TERRY GROSS, host:

This is FRESH AIR. I'm Terry Gross.

Our interview today is about discoveries in physics and cosmology, but it may sound more like an episode of this.

(Soundbite of television program, "The Twilight Zone")

(Soundbite of theme music)

Mr. ROD SERLING (Writer/Narrator): You're traveling to another dimension, a dimension not only of sight and sound but of mind, a journey into a wondrous land whose boundaries are that of imagination. That's the signpost up ahead: Your next stop, the twilight zone.

GROSS: If "The Twilight Zone" expanded your notions of reality, wait 'til you hear what Brian Greene has to say. His new book is about parallel worlds, the theory that our universe might be one of several universes. Our universe may be just part of a multiverse.

Although the idea of hidden realities may sound like science fiction, it comes out of very advanced mathematics. Brian Greene is a professor of physics and mathematics at Columbia University. He's conducted important research on string theory and wrote a bestselling book explaining string theory, called "The Elegant Universe." It was the basis of a PBS series and a finalist for the Pulitzer Prize. Greene's book "The Fabric of the Cosmos" was also a bestseller.

Brian Greene, welcome back to FRESH AIR.

Professor BRIAN GREENE (Author, "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos"): Thank you.

GROSS: So does everyone compare the concept of multiple universes to an episode of "The Twilight Zone"?

Prof. GREENE: You know, not everyone, but it certainly is the case that the science we're talking about is touching upon things that science fiction has explored, has made use of, in a great many different environments.

So the kinds of science we're talking about is, in some way, bordering along science fiction.

GROSS: And that's what makes it sound both possible and absolutely impossible to me at the same time. Possible because I've heard that story in fiction and impossible because - but that's fiction!

Prof. GREENE: But the wonderful thing about fiction is, if you look at some of the ideas that have come out of science fiction, oftentimes they are just ahead of their time. It's fiction in the time that the piece is written, but some science fiction becomes science fact.

GROSS: So I'm having a lot of trouble wrapping my mind around the concept of parallel universes because it seems like a contradiction in terms. As you point out in the book, I mean, I thought universe meant everything. I thought the universe was infinite, and that infinite was beyond any amount that you could possibly imagine: Everything, everything, everything was included in infinite. So how can there be more than one of those?

Prof. GREENE: Well, to some extent, it is a question of language. If you define the universe as truly being absolutely everything, then you're right, to talk about more than one universe would be a contradiction in terms.

The reason why we have introduced a new term called the multiverse -which basically means multiple universes - is because as we have studied physics ever more deeply, we have found that what we have long thought to be everything is only a small part of a grander whole, only one piece of a much wider cosmos. And to really kind of communicate that idea, we've introduced this new terminology that our universe is just one of many universes populating, possibly, a grander multiverse.

GROSS: And when you talk about many universes, there's lots of different theories about what those universes might look like, right?

Prof. GREENE: Yes, the wonderful thing about the subject is that there's not one monolithic notion of what a multiverse would be. As we have studied a whole variety of different areas of physics, from relativity, quantum mechanics, cosmology, unified physics, it seems to be the case that whenever we follow the mathematics of these deep theories sufficiently far, we bump into one or another variety of parallel universe idea.

And to me what makes it so compelling is, it's not that we physicists are sitting at our desks saying what kind of crazy idea can we introduce into science now - it's not like that at all. What we're doing is sitting at our desks, trying to do what we always do, which is trying to understand the universe, come up with theories that can describe our observations, our data. And when we follow those theories far enough, we come across some version that our universe is one of many.

GROSS: Okay, so in one model of parallel universes, of a multiverse, in one model, I am interviewing you right now in another universe.

Prof. GREENE: Absolutely.

GROSS: What's that model?

(Soundbite of laughter)

GROSS: And how's our interview going there?

(Soundbite of laughter)

Prof. GREENE: Well, I hope it's going well. But there are a couple of variations on the multiverse theme which would be compatible with that idea. The simplest is basically the idea that you began with, that our universe may be infinitely big, that is, space may go on forever. And if that's the case, it turns out that you can establish, using pretty basic mathematics, that there's only a finite number of different ways that matter can arrange itself.

So if you have an infinite expanse with only a finite number of different possibilities, the possibilities have to repeat. I mean, if you think about having a deck of cards, when you shuffle that deck, there are just so many different orderings that can happen.

So if you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe and only a finite number of different complexions of matter, the way in which matter arranges itself has to repeat.

