WGBH

Our own planet twists the fabric of space-time, as shown in this animation from "The Fabric of the Cosmos."

Theoretical physicist Brian Greene admits that the world he describes in his new public-TV documentary series, "The Fabric of the Cosmos," is nothing like everyday experience. He's not even sure some of the things he describes are for real. For example, how can we possibly know other universes exist? Believe it or not, there are ways to find out.

The four-part "Nova" series makes its debut on PBS stations on Wednesday night with an episode that delves into the mysteriously substantial properties of empty space. "As it turns out, empty space is not nothing," the Columbia University professor says at the start of the show. "It's something. ... So real, that empty space itself helps shape everything in the world around us, and forms the very fabric of the cosmos."

That episode is already available for watching over iPhones, iPads and iPod Touch devices, as well as through Amazon Prime instant video. And if you miss seeing it on TV on Wednesday, you'll be able to catch up with it later online. Over succeeding weeks, Greene addresses not only space, but also the nature of time, the weird world of quantum mechanics and the possibility that our universe is just one bubble in the cosmic ocean (or raisin-bread loaf, or cheese wedge) of the multiverse.

Most of the substance in "The Fabric of the Cosmos" comes from Greene's book of the same name — but the part about the multiverse is more speculative, and is derived from Greene's follow-up book titled "The Hidden Reality." So of course that's where I had to start when I had a chat with Greene this week. Here's an edited transcript of the Q&A:

Cosmic Log: You must get this question all the time: What sort of proof do you have that any of this stuff is true?

Brian Greene: Well, the first three episodes — focusing on space, time and quantum mechanics — are much more closely tied to observations and experiments that have already been done. Much of what we describe in those programs is firmly rooted in science that is now largely accepted, even though it's weird. The fourth program is different in that regard, because as the last program in the series, it is looking beyond what we currently know, and surveying the landscape of possibilities that may in the future become accepted science. But not yet.

That's the multiverse. The multiverse is hard to test because we have access to this universe, and the theory proposes that there are other universes. We can't directly see them. We can't visit them. So how would you ever prove that idea?

In the program, we tackle that issue head-on. We describe how the multiverse naturally emerged from investigations that were rooted in observations and experiments: things to do with questions of the origin of the universe, the cosmic microwave background radiation, issues surrounding the puzzles of the big bang that can only be resolved through an inflationary view of the universe — which then yields the multiverse. But it won't be a satisfying explanation until we have some kind of direct confirmation.

To my mind, there's really one main way that could happen in the near future: In this proposal, different universes are like different expanding bubbles in some larger cosmic environment, like bubbles in a bubble bath. And when bubbles in a bubble bath expand, they can run into each other. Similarly, expanding universes can collide. The math indicates that if and when they do, the collision can send ripple-like disturbances through the microwave background radiation — the heat signature left over from the big bang. Those temperature differences of that particular sort are something that people are looking for. Some even claim they've seen the first tentative signals of the pattern. I'm highly skeptical about that, as everybody should be. But this could be a way to subject an idea that seems so foreign to an observational test.

Q: There's also been some talk about possible observations at the Large Hadron Collider that might suggest energy was "leaking" into other dimensions.

A: Yes, the notion that there are extra dimensions provides another way in which you could have other universes. Our universe might be one piece of bread in a big cosmic loaf, where the other "slices" are displaced from ours in a new direction, and are actually other universes. One way to check that idea would be to have a very energetic collision of particles in our universe, on our slice of space. The math shows it's possible for energy from those collisions to be ejected off our slice and migrate into the wider cosmos. We would notice that here by seeing that energy was not conserved. The energy after the collision would be a little less than it was before, because some of the energy would have crossed beyond our universe.

The point is that there are strange ideas about the universe that can nevertheless yield evidence, if we know where to look.

WGBH

In "The Fabric of the Cosmos," physicist Brian Greene graphically shows how the "Mona Lisa" ... and even Brian Greene ... could exist in more than one universe.

