break, and they create so many collisions every second, you know, t trillions and trillions. >> yes. >> and the data accumulates. so the first thing they ruled out at this energy level is extra dimensions. they're not saying they don't exist, but they haven't found it. >> sure. >> i want to lead in another direct in that, at least for a short while, i just heard from certain that they haven't found any proof of supersymmetry either. >> that's correct. >> just happened now. so as of now with all the day they've checked in the year at half the energy they can reach, they haven't found supersymmetry, and i think supersymmetry is another place

where the mathematics and the physics might diverge. so let me add something. i'm not here to play your psychologist -- >> i'm a little bit worried, how many people think it's weird that he keeps -- >> you don't trust me. >> let me explain. [laughter] i don't know, you live here. they can come and visit you, i just come here once in a while. [laughter] so the full name of string theory is superstring -- >> wait. i'm talking about supersymmetry, i'm not talking about string theory. >> now, what is it? well, supersymmetry is a fantastically interesting mathematical symmetry that relates things that previously we thought were totally unrelated. you know, what is symmetry? if i take this grass, and i begin to take this glass around, it's highly similar metic which

means no matter how i turn it, each point is related to every other point in a way that suggests none is special. each can be turned into the other point by simply rotating it. similarly, there are a class of particles in the world that makes us up, electrons and corks, things that make up protons and neutrons. those particles seem to be very different from a class of other particles by virtue of the fact that they actually ship around differently. those particles that we know to be turn half, but there are other particles that we know about that have spin one. that's like the foe on the or the particles that commune candidate the nuclear forces, and there are some hypothetical forces that would have spin zero. supersymmetry is a mathematical symmetry that would relate all of those parols. -- cls -- particles. if that's the case, for that to

be true, there would have to be a certain other class of particles not yet observed that the known particles we know about would turn into under this kind of similar metic rotation. those with the supersymmetric particles. it's known as the supersymmetrical electron. [laughter] i don't name them. for every known particle, there's a cousin called a sport cl. so we're now looking for the sport cl. if they're there, it will confirm this idea. if they're not, it either means that we don't have sufficiently powerful accelerators to create these sport cls, or it may mean they don't exist. >> right. it's a beautiful theory, but we don't know if it has anything to do with the real world. >> we don't. >> the problem with mathematics and fizzings go -- physics was

united with a special theory for relativity, and when that was done in 1928, i think something like that, looked at his equations. now, each going to sign like brian. maybe in another universe, i'm brian. so i'm talking, what brian says is we trust the mathematics. >> met me -- >> let me finish. >> i have to interrupt you if you're putting words in my mouth. >> [inaudible] >> hang on. mathematickings can be a to text guide to what we should consider interesting, but until observation, until experiment confirms it, no, i don't trust it. i only trust observation and experiment. >> fine. [laughter] so he looks and rerealizes a way of uniting special relative with quantum theory. when he does that, he gets his

mathematics, and i'm not going to put words in his mouth, and he looks at the mathematics, and the mathematics tells him that there are negative energy levels for the electron. and he says, well, maybe anybody else looking at it would have said this is just the math. it's like when you solve an equation and you get two solutions, one is imaginary, and one is real. i'm going to ignore the imaginary, it's only the real one that is good for me. but derock didn't do that, he said there must be a particle that has these negative energy revels. first he thought it was a proton, then he realized it was another whole new particles. and that's particle, thes position electron was discovered experimentally a few years later. so the point is sometimes it works, but it doesn't work all the time. >> exactly. >> so i'm glad you're open-minded yo and you're saying we want to follow the

mathematics. we are an experimental science, we want to see where it leads us. the problem is it's sort of a point to a lot of physicists because a lot of physicists believe in supersymmetry or follow a lot more than other theories. so if we don't find these particles, that means here's a symmetry, a beautiful mathematical construct that may have absolutely nothing to do with this universe or any other universe. >> that's right. so, ultimately, nature speaks through experiment and observation. but, you're right, this is a large segment of the theoretical community that takes this idea very seriously. we have been work aing on it since the 19 -- we have been working on it since the 1970s. so if these particles are found, scientists around the world will be popping champagne corks. the example you just gave would be recapitulated in a big way. if they're not found, we will go back to the drawing board, and