So our collection of matter right now, with you interviewing me, that is repeating itself out there in the cosmos.

GROSS: But isn't that a kind of Earth-o-centric notion of infinity, that, like, space is endless, but the ways that matter can organize itself is finite? I mean, if space is endless, isn't there matter beyond our comprehension? Aren't there particles or ways of organizing particles beyond our comprehension?

Prof. GREENE: You are right. There are potential loopholes to this way of thinking.

(Soundbite of laughter)

Prof. GREENE: We are definitely imagining that the physics that we have understood here on Earth does apply everywhere throughout the cosmos. And the reason we believe that is when we look out as far as we can in the cosmos, and that's pretty far, we can look out billions and billions of light-years from Earth, very, very far away, everything we see seems to be describable using the laws of physics that we humans have been able to develop.

And what we're basically saying is: Let's therefore extrapolate and assume that those laws really do work everywhere and, really, everywhen. And if that's the case, then the reasoning that yields this idea that matter must repeat is something that's a consequence of that mathematics.

GROSS: On the other hand, isn't there another theory of the multiverse that says in other universes, there are different laws governing matter and space and time and gravity? There might be principles we don't even know about.

Prof. GREENE: Yes, there's another variation on the multiverse theme that comes from our thinking about cosmology, the origin and the evolution of the universe, that does yield a somewhat more robust version of the multiverse than the one that we're describing.

And it's coming from something called inflationary cosmology. Now, most people have heard of the big bang theory, this idea of how our universe began and a very, very small nugget that, about 13.7 billion years ago, erupted with space and time being flung outwards, matter and energy coalescing into stars and galaxies over the course of billions of years of cosmic evolution.

But the thing that we don't often stress enough about big-bang cosmology is the big bang leaves out the bang. The big bang theory does not tell us anything about what actually happened at time zero itself. It doesn't tell us what caused that explosion, that outward push, to happen.

And this new theory, inflationary cosmology, is what fills in that detail. It tells us that there was a configuration of energy in the early, early moments of the universe that could give rise to something that sounds very strange, something called repulsive gravity.

We're all used to that gravity is attractive: You let go of something, it falls to the Earth. Earth pulls things toward it. But there's a kind of gravity that does the reverse. Repulsive gravity pushes outwards. And we believe that in the early, early universe, repulsive gravity was in operation, and that repulsive push is what drove everything apart.

Now, the reason I bring this all up is when we study that repulsive push in detail, we find that it's not a one-time event, the kind of big-bang outward push. There could be many big bangs, many outward pushes at various and far-flung places throughout this wider cosmos, giving rise to different universes. It's like a cosmic bubble bath of universes. Our universe is one bubble in this big cosmic bubble bath.

GROSS: So there are lots and lots of big bangs? One bang led to another bang, to another bang, and each of those bangs created a different universe?

Prof. GREENE: Yes, exactly, and to get back to your original question, when you study those universes in a little bit more mathematical detail, you do find that their features can differ widely.

They don't have to have the same kind of particles. We know about electrons and quarks, protons and neutrons. Those are the kinds of particles we are familiar with, and those other big-bang universes, they don't have to have those particles. They don't have to have the forces that we know about, the electromagnetic and the nuclear forces, for instance. They may not be in operation in those universes. Other forces, instead, may take their place.

GROSS: So in this repeating big bang theory, I am not interviewing you in another universe.

Prof. GREENE: Well, it doesn't rule out that there could be other bubble universes in which the laws are very similar, maybe identical to ours. So we could in fact be having this conversation out there. But it allows for a wider variety of possibilities because the laws can, in principle, be different.

GROSS: Now, you said something that really baffles me. You said: When we study those universes in mathematical detail - what do you mean by that? I mean, we don't even know those universes exist. So when you say when we study them in mathematical detail, what are you talking about?

Prof. GREENE: Well, that is a confusing idea, I think, for people who don't actually engage in the kind of research that I'm talking about because what we do is we sit down with equations, equations that describe space and time, equations that describe how matter can move through space and time.

And using those mathematical equations, we can get a sense of what it would be like to be in one of those other universes, even if we can't actually visit or see or interact with that universe in any real sense. That's the power of mathematics.

And I have to say, underlying everything that we're talking about, in fact underlying everything I do with my entire life, pretty much, is a firm belief that mathematics is a sure-footed guide to how reality works. If that's wrong, then all bets are off.

