Cool segment from yesterday's Science Friday on NPR's Talk of the Nation. Brian Greene is the author several excellent books, including his newest book on the nature of a possible multiverse, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.

This is some mind-bending stuff - I love it. Here is a section that seems to be a summary of the main ideas of Greene's multiverse approach:

So there's a version of multiple universes that comes from string theory, and it's called the Brane Multiverse, B-R-A-N-E. And that comes from the word membrane, which is one of the recent realizations in string theory is that the ingredients in the theory are not just the little, tiny vibrating strings that we've, for instance, spoken about in the past on this program. But the theory also has giant membranes which may have two dimensions, like a giant sheet, or three dimensions, a three-dimensional membrane. We call it a three-brane.And that suggests the possibility that all we know about our universe may exists on a giant membrane. I like to think of it as a giant slice of bread. So imagine every star, every galaxy's on a giant slice of bread, with other slices of bread out there, other universes, other brane universes nearby in a giant, cosmic loaf.

I have enjoyed Greene's earlier books (particularly The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory and The Fabric of the Cosmos: Space, Time, and the Texture of Reality), so I'll have to read this one as well.

In a new book, mathematician and theoretical physicist Brian Greene explains the concept of multiverses, and why some physicists believe there could be more than one universe. Plus, a look at the hidden universe of Greene's desk, this week's "Desktop Diary" video pick.IRA FLATOW, host: This is SCIENCE FRIDAY. I'm Ira Flatow.

Up next: physics to make your hair hurt in a, really, a good way. Now, you're going to want to hear about this. For example, think about this: Is the universe we are living in the only one? What if, when you got into your car this morning to drive to work, another you in another universe got into a different car and drove to the beach instead?

Seems hard to believe and maybe more like science fiction, but some theoretical physicists say we might exist in one of many universes, or in a multiverse, as they call it. So what if the universe, as we know it, is really just one of many universes? What does that tell us about the laws of physics here in the universe we know of? Can we ever test the theory of a multiverse? And ultimately, does this mean we need more than one Miss Universe pageant?

Joining me now to talk about it is my guest - Brian Greene, having a good laugh with me, here. He is professor of physics and mathematics at Columbia. He's also author of a new book, "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos," back in our New York studios.

Thanks for being with us here today.

Professor BRIAN GREENE (Physics and Mathematics, Columbia University; Author): Oh, it's my pleasure.

FLATOW: Do we need another Miss Universe, kind of?

(Soundbite of laughter)

Prof. GREENE: I guess so. I didn't - I never thought of it that way.

(Soundbite of laughter)

FLATOW: This is a serious idea, right? It's all through your book that there could be more than one universe.

Prof. GREENE: Yes. It's a strange idea, because in the old days, which means like two-and-a-half years ago...

(Soundbite of laughter)

Prof. GREENE: ...the universe meant everything, every star, every galaxy. So what would it mean to have more than one everything? But from a number of directions, our mathematical research is suggesting that what we have thought to be everything in the past, well, that may be a small part of a much grander whole that may have other realms rightly called other universes.

FLATOW: And where would they have originated?

Prof. GREENE: Well, it depends on the proposal. Actually, in the book, I go through, actually, nine different ways in which physics has come upon this idea. The simplest is perhaps there could have been more than one Big Bang. We think of the Big Bang as creating our universe, but as we have studied the Big Bang in more and more detail, the math is suggesting that the Big Bang may not have been a unique event. There may be many Big Bangs that happened at various and far-flung locations, each creating its own swelling, spatial expanse, each creating a universe -our universe being the result of only one of those Big Bangs.

FLATOW: And you talk about three different kinds of universe.

Prof. GREENE: Oh...

FLATOW: We don't say universi(ph), I guess - universes.

(Soundbite of laughter)

Prof. GREENE: Yeah. We have definitely gone with the S as the plural.

FLATOW: Yeah.

Prof. GREENE: But there are actually are these nine versions. One is from this multiple Big Bangs. Another very simple one to think about which aligns with your remarks about someone driving to the beach instead of going to work, arises from imagining that space goes on infinitely far. We don't know that it does, but that's an idea that people take very seriously, that it may go on forever.

If it does, there's a startling conclusion that really hasn't received as much attention as I think it deserves, which is in any finite region of space, matter, particles, can only arranged themselves in finitely many different configurations.

