reason for the child-man is female independence. the young man reaches the age where in any other period of history he would be defining himself as potential husband and father with the understanding that he had a clear and important social role. today provider husbands and fathers are optional with reproductive technology. women can simply buy sperm and forget about the man who delivered it. meanwhile men have been cast out of their home and separated from their children. no wonder they look around at the culture, shrug and do their own thing. >> you can watch this and other programs online at booktv torg. >> 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 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. >> yes. [laughter] in this universe i am, that's true. >> you stole my next question. >> sorry. >> whether 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 the woman next to me, i ordered vegetarian, and the woman next to me said, would you be offended if i ate meat? i said, i don't care what you eat. i see you're offended by your doppelganger. >> he's not offended by me.

>> so tell me something else. i understand you have a number called, what is it? >> oh, the air dose -- >> yes. can you explain about that? >> yeah. you know, 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 be have their own version of kevin bacon which is the man who collaborated with many, many mathematicians, so the question is how far are you away from having written a paper with air attorney? people said let's put it all together and see how far an individual is from kevin bacon and the mathematician. as you can imagine, there aren't too many people, but there are a handful of us, and i'm one of

them. >> how many are you? >> i used to be the world leader. >> what's your number? >> number five. i've been overtaken. >> five in what? >> five total. two from the mathematician and three from bacon, or something like that. [laughter] but i think, like, gwyneth paltrow has taken -- she wrote a paper. yeah, i'm not sure. there are definitely people who have taken over. >> i see. in another universe, you're number one. >> that's always going to be the case, yes. >> so what is this -- let me ask you, we all think that there's 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] the word universe meant just what you are saying, it 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 are suggesting 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] multi cereal. >> you have a food thing going on here, don't you? >> i know. so tell me, i understand that physics is a science experiment. >> yes. >> sounds more like a religion to me. this universe and another

universe, 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 the two parts. one is, in some versions of the multiverse, and i should emphasize there's not one proposal, there are a number of proposals. in some there can be subtle connections that might allow us a 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 founded upon, really, hundreds of years of experience that moth can provide -- math 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, einstein is the greatest

example, right? the he wrote down his equations of the general theory of relativity way back in 1915. others looked at those and said it seemed to say the universe is expanding. einstein himself said, no, i don't actually believe that, but 12 years later observations showed the universe is expanding. the math was confirmed by observation. other examples are black holes. again, einstein's math gives rise to them. einstein didn't believe it. observations 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. >> i think the operational word was can. sometimes the mathematics works, and sometimes it doesn't. >> exactly. >> you can go back and say that cycles were used by mathematicians. so here's mathematics that's valid as mathematics -- not very complicated -- >> yes. >> that 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 who were looking at the motion of the earth and planets and coming to certain conclusions that we now know to

be erroneous, conclusions about how things were working. there were other physicists, mathematicians who looked at that math and said this is so complicated and 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 east. 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 by following the math, confirming it through observation. we may be on the threshold of the next emotion by following exactly the same pattern. earth is not the center, sun is not the center, galaxy's 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 the mathematics is always simpler, in a sense, for --

>> 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 can understand where you might come to the conclusion because if we get into any of the details, 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 equations, the starting point, it's actually simple. >> how many theories are there? >> there's one now. wonderfully, in the last decade the math has come together, and we've realized that what we thought were different theories are actually all the same, just expressed in this a slightly different language. so everything has been simplifying. you know, if you take -- even as a good example, darwinian evolution. the principles of evolution are pretty straightforward, right? nevertheless, they 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, say string theory in we get into it, extra dimensions, vibrating strings, it seems complicated, but that's like the rich life coming from evolution. the starting point, pretty straightforward. >> i see. so, tell me, what are some of these theories that lead to the multiverse? in your book you describe many of them. i couldn't find the anti one, that's my favorite. your anti person is positive terrors -- >> star trek there. >> right. 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 surface if you took a similar journey? that's possible. or would you simply go on 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 in finitely many different -- >> very large. >> a finite number. similar to, like, if i take a deck of cards, there's only finitely orders of the cards. so if i shuffle the deck enough times, infinitely many times, the order of the cards has to repeat. simply, in infinite space, the order of the particles has to repeat too. now, what would that mean?