Q: One of the points of the series is that there's a deeper level to reality that what we see in everyday life, suggested by mathematics and physics. You use all sorts of animations and graphics to convey a sense of the underlying fabric of the cosmos. Do you have some favorite tricks that you've used in the TV series?

A: When you're dealing with subjects that are abstract ... these are mind-bending ideas, but what do you point the camera at? That's a funny thing, because everything we do takes place within space, within time. The concepts of space and time are so profoundly interwoven with reality as we know it, and yet science has revealed that there are features of space and time that run completely counter to our intuition — if you examine them on non-human scales, that is, scales that are very tiny, or very big, or when you're moving very fast, or if you're near a very strong, massive gravitational object.

Since we can't actually go to those exotic realms, we use animation to show what it would be like if you could shrink down to a billionth of a billionth of a meter ... or what it'd be like if you could travel at just a tiny fraction less than the speed of light ... or what it would be like to hover near the edge of a black hole and then come back to Earth. And we use animation to show the largest bird's-eye view of the cosmos if some of these multiverse ideas are correct. That really gives you a visceral understanding of the concepts.

Q: In fact Einstein used these types of thought experiments as well when he worked on his theories of relativity. He imagined what it would be like to ride a light wave, or to be falling through space in an elevator...

A: If only Einstein had the tools of animation, who knows how far he would have gone!

But there's a serious point here: When I do my own work, I'm constantly trying to build a mental image of what's going on. I'm never comfortable if my understanding is just completely in the equations. I feel like I have a storehouse of imagery built up just from the scientific research itself, which then leads to a form that will work in a book or on TV, which requires dressing it up in various ways. The whole idea of trying to visualize abstract equations is something that many of us do as part of our second nature, as researchers.

Q: Are there any favorite visualizations you keep coming back to?

A: Well, sure. A lot of my work has to do with extra dimensions of space. And I readily admit that I cannot picture anything more than three dimensions. So in my own work, I'm constantly doing what we do in the television program, which is to use lower dimensions as analogies — two-dimensional analogies that you can draw and manipulate. You use those as a guide to what's happening in higher-dimensional settings, where the equations of string theory reside.

You have to be careful. Sometimes a lower-dimensional analogy can be misleading. But you begin to build up the art of knowing what aspects of those visualizations you can trust when you're taking the leap to higher dimensions, and which aspects make you say, "No, no, no, that won't give me insight into my real interest."

Q: What do you hope viewers will take away from the show?

A: The main goal is for people to leave the program with a more complete sense that when it comes to the universe, what you see is not what you get. There are layers upon layers of reality that we are unaware of in everyday life. Intuition is built up from experience, and our experiences since we appeared on the planet has been largely dictated by what is beneficial for our survival. Understanding the quantum world, and understanding the possibility of other universes, and understanding the deep nature of time don't help you get the next meal. So there hasn't been any real evolutionary pressure for us to gain intuition about those things. But when we have the luxury of thinking about them mathematically, we learn that there's much more to the universe than meets the eye.

It's absolutely thrilling to learn that time for me is not the same as time for you; that out there in space, time is elapsing at a different rate near the edge of a black hole; that in the depths of space, there is unavoidable, ferocious quantum activity; that the world is governed by probabilities, not certainty; and then there's entanglement, the idea that what you do over here can have a direct effect on something over there. Wow!

Wow indeed. The TV show is just the tip of the iceberg: The "Fabric of the Cosmos" website offers tons of videos, interactives, intereviews and links to online resources. More than a dozen "Cosmic Cafes" have been organized nationwide to talk about space, time and the multiverse. And the World Science Festival has organized a screening of the first episode at Columbia University at 9 p.m. ET Wednesday, to be followed by a forum featuring Greene, theoretical physicist Leonard Susskind and Nobel laureate Saul Perlmutter.