The metaphor I like think of is a deck of cards. You shuffle the deck, the cards come out in different orders, but there are only finitely many different orders. And that means if you shuffle the deck enough times, the order of cards must repeat sooner or later.

FLATOW: Ah.

Prof. GREENE: Similarly, in infinite space, the arrangement of the particles must repeat sooner or later, too. And, look, you and me, we're just collections of particles. So is the Earth and the sun. So if the particle arrangement repeats, then we are out there having this conversation.

And to speak to your introductory remarks, it's even easier for the particle arrangement almost to repeat, but not repeat exactly. That would mean that you may be doing something else. Maybe in that other universe, you have a mustache, or my name is Jonathan or...

FLATOW: Right.

Prof. GREENE: ...there's all sorts of crazy stuff happening out there, and that is all compatible with this idea of space going on infinitely far.

FLATOW: Is it possible to find any evidence of these universes at all?

Prof. GREENE: That, of course, is the key question. And I should say that the book itself is not really a cheerleading book for the multiverse. It's not a multiverse manifesto. It's taking the idea seriously, because a large fraction of the theoretical physics community is taking this idea seriously and looking at it from all angles and asking those kinds of difficult questions. Can you test it? And there are some ways in which this notion could be tested.

For instance, in the multiple Big Bang version that gives rise to different universes, it's possible that the different universes, as they expand, could collide with one another, sort of like - think of a bubble bath. Each of the Big Bangs gives rise to one expanding bubble in the cosmic bubble bath. Those bubbles can smash into each other.

And if they did, if our universe got hit by another, had a fender-bender with another universe, that would send ripples going through the cosmic microwave background radiation - heat left over from the Big Bang. And astronomers are now looking for patterns in the microwave background radiation that might suggest that we did have that encounter in the past with another universe. That'd be a very direct way of establishing that other universes are out there.

FLATOW: 1-800-989-8255. We're talking with Brian Greene, author of "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." You can also tweet us @scifri: @-S-C-I-F-R-I.

And talking about multiple universes here, the Large Hadron Collider, it's going to do powerful experiments. Any possibility of seeing evidence of multiple universes there?

Prof. GREENE: Yes. Absolutely. So there's a version of multiple universes that comes from string theory, and it's called the Brane Multiverse, B-R-A-N-E. And that comes from the word membrane, which is one of the recent realizations in string theory is that the ingredients in the theory are not just the little, tiny vibrating strings that we've, for instance, spoken about in the past on this program. But the theory also has giant membranes which may have two dimensions, like a giant sheet, or three dimensions, a three-dimensional membrane. We call it a three-brane.

And that suggests the possibility that all we know about our universe may exists on a giant membrane. I like to think of it as a giant slice of bread. So imagine every star, every galaxy's on a giant slice of bread, with other slices of bread out there, other universes, other brane universes nearby in a giant, cosmic loaf.

Now, to speak to your question: At the Large Hadron Collider, when protons slam against protons at very high speed, the math suggests that some of the debris created in the collisions could be ejected off of our brane, off of our universe. How would we know that? Well, the debris will take some energy with it, which means there'll be a little less energy in our detectors after the collision than before, a missing energy signature that could give evidence that we're living our lives on a brane, that there are other branes out there, that we are part of this multiverse from string theory.

FLATOW: And some of these particles that disappear from - could be going into a - one of those dimensions, which string theory allows for.

Prof. GREENE: They would - precisely right. So the other universes would be displaced from us. They'd be away from us by perhaps a millimeter or less, but in another dimension. So the dimensions that we know about would be on our piece of bread. The orthogonal directions that go from one brane to another would be the extra dimensions that string theory requires.

The remarkable thing is, you say, well, if they're out there, why don't we just see those other universes that are right nearby? And the thing is, in string theory, light, the medium by which we see, can easily travel along our brane, our universe, but it can't jump off of our brane and travel between the brane universes.

FLATOW: So it has to stay at the edge of the membrane.

Prof. GREENE: It stays on the edge when you're thinking about the edge in those extra dimensions. But it can move all around universe. I can see you. I can see stars. All of those are light beams that are going along our slice of bread. You try to make the light jump off our slice of bread so you can see the other universe, the light won't do it.

FLATOW: It just doesn't want to cooperate.

Prof. GREENE: Exactly.

FLATOW: 1-800-989-8255, talking with Brian Greene, author of "The Hidden Reality."