well, as we heard in the introduction, it would mean something pretty strange. you see, you and i, we're just a configuration of particles, everybody in this room as is the earth and the sun and so forth. if configuration of particles repeats some place out there in the cosmos, it means all that we know is repeating. we are out there, and that's a very straightforward mathematical conclusion from a simple starting point. space is infinitely far. >> but you're leaving out an important thing, the measure of that is zero -- >> it doesn't matter. >> the possibility of us speaking in another universe is -- do you want to go there? >> oh, absolutely. in fact, i don't need to frame it in probability terms. if i had that deck of cards and shuffled it over and over again, do you agree that sooner or later the order of the cards will repeat? not the probability -- >> the deck is too large. >> no, it's 52 cards. >> no, that's easy. you're taking the easy way out. i'm talking about universe -- >> no, you're discounting the

power of infinity. infinite space. this is the supposition. you can challenge that, but let's not just to get to the end of the argument. if you take onboard this idea which i think most cos molingses and physicists have that space goes on infinitely far, then you've got a lot of room for this to happen. >> i have a problem with space going infinitely far. >> okay, fine. that's a good place to try to poke a hole in it. >> in mathematics, dimensions go infinitely far, but in physics, the way i understand physics, these three dimensions in which we live and the fourth of time which einstein taught us is related to the other three was created in the big bang. so i think if you think as a physicist, we're not expanding into another space, we're creating space as we're going out. as the galaxies are expanding,

we're creating more space. so where's the other universe -- as mathematicians, okay, there's this dimension that goes on forever, call it x, y and z. >> yes. >> but i think in one of your universes, you've got one here and one here and one here and one here ask that's okay, but does it really exist from a physical point of view? space and time were created in the big bang. >> right. i do need to correct you a little bit, with all due respect. >> that's what i'm here for. >> there is an incorrect image that people have in mind. when we think about the big bang, typically we imagine further and further back in time the entire cosmos was smaller and smaller, and way back toward the beginning the universe we sort of intuitively think was very small, and run that forward, space is created from that big bang, so how could it be infinitely big if it was

small in the past? if that were the right picture, you would be right. but that was not the picture that's compatible with an infinite universe. in this an infinite universe as you head ever further back in time, the universe is still infinitely big. if you go back in time and the universe is half as large as it is today, half of infinity is still infinity. one-third of infinity -- >> so what does it mean here? >> the traditional one. >> the universe is infinite? >> yes. >> so what's the radius of 13.7 -- >> ah, that's the observable universe. >> so the universe goes beyond that? >> absolutely. >> what is the big bang? >> that's the key point. the big bang is an event that gave rise to our realm, but if universe is infinitely big, then our part, the part that we have access to is only a piece of the entirety. so you need to maybe -- >> and the others are expanding as well? >> exactly. so you need to make a distinction between the

observable universe and the entirety. the observable is just the part that we can see, and you're right, we can't see further back that roughly 13.7 billion light years because that's the amount of distance that light can travel since the beginning. but we, almost nobody believes that the universe ends at that point. most everyone believes it goes on at least a far distance beyond that, and the supposition of this particular example that we're saying is that it goes on infinitely far. >> brian, you're a magician too. you pulled infinity out of the hat. [laughter] what does infinity have to do with anything -- everything we learn about physics is really 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 definition is the real line extended in exactly the way that you know about from when you

took mathematics at a young age. it goes on without bounds. if universe is not infinitely big, what happens when you travel out? >> i interviewed steven weinberg a few months ago. >> yes. >> and i asked him, the big bang is believed to be a quantum fluctuation that created our universe. what was it a quantum fluctuation in? what was the medium in the which we respond, if you will? and he said, that we don't know. we can't go there. >> that's right. >> so, 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 exists la on theically. >> no, let me ask you a question. if you build a spaceship and you go out and you just keep on going, what happensesome. >> well, if i take physics the way physics has been done,

here's the big bang, it started here. 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 created with the big bang. >> so if you go onto a rocket ship and you head out into space and you keep on going, what happens? [laughter] >> you know this, you can't. >> what do you mean you can't? do you hit an end? >> no. brian, you know pretty well if 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 the speed faster than light because of the expansion of the universe. you don't even need acceleration. so that part doesn't talk with this part. how are you ever going to get from one part to another with a spaceship that travels less than the speed of light? >> so if you get in that ship,

what will happen? >> i don't know what will happen. i'd be lost in space. [laughter] >> you know, it's a mathematical question which would be what's the overall to poll? >> that's where i disagree with you. i think toe polly exists in a mathematician's mind as a platonic kind of thing to weigh varieties or mote motives or things that may have nothing to do with the real world. >> well, this is a good point. >> when you as a physicist take parts of mathematics, you emphasize my key point which is mathematics is not physics. i'll give you an example. >> yes. >> we -- i'll give you another example. >> good. >> i want to jump up to the same level. >> okay. >> one of the fathers of quantum mechanics, you know, in this

