MR. KOJO NNAMDIFrom WAMU 88.5 at American University in Washington, welcome to "The Kojo Nnamdi Show," connecting your neighborhood with the world. Imagine our universe as a big slice of bread, right next to it is another slice of bread and another and another. Those slices are other universes, places where infinite versions of me are speaking into the microphone right now. It's just one of the ideas that physicists are toying with these days. And while it may sound like science fiction, a growing number of researchers say there's evidence that not only are we not alone but that there may be many universes rather than just one. The idea is just that at the moment, an idea, but like many other ideas in physics, it has the potential to transform the way we view our world. Luckily, physicist Brian Greene can explain it better than I can, and he joins us now in studio to talk about these new ideas about multiverses. Brian Greene is a professor of mathematics and physics at Columbia University and author of "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." Brian Greene, good to see you again. Thank you for joining us.

MR. BRIAN GREENEGood seeing you. Thank you.

NNAMDII'd like to start by reading a bit of the blurb on this book jacket, which describes some of the different multiverses that are out there, including, quote, "a multiverse comprising a vast ocean of bubble universes of which ours is but one, a multiverse that endlessly cycles through time, or one that might be hovering millimeters away, yet remains invisible, or perhaps strangest of all, a multiverse made purely of mathematics." In the plainest of terms, how do we define a multiverse?

GREENEWell, a multiverse is a word that's really a contraction. It's short for multiple universes. And when you hear that term, at first sight, it sounds almost like a contradiction in terms. Universe, we usually think of as everything. So how could you have more than one everything? Well, what we have found in the last few decades in physics is that our research is suggesting that everything is much bigger than what we thought it was in the past. And that has led us to this new notion of our universe being one of many universes, the multiverse.

NNAMDIIs the idea of multiverses widely accepted now in physics, or is it still a controversial idea?

GREENEHighly controversial, but not a fringed subject. It is an area that many top-flight scientists -- physicists are taking seriously and investigating. It's an idea that, as you say, has not been experimentally proven. It comes from mathematics, but we have found in the past that math is often a pretty good guide as to the true nature of reality.

NNAMDIBefore we get too deep into the topic and talk more about how the reality we live in may not be reality at all, we need to spend, I guess, a little time reviewing quantum mechanics. How can we use quantum mechanics to understand that every action we make may actually result in many alternative actions we're not even aware of?

GREENEYes. Well, the new idea that quantum mechanics brought into physics is that when you try to describe the world, you can't predict with absolute certainty what will happen in any given experiment. The best you can do is predict the probability that things will turn out one way or another. You might say there's a 50 percent chance that an electron will be here, and a 50 percent chance that it will be over there, and that's the best you can do according to quantum physics. The weird thing is that for almost a century now, we have been struggling to determine how do you go from this probabilistic description, a sort of fuzzy, hazy description of reality to the definite reality that we see when we make an observation. When you look around the world, you don't see half of me over here and half of me over there. You see one of me right here. How do you bridge that gap? Well, many years ago, a wild suggestion came forward from a guy named Hugh Everett at Princeton University.

NNAMDIYeah.

GREENEHe said, look, if the math is saying that the electron could be here or it could be there, then it actually is in both locations, just in two separate universes, and there are two of you looking at the electron in each of those universes and thinking incorrectly that that single reality that you're perceiving is the only reality. There are other realities, multiple realities. That's how quantum physics gets its version of multiple universes into the lexicon of physics.

NNAMDIWe're talking with Brian Greene about multiverses. If you'd like to join the conversation, call us at 800-433-8850. Do you believe there are other dimensions? 800-433-8850. Or do ultimate universes sound like science fiction to you? You can also join the conversation at our website, kojoshow.org, or you can send us a tweet, @kojoshow. Brian, does this now mean that, for example, as I speak into this microphone right now, maybe in another parallel universe, I'm actually staying silent or screaming or even walking out of the studio?

GREENEYes. That is one of the implications of some of these versions of multiple universes. You know, in the book, I cover actually nine versions. You mentioned...

NNAMDIOh, yeah.

GREENE...a few of them at the outset. But, yes. And some of those, what you're describing would be happening, and frankly, I'm glad that I'm not in -- at least for this consciousness, in my head, I'm glad I'm not in the one where you storm out of the room.

NNAMDIWell, we'll get to one of those nine in a -- a little bit later. What's the difference between a universe and a dimension?

GREENEWell, a dimension is a way of describing the expanse of space. So when you, for instance, have a dinner party, you need to give three pieces of information in order that people know where to go. You have to give a street, a cross street and a floor number. And that means that there are three dimensions of space. You need those three pieces of information. When we're talking about multiple universes, each of those universes would have an expanse of space. Each would have a certain number of dimensions, but they're really two different ideas, even though they are related.