You know, we hear - one of the most exciting things in astrophysics is talking about this dark energy stuff. My pet theory - and I have absolutely no reason to state(ph) anything with it - is if there's another universe someplace, could that not be attracting - that the - so it's an attractive force from the outside. It looks like it's a pushing force from the inside.

Prof. GREENE: It's an interesting thought. And people have played with these kinds of ideas. I've never seen a version of that story that seemed convincing.

FLATOW: Yeah.

Prof. GREENE: But people have done a version of that with dark matter that, I think, holds, perhaps, a little bit more promise. You know, we know that there's matter out there that doesn't give off light, but affects us gravitationally. Could that be matter, say, on a nearby brane, a nearby slice of bread universe? Gravity can travel from one brane universe to another. Could it be responsible for the additional gravity that we know must be out there causing these stars and galaxies to move as they do? But the matter would be invisible, because light doesn't travel from brane to brane. That's a proposal, too, that people have studied.

FLATOW: Mm-hmm. Tell us about this - one of the kinds of universes that you have in there is the holographic universe - hard to imagine, for many of us.

Prof. GREENE: Yes. That is, in many ways, the strangest proposal of all, but it is one that may have the chance of being tested in the next few years. In fact, we're doing tests right now at the Relativistic Heavy Ion Collider. Experiments there are actually probing this idea.

And the idea is this: All that we know about in this three-dimensional world around us, this proposal suggests, may actually be a holographic-like projection of laws of physics that exist on a thin-bounding surface that surrounds us. Just like an ordinary hologram, that's a piece of thin plastic. You illuminate it the right way, it creates a realistic, three-dimensional image.

The math of string theory and the math of black hole physics suggest that everything we know about may be a similar holographic projection of fundamental information that exists on a large surface that surrounds us. Now, you may wonder: Where does this crazy idea come from?

(Soundbite of laughter)

FLATOW: You noticed the silence.

(Soundbite of laughter)

Prof. GREENE: It comes from an interesting puzzle, which comes from black holes. When you throw something into a black hole, we know it disappears. But the puzzle has been: What happens to the information that the object may contain? Let's say you throw your laptop or your iPad, whatever, you throw it into a black hole. Where does the information that that object contains go? Now, one suggestion from Stephen Hawking a long time ago is it simply disappears. The problem is, that conflicts with quantum mechanics. It creates tension with quantum mechanics.

So people like Leonard Susskind and Gerard 't Hooft, you know, they study this for a long time. And they concluded that what actually happens to the information is it gets smeared out on the surface of the black hole.

FLATOW: Mm-hmm.

Prof. GREENE: So your iPad, whatever, it goes into the black hole, but a copy of the information is smeared on the surface. That means that information on a bounding surface can describe a three-dimensional object that lives inside. And we believe what's true for black holes may be true for space more generally. We may be three-dimensional objects described, just like your iPad, that go into a black hole by information on a big, two-dimensional surface that surrounds us.

FLATOW: Do you think we'll ever know the real answer to any of this?

Prof. GREENE: Well, this particular proposal allows us to perform certain calculations that are otherwise completely impenetrable, to do with what will happen when gold nuclei slam into each other at very high speeds - which is what happens at the Relativistic Heavy Ion Collider out there in Long Island. The calculations are too hard to do in the traditional rote. But if you use this holographic version to translate the calculations into this bounding surface on the interior of this framework, you can do the math. The math makes predictions. And so far, the predictions are closer to the experimental data than any other approach that we have.

FLATOW: Wow. Talking with Brian Greene, author of "The Hidden Reality," on SCIENCE FRIDAY, from NPR.

And joining us now is Flora Lichtman, our digital video editor, who -you look - with our Video Pick of the Week. And it's a special one, Flora. Why don't you introduce it?

FLORA LICHTMAN: It is a special one. Hi, Dr. Greene.

Dr. GREENE: Hi, there.

LICHTMAN: This week's video pick is second in our series of desktop diaries. And in this desktop diary, we explore the hidden reality of Dr. Greene's desk.

(Soundbite of laughter)

FLATOW: So you...

(Soundbite of laughter)

FLATOW: It wasn't very hidden, was it, the desktop?

LICHTMAN: No, it wasn't very - it's a very tidy workspace, Dr. Greene, very, very tidy, indeed. In fact, we have a little piece of audio from that interview.