that, to me, is thrilling. >> good. fair enough. how about some of the other theories? >> well, another simple one is one that comes out of thinking very carefully about the big bang. so, again, we've touched on the big bang earlier which is this quod that the universe underwent this rapid expansion early on, but one of the things that perhaps we don't emphasize enough when talking in general context is that the big bang theory leaves out something pretty important which is the bang. the big bang theory tells us how the universe evolved from a split second after what happened the swelling to happen in the first place, but it doesn't tell us what caused that swelling to actually occur. now, people have been working very hard to fill in this gap p, and the reason i bring this particular gap up is because there is a proposal now for what caused the outward swelling. it's basically, the recognition

that goes back to einstein that gravity on certain circumstances can be repulsive. you drop the glass, et falls because the -- it falls because the earth attracts it. that's what gravity does. but actually einstein showed, surprisingly, that under exotic circumstances gravity can push things apart. the belief is, the possibility is that in the early universe that environment was realized, there was an energy-sufficient fusing space that gave rise to repulsive gravity, that's why the universe started swelling in the fist place. the thing is when you study this theory in detail, it seems to show that this outward swelling would not have been a unique, one-time event. it says there could be many of these big bang-like beginnings each giving rides to a swelling rell. , each giving rise to a universe that people like us could

inhabit. this is the inflationary multiverse. and the nice thing about this approach is that the idea that space underwent this rapid swelling early on from this repulsive gravity, that has been subjected to some very interesting observational tests. as the universe went through this rapid swelling early on, here's what would happen. little tiny quantum jitters, quantum fluctuations in the young universe would be stretched out by the rapid swelling and smeared out across the sky. an analogy is if i had a balloon with a fine-tip pen. i wrote a little message, you couldn't see it, it's too small. as the balloon stretches, my message gets smeared out, now you can see it. now, the tiny can funnel jetters are like the little message, and that space underwent this rapid expansion. that message gets smeared out across the sky, and tiny

temperatures differences in the heat left over from the big bang. and we have measured this heat left over from the big bang, and the way the temperature varies from point to point is exactly in line with the mathematical calculations. >> but does -- >> and that is a very convincing piece of evidence for taking this theory seriously. >> i think the here theory's seriously, but most -- >> oh, yeah. >> does it really imply the existence of something that's unobservable as of now which is a multiverse? i think that those microwave, you know, fluctuations as they expand, and i think there are galaxies that are spawned from them as well. >> sure. >> does that really imply other than the mathematics? you keep going back to the mathematics. does the mathematics really tell you that if you see this picture of the microwave background

radiation in space you must -- >> no. no, no, no. >> okay. >> not must. and that's why i'm not here saying these ideas are proven. you may recall i emphasized these are speculative ideas that come from our investigations. and until we have observation of them, we can't believe that it's real. >> let me ask you -- >> let me just take it a little bit further. but what happens in the subject is when you have a theory that is able to describe things that you can see, it bolsters your confidence to follow the theory further. that's where the confidence comes from, to follow the math further. now, does the matthew anemically imply -- uniquely imply other realms? they're very hard to come by. they're very cumbersome, they feel contrived from a mathematical standpoint. they could be reek. but the ones that don't have that con tried qualities do give

rise to other universes. sol do we know that they're there? absolutely not. if you had these expanding rem ms. >> id shut up if you want me too. [laughter] >> imagine it, you know, as a big cosmic bubble bath of different universes with our universe being one of those bubbles. now n a bubble bath, the bubbles can collide. similarly, these universes as they expand can collide to. if they form close enough together, they can smash into each other. how would we know that? well, that collision can send a ripple through this heat left over from the big bang, this cosmic microwave background information once again. so scientists are looking to try to find finer patterns in the temperature variations in space that might indicate that we got

hit by another universe. is there any positive evidence yet? no, not yet. the collisions maybe never happened, but that' the way in which in principle you could have objection invitational evidence of a universe you can't see. you see its effect in our universe. [laughter] >> so how would you know? there have been several generations of satellites looking at the -- >> exactly. >> and we know a lot about the microwave background radiation. in fact, it's uniform to -- >> yes. >> it's very, very small. >> right. >> how would you be able to tell? you've got to give us something concrete. you know, you lost me at the beginning because i don't think another universe can exist on this axis because of the fact that we created this space. what is this space? you haven't answered my question on that, but let's leave that

out. >> [inaudible] >> let me finish. >> you've given me the impression that something is missing, and the missing part is actually you're not fully comprehending the idea. >> i know what you're thinking. there's the hyperspace -- >> no, no hyperspace. >> knowledge can be a dangerous thing. you sort of know too much right now. [laughter] this has nothing to do with hyperspace, nothing to do with -- >> okay. >> bread and butter cosmology that takes place in the ordinary dimensions. >> fine. >> let me just describe it. so the wider cosmos that you're having trouble grasping, think of it as a big sauna, a steam bath. >> it's in -- >> three dimensions. yes, let's just stay simple. filled with this dark energy that causes the outward repulsive gravity that i was referring to. what happens is region by region in this big cosmos the energy can degrade, and as the energy degrade, holes open up in this wider cosmos where the energy

