A. Cross / J. Dunn / Edgeworx for NOVA

Columbia physicist Brian Greene inhabits a multiple-perspective landscape modeled after M.C. Escher's artwork in a scene from the NOVA series based on his 1999 best-seller, "The Elegant Universe." Greene says his latest book, "The Hidden Reality," ranges over an even broader cosmic canvas. Click on the image to watch the NOVA program.

Is it preposterous to consider the existence of parallel universes? Or is it preposterous not to? Physicist Brian Greene would tend toward the latter view.

The Columbia University theoretical physicist's latest book, "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos," follows up on his two earlier books for popular audiences, "The Elegant Universe" and "The Fabric of the Cosmos."

Those works presented step-by-step guides to string theory and space-time, respectively, leavened with pop-culture references and analogies drawn from everyday life (that is, if your idea of "everyday life" involves watching ants crawl on a power line). "The Hidden Reality" follows a similar formula, using slices of bread, "South Park" and the Wizard of Oz to explain weird ideas such as brane theory, the inflaton field and the holographic universe.

Greene doesn't explain just one scenario in which unreachable universes co-exist alongside our own. He delves into nine possibilities, drawn from different corners of scientific speculation. Is that too much speculation? Some folks think so.

Scientific American's John Horgan wrote that he used to get fired up over the idea that our universe was just one of many making up a grander "multiverse." But not anymore:

"Now, multiverse theories strike me as not only unscientific but also immoral, for two basic reasons: First, at a time when we desperately need science to help us solve our problems, it's irresponsible for scientists as prominent as Greene to show such a blithe disregard for basic standards of evidence. Second, like religious visions of paradise, multiverses represent an escapist distraction from our world."

Over at the Not Even Wrong blog, a colleague of Greene's at Columbia, mathematician Peter Woit, has his own set of moral qualms:

"My own moral concerns about the multiverse have more to do with worry that pseudo-science is being heavily promoted to the public, leading to the danger that it will ultimately take over from science, first in the field of fundamental physics, then perhaps spreading to others."

Woit goes on to catalog all the books that have come out or are about to come out that make reference to the multiverse, including "From Eternity to Here," "In Search of the Multiverse," "The Grand Design" and "Visions of the Multiverse" just in the past year. Does the world really need another book on the subject?

As a string theorist, Greene is used to such criticism. Like parallel universes, the idea that matter's fundamental building blocks are tiny vibrating strings or multidimensional membranes has often been knocked as unprovable, unverifiable, unfalsifiable speculation. Lawrence Krauss, a theoretical physicist at Arizona State University, is fond of saying that string theory's vision of a "theory of everything" is actually a "theory of anything" that turns out being a "theory of nothing."

"That's provocative nonsense," Greene told me last week. Theorists are not just pulling this stuff out of thin air, he said. Rather, they're being led to seemingly wild conclusions while working within what he called "the tight straitjacket of mathematics."

Random House

"The Hidden Reality" is Columbia physicist Brian Greene's latest literary excursion to the frontiers of physics. Click on the image to read an excerpt.

In a telephone interview conducted during his book tour, Greene addressed the suggestion that multiverse theory was an empty exercise, and explained why scientists have to take parallel universes seriously. Take a look at this edited transcript of the Q&A, read an excerpt from the book, and then let me know what you think in the comment space below:

Cosmic Log: Some people have said, "Oh, no, not another book about the multiverse ... all these things we can't see, all these claims that we can't prove. Why do we need another book about this subject when there have been so many already? And isn't it all speculation anyway?"

Brian Greene: Well, when we are doing mathematical investigations in physics, we as theorists allow the math to take us where it will go. We have seen, time and again, that math is a very potent guide to revealing the true nature of reality. That's what the past couple of hundred years have established. So all we're doing is following the same kinds of procedures that we always have. And as we follow the procedures, as we push the mathematics forward, the math is clearly suggesting that there may be other universes out there.

That does not mean that there are. It does mean, however, that there's a compelling enough reason to take these ideas seriously, develop them further, and try to make contact with observation and experiment. I fully agree that none of these hypothetical ideas can be put within the canon of established physics until there is some kind of observational confirmation. But you can't get to that point unless you understand the theories extraordinarily well. And that's what a lot of cutting-edge physics is now doing.

Q: In your book you talk about several types of parallel universes. What do you mean by the term? Often people have the conception of traveling back in time, or living in a quantum world where you're having a drink at a bar and yet not having a drink. In the TV series "Fringe," there are parallel universes in conflict with each other. People have a lot of conceptions about what a parallel universe means, but what does it mean to a scientist?

A: We have for a long time had a conception of what a "universe" is. Look out at the cosmos, and it's the totality of the stars and the galaxies that are out there, everything that we in principle can see. But we have learned, through a variety of approaches in physics, that that notion of "everything" is possibly a small part of a far larger cosmos, a far grander reality.

I like to make this concrete with a simple example that I think helps ground the physics about this. We all know about the big bang, which is basically how our universe got started. The universe was very small in the distant past, it underwent a rapid expansion, and in the course of that expansion, the universe cooled down and allowed matter to coalesce into stars and galaxies.

Now, many people don't fully appreciate that this story of the big bang leaves out something very important: It leaves out the "bang." It leaves out the physical process that started the outward swelling of space in the first place. As we have developed mathematical tools to fill in that gap, to really understand what happened at the beginning, the math has indicated that the big bang may not have been a unique event. There may have been, and may continue to be, many big bangs — each of which gives rise to its own expanding universe, our universe being but one among many. In that sense, we are part of a multiverse.

Q: One of the more provocative ideas that you put forward in your book is the suggestion that there could be other versions of Brian Greene or Alan Boyle that are just slightly off, existing in some other quadrant of the multiverse. Have you gotten some raised eyebrows over that?