(Soundbite of archived recording)

Dr. GREENE: When I was five, six, you know, my dad would send me these 30-digit-by-30-digit multiplication problems. And I would spend a weekend doing them, you know, these huge pieces of paper, like, 30 digits by 30 - you know, that's a lot of writing.

But to get to answer at the end that no one had seen before was exciting. Now, look, no one had seen it before because there is no point in multiplying those numbers together. But I didn't care. It was just this big, spectacular game.

LICHTMAN: So I was curious, you know, is there an element of this spectacular game in the kind of work that you do now?

Dr. GREENE: Well, in a sense, yes, in that much of what we're discussing here and much of what I do in my day-to-day life is mathematical calculations. But there was a moment when I was growing up - older than five or six, but 12 or 13 or so - when I realized, because people taught me, that math can actually describe what's out there. And that changed math for me completely. It became much more than a game. It became a gateway to try to understand what's actually out there. And that's what really thrills me about these ideas. Yes, they come from math, but time and again, we have found that the math really can explain what we see.

FLATOW: And it does very - your desk, we would send Flora to do - your picture of your desk, and she came back with this wonderful, wonderful video, showing your environs that you work in.

LICHTMAN: And one thing that was surprising to us is that you do a lot of your calculations with pencil and paper...

Dr. GREENE: Yes, I do.

LICHTMAN: ...sort of a throwback.

(Soundbite of laughter)

Dr. GREENE: You know, when they get too complicated, we do turn to computers, you know? And I have good graduate students and post-docs, as well, and they're great calculators. But I was worried when you were coming and doing this desktop thing, because as I told you, there's nothing on my desktop because...

LICHTMAN: You didn't clean up for us?

Dr. GREENE: No, I did not clean up for you. I can't stand clutter. I can't stand piles of stuff. And whenever I see it, I basically just throw the stuff away.

LICHTMAN: But that's a change. That's a recent - or maybe not recent, but...

Dr. GREENE: Oh, it is a change. Yeah. When I was in college, it was completely different. In fact, there's a picture of my dorm room in the college yearbook as the most messy, most disgusting room on the Harvard campus, where I was an undergraduate. But I got to a point where I get fed up with it all, and I'm very digital. I'm this or that. And now, I'm Mr. Neat.

FLATOW: And you have multiple computers on your desk.

Dr. GREENE: I do. I do. That's correct.

FLATOW: Does each one have a different function, or do you just use them all?

Dr. GREENE: No. There are some that have more powerful programs installed on them, like Mathematica, which when I do calculations that are not really amenable to pen and paper, pencil and paper, are done in those kinds of calculational environments. So some are more powerful for that sort of an undertaking.

LICHTMAN: You know, one thing I wanted to ask you, because you use math to sort of tell this story or follow it to these, you know, speculative stories, do you feel like math is a creative act?

Dr. GREENE: I think math is a hugely creative field, because there are some very well-defined operations that you have to work within. You are, in a sense, straightjacketed by the rules of the mathematics. But within that constrained environment, it's up to you what you do with the symbols. How you go to the next step?

So rather than allowing your imagination just to run freely and wild -as you can in some areas, to great effect: screenwriting and writing fiction and things of that sort - here, you can allow your imagination to run, but it has to run within these very strict confines.

And that's why when the math says there may be other universes - you know, the things I describe in the book - we physicists sit up and take that seriously. We don't believe it until there's observational support. But when the math tells us something, we listen.

FLATOW: Do you think of yourself as the Stephen Hawking of our time?

Dr. GREENE: No, no. I mean, you know, you're talking about, you know, genius upon genius. And, you know, I try to make my contributions, however small they may be. But it's just exciting to be part of what I consider a species-wide quest that we have been on for 2,500 years, to understand reality better.

FLATOW: And if you want to understand Brian Greene's reality better, I suggest you pick up a copy of his book, "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." It explains all these different things we've been talking about. And also see our Video Pick of the Week, up there on our website. It's Brian's desk, joining some other great real estate up there.

LICHTMAN: Go right in to Brian's office.

FLATOW: Right there.

LICHTMAN: See where the magic happens.

FLATOW: Thank you, Brian.

Dr. GREENE: Thank you.

FLATOW: And good luck with the book. And thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: We'll go - we'll be assigning you to the next desk, as it comes up.

LICHTMAN: Can't wait.

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