turns into particles that make star and galaxies. so our universe is simply one of these reasons where the energy has degraded. the imimage that works pretty well is imagine a block of swiss cheese. it's forcing thingsing to experience gravitational repulsion. the holes in the cheese are places where stars and galaxies can form. so the different universes i'm talking about -- >> so they're really one universe. >> whatever language you'd like. again, as i said earlier on -- no, let me just say, the language is confusing. >> we're talking about experimentally detecting evidence of the multiverse. >> request yes. >> whatever the multiverse may mean. >> exactly. >> cheese. >> yeah. yeah, yeah. >> so you've got these two group verses colliding. >> yes. >> and he's the background, radiation. it's fluctuating.

>> yes. >> how do you know it's from that and not from something else? >> that's a question you face with all experimental data. you look at data, and you try to rule out all competing proposals, and the proposal that stands up and is the best explanation is the one with you gain confidence in. so we've done calculations, actually, aye not done these calculations myself, others should get the credit for it, but other physicists have done credit of what would happen under this process. and they have explicit predictions for what would happen to the radiation, plus how the temperature would vary from place to place. and if you find temperature variations in line with those predictions and there's no other competing explanation, then, indeed, your confidence in this possibility would rightly grow. that's the way sykes works. >> final. let's assume it will happen someday, and then we'll have proof of it. >> i agree with you completely. >> so tell us about some of the

other multiverse theories j. what time is it? because i think we've been going on -- >> it's 8:10. [laughter] >> no, i want to make sure people can interface. i'm happy to keep on going, i've got no place to go tonight, but whatever you want to do. [applause] >> i guess you got your answer. >> anybody have a question, want to throw anything out? >> he's tired of my questions. >> we'll save time for some questions now. we have two museum staff members with microphones who will be walking up and down the aisles. we'll select you, and when we do select you, please, stand up and don't begin talking until you have a microphone. so we're ready for some questions now. first question down here. >> i know this field moves very quickly, but in 2006 lee smolin,

theoretical physicist at the institute in canada -- >> yes. >> -- wrote a book entitled "the problem with physics. ". >> the trouble with physics. >> excuse me. >> yeah. >> it seems to me that he has, basically, abandoned string theory. so -- chiefly because of lack of experimental confirmation. >> yep, yep. >> so my question is has he abandoned it too early because of this? or can this carry on to future -- >> yes, a good question. and, you know, lee's a good friend of mine, and when i speak to him he says largely he thinks his books have been somewhat misinterpreted. what he claims he was meaning to say was string theory is not the only approach to putting together quantum mechanics and general relativity. in fact, he's a champion and has been one of the founders of a competing approach called loop

quantum gravity. and part of what he was saying was he feels that too many people work on string theory, and the health of the field would be advanced if there was a more balanced approach where more people worked on these other approaches and string theory wasn't sort of the primary one that was looked upon as the solution in the physics community. you know, i agree with that. i feel that health of a field is evidenced by all sorts of different ideas. the reason why more students work on string theory, frankly, is i think it's a more attractive and appealing and promising approach. i think that's how graduate students make their decisions. but, you know, i full well agree that it'd be great to have active centers of research in all these approaches. and he helped found the perimeter institute that you mentioned, and there are a lot of people there working on loop quantum gravity. so the idea that he abandoned --

he's one of the folks that really tried to cross over, melding them together. he and i have discussed this. that would be great if that happened, but his main point is that there are other approaches and they deserve anticipation, and on that i would have to agree. >> we have a question over here. >> hi. yeah, my question's related to the be worlds theory. basically, about the fact that right now in another world like, i don't know, i could be asking this question to someone else. but whose world is it? like, if we're making these choices, like, are we creating these worlds? so whose world is this, whose world is this? is it yours? is it mine? >> yeah. well, according to the bread and butter many worlds approach as others have developed it, if you're in a situation where quantum mechanics says there's a possibility of this, a possibility of that, a possibility of -- and so forth,