'20s who built this theory, you know, everybody knows about the uncertainty principle and maybe also about matrix mechanics, and then he went a step further. and he thought he's going to go into something else, and he said here's a proon the, and i need three -- pro on the, so you've got a symmetry between them. and he called it su2 which, of course, you know. and then we won't go where it went from there, but that assumption was wrong. that was taking mathematics that makes a lot of sense this your mind as a mathematician but has nothing to do with the real world in the sense of the proton and electron. one of them is a lot heavier than the other in absolute terms, but when compared to the mass of the two, then you think they're really very, very

similar. and he went into symmetries. now, of course, you and i know that jung and wilson did all kinds of things, and the math mathematics sort of came back. but at that moment what you have is very powerful and absolutely useless. >> yes. [laughter] >> i rest my case. [laughter] >> no, no. it's a case that i agree with. what i would say is that mathematics opens up the realms of possibility. thank you very much. what the art of physics is, is being able to sniff out which mathematics is relevant for reality and which isn't. now, experiments and observation are a key part of that story. ultimately, it was observation and experiment that dictated that map wasn't the right way to go. so what we need to do and what we spend our professional lives doing is trying to understand

which body is relevant and which isn't. now, in this particular case that we're talking about, the argument makes the assumption that a certain body of mathematics is relevant to reality. if that's not right -- and it may not be, i'm the first to say that 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 motive thinking about many of 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 the horizon 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 help us here. so people have asked themselves, if space doesn't go on infinitely far, could we, perhaps, objection invitationally publish that?

one way to do that is if it doesn't go infinitely far and if it does have a shape like the surface of the earth where it comes back on it, well, as you know, there are structures in space that give off life -- background radiation and so forth -- if universe has that shape, light that comes from a distant source can hit our eyes, but it 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. no such evidence yet. that doesn't mean it's infinite, could be just big. that's exactly what physics is about. 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] >> about the many worlds. >> many worlds. >> i can't even say. [laughter] >> many worlds is a somewhat

different character of proposal, and you may note that in the book it's actually one of the latest chapters because -- >> yeah, i was worried about that. >> you're right, chronologically, it's the earliest, you're right. marching through the developments chronologically doesn't give you the most pedagogically sensible way of thinking about where we are today because in particular quantum mechanics stands outside some of the ideas of string theory. but it is an interesting proposal, and that's why i have a chapter devoted to the it. >> it's weird though. >> you're right, it is weird. and you'll note in that chapter i basically come to the conclusion that i don't think it works. >> whew. laugh. >> but that doesn't mean it doesn't. if you're talking to other people like david deutsche from ox nord or -- oxford or various other researchers, david

wallace, they would sit here and say it absolutely does work. well, here's the idea. so the new idea of quantum mechanics in the early part of the 20th century was that 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 the observations established that that was a way of thinking about things that was very accurate when applied to everyday objects like glasses or to the moon's motion or to a rock that you throw. newton can tell you what will happen, you do the observation, and it does happen. when people began to probe the microscopic realm, that whole structure began to fall apart. >> different universe there. >> completely different. different realm. let's not use the word universe in the too many different ways tonight, but a completely different environment. why should the laws that work on everyday scales also work on

tiny scales? and it turns out that they don't. the new laws, the laws of quantum physics, and the new idea of cab tunnel physics is that you can only predict the hikely hood, the probability of one outcome or another. so if i'm not dealing with a rock or the moon and i want to know where it is, the quantum physics say there's a 50% chance it's over here or over there -- >> or both. >> well, no, a 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 an observation of the electron, you always find it either here or there. there's never sort of some melding of the two. so the puzzle has been for 80 years even though the probability of quantum mechanics are confirmed by doing an experiment over and over again, finding the electron 50% of the time here and 50% of the time here, how do you go from the