Even though the in-person event sold out almost immediately, you can still tune in to live streaming video and join the discussion via Facebook or Twitter. I have an alternate suggestion: Watch the episode in advance, or save it for later, and tune in to "Virtually Speaking Science" at 9 p.m. ET Wednesday for my chat with interstellar-travel expert Marc Millis. Then, at 10 p.m. ET, switch on over to the World Science Festival's forum.

More cosmic contemplations from Brian Greene:

• A black-hole fairy tale for kids
• 'Fabric' takes on the space-time continuum
• Hidden universes revealed
• String theory gets time on prime time
• Elegant physicist makes string theory sexy
• Space.com Q&A on 'The Fabric of the Cosmos'
• Read an excerpt from 'The Hidden Reality'

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds. 

Want to get a head start on a mind-bending TV miniseries about space, time and the multiverse? There's an app for that.

Eight years after PBS aired "The Elegant Universe," a series based on Columbia physicist Brian Greene's best-selling book about string theory, the public-TV network is gearing up for the sequel. "The Fabric of the Cosmos," a four-parter from the "Nova" documentary team, focuses on the mysteries surrounding all the cosmic stuff that surrounds us.

The show premieres on Nov. 2, and it'll be streamed on PBS' video website — but if you have an iPhone, iPad or iPod Touch and are of a mind to download the PBS app, you can watch the first hour right now.

"Brian Greene's 'The Fabric of the Cosmos' is an amazing journey into some truly astounding theories of our universe," Jason Seiken, senior vice president for interactive, product development and innovation, said today in a news release. "On mobile, viewers get a sneak preview of the series' futuristic concepts and graphics leading up to the broadcast premiere and can continue their scientific exploration throughout the series."

It's been seven years since book version of "The Fabric of the Cosmos" was published, but the theme of the TV show is basically the same: Everything you know about space and time just might be wrong.

"We really see how our understanding of space and time from Newton until today has gone through remarkable changes," Greene told me back in 2004, "and most importantly, how so many things that we have in our intuition about space and time, their properties and so forth, are just not true to how the world actually works."

For example, consider space. Most of the universe is made up of empty space, and I'm not just talking about outer space. During the program, Greene uses computer graphics to bring the point home: If you could remove all the empty space from New York's Empire State Building, you would be left with a clump of smashed-together subatomic particles that was no bigger than a grain of rice — but still weighed hundreds of millions of pounds.

Greene isn't the only one gob-smacked by the weirdness of the space-time continuum. During the program, University of Maryland physicist S. James Gates says the nature of space "is one of the deepest mysteries in physics."

During the course of the miniseries, Greene manages to work in some of the ideas from "The Hidden Reality," the book that came after "The Fabric of the Cosmos." The last episode dwells on the concept of the multiverse — the idea that our universe might be just one of the myriads of cosmic bubbles floating in an larger extradimensional reality. Some of those bubbles might even be exactly like the one we inhabit — except, perhaps, that I'm the brainy physicist and Brian Greene is the befuddled journalist.

In this cosmic bubble, Greene and his brainy friends are planning lots of activities tied to the series. The World Science Festival, "Nova" and Columbia University have set up a special screening of the opening episode at 9 p.m. ET Nov. 2 at Columbia's Miller Theatre. After the show, the World Science Festival is planning a live webcast of a conversation with Greene and other guests, including newly named Nobel laureate Saul Perlmutter.

"Nova" has also teamed up with the American Society of Physics Students to create a special series of science cafes, focusing on the out-of-this-world ideas raised by "The Fabric of the Cosmos." Check out this map to find the nearest Cosmic Cafe. Maybe I'll see you at the Seattle event.

More about space, time and the multiverse:

• What? Could our universe be just one of many?
• Probe confirms that we live in a space-time warp
• Interactive: Looking beyond the big bang
• Can we dodge the arrow of time?
• Physics prize highlights cosmic puzzles
• Physicist introduces you to 'The Hidden Reality'
• YouTube: Brian Greene at New York Comic Con

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to a circle on Google+. And for something completely different, check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

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."