NNAMDIOnto the telephones. We will start with Gary in Washington D.C. Gary, you're on the air. Go ahead, please.

GARYGood afternoon. I have a certain problem with a lot of these ideas in physics, not that I disbelieve them, but we -- at one time, we had Newtonian physics, which seemed absolutely true and it worked really well -- still works really well. Though these days, we say that Newtonian physics is incorrect, even though it's useful. And how might it be that the ideas upon which we're basing a lot of these ideas in physics are convenient based upon what we know now but may ultimately turn out not to be true. And I have -- and my biggest problem is that that scientists often speak in terms of truth as opposed to in terms of theory and ideas.

NNAMDIHere's Brian.

GREENEThat's a great question, and I -- if I ever slip into the language of speaking of truth as opposed it, Gary, please call back, because that would be a mistake. You're absolutely right. What we do is we try to write down descriptions of the world that match the data that we have access to. Newton had access to certain data that basically he could see with his eyes -- the motion of the moon, the motion of a rock. He wrote down equations that could describe that motion spectacularly well. In the 20th century, we could go further than Newton. We could examine molecules and atoms. And in that domain, we found out that Newton's mathematics didn't work.

GREENEIt made wrong predictions. So we developed new mathematics that could describe that realm. And what we're doing today is the same thing. We're looking at data, developing mathematical theories. And when we pursue those theories -- and this is the main point of what we're talking about here -- when we pursue those mathematical theories, they bump into one or another variety of parallel universes. It doesn't mean this idea is correct. We can only believe things once they're experimentally confirmed, but it is compelling that when we think about our theories sufficiently deeply, we come upon this idea of parallel universes. It comes to us. We don't go chasing it.

NNAMDIThank you very much for your call, Gary. You too can call us. 800-433-8850. I'm glad you brought up mathematical theories because string theory might be related to this. How are these parallel universes being created? What are they made of?

GREENEWell, string theory is this approach to building a unified theory, which is Einstein's long-held dream, one master equation that might describe everything in accurate and precise detail. And as we have been developing this mathematical theory or string theory over the last few decades, we have found -- to go back to your previous question -- that we need more dimensions of space than we thought. We need six or seven additional dimensions of space. So if you have a dinner party giving a street, cross-street or floor number according to string theory, wouldn't actually be enough. You need to give additional pieces of data to really delineate where things happen.

GREENEThose extra dimensions have to be curled up very small all around us. That's why we don't see them. The puzzle has been -- you can curl them up into many different shapes. We don't know which shape the extra dimensions take in our universe. So one of the ideas is maybe there are many universes, each with a different shape for the extra dimensions. And we live in that universe with the shape of the extra dimensions yields physical properties compatible with our biological makeup. This is one of the ideas of string theory.

NNAMDIWe're talking with Brian Greene. He's a professor of mathematics and physics at Columbia University. His latest book is called "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." What does this do to our understanding of the here and now? Is there a now, or are there many versions of it?

GREENEWell, even Einstein back in relativity challenged the notion of what does now mean? Because in relativity, different individuals that are moving relative to one another, they're clocks. Tick off time at different rates. So what you consider to be now, if I was moving relative to you, I might consider the future or I might consider the past. So the demarcation to past, present and future is something that we have some difficulty really nailing down in physics, even Einstein in consoling the widow of a close friend. He said the difference between past, present and future is only an illusion, however, persistent. When you go to a multiverse, the issue only becomes greater because the notion of time in these different universes can be independent of one another. So there may not even be a notion of now that applies across all of the universes.

NNAMDIWell, now, I'm gonna go back to the phone, and that will be in the future. Here is David in Suitland, Md. David, you're on the air. Go ahead, please.

DAVIDOkay. This might be jumping the conversation ahead a little bit, but one of the spinoff postulations that I've heard about the multiverse hypothesis or theory is that that there could exist in the multiverse advanced civilizations, and that these advanced civilizations might be so advanced that they're capable of running simulated universes, and that each one of these advanced civilizations are probably not just running one simulation but maybe hundreds or thousands or zillions of simulations. And so what I've heard from people talking about the multiverse theory is that there actually could be more simulated universes out there than actual universes.

GREENEYes.

DAVIDThe only -- okay. But this is where I get confused. (laugh) If a -- suppose I want to figure out the percentage of actual universes that are out there as opposed to, you know, compared to all universes, both simulated and actual. So I take the number of, say, real universes, which is infinity, and I divide that by the total number of universes, both real and simulated, which is infinity, and, well, I'm not too sure what number that is, but I'm not too sure that that means that there are more simulated universes than actual universes, unless some infinities are larger than other infinities. So I guess that's my question...

GREENEGreat...

DAVID...are some infinities larger than others?