fuzzy, hazy, probablistic 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 comes from 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 follow it through and apply it to the experimenter as well, the math seems to say that when you do the observation, you find the electron here, and you find the electron here just in two different universes. in each universe there's a copy of you thinking incorrectly that there's a single definite outcome. but from the bird's eye view, there are two of you thinking that, and that's just a single example with an electron. 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 the any of the analogies 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 observations. that link, i don't think, has been established. >> i think we just don't understand quantum mechanics. most people -- >> 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. >> will well, it just doesn't appeal to our understanding of the universe because we are live anything a space where things don't happen the way they happen in the microworld. >> let me make a small footnote to that. >> mostly, except for -- [inaudible] things like that. sometimes we can see large

objects behaving quantum mechanically. >> whether i wasn't going there, i just want to emphasize explains why quantum mechanics is counterintuitive. >> it's worse than that. [laughter] it's crazy. >> whatever word you like, whatever word you like. >> einstein couldn't accept it. >> um, exactly right. but why is that? and there's two parts -- >> why? >> two part toss the story. no, i wasn't actually asking a question. [laughter] >> no, but i want to answer it. >> 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. >> right. >> but if these crazy ideas have been fully linked out mathematically and the link to observation has been made, which it hasn't yet, then we'd have to accept that our intuition has been built up from thousands of years of living in 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 matters if you understand newtonian dynamics of 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 really be newtonian. if i took this glass, okay, and i took the water out and i threw it, somebody could catch it. they would be doing the newtonian calculation because it's intuitive. if i were to do the same thing with an electron, they wouldn't be able to catch it because they don't have the same intuition. the real problem with quantum mechanics is that there's a real puzzle that we haven't answered yet. how do you go from the probablistic math to the definite reality? that has not been solved. >> but, brian, why do you have? are you a gambling man? we talked about your food habits, do you gamble? have you been to a ca see though? >> yes, i've been. >> okay. so you have something that rolls

around and falls on one number. there's 36 numbers and 0 and 00, but it chooses one number. >> yes. >> do you have a problem with that? >> do i have a problem with that? >> yes. do you have a problem with that? >> no. >> okay. why do you have a problem with the activities of an electron? >> not in a world that's based in realities, i do have a -- ion tine. >> not lowellty and interpretation and all kinds of other things even though he had the vision to actually understand 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. [laughter] but gambling, you have no conceptual problem.

as the guy on the prairie hunting mast done or whatever -- >> yes. >> you have no problem with the wolf or whatever you're hunting going another way and then another time you're chasing an animal, social it's going anoth. that's newtonian in a sense. would you need to see a shrink if wolf went one way -- >> if wolf looked like my mother or father, i might. [laughter] >> you have no problem with it. >> i'm not sure of the point you're making. >> okay. here is my point. you do an experiment, and when you open the box, the electron goes one way. it could be to the right and another universe, and to the left. but when you don't, the electron goes both ways, right? we know that. we're not neanderthals, right? >> where we're trained. >> yeah. it's okay for us. box is closed, it goes both ways and interferes with itself. typical experiment with one particle. you have no problem with that at

all. when you open the box, you collapse the wave, so to speak. you toss the coin, you roll the roulette wheel, and it goes one way or the other. and, by the way, the problem is not mathematics, and you know that. >> just to get a sense, how many are familiar with hilbert space in this room? about three. [laughter] >> i was talking to you. >> i think we're going a little bit far afield. let me be clear here. my problem has nothing to do with the fact that it involves probabilities. i'm happy with -- >> so there are no more mini worlds. >> somehow we're talking at cross-purposes. >> that's the alternative to the probabilities. >> no. no, no, no, absolutely not. >> okay. >> people who believe in many worlds also believe in probabilities. they're just trying to make a link between the probablistic predictions and the fact that when you make an observation, you say a single definite

reality. >> right, okay. >> and that link is a subtle one that has resisted solution now for about 50 years. so if you were talking to a person who does believe that there are many universes is quantum mechanics, you would, ultimately, find that they're trying to explain the very same probabilities that middle east boor was trying to explain back in the old days. 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 formalism. but my suggestion is that we move on from this because this is simply one variation -- >> 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. >> right, okay. >> which is one way of thinking about the summit. and from that there's a multiverse that comes from string theory which i find particularly exciting along these lines which is called 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 matter, little tiny parol accounts and the old way of thinking of things, the new idea of string theory is that within these little tiny particles there's something else which is a little tiny filament that vibrates in patterns. the idea is that deep in the heart of matter there's 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. within this theory there are not only these little, tiny