all of those possibilities rap. not really a matter of choosing which happens. the mathematics doesn't allow any possibility to go unrealized. all roads are traveled in the quantum universe. i'm teaching this right now, and i'm literally this week talking about the many worlds approach, and we set it up last week. and if you actually go through the mathematics of it which have few people i found actually do. few people go back to the thesis that hugh everett wrote back in the '50s. have you actually read his thesis? his thesis is a mathematical gem where he makes a potent case for this idea from a modern perspective -- i don't want to sort of open it up again. i think there's still thing missing, but when i read his thesis, i'm taken along, and

it's an idea that i don't think is right, but i'm taken along to sort of the last step. and the last step i think he didn't quite get right, and i don't think anyone, in my opinion, has filled it in. others disagree with me. but if it's correct, all possibilities allowed by can tunnel physics actually happen. sometimes i'm asked is there one universe with sarah sarah palin is president? and i tell them it has to be compatible with the laws of physics. [laughter] [applause] >> next question over here. >> so you mentioned some of the other potential quantum gravity unifications. >> yes. >> do any of them have the implications as far as the multiverse goes? >> you know, t a good question. i don't know enough about them, i have to say, to answer that with any degree of confidence.

in all of them, quantum mechanics is part of the story. so if quantum multiverse is true, then i think all of them will likely embrace it in the manner that we've been discussing. so from that perspective, yes. in terms of the other multiverse ideas that we've discussed here today, i'm actually not sure what they have to say ant it. >> question over here. >> in your many-bubbled world -- [laughter] >> yes. >> we know that after the big bang certain specific criteria had to be met or the universe would have flown apart. >> yes. >> so in the other worlds, did they have to follow our laws in order to succeed, or did some of them die or how does that work? >> yes. so one of the deep questions that we have faced over the last 15, 20 years is aligned with exactly what you're asking. we've measured certain features about universe, certain numbers, certain parameters like the strength of the electromagnetic

force, the strength of the gravitational force, the masses of the core and so forth. what we found is we understand the numerical values that the experiments are revealing, but we haven't been able to explain why those particular values have been found. now you might say you shouldn't really worry antibiotic, but you should for exactly the reason you ask. if those numbers had been somewhat different than the universe as we observe it, if i had a machine with 20 dials and i call on someone randomly to come up, the group verse does not evolve in the way that we know. stars don't form, planets don't form, and t hard to imagine life would exist in such a universe. so the keep question has been why do those numbers have just the right values to give rise to the your venn that -- universe

that we are far with? we have hit a dead end so far in the trying to answer that question. the multiverse has a different way of thinking about that question. it's along the lines of what you suggest. the idea is maybe there are many be, many, many universes in which those numbers vary from universe to universe to universe, and in most of them we couldn't exist because the stars wouldn't be there, the planets and so forth. and we wouldn't observe any other value. we couldn't exist in those other realms. and that is an approach that may, ultimately, hold water. now, let me just give you a little analogy on this that happened to me two years ago with my 4-year-old which i think helps one understand this a little bit more. you know, my son is 6 years old now, he was act 3 and a half. and we went to the shoe store. this was the first time he was really old enough to think about what was happening. the guy measures his shoe, goes

in back, puts on his shoe, it hits -- fits, we leave. my son turns to me in the street and says, wasn't it lucky that they had my shoe size? [laughter] and as i probed further, what he had in mind was that shoe store had a single shoe size, and it just so happened that it fit his foot. [laughter] when i explained to him back in the stockroom there were many, many different shoe sizes, themy ri went away. what's the moral? the moral is if you think there's a unique object that you're trying to explain, that can be mysterious. if you realize it's one of a vast collection, the mystery can e sap wait. that may with true with these parameters. just as we found the shoe size that fit his foot, we found parameter in our disbs, and that may be the answer.

>> all right. >> sorry, i don't -- >> i guess he's a theoretical physicist. >> you're thinking of -- >> in connecticut, right. yeah. um, i guess he postulates that if you twist light enough, that you can twist space-time sufficiently to create, like -- >> you can go back in time? >> well, or at least a subatomic particle stream back in time to when the machine would turn on. can't go back -- right. but if such a machine were actually built, could some of that be used to test some of these theories? >> oh, boy. you talk about speculation, we're now in speculation squared or something. [laughter] you know, how would time travel interface with some of these ideas? let me just turn it in that

direction, and i'll simply say this: one of the big puzzles with time trail, of course, is you go back in time and affect things in a way that maybe effects your own existence. in back to the future, you know, hollywood loves this idea. you know, there's a variation on the pair ro dock -- paradox which comes from the following idea. imagine that you travel, imagine you can travel to the future, you know? imagine i travel to the future, let's just say, and i want to see what's happened in string theory, has it been proven or not? so i go to the library, and i see that, surprisingly, the theory has made a major advance, and the author of that paper is my mom. and i'm like, that's weird because my mom department know anything about physics, wants me to be a doctor, not this kind of doctor. and i look to the