A: Well, it's a staggeringly strange idea, but again, we need to emphasize that it doesn't emerge from some scientist sitting in a dark room and letting his imagination run wild. This idea comes from the notion that the expanse of space goes on forever — that it's infinitely large. That's an idea that people have contemplated for a long time. In fact, I would say that the majority of physicists and astronomers, when they speak about space, they do envision it going on forever. Then it takes but a simple little mathematical exercise to establish that, in any finite region of space, matter can only arrange itself in finitely many configurations.

The analogy I like to use is a deck of cards. When you shuffle the deck, the cards come out in different orders, but there are only finitely many different orders of the cards. If you shuffle that deck infinitely many times, the orders necessarily will repeat. Similarly, in an infinite spatial universe, the arrangements of particles have to repeat, too. If they repeat, then indeed, things that we are familiar within the world around us — you, me, Earth, the sun, everything else — would repeat as well.

When one explains this idea to someone who hasn't heard it before, it is shocking at first — you're absolutely right. But when one takes in the mathematical argument and mulls it over, it becomes clear this is what would happen.

Q: That's just one of the nine options suggested for the existence of parallel universes. Do you have a favorite scenario?

A: It depends on how you measure the "favorites." The measure I'm most fond of is, "Which of these stands the greatest chance of receiving some experimental support in the not-too-distant future?" By that measure, I like to focus on the "brane multiverse" theory. That's this idea that string theory doesn't just contain strings. It also contains membranes — two-dimensional objects — and three-branes, which are three-dimensional objects, and so forth.

The brane multiverse imagines that all we have thought to be the universe actually takes place on one of these three-branes, with other three-branes potentially out there. The analogy I like to use is a loaf of bread, where our universe is one slice, but there are other slices out there populating this grander cosmos. And this idea of a brane multiverse can be tested at the Large Hadron Collider.

When you have powerful proton collisions, the math suggests that some of the debris from those collisions can be ejected off our brane, and we would notice that by virtue of having less energy after the collision than before — because the debris would take some of the energy away with it. People are looking for these kinds of missing-energy signatures. If the results prove positive — which, I absolutely need to underscore, I consider a long shot — then it would be evidence that we are living on one of these branes. If we are living on a brane, then there's really no reason to anticipate that our brane would be the only one. There would be other branes out there, other universes.

Q: What energy level would be required to see that sort of evidence?

A: It all depends on the size of the extra dimensions within which all these branes would be embedded. If the extra dimensions are very small, it takes increasingly large amounts of energy to get debris from the collisions to leave our brane and go into this tiny extradimensional space.

That's the unknown: If the dimensions are big enough, then the energies required would be within reach of the Large Hadron Collider. If the extra dimensions are small, then the Large Hadron Collider would not be able to cause this process to happen. So the best we can do is get some evidence that confirms the brane multiverse idea. It's pretty hard to get evidence that would flatly rule it out.

There's one point I want to get out about the book: It's not a "multiverse manifesto." It's not trying to say, "Look at this wonderful idea, and it's true." No. I'm saying, "Look at this curious idea that many leading scientists are thinking about" — including me, I do work on this stuff right now. Let's ask ourselves, "What's this all about? What's the mathematical motivation for thinking about it?" And I ask the question "Is this science?" How can we verify these ideas? What other insights do we need to acquire going forward, in order to make the multiverse idea something that fits squarely within confirmable or falsifiable science?

This idea is controversial for good reason. It is at the cutting edge — not only the cutting edge of science, but also the edge of the kinds of ideas that we want to embrace in science. That's what makes it exciting.

Q: You make the point that it's very difficult to have any sort of direct contact with other universes. The differences are just so great. The only way to conceptualize other universes, I suppose, is through mathematics and the bits of evidence that can be gleaned from particle collisions or the cosmic microwave background radiation. Is there any possible avenue to get substantive information about the bigger picture, or are we pretty much stuck in our own little corner of the multiverse?

A: I think we're certainly stuck physically. But I would not underestimate the power of mathematics to provide the kinds of insights you are referring to. We are definitely at a rudimentary state in our understanding of these multiverse proposals. But if we can refine that understanding, we could produce detailed "universe demographics." We could gain a very detailed understanding of the percentage of universes that would have this or that quality.

In fact, we might get lucky with a well-developed multiverse theory. We might find that universes differ in substantial ways, but we might also find that there are certain common features that all universes share — like a certain class of particle, for instance. Then, to adjudicate that multiverse proposal, all we would need to really do is look for those particles here in our universe. We're part of this multiverse, after all. If we fail to find those particles, we could rule out that proposed theory. It's falsifiable, even though we can't actually see the other universes. If we do find those particles, that would bolster our confidence that the theory is correct, as would be the case for other fields of experimental science.

My point is, I'm laying out the way in which various multiverse proposals could rise to the level of being testable, of being falsifiable. The mere fact that you can find ways to do that shows quite clearly that the subject can't simply be written off.

Q: You've had quite the range of experiences during your book tour — including an appearance on "The Colbert Report." [During his chat, Greene told host Stephen Colbert that he could be described as "a bag of particles governed by the laws of physics," leading Colbert to quip, "That is a great pickup line."] In a parallel universe, is there anything you'd want to change about the past few weeks?

A: Oh, goodness. ... If I could get a couple more hours of sleep in the day, that would be welcome. But that's about it.

... And that's about it from the interview as well. Read an excerpt from Chapter One of "The Hidden Reality" and let me know what you think.

More about the shape of the cosmos:

• Elegant physicist makes string theory sexy 
• Interactive: Beyond the big bang
• Interactive: The symphony of everything
• Scientists say cosmos is mind-bogglingly big
• So what's the universe expanding into?

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."