(Break)

GROSS: Earlier in our conversation about the multiverse - the idea that there are parallel universes, our universe is not the only universe - you were describing how, in one model of the multiverse, everything that we are doing now is happening in another universe. So the interview that we're doing now is happening in another universe. What is the theory that backs up that model of the multiverse?

Prof. GREENE: Well, there are two. I mean, one is, I wouldn't quite call it a theory per se, but it's the notion that space may go on for infinity. And that fits within the general theory of relativity, which is Einstein's theory of gravity, which is the force most relevant on the largest scales of the cosmos.

And according to that theory, the universe could be, doesn't have to be, but it could be infinitely big. It could also be that if you head out into space, you might sort of circumnavigate the cosmos and return to your starting point, much like what would happen if you walked on the surface of the Earth in one fixed direction: You'd come back. You wouldn't keep on going forever in one direction.

If that's the case, then space wouldn't be infinite, and the ideas that we're talking about wouldn't be true.

There is another way that you can come to the conclusion that variations of this conversation are having a realization out there in the cosmos. And that comes from a theory called quantum mechanics, a completely different set of laws that are not as relevant, at first sight, to the largest things in the universe, they're relevant to the smallest things in the universe.

GROSS: Yeah, quantum mechanics is the study of all those subatomic particles that make up matter.

Prof. GREENE: Yes.

GROSS: And this is where, I guess, the laws of probability come in?

Prof. GREENE: That's the key idea. The sharp break from the older, classical physics that arose when we learned about quantum mechanics, is that in Newton's day, his way of thinking about the universe was: You tell me how things are right now, and I will tell you with absolute precision how they will be in a minute or five minutes or an hour from now. It was absolute, definitive predictions.

Quantum mechanics came along in the early part of the 20th century and said: Actually, that idea is only approximate. That idea is not fully correct. When you study the universe with greater precision, you learn that you can't make those kind of definitive statements. The best you can do, according to quantum mechanics, is predict the likelihood, the probability of one outcome or another.

So if you're studying, say, the motion of a little particle like an electron, the math of quantum mechanics might say there's a 50 percent chance that the electron is over here and a 50 percent chance that it is over there. And that is the best you can do in terms of delineating where that particle will be.

Quantum mechanics says there's this inevitable portion of the world that's described in terms of probabilities.

GROSS: And how does that connect to the idea of multiple universes?

Prof. GREENE: Well, here's the puzzle. The idea that the world is governed by probabilities is strange enough. When you actually do the experiment to, say, figure out where that electron is, you always find it in one location or another.

And indeed, if the math said there was a 50 percent chance it was one place and 50 percent at another, if you do that experiment 100 times, then pretty much, 50 times you find it one location and 50 in the other. So the math is borne out by the experiments.

The little dark secret that doesn't get, perhaps, as much play as it should: When we study the mathematics of quantum mechanics, we still do not understand how to go from the fuzzy probabilistic description that the particle might be here, and it might be there; we don't know how to go from that description to the single, definite, absolute reality that we see when we do the measurement.

We never find the particle partly here and partly there. We always find it definitely here or definitely there. How do we go from the probabilities to the definite outcome?

People have struggled with that. They continue to struggle with that. I've struggled with this problem.

Back in the late 1950s, a fellow named Hugh Everett suggested a radical way to deal with this problem. He said: The idea that only one outcome happens in a given experiment, that's just not right. He said: Follow the math of quantum mechanics. Take it very seriously. And it is telling us that there are two possible outcomes. The particle can be here or there.

Therefore, what happens is there are two universes. In one universe, the particle is here; in the other universe, it's over there. And there's a copy of you in each universe measuring that particle and thinking, incorrectly, that that particle's unique location is the only reality. But, in fact, there are two of you thinking that. There are two realities, two parallel universes.

Now, I should say when you frame this in terms of the position of electron, it might sound kind of curious, but it also might seem, well, not that relevant to everyday life. Who really cares about where one electron is, here or there?

But when you take account of the fact that everything you think, everything you do, everything you experience amounts to particles moving around inside your body, moving around inside your brain, every aspect of reality has to do with how particles move, what we're learning from quantum physics, if you take the math seriously, is that every possible reality consistent with the laws actually happens in its own separate universe.

GROSS: Now, are you convinced by that?

Prof. GREENE: No, I'm not convinced by that, not yet. I find it the most attractive way of dealing with this puzzle in quantum mechanics - going from the fuzzy probabilities to the definite outcomes - but when I study this theory in detail, mathematically, I find various holes in the mathematics.

Holes is perhaps too strong a word. I find various points in the mathematics that I'm not yet convinced that this is the right way of dealing with the issue.