acknowledgments in the paper and she thanks me for teaching me all this physics. and i think, holy crap, i gotta get back, she doesn't know very much. i start to tutor my mother, and, man, it's not going well. i'm like, how in the world is she ever going to write that paper? and then i say to myself, i read it. let me just tell her what to write. [laughter] so i tell her what to write, and she writes the pay or -- now, the question is, who gets the credit? [laughter] it's not a question of credit really, it's a question of where did the information come from? did she think of it? no, she got it from me. did i think of it? no, i got it from her paper. so ideas seem to just pop in from thin air. now, how does this relate to multiple universes? here's the fanciful idea that people floated. imagine that when you travel to the past, for instance, you

never come back to your own universe. you come back, say, in the quantum universe. you come back to one of those other copies of our universe. so if i go back in time and till my participant before i'm born, i wouldn't be born in that universe, but so what? my parents would be unaffected in the universe which e started. so that's sort of, again, a little far afield, but at least that's some interaction with time trial. time travel. >> question over here. >> i've just had a question of something i'm recently aware of, the condensation theory. >> yes. >> and it's something that physicists, we are so lucky to live in a time where people can prohughes this -- produce this. now, hypothetically, if they can create that instant in a room, let's assume they can, do all the basic theories of quantum

mechanics break down? >> do you want me to answer? >> sure. >> i don't think it does that. brian will be the final arbiter, but i think it was created right here at m irk t. it was also created in colorado. so around the same time. it's just you cool something, you cool some atoms to a very, very low temperature, and what happens is the way, the very particles of wave also, so the waves overlap, so you're really creating quantum mechanics for a large object, in this case a collection of atoms. i don't think it relates to anything else we've been talking about. >> yeah. and it really comes out of basic can tunnel mechanics. >> jumped to one point. i mean, that's the basic principle. >> pardon me? >> you actually reach a

boaz-einstein -- >> that's what the physicists at mit say. >> i personally wouldn't describe it that way -- >> well, mit described it -- >> yeah, i'm not sure exactly what they had in mind. >> well, they described it that way. >> okay. [laughter] >> we have time for one last question over here. >> yes, i'm a diehard mini wolzer, i was impressed by an observation in your first book where you noted a duality between length and a one-over time which seemed to have a special meaning, if i understand this correctly, after the big bang when the energy for a wound and an unwound string are about the same. and there was something, i don't recall the details, but something you said in the notes prompted this idea, and i'd like to know if anyone's pursuing anything like this.

>> with okay. >> and if you imagine at this moment there's this one moment in time of perfect similar industry, and i mean a finitely describable universe that is now going to evolve in the vacuum states or whatever it takes to get us forward to all the different versions of us here now and shortening over this dead cat and this live cat. you'd expect the same thing to be happening in that one-over interpretation going back towards the moment of the so-called big bang now that singularity turns into an illusion. it's like the singularity of the north pole. an inappropriate set of axes to describe that happen, that first little thing. and now we have another doppelganger which is the whole multiverse repeated back many that first moment of time. curious if anyone's pursuing ideas anything like that. >> so it is up with of the more

surprising features of string theory which shows under the circumstances largely that you're recount ago universe that -- recounting a universe bigger than the plank length and expanding is actually one-over world that you're talking about. i wouldn't use the word doppelganger or imimagine to describe these two rell 78s. -- realms. they're distinct mathematical descriptions of the same reality even though they seem vastly different. but as to your question how old cosmology hook and how would the singularity look in this picture, yes, in fact, right here at harvard robert brandenburger, they studied cosmology in the context of one-over symmetry, and they did find something along the lines that you're suggesting.

so if you run the universe back and back in time, it gets denser and denser way back to infinity. they found that in this setup when the universe gets smaller than a plank length, when it gets smaller than the plank length, the temperature levels out, and as the universe gets smaller, the temperature starts to turn down. because of this various symmetry that you're talking about. it never spikes -- excuse me, there's never a time when the density grows infinitely big. so this is a cosmo logical model that has been proposed based on that symmetry. this is much work that would need to be done to take it fully seriously, but as a toy test case, yes, it's one of the most potent ones that's come out of string theory. >> do we have time for another brief comment? question? i just wanted to point out that if you're taking that kind of a