filaments, there can also be what we call membranes, giant sheets that can have two dimensions or even three 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, is living out its life on one of these membranes. let me just do a two-dimensional analogy. and that's like 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 grand cosmic loaf -- to use the metaphor -- with our universe just being one slice of bread in this collection. now why do i find this particularly exciting? 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 there's enough energy, if they're moving fast enough, when the protons collide, they can create some debris that will 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 missing in the way that the math suggested it should be, this would be interesting evidence that this brane picture is correct suggesting there might be other -- >> have you been depressed recently? >> why do you ask? >> was you know that the lhc hasn't found something, so maybe they'll find it, but right now as a lot of people may have heard, the results are negative on that. and they're also negative on something else which i want to -- >> let me just respond to that too. you know, it's 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. but 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. if we could rule out string theory, just let me be on the record, would i be depressed? i would jump for joy because i'm not whetted to a particular theory. i'm whetted toward working toward truth. 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 yesterday. so it's not a matter of having a certain emotional investment. >> oh. >> in one outcome or other. 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 way, winnow down the possibilities.

so depression, no. excitement. >> good. so you'll always be excited whatever they find. >> that, to me s the nature of reality, the nature of the universe. >> been running for a full year now. i think the end of march, 30th of march is when they started. of course they start with a break, and they create so many collisions every second, you know, trillions and trillions. >> yes. >> and the data accumulates, and they haven't found anything. so the first thing they ruled out, actually, at this energy level is extra dimensions. they're not saying they don't exist, but they haven't found it. i want to lead in another direction, and that at least for a short while. i just heard that they haven't found any proof of supersymmetry either. >> that's correct. >> just happened now. so as of now with all the data they've collected in the year at half the energy they can reach, they haven't found any supersymmetry. and i think supersymmetry is another place where the mathematics and the physics might die verge. so let me add something.

i'm not here to play your psychologist -- >> but i'm a little bit worried. how many people are -- >> i'll explain it. don't worry. >> okay. let me just explain it first. [laughter] >> you don't trust me. >> it's not that i don't trust you, it's just, you know, you live here. they can come and visit you. i just come here once in a while. >> all right. >> with so the full name of string theory is super string theory. >> wait. hang on, i'm not talking about string theory. >> and the full name of super string theory, supersymmetry. now, what is it? well, supersymmetry is a fantastically interesting math mathematical symmetry that relates things that previously we thought were totally unrelated. you know, what is the symmetry? if i take this glass and i begin to turn it around, it's highly similar metic which means no matter how i turn it, it pretty much looks the same. each point is related to every other point in a way that suggests that 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 are very important to us that are particles that make us up. electrons and corks and things that make ip protons and neutrons, those particles seem to be very different by virtue of the fact that they actually spin around differently. those particles that we all know about turn out to have something called spin aft, but there are other parol accounts that we know about that have spin one, that's like the foe on the or the particles that communicate the nuclear forces, and there are some hypothetical forces not yet seen that wouldn't spin around at all. supersymmetry is a mathematical symmetry that would relate all of those particles. it would say that in some sense each of those particles can be rotated into the others. now, if that's the cays, for -- case, for that to be true, there would have to be another class

of particles that the known particles we know about would turn into under this kind of symmetric rotation. those are the supersymmetric part calls. it partner under this kind of symmetry is known as the selectron. i don't name them. [laughter] for every known particle, there's a cousin called a sparticle. if they're there, it will confirm this idea. if they're not, it either means that we don't have sufficiently powerful or accelerators to create these sparticles, or it may mean they don't exist. that's the current state. >> 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 math mathems and physics dose back to an english physicist who united can funnel mechanics with a special theory of relativity.