GREENEYeah. Well, it's a great question. Let me just give two quick responses. So, first of all, the idea that there may be simulated universes is one of the multiverses that I cover in the book. It's one of the later chapters. I wanna emphasize at the outset in addressing your question that it's the most fanciful possibility. This book is not a book about science fiction. It's really about science as we are understanding it at the cutting edge of research today but as sort of light relief at the end of the book I take up some of the issues that you're talking about -- they're mind tickling. They're fascinating.

GREENESo it is possible that there could be simulated universes like "The Matrix," and the interesting point that a philosopher at Oxford made a couple of years ago is the one that you're making. If you could go home at night and sort of kick back and turn on your universe simulator and to simulate universe upon universe, well, there'd be many, many more simulated universes than real ones. And that would suggest that what we're seeing in the world around us might be simulated. That is how the idea went.

GREENEOne of the issues is exactly the one that you're raising. If there are, perhaps, are infinitely many of these simulated universes and, maybe infinitely many real ones too, how do you compare the different infinities? This is one of the main puzzles in this whole subject that we are grappling with today. In fact, there was a research gathering at Princeton last week that was focused on exactly this issue. How do you compare infinities in this context. We have not solved it yet. Typically, what one does is, one only looks at a finite subset of these infinite universes and makes comparisons between the number of simulated ones and the number of real ones there. But you have to be certain that as you allow the cutoff to include all of the universes, that it doesn't somehow bias your result, that it simply was determined by the technicalities of how you happened to do the calculation. So the bottom line is, the question you raised is a real one that we have not yet resolved.

NNAMDIDavid, thank you so much for your call.

DAVIDWell, thank you for clearing that up. (laugh)

NNAMDIWe're gonna take a short break. When we come back, Brian Greene will clear some more things up about multiverses and parallel universes. But if the lines are busy, you can go to our website to raise your question or make a comment. That's kojoshow.org. Or you can send us a tweet, @kojoshow. I'm Kojo Nnamdi.

GREENEOur guest is Brian Greene. He's a professor of mathematics and physics at Columbia University and author of the book "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." A lot of people are calling and e-mailing. I get the impression they just want me to get the heck out of the way so they can ask you questions themselves, but let me just get in a couple of questions before that happens. What do these multiverses look like and why can't we see them?

GREENEWell, the multiverses could have a number of different complexions. Some might be similar to the world that we see around us right now. Others could be governed by completely different looking laws of physics. It could be that those other universes don't have the kinds of particles or the kinds of forces that we have and experience in this universe. So the range of possibilities is large. But again, the point that I keep coming back to, I don't mean to beat a dead horse, we don't sort of sit in dark rooms and say, what kind of nutty idea now can we think about in physics? We follow our mathematical theories that we developed to describe the things that we can see in the world around us, and we allow those laws. We allow those laws to tell us what those other universes might look like.

GREENENow, why can't we see them? Well, some of these universes would be so far away that light that they give off could never traverse the intervening space between us. And that's one way that we couldn't see them. In the slice of bread version of the multiverse that you started with early on, when you follow the math in those theories, you find that light can only move along your slice of bread. It can't get off your slice of bread, and that's why you can't see the others. So the different multiverses have different reasons why the other universes would be invisible to us.

NNAMDIPlease keep calling. Or if the lines are busy, go to our website, kojoshow.org, or send us an e-mail to kojo@wamu.org. But there are a few concepts I'd like to get to before we get back to the phones. You mentioned earlier that scientists have come up with nine versions of the multiverse concept. Can you explain the Brane or B-R-A-N-E version to us?

GREENEAbsolutely. So actually, the Brane version is the technical name for the slice of bread version, and it comes out of string theory. You would think by the name string theory that the ingredients in the theory are strings, little tiny vibrating strings, and that is the new view of matter and microscopic scales that comes out of this hypothetical, unified theory called string theory. But as we studied the math of string theory, we found that strings are not the only ingredient. There can be these membrane-like surfaces that have two dimensions or three dimensions. And from the word membrane, we get the idea of a brane. A two brane has a membrane. A three brane has three dimensions.

GREENEAnd the wonderful thing about this approach is that everything that we see in the world, according to these equations, could be existing on one of these branes. And that's why I used the slice of bread metaphor. If we are living on a brane, it would be like a slice of bread, and there could be other slices out there, as you mentioned, which would be other universes, a brane multiverse.

NNAMDISo we would be living in a three-brane universe so to speak?

GREENEPerfect. Yes, a three-brane universe.

NNAMDIDo these branes ever touch or collide?