There are other physicists in the world who, if you were talking to them, they would say: I am absolutely, positively convinced there is no other way that this problem can be solved, this is the right answer. I have not gotten to that point yet.

I find this a wonderfully compelling idea, but I am not a full convert.

GROSS: So, you know, we've been talking about this idea that we live in one of several universes, there are other universes out there that we can't detect, but the math, the very, very, very high-level math, is showing that there really might be other universes.

When you write about this, when you do research on it, do any of your fellow physicists or mathematicians think that you've flipped, that you've kind of gone over to the other side? Or is this idea of a multiverse pretty commonly accepted in your field?

Prof. GREENE: I'd say it's highly controversial, but there are a lot of people on both sides of the aisle. It's not as though it's a fringe theoretical study with just a small number of people thinking about the possibility that we might be part of a multiverse. There are many researchers, many top researchers, who are taking the idea seriously.

I mean, the reason why the people who are not fond of this idea are critical of it is quite sensible. We're used to science giving explanations of a different sort than a multiverse can give.

We're used to sitting down, looking at data and coming up with theories, doing our calculations and showing that our mathematical calculations yield the answer that agrees with what the experimenter or the observational astronomer has found. That's the way science has progressed for a very long time.

If we are imagining we're part of a multiverse, we're changing, in some sense, the way in which our theory and our observations affect one another. After all, we can't see those other universes, we can't touch them, we can't visit them, and that is uncomfortable to many physicists and scientists who are used to the more traditional way of doing science.

GROSS: But if it is true, if you and others are doing research now that will show we live in a multiverse, not just in a universe, then you're on the verge of a scientific revolution.

Prof. GREENE: This would be, of course, a huge revolution. It's a revolution in a way that would complete a metarevolution that's been in the making for five centuries.

I mean, a long time ago, we all know that we thought that the Earth was the center of everything. Then Copernicus comes along, and we learn that no, the Earth is going around the sun. And then later, we learn the sun is one of many stars in our galaxy, one of hundreds of billions of stars in our galaxy. Then we learn that our galaxy is not the only galaxy, there are hundreds of billions of galaxies out there.

If you take the Copernican revolution further, it would suggest that what we have long thought to be the universe might also just be one of many universes in a grander cosmos.

(Break)

GROSS: Is there a multiverse theory that you find most convincing?

Prof. GREENE: It's a great question and I think it really does speak to what makes the whole subject of interest to me, which is as I was saying before, you almost can't avoid having some version of multiverse arise in your studies if you push deeply enough in the mathematical descriptions of the physical universe. And that to me is really the hallmark of what makes this an interesting subject.

I mean the stakes are very, very high because there are many of us thinking about one version of parallel universe theory or another. If it's all a lot of nonsense then there's a lot of wasted effort going into this far-out idea. But if this idea is correct, this is a fantastic upheaval in our understanding. Which of the ones is most likely to be, say, tested in the next few years, which is the only way that I'll be convinced of any of these is that you really can have some sort of experimental support behind them, is a version of the multiverse that comes from string theory.

GROSS: And that's your specialty.

Prof. GREENE: That's the theory that I've been working on for now 25 years. Yes. And that's a theory that is attempting to realize Einstein's dream of a unified theory of physics, that is in essence one master equation that might be able to describe the big, the small, and everything in-between. And as we studied this theory, we have run into the idea that everything that we know about, again everything that we have long thought to be the universe, might actually be taking place on a membrane. And the image that I like to have in mind is, imagine that our universe is like one slice of bread in a much grander cosmic loaf with the other slices of bread being other universes. It's called the brane multiverse, and again it comes directly from the mathematics of this attempt to realize the unified theory that Einstein sought but never found.

GROSS: Okay. Let's backtrack just a little bit. So the unified theory that Einstein sought and never found, that's a theory that would explain both subatomic particles but also explain, like, the laws of gravity and speed and light and the cosmos and make the large coincide with the small.

Prof. GREENE: That's exactly right. What we have found is that in the 20th century there are two major developments in physics. One as you mentioned, general theory of relativity, Einstein's theory of gravity. It does a fantastic job for big things, stars and galaxies and so forth. The other development we were talking about, quantum mechanics, and it does a fantastic job at the other end of the spectrum for small things -molecules, atoms and subatomic particles. The big problem for 70 years is that each of these theories does fantastically well in its own realm, but whenever these theories confront one another, they are ferocious antagonists. The math completely falls apart.