and when he did that in 1928, i think something like that, he looked as hi equation. now, i'm going to sound like brian. maybe in another universe i'm brian. what brian says is we trust the mathematics, and that's what -- >> let me -- >> let me finish. >> i have to interrupt you if you're putting words in my mouth. >> okay. >> hang on, hang on. mathematics can be a guide for what we should consider interesting, what we should invest further, but until observation, until experiment confirms it, no, i don't -- >> [inaudible] fine. so paul was sitting in front of a fireplace at cambridge, and he looks, and he realizes a way of uniting special relativity with quantum theory creating quantum field theory. when he does that, he gets his mathematics, and i'm not going to put words in the his mouth. and he looks at the math mathem,

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, one is real. it's only the real one that's good for me in this particular real world example. but he didn't do that. he said there must be a particle that has these negative energy levels, and it turned out -- fist, he thought it was a proton, and then he realized it's another whole new particle. and that's particle, the positive electron was actually discovered experimentally sometime later, a few years later. so the point is sometimes it works, but it doesn't work all the time. >> exactly. >> the example. so i'm glad you're open-minded and your saying we want to follow the mathematics and we are an experimental science, and we want to see where it leads

us. but the problem is, and i think it's sort of a point to a lot of physicists because a lot of physicists believe in supersymmetry more than or follow supersymmetry a lot more than other theories. so if we don't find these particles, that means here's a symmetry that's 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, and it speaks through experiment and observation. but you're right, there's a large segment of the theoretical community that takes this idea very seriously. we have been working on it in one way or another since the 1970s. so if these particles are found, scientists around the world will be popping the champagne corks. this would be an exciting moment where the example you just gave would be recapitulated in a very big way. if these particles are not found, we will accept that as the way the world works and go back to the drawing board. and that, to me, is thrilling. >> good, fair enough. so how about the other theories?

tell us about -- >> some to of the other ways you can get to multiverse theory. well, another simple one is one that comes out of thinking very carefully about the big bang. again, we touched on the big bang earlier which is this idea that the universe underwent this rapid expansion early on, but one of the things that perhaps we don't emphasize enough when talking in if general context is that the big bang theory actually leaves out something pretty important which is the bang. the big bang theory tells us how the universe evolved from a split second after whatever started the outward 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, and the reason i bring this particular gap up is because there is a proposal now for what caused the outward swelling. it's called inflationary cosmology. it's basically the recognition that goes back to einstein that gaf gravity under certain circumstances can be repulsive.

you drop the glass, it falls because the earth attracts it, the earth pulls things together. that's what gravity does. but actually einstein showed that under exotic circumstances, gravity can push things apart. the belief is that the, the possibility is that in the early universe, that exotic experiment was realized. there was an energy-suffer fusing space that pushed everything apart. that's why the universe started swelling in the first place. 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 in a much larger cosmos, each giving rise to a swelling realm, giving rise to an observable universe that people like us could inhabit, but there'd be universes upon universe upon universe.

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. if 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 little balloon with a fine-tipped pen, imagine i wrote a little message on the surface of the balloon. you couldn't actually see it, it's too small. as the balloon stretches, my message gets smeared out across the surface of the balloon, now you can see it. now, the tiny quantum jitters in the early universe are like the little message, and as space underwent this rapid expansion, the message gets smeared out across the sky at tiny temperature 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 -- >> and that is a very convincing piece of evidence for at least taking this theory quite seriously. >> i think the theory's taken very seriously by most not only cosmologists, but even astronomers and physicists. >> 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 ec pend, i think -- expand, there are galaxies that are spawned from them as well. >> sure. >> does that really apply other than math mathematics? does the mathematics really tell you that if you see this picture of the microwave background radiation space, you must have -- >> no. no, no, no. not must, and that's why i'm not

here saying these ideas are proven. you may recall when we started out this conversation i emphasized that these are speck la -- speculative ideas that come from our investigation. and until we have observation of them, we can't belief 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 that there -- uniquely imply there has to be these other realms? no. they're very hard to come by. they're very cumbersome, they feel very contrived from a mathematical standpoint. that doesn't mean they're wrong, they could be right. but the ones that don't have that contrived quality are the ones that do give rise to these other universes. so do we know that they're there? absolutely not. but does this suggest it as a

compelling possibility that's worth future study? yes. here, again, rhetorical. [laughter] if you had these expanding -- >> i'll 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. in a bubble bath, the bubbles can collide. similarly, these universes can collide too. 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 trying 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 could yield a signature that's too small for our current level of technology to access, or maybe it never happened. but that's the way, at least in principle, you could have observational evidence of a universe you can't literally see. you see it effect in our universe. [laughter] please. >> so how would you know? there have been several generations of satellites looking at the microwave background -- >> exactly. >> and we know about a lot about it, in fact, it's uniform to one in ten or six, something like that, very, very small. >> right. >> how would you be able to tell? you've got to give us something concrete. >> yes. >> 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. >> well -- >> hold on, let me finish.