GREENEThey could. And, in fact, one of the other multiverse ideas that comes out of this approach is one in which the other universes are not out there in space, they're out there in time. How would that work? Well, two of these Branes can approach one another and collide, slam into each other like two pieces of bread in the sandwich just smacking into each other. And if that happened, calculations show that there'd be a kind of Big Bang-like eruption that would happen on each of those branes. Each of those universes would be started anew in this fiery collision from these two branes. So the idea is these branes could collide, bounce off, collide again, bounce off, collide again, giving rise to universe upon universe upon universe happening in time, not in space. Now the timescale go would be big, trillions of years. So the idea would be maybe trillions of years from now, we will collide with another brane that's hovering out there slowly coming toward us and that would initiate a new universe in this brane that we now occupy.

NNAMDIHave you discussed this concept with Jon Stewart on "The Daily Show" yet?

GREENENo. (laugh)

NNAMDII can't wait to hear his responses. Here is Steve in Arlington, Va. Steve, you're on the air. Go ahead, please.

STEVEYes, thank you. I always enjoy Mr. Greene on your show, Kojo. And I'm sorry he didn't get a few minutes during the State of the Union address last night. I think next year the President should definitely invite him. But my question is, early in the show he said we discovered that everything is much bigger than we thought it was. I didn't really understand the sense in which he meant big. If I understand that, maybe I can cope with the idea of the multiverse a little easier. So maybe he can clarify and I'll take my answer off the air.

NNAMDIThank you, Steve.

GREENEI think one way of clarifying is to be concrete with an example. So let me use this, we all know of the Big Bang as the theory about how the universe started. It imagines that our universe was, in the distant past, a very, very small, very, very dense and then erupted with matter and energy all spewing outwards, expanding, giving rise ultimately to the stars and galaxies that we see. The little dark secret about the Big Bang that we perhaps don't emphasize enough is the Big Bang says nothing about what happened at time zero itself, at the very, very beginning. And we've been struggling to figure out what the bang was that got it all going.

GREENEThere is a theory for what the bang is. It's called inflationary cosmology. There is a kind of gravity it turns out that's repulsive that may be what drove everything apart at the first instant of time. The reason I bring this up is, when we studied this theory in mathematical detail, we find that the Big Bang-like explosion is likely not to have been a one-time event. There may have been many, many Big Bangs happening at various and far-flung places throughout a bigger cosmos. So our universe would be one bubble in this grand cosmic bubble bath of universes. It's in that sense that the universe, the whole thing, the multiverse would be bigger than what we thought. Our universe would only be one of those expanding bubbles. There'd be many other bubbles out there, a bigger cosmos.

NNAMDIBrian Greene's latest book is called the "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." We're inviting your calls at 800-433-8850, or you can simply send us a tweet, @kojoshow, with your comment or question. Here is Stanley in Upper Marlboro, Md. Hi, Stanley.

STANLEYHello. I was thinking -- listening to the different things we're speaking of earlier with the other caller such as the slices of bread and such, where the Big Bang originated from, and how quantum mechanics in math does not work with our mathematics such as in the beginning of time, according to Stephen Hawking, when the Big Bang was the size of a grapefruit, the expanse was a trillion times faster than a speed of light. Well, our mathematics says that would go back in time. But according to Hawking that they're not true because there was no time. But particularly, I'm interested in that slice of bread thing, where things came from about white holes and black holes. Do you know anything about white holes?

GREENEYes. A white hole is a theoretical entity that would be a kind of reversal of a black hole. Black hole is a region of space where the gravity is so powerful that anything that gets too close gets sucked in and can't get out. A white hole would be the reverse. Rather than pulling things in, it would be spewing things out. So in some sense, you can think of, roughly speaking, as the Big Bang is sort of what a white hole would be like -- everything rushing outward as opposed to coming in. We don't know if white holes exist. There are reasons for believing that they don't exist in the universe around us. But this is an active area of thought.

NNAMDIStanley.

STANLEYYes. Fantastic. I was thinking about the unified field theory in the sense that when I was in the fifth grade, my teacher in science told us that perhaps our cosmos makes up the molecular field that makes a dust particle that's on a badge of a policeman on a dirty street corner in a macro existence in the aspects of macro and micro.

GREENESure. And that's an idea that you'll also find in "Animal House" when the frat boys are wondering where there's a whole universe taking place in their fingernail. But it is an interesting possibility that all that we see is actually something that's tiny, tiny in a bigger universe, and some of these multiverse theories as we're discussing brush up against that weird-sounding idea because all that we see would be, in fact, a small part of a universe or multiverse that would be a lot bigger. One thing I just wanna go back to quickly, we're talking about how fast space is expanding early on as Hawking had noted, you know, trillions of time faster than the speed of light. That may sound contradictory to the usual notion that nothing can go faster than the speed of light.

