The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos

The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos

by Brian Greene


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The bestselling author of The Elegant Universe and The Fabric of the Cosmos tackles perhaps the most mind-bending question in modern physics and cosmology: Is our universe the only universe?

There was a time when "universe" meant all there is. Everything. Yet, a number of theories are converging on the possibility that our universe may be but one among many parallel universes populating a vast multiverse. Here, Briane Greene, one of our foremost physicists and science writers, takes us on a breathtaking journey to a multiverse comprising an endless series of big bangs, a multiverse with duplicates of every one of us, a multiverse populated by vast sheets of spacetime, a multiverse in which all we consider real are holographic illusions, and even a multiverse made purely of math—and reveals the reality hidden within each.

Using his trademark wit and precision, Greene presents a thrilling survey of cutting-edge physics and confronts the inevitable question: How can fundamental science progress if great swaths of reality lie beyond our reach? The Hidden Reality is a remarkable adventure through a world more vast and strange than anything we could have imagined.

Product Details

ISBN-13: 9780307265630
Publisher: Knopf Doubleday Publishing Group
Publication date: 01/25/2011
Pages: 370
Product dimensions: 6.40(w) x 9.30(h) x 1.60(d)

About the Author

Brian Greene received his doctorate from Oxford University, where he was a Rhodes Scholar. He is currently a professor of physics and mathematics at Columbia University, and is highly regarded for a number of groundbreaking discoveries in superstring theory. His first book, The Elegant Universe, was a Peabody Award–winning PBS series, a national best seller, and a finalist for the Pulitzer Prize; his second best-selling book, The Fabric of the Cosmos, is also currently in production as a PBS series. He lives in Andes, New York, and New York City.

Read an Excerpt

Chapter 1

The Bounds of Reality

On Parallel Worlds

If, when I was growing up, my room had been adorned with only a single mirror, my childhood daydreams might have been very different. But it had two. And each morning when I opened the closet to get my clothes, the one built into its door aligned with the one on the wall, creating a seemingly endless series of reflections of anything situated between them. It was mesmerizing. I delighted in seeing image after image populating the parallel glass planes, extending back as far as the eye could discern. All the reflections seemed to move in unison—but that, I knew, was a mere limitation of human perception; at a young age I had learned of light’s finite speed. So in my mind’s eye, I would watch the light’s round-trip journeys. The bob of my head, the sweep of my arm silently echoed between the mirrors, each reflected image nudging the next. Sometimes I would imagine an irreverent me way down the line who refused to fall into place, disrupting the steady progression and creating a new reality that informed the ones that followed. During lulls at school, I would sometimes think about the light I had shed that morning, still endlessly bouncing between the mirrors, and I’d join one of my reflected selves, entering an imaginary parallel world constructed of light and driven by fantasy. It was a safe way to break the rules.

To be sure, reflected images don’t have minds of their own. But these youthful flights of fancy, with their imagined parallel realities, resonate with an increasingly prominent theme in modern science—the possibility of worlds lying beyond the one we know. This book is an exploration of such possibilities, a considered journey through the science of parallel universes.

Universe and Universes

There was a time when “universe” meant “all there is.” Everything. The whole shebang. The notion of more than one universe, more than one everything, would seemingly be a contradiction in terms. Yet a range of theoretical developments has gradually qualified the interpretation of “universe.” To a physicist, the word’s meaning now largely depends on context. Sometimes “universe” still connotes absolutely everything. Sometimes it refers only to those parts of everything that someone such as you or I could, in principle, have access to. Sometimes it’s applied to separate realms, ones that are partly or fully, temporarily or permanently, inaccessible to us; in this sense, the word relegates ours to membership in a large, perhaps infinitely large, collection.

With its hegemony diminished, “universe” has given way to other terms introduced to capture the wider canvas on which the totality of reality may be painted. Parallel worlds or parallel universes or multiple universes or alternate universes or the metaverse, megaverse, or multiverse—they’re all synonymous and they’re all among the words used to embrace not just our universe but a spectrum of others that may be out there.

You’ll notice that the terms are somewhat vague. What exactly constitutes a world or a universe? What criteria distinguish realms that are distinct parts of a single universe from those classified as universes of their own? Perhaps someday our understanding of multiple universes will mature sufficiently for us to have precise answers to these questions. For now, we’ll use the approach famously applied by Justice Potter Stewart in attempting to define pornography. While the U.S. Supreme Court wrestled mightily to delineate a standard, Stewart declared simply and forthrightly, “I know it when I see it.”

In the end, labeling one realm or another a parallel universe is merely a question of language. What matters, what’s at the heart of the subject, is whether there exist realms that challenge convention by suggesting that what we’ve long thought to be the universe is only one component of a far grander, perhaps far stranger, and mostly hidden reality.

During the last half century, science has provided ample ways in which this possibility might be realized.

Varieties of Parallel Universes

A striking fact (it’s in part what propelled me to write this book) is that many of the major developments in fundamental theoretical physics— relativistic physics, quantum physics, cosmological physics, unified physics, computational physics—have led us to consider one or another variety of parallel universe. Indeed, the chapters that follow trace a narrative arc through nine variations on the multiverse theme. Each envisions our universe as part of an unexpectedly larger whole, but the complexion of that whole and the nature of the member universes differ sharply among them. In some, the parallel universes are separated from us by enormous stretches of space or time; in others, they’re hovering millimeters away; in others still, the very notion of their location proves parochial, devoid of meaning. A similar range of possibility is manifest in the laws governing the parallel universes. In some, the laws are the same as in ours; in others, they appear different but have shared a heritage; in others still, the laws are of a form and structure unlike anything we’ve ever encountered. It’s at once humbling and stirring to imagine just how expansive reality may be.

Some of the earliest scientific forays into parallel worlds were initiated in the 1950s by researchers puzzling over aspects of quantum mechanics, a theory developed to explain phenomena taking place in the microscopic realm of atoms and subatomic particles. Quantum mechanics broke the mold of the previous framework, classical mechanics, by establishing that the predictions of science are necessarily probabilistic. We can predict the odds of attaining one outcome, we can predict the odds of another, but we generally can’t predict which will actually happen. This well-known departure from hundreds of years of scientific thought is surprising enough. But there’s a more confounding aspect of quantum theory that receives less attention. After decades of closely studying quantum mechanics, and after having accumulated a wealth of data confirming its probabilistic predictions, no one has been able to explain why only one of the many possible outcomes in any given situation actually happens. When we do experiments, when we examine the world, we all agree that we encounter a single definite reality. Yet, more than a century after the quantum revolution began, there is no consensus among the world’s physicists as to how this basic fact is compatible with the theory’s mathematical expression.

Over the years, this substantial gap in understanding has inspired many creative proposals, but the most