Now you might say when would they ever confront each other, one's for the big, the other is for the small? But there are realms in the cosmos, such as at the center of a black hole, where an entire star is being crushed to a very small size. A star is big and heavy. You need the theory of gravity. It's being crushed to a fantastically small size. You need quantum mechanics. In that domain you need both of these theories and when you bring them both to bear, everything falls apart.

GROSS: So - yeah.

Prof. GREENE: String theory is an attempt to fix that.

GROSS: How?

Prof. GREENE: Well, we have found rather surprisingly that a seemingly modest change to our picture of how the world is constructed allows us to sidestep the problem. In the older days, the older theories of physics, we envisioned that when you spoke about molecules and atoms and subatomic particles, when you got down to the particles, the electrons and the quarks inside the nucleus of atoms, we envisioned that those particles were little tiny dots that had no structure, no size. They were really infinitesimal.

If we change that idea and envision that these particles are actually not little tiny dots but little tiny loops, little loops of string, a little piece of string that can vibrate at different frequencies, that change from a dot to a string is able to cure the mathematical inconsistencies between general relativity and quantum mechanics at least on paper. We haven't tested. We have not been able to test these ideas yet. That's the big issue. But at least on paper, that modest change from a dot to a loop cures the problem.

GROSS: How does it cure the problem?

Prof. GREENE: That's a very interesting and difficult question, but I'll...

GROSS: Yeah, I figured it would be difficult.

(Soundbite of laughter)

Prof. GREENE: I'm absolutely willing to give it a shot.

GROSS: Okay.

Prof. GREENE: So first let me just give you what the problem is in a touch more detail. Einstein's vision of space was that it was malleable, it was flexible, sort of like a trampoline. That's a metaphor that we physicists typically use. So the reason, for instance, that the Earth goes around the Sun is that the Sun sort of like sits like a bowling ball on the trampoline of space. And because it creates an indentation in space, as the Earth moves, it's nudged around by the curved surface of the warped space that the Sun creates. That's the way gravity, according to Einstein, works: warps and curves in space.

Now go to quantum mechanics. Quantum mechanics on the very small scale says there's something called the uncertainty principle at work. And the uncertainty principle basically says that you can't ever fully know both the positions and the speeds of all the particles in the microscopic realm so there's a certain amount of chaos, a certain amount of tumultuous, frenetic behavior happening in the microscopic realm because you can't ever fully nail down what's going on.

The frenetic behavior of quantum mechanics is very much at odds with the nice gently curving space picture that Einstein had for general relativity. And in fact, if you follow quantum mechanics and examine a little patch of space, magnify it with a fantastic magnifying glass, it says that if you magnify space to fantastically small scales, way down on small scale, space is not gently curving, as Einstein had in mind; it looks more like the surface of a violently boiling pot of water, a completely different image of space and one that makes Einstein's mathematics fall apart. That's the problem. The wild jitters of space on microscopic scales.

Now how does string theory fix that? It basically spreads things out. When you go from a point particle to a string, you're spreading it out. You're spreading that point particle out into a loop. And when you spread anything out, you dilute it. Similarly, as you spread out the particle to a string you spread out space and the wild undulations of space that were the cause of the problem, they get spread out, they're still there but when they spread, they dilute. They dilute to a level that allows the math of general relativity and quantum mechanics to harmoniously coincide.

GROSS: Okay. So I'm really working hard to absorb all of this.

(Soundbite of laughter)

Prof. GREENE: Me too.

(Soundbite of laughter)

GROSS: And now I want to take it a step further and have you connect everything that you've been saying about string theory and a unified theory and relativity and apply that to the multiverse.

Prof. GREENE: Sure.

GROSS: To the vision of the multiverse that arises out of your understanding of string theory, and if anybody understands string theory, it's you.

Prof. GREENE: Well, there are a couple of multiverses that come out of our study of string theory. One is something that emerged, oh, about 10 years ago or so, which was a realization that within string theory the strings that we're talking about are not the only entities that the theory allows. It also allows extended objects that look like membranes, which are two-dimensional surfaces. There also are three-dimensional surfaces within string theory and so forth. And what this has opened up within string theory is the possibility that we might be living on one of those gigantic surfaces. And that gets back to the metaphor that I was mentioning before where we are imagining our universe being a slice of bread in this big loaf.

What that meant is our universe is living on one membrane and there can be other membranes floating out there in space. And that is what is known as the brane multiverse, that what we have long thought to be everything is actually confined to living on one of these giant surfaces and there are other giant surfaces out there. That idea actually may be testable in the next few years with the Large Hadron Collider, this big accelerator in Geneva, Switzerland.

GROSS: How would you test that idea on this accelerator?

Prof. GREENE: Well, if we are living on one of these giant membranes, then the following can happen: when you slam particles together, which is what happens at the Large Hadron Collider, protons are slammed against each other violently. They're sped up near the speed of light and they have these head-on collisions. Some debris from those collisions can be ejected off of our slice of bread, off of our membrane and be ejected off into the grander cosmos within which our membrane floats.

If that happens, that debris will take away some energy, which means there'll be less energy for our detectors here on our slice of bread to measure. So if we measure the amount of energy just before the protons collide, and compare it with the amount of energy we record just after they collide, if there is a little less after and if it's less in just the right way, that would indicate that some had flown off, indicating that this brane picture is correct.

GROSS: Wow, okay. So it must be so both exciting and frustrating for you. You're theorizing that there could be other universes. And it's conceivable that you could mathematically prove that. But you wouldn't be able to get there to actually experience it with your own senses.

Prof. GREENE: Yes. So you can ask yourself, is it really worth thinking about if it's something that is purely intellectual? It seems esoteric and perhaps even standing outside of what we'd want to call nuts and bolts science.

And the reason why we are compelled to think about these ideas is, well, one, it's just wondrous and interesting. But beyond that, there are certain problems, certain problems that we have struggled with for decades, which when you reframe them in the context of a multiverse theory, some of those problems simply evaporate. And that capacity to go from complete lack of understanding on certain problems to the problems simply going away when you think about them in this other setting is quite compelling.

(Break)

GROSS: One of the things people love about space travel is that it gave us Teflon and... Tang?

(Soundbite of laughter)

GROSS: And all kinds of, you know, like materials that are used now. It had practical applications in addition to being like fantastic and, you know, mysterious and all of that. Are there practical, I think a lot of people would want to know, are there practical applications of the kind of really abstract research that you're doing?

Prof. GREENE: Really hard to answer. I certainly can't think of any, because the stuff that we're talking about is so far removed from the everyday. But what I think needs to be said is if you were to have asked that very same question to the people that were developing quantum mechanics in the early part of the 20th century, people like Niels Bohr or even Einstein or Schrodinger, I think they would've said I don't see any applications of these ideas. We're talking about atoms and subatomic particles. That's just too far away from everyday life to really affect anything that we do in everyday life. But now 80 years later, 90 years later, the equipment that we make use of, the fact that you have a cell phone, your personal computer, all manner of technology that has an integrated circuit relies upon quantum physics. Without quantum physics there wouldn't be any of that stuff.

Someone actually estimated that something like 35 percent of our gross national product comes from quantum physics, which is all just to say you don't know where science will go 80, 100 years, 500 years after fundamental discoveries are made. So this kind of science, if it's correct, and I really need to emphasize, this is cutting-edge stuff. We don't know that it's correct. But if it is, it could have a major impact on the way we live.

GROSS: What got you interested in investigating this kind of cosmological level of physics? Was it the math itself, or was it a desire to find answers to really profound questions?

Prof. GREENE: There are people who are driven from different angles. Some are driven by the mathematics, and it just takes them wherever it does and they just find it exciting to follow the equations. I'm not like that. For me, it is the ideas. I mean, I love the idea that what we're studying is something that is so big, so transcendent. We're trying to talk about not even the universe. We're talking about, perhaps, other universes, but all within a rational, logical framework that allows us to make some definitive statements. To me, that's enormously exciting, to step outside the everyday and really look at the universe in these mathematical terms on its grandest of scales.

GROSS: What do you most hope to learn in your lifetime?

Prof. GREENE: I'd love to understand how our universe began. If indeed there is one universe, or if there are many universes, I'd still like to know how this one got started. And associated with that is the most puzzling question to me of all, which is: What really is time? What is the nature of time? I mean, we all think about time. We all live within time, but we are still struggling to figure out what time actually is. And if our mathematics, if our theories could answer that, I think that would be a profound step forward.

GROSS: Well, Brian Greene, it's been great to talk with you.

Prof. GREENE: Thanks.

GROSS: Brian Greene's new book is called "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos." You can read a chapter on our website, freshair.npr.org.

Greene is a professor of mathematics and physics at Columbia University.

Coming up, our rock historian Ed Ward talks about a Memphis record label that produced great soul records in the '60s, but remains almost unknown.

This is FRESH AIR.

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