GREENEBut you really need to bear in mind that when Einstein said nothing goes faster than the speed of light, he really meant that no object can move through space faster than the speed of light. Space itself is not constrained by that. It can expand at any speed it wants. It can expand faster than the speed of light. And our theories, many of which are backed up now by astronomical observation, suggest that they did expand faster than the speed of light and may, in fact, be doing so today in far distances from where we're now sitting.

NNAMDIThank you very much for your call, Stanley. I'm glad you brought up "Animal House" and popular culture because we got this e-mail from Kurt in Vienna. "As a comics fan from way back, the concept of many universes is an old one. John Broome, a science fiction and comics writer, started the annual crossovers between Earth-One's Justice League and Earth-Two's Justice Society. This was a way of letting the Silver-Age Justice League meet with the Golden-Age Justice Society. As time went on, it got very complicated with the multitude of Earths for all sorts of groups of heroes. DC Comics tried to cut the number back in the '80s to one universe. But in the odds, the multiverse came back, just because the idea is so much fun to play with. I was wondering if Brian had run into the comics."

GREENEI have not, unfortunately. But it is the case that there is a lot of fiction that has been inspired by the possibility of other universes. I mean, even in the "Wizard of Oz," there is a notion of an alternate reality. If you look at any of the wonderful short stories, a number of short stories of Borges. He explores this possibility about the universes. Things like, you know, "Sliding Doors," a recent popular film examines this idea. So I think we're really well-equipped emotionally to think about this idea. We, I think, are searching often for a reality that lives beyond what we can see directly with our eyes and directly feel with our fingers and senses. The remarkable thing is that even though scientists are not driven by that kind of urge that comes from some place very human, we find that our math is leading us to variations on this idea.

NNAMDISpeaking of skepticism, let's talk with David in Arlington, Va. David, you're on the air. Go ahead, please.

DAVIDHi. I would just like to say that I think this is a good example of how theoretical physics is sort of destroying physics and how it's becoming very much a mystical sort of philosophy. And I'm curious if you've ever heard of David Harriman, H-A-R-R-I-M-A-N, and his lecture, Crisis in Physics, how he critiques the -- what's happened to physics over the last 100 years. And that can be viewed online for anyone who wants to after the show. Thank you.

GREENEWell, I can sort of say two things. Number one, let's be real clear. We are talking about research at the cutting edge of physics, which I have emphasized before, but I'll emphasize again. We do not know if these ideas are correct. We can only believe them once they make experimental predictions that are confirmed by observation or by data. So we are at the edge. Having said that, we are not in any way just dreaming up wild ideas. We are doing what scientists have always done, which is try to come up with theories that can describe the things that we can see, that are verifiable.

GREENEFor instance, in this multiverse with the bubble universes and this grandeur cosmos that we were talking about just a moment ago, that theory gives rise to predictions for tiny temperature variations in the depths of space. Heat left over from the Big Bang, the cosmic microwave background radiation. We have built powerful telescopes. We have measured the heat in space. And the predictions from the theory and the observations matched to fantastic precision. This is not wild-eyed imaginings. This is coming from very conservative coherent thinking about the nature of reality coming from our mathematics.

GREENEYou know, back when Einstein introduced relativity, it was a kind of crazy idea at first, talking about how time would be different based upon how you move or the gravity you feel. And then we did experiments and we confirmed it. I'm not saying that that is happening yet. I don't know when or if it will happen with these perhaps more far-out ideas. But we are following exactly the same procedure. Math has proven to be a surefooted guide to the nature of reality and we are using it as a guide now.

NNAMDIAnd there may not be any proof right now, but it is my understanding that there is the potential proof in the possibility of what the Large Hadron Collider in Geneva can do.

GREENEYes. That's a great point. So this slice-of-bread universe that we are talking about, the brane multiverse is one that could receive experimental support at the Large Hadron Collider. The idea is this, the Large Hadron Collider slams proton against proton at very high speed. And the mathematical calculations we do suggest that in those collisions, debris can be created that might be ejected off of our brane, off of our slice of bread. How would we know that? Well, if the debris goes away, it'll carry away energy with it. So there'll be less energy for our detectors to detect after the collision than before. There'll be missing energy, because some have drifted out into that bigger, wider cosmos.

GREENEPeople are now looking for these signatures. They have not found them yet. It's still early in the game. But if we do find evidence that there is energy missing in the manner that I'm describing, that'll be indirect experimental evidence that we're living on a brane, indirect experimental evidence there for that there should be, perhaps, other branes out there.

NNAMDIBased on what we have seen so far, are you convinced that these multiverses do exist?

GREENEAbsolutely not. I am only convinced of things for which there is experimental evidence, observational evidence, observational data. I get excited about these ideas, because they're so wondrous and mind blowing and they are rooted in our traditional mathematical analysis of the world around us. But you can't believe anything until there's experimental proof.