>> you've given the impression that there's something missing, and the missing part is actually you're not fully comprehending the idea. >> i know what you're thinking. there's the hyperspace there -- >> no, no hyperspace. knowledge can be a dangerous thing. [laughter] you sort of know too much right now. >> [inaudible] >> 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. >> it's in three dimension? >> yes, three dimensions. let's just stay simple. the three dimensions that's filled with this dark energy that causes the outward repulsive gravity. what happens is region by region in this big cosmos the energy can degrade, and holes open up in this wider cosmos where the energy turns into particles that makes stars and galaxies.

so our universe is simply one of these regions where the energy has degraded. the image that works pretty well is think of a block of swiss cheese. imagine that the cheesy part of the swiss cheese is where this energy exists, and it's forcing things to experience gravitational repulsion. the holes in the cheese are places where the energy has degraded where stars and galaxies can form. so the different universes that i'm talking about are just the ditch holes in this big block -- >> so they're really only one universe. >> whatever language you want. as i said early on, the language is confusing. >> fine. let's leave it at -- we're talking about experimentally detecting the evidence of the multiverse. >> yes. >> whatever the multiverse may mean. >> yes, exactly. >> swiss cheese. >> good. >> so you've got these two universes colliding. >> yes. >> and here is the background radiation. >> yes. >> it's fluctuating. how do you know it's from that and not from something else. >> >> that's a question you face

with all experimental data. when you look at data, you say what's the best explanation for it? and you try to rule out all other competing proposal, and the proposal that stands up and is the best explanation is the one that you gain confidence in. so we've done calculations and, you know, actually, i've not done these little calculations myself. others should get the credit for it. but other physicists have done calculations of what would happen to the microwave background radiation under this process. and they have very explicit predictions for what would happen to the radiation plus of 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 right ri grow. rightly grow. that's the way science works. >> fine. let's assume it will happen someday, and then we'll have proof of it. but until then, of course, we don't know. >> i agree with you. >> tell us about some of the other multiverse theories. >> what time is it?

because i think we've been going on -- >> 8:10. [laughter] >> only because i want to make sure people get a chance to interact if they want to. i don't know what the format is here, but you tell me. i'm happy to keep on going, aye 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 question -- >> we'll save some time for 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

smollen, theoretical physicist in canada -- >> yes. >> -- wrote a book entitled "the problem with physics." >> the trouble with physics, yeah. >> excuse me. and it seems to be that he has, basically, abandoned string theory. so, chiefly because of lack of experimental confirmation. >> yeah. >> so my question is, has he abandoned it too early because of this? or can this carry on to future -- >> yes, it's a good question. and, you know, lee's a good friend of mine, and when e speak to him, he thinks his book is somewhat misinterpreted. what he claims he was really meaning to say in that book is string theory is not the only approach to putting together quantum mechanics and general relativity. there are other approaches. 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 too many people work on string theory, not enough people work on loop quantum gravity, and the health of the field would be more advanced if there was a more balanced approach where string theory wasn't sort of the primary one that was looked upon as the solution in the physics community. you know, i, i agree with that. i feel the 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, a more appealing, a more 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 if all these approaches. and he helped found the institute that you mentioned, and there are a lot of people that are working on loop quantum gravity. he's not really a string theorist. he's worked on it from time to time because he's trying to cross over.

maybe there's a way of 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 attention. and on that point i would agree. >> we have a question over here. >> hi. yeah, my question's related to the many worlds theory. basically, about the fact that, like, right now in another world, like, i don't know, i could be asking a question to someone else. but who's world is it? like, if we're making these choices, like, are we creating these worlds and, like, so, whose world is this? whose world is the other world? is it yours, mine, someone else's here? >> yeah. well, according to the bread and butter other world approach as developed since the 19 oohs, if you're -- 1950s, if you're in a situation where quantum mechanics says there's a possibility of this or that, all of those possibilities happen.