NNAMDICan't believe, he'd rather know. Here is Dennis in Chevy Chase, Md. Dennis, you're on the air. Go ahead, please.

DENNISHi. Well, I'm amazed, aghast that I don't know so much as you folks, but I'm amazed that there's such complexity. Now, the question though is what happens if evolution between these universes occurs, and really result of the evolution is that only one actually survives, the strongest or the one that is less affected? What happens in our thinking if all of your mathematics actually does reduce to one final number, which gives our equation? Have you studied the possibilities to that? Have you studied the possibility of time going both ways and actually ending up at some point that's actually in the middle?

GREENEWell, it's a very good question to wonder about, are these different universes in competition in some sense. Is there a way in which Darwinian evolution applies across the multiverse and only the most fit universes would survive? It's an idea that has been taken up in some multiverse proposals. My friend, Lee Smolin, you may be familiar with his book, took up this idea with black holes being the place where new universes would be produced. And he had a version of Darwinian evolution that would happen there.

GREENELet me stress, though, that these universes really are not fighting against one another. They're king of coexisting. The only way in which one universe would kind of win out in a Darwinian sense is there might be more universes of a given type in this grand multiverse. Therefore, there'd be more universes in which certain properties were manifest. And if those properties agreed with the properties that we see here, that would suggest that we are just an average garden-variety universe in this big multiverse, and that in fact would be one way of testing a theory like that. If a multiverse theory says that almost all the universes have an electron with certain properties, and we see that electron with those properties, that would be a confirmatory piece of data for that multiverse. If a given multiverse proposal says almost all universes have, you know, people with square heads, and we look around and we don't see people with square heads -- and, you know, it's a metaphor for, say, a particle with different properties -- then that multiverse would be disconfirmed. That's the way in which science would progress in a multiverse setting.

NNAMDIDennis, thank you very much for your call. We've got to take another short break. If you have already called, stay on the line. We'll try our best to get to your call. Brian Greene is author of "The Hidden Reality." He's a professor of mathematics and physics at Columbia University. He joins us in studio. You can also join the conversation by sending us an e-mail to kojo@wamu.org or by going to our website kojoshow.org. I'm Kojo Nnamdi.

NNAMDIWe're talking about multiverses or parallel universes with Brian Greene. His latest book is called "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." Brian Greene is a professor of mathematics and physics at Columbia University. You mentioned your colleague, Lee Smolin. We got an e-mail from Lynn who says, "Please have Brian Greene respond to the criticisms of Lee Smolin, a theoretical physicist and author of "The Trouble with Physics." He writes about the failure of string theory to live up to its claims and discusses the strong bias in favor of funding string theory research in academia rather than a variety of other realms of physics. I loved Greene's book, but it does seem to be that string theorists have come up with very little proof in decades."

GREENEThis is absolutely true. We are in the midst of developing string theory. It's largely a mathematical undertaking at the moment, and we would love to have experimental proof. It's a very difficult theory because we are dealing the most extreme realms of physics. String theory puts together general relativity, the laws of the big things like stars and galaxies together with the laws of the small things like molecules and atoms. And theories that put those two very, very diverse realms together into one package only really have a direct experimental signature in the most extreme realms, the center of the black hole, the moment of the big bang. It's very hard to test these theories. But let me emphasize that's not a problem with string theory. That's a problem with any approach to put together the big and the small. Any quantum theory of gravity faces exactly the same challenge, even the approaches that Lee Smolin has been following. They haven't made any experimental predictions either, I absolutely assure you of that.

GREENEYou know, Lee is a good friend of mine. And largely said that he's been misunderstood in his book, that his only point really was that it's more healthy for a very deep problem to be approached from a variety of different angles, not just string theory. And I have to say I completely agree with that. I am thrilled that there are people that work on string theory, that there are people that work on Lee Smolin's approach with his colleagues. And there are people are working at another approaches, dynamical triangulations and all sorts of other ideas. That is a great thing. So I completely agree with him on that point. We want to approach this deep, important problem. Quantum mechanics and gravity, put them together, we want to approach it from a variety of different perspectives.

NNAMDIIn some ways, this book seems to be about the power of math because it's math that gives us the vehicle to even put these ideas forward. Are there any limitations to where the math can take us with these big ideas?

GREENEYou're absolutely right. The one hypothesis that underlies everything I described in this book, in fact, the reason I called this book "The Hidden Reality" as opposed to hidden realities, plural, the reason I took singular is because there is one idea underlying all these multiverse proposals, which is that math can take us to the next step to understanding the universe. Why do we believe that? Well, it's done that in the past. We have developed theories for how the electron operates. We do our calculations. We get certain features of the electron, its magnetic properties, we calculate it to 10 decimal places, we do the experiment. The math and the experiment agree to 10 decimal places. Holy moly, that's incredibly impressive that math can do that. And our belief is that it can take us to the next step as well. Are there limits on math? There may be. Math may be a language that has been developed by humans and is really, really good at describing patterns in the universe. But maybe there's another approach.

GREENEYou know, I was once in a public debate and someone asked a similar question. And I said, well, I can imagine that one day we'll be in contact with some alien civilization -- not sci-fi, I'm just trying to make a point. We might be in contact with some alien civilization and they tell us, how have you been going about understanding the universe? And we say, well, we have these mathematical equations. We show them what we've done, they say to us, yeah, math, we tried that. (laugh) You know, it worked for a while, but here's the real way of thinking about the universe. But if you push me further and say, what would the aliens say next? What is the real way? I have no idea, and I don't even really know how to step outside mathematics. So I don't know the answer to the deep question that you just asked. It's possible there are limits. I'm not sure.

NNAMDIHere's Cathy in Bethesda, Md., who may have another way of looking at it.

CATHYHello.

NNAMDICathy, you're on the air. Go ahead, please.

CATHYHello. Just to tell you where I come from, I'm about 50 years old and I'm a painter. I'm always interested in science since childhood. I think one of the best books ever written that would combine my interest was written by Leonard Shlain called "Arts and Physics." How -- he has put out how arts and physics have been paralleled in developing ideas. And I just wanted to tell you that where I am myself, if I can just say, that there are parallel universes in our own world that after all this search that I have to find the meaning of life, I find that in the end, my painting just shows that the search for the self. And I think parallel universes exist among people that if you want to enter a parallel universe, all you have to do is talk to another person 'cause each one of us lives in his own universe. That's just my comment. I enjoy your program, Kojo. Thank you for taking my call.

NNAMDIHere's Brian Greene, Cathy.

GREENEWell, I think that's great. And, you know, Leonard wrote a very nice book on art and science, which I read and enjoyed it very much. But one thing I do wanna stress is the kind of parallel universe you're talking about is an important one. You know, you talk to different people, they have different perspectives. And different cultures have different perspectives so you can think about these as parallel worlds. That's all great. What I'm talking about here in the book is completely different. What I'm talking about here in this book is really literally other universes in the real most basic sense of a physical reality that would be beyond the reality that we can see in the world around us. So while it's great to draw connections between physics and what we experience in doing our everyday lives, I just do wanna draw that one sharp distinction.

NNAMDIHere is Gerald in Northwest Washington. Hi, Gerald.

GERALDHi, Kojo. Great show. Great show. I've been listening, and my question, initial question, has changed a little bit. At the Big Bang, when quarks were created, when the quarks were created, could possibly that extra dimensions of space and time, the universe that we're in, that we perceive right now, did those quarks, as they began to form electrons and protons, could create -- could have created another universe? And the other thing that I'd like for you to talk about a little bit is M-theory, the -- which predicts the 11 dimensions of space and time.

NNAMDIHere's Brian.

GREENESo to begin with the second question first, so M-theory is an extension of string theory that has come from developments since roughly the middle of the last decade, the mid-1990s. And one of the big ideas of M-theory, one of the things that M may stand for is membrane. So this brane multiverse that we're talking about, more properly, does come from M-theory even though I use the name string theory. M-theory would be the more precise way of describing where the brane multiverse sits in our logical understanding of cutting edge physics.

GREENEThe other question you asked in terms of could the way in which quarks combine yield other universes, other realities, well, yes. In these different bubble universes that we were talking about before, just to give a concrete example, there could be quarks in those other universes. And they might come together in different ways due to slight variations in the way physics and the forces manifest themselves in those other bubbles. And therefore, properties in those other universes could be different from the properties we are familiar with here. Maybe there's no protons in those other universes, or no neutrons or no atoms. Or maybe there are other kinds of atoms that we've never heard of. All this is compatible with the ideas that we are now exploring.

NNAMDIThank you very much for your call, Gerald. We got an e-mail from Lisa in Silver Spring, who says, "The President spoke last night about how he's worried that we're falling behind in math, science, the tools that lead to these kinds of theories and discoveries. What got Brian sucked into the scientific world, and what concerns does he have about whether young American students are going to continue to carry the flag for our best and brightest in the scientific world?"

GREENEWell, I think, like many practicing scientists, what got me involved in this kind of work in the first place was, when I was young, I had some really great teachers that got me excited about these wondrous ideas. So from seven or eight or nine years on -- it started with my dad. But then in the more formal process of education, I was very fortunate to have people around me that were excited by science. And that has to be the case going forward if we are, as the president said, to stay competitive. It's one way of thinking about it.

GREENEBut, you know, we, as a collective species worldwide, there are some great questions that we are on the verge of addressing, and we need to have the next generation of scientists, American or from the rest of the world. It doesn't really matter to me. I just want the collective understanding to press forward the river of knowledge. We need to participate as a global community, and that requires getting kids excited about science. You need to have great teachers, and you need to have other ways, beyond the formal education system, for getting kids excited.

GREENEYou know, that's why I write books. That's why I'm filming a NOVA television special about space and time and quantum physics. It's why we started something called the World Science Festival in New York, where, five days each year, we have events all around New York where kids and adults can just immerse themselves in the wonders of science. All of this is vital to our future, our collective future.

NNAMDIThank you. Here's Jim in Carrolton, Texas. Jim, your turn.

JIMOh. Thank you for taking my call. I have two questions, really, that I think you can answer with one answer. The first is since quantum mechanics and relativity contradict each other, doesn't that mean that one or the other has to be wrong? And specifically, Einstein predicted a particle called the graviton. Wouldn't we have detected the graviton if it existed by now?

GREENESo it is a good question. And you're right. If two theories -- quantum mechanics and general relativity -- don't go together, one of them has to cry uncle. One of them has to change. And in string theory, the one that cries uncle is Einstein's general relativity. String theory modifies the mathematics that Einstein wrote down, not in ways that would affect Einstein's predictions when it comes to the motion of the moon or the sun or the galaxy, but predictions that would be different if you're applying his ideas to the microscopic realm. That does not affect the possible existence of a graviton. That still comes out of the mathematics. And we're pretty confident on why we haven't seen the graviton. Gravity is the weakest of all forces. Graviton is the smallest particle of the weakest force. That makes it a huge challenge to be able to detect a particle like that. We haven't done it, and we may not for a very long time.

NNAMDIIf the theories about multiverses are ever proven to be correct, how would that challenge our current ideas about science?

GREENEWell, you have to do science in a somewhat different way because you have to take into account that there are realms that you'll never see, you'll never experiment with, you'll never visit. And that's not usually how we think about science. You wanna understand the micro realm, you go in there with powerful microscopes. You wanna understand the far reaches of the cosmos, you look at them with powerful telescopes. We will have to have a more indirect approach to testing these theories, a more indirect approach to making predictions. It's a slightly different kind of science, but I assure you, it's still will only be convincing if it makes predictions that we can test.

NNAMDINow here is Mindy in Laurel, Md. Mindy, you're on the air. Go ahead, please.

MINDYRight. Thank you. Yeah. I'm -- I find it very interesting. When I was a kid, I remember watching a show. It was a Carl Sagan show, and specifically he was talking about different dimensions. And even today, from that, what I gathered was that because as humans we see in 3D, who's to say there's not another dimension, a fourth dimension, out there that we can't see because we are made to see in 3D, but that could exist right here with us, you know, on our own little slice of bread? (laugh)

GREENEYes, it's absolutely right. I know the episode of "Cosmos" that you are referring to, where Carl Sagan is describing using the technique that comes from a book called "Flatland," where you have beings that live in a two-dimensional world. He's describing what it would be like for them to live in a world that actually has a third dimension that they can't see because they're not constructed to be able to see in that third dimension. And you're right. We really are taking that idea and expanding it -- in string theory, in particular, since we need extra dimensions. The explanation for why we don't see them is we are built to see the three dimensions in the world around us. The other dimensions are here. They're all around us, but we can't see them because we are not built, biologically, to have direct access to them.

NNAMDIHow do you take these theories forward now after you've done explaining them to an audience that may have started out being baffled but now has a better understanding, both from either reading your book or listening to you?

GREENEWell, you mean like my own work?

NNAMDIYeah.

GREENEYou know, I am doing research right now on the string theory versions of parallel universes. And we are trying to understand how cosmology, how this whole collection of universes would evolve over time. And we are doing various calculations, some of which I described in the book, of how a universe of one form with one shape for the extra dimensions might go through a quantum process and the shape of the extra dimensions might change, giving rise to universe inside of universe inside of universe, sort of like Russian dolls that we're familiar with and playing with them as kids. So I'm deep in the research on this subject. And basically, this is a stepping outside to give people a snapshot of our understanding on some of these far out ideas.

NNAMDIWell, I'd like to thank you for the part of your work that you do that attempts to make these very complicated scientific theories understandable for the lay reader or for the lay listener.

GREENEThank you.

NNAMDIBrian Greene is a professor of mathematics and physics at Columbia University. His latest book is called "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." Thank you very much for joining us.

GREENEThank you.

NNAMDIAnd thank you all for listening. I'm Kojo Nnamdi. Brian Greene will be appearing at Politics and Prose bookstore tonight at 7:00 p.m. He'll be signing books. That is tonight, as in Wednesday, Jan. 26, 2011. Again, thank you all for listening. I'm Kojo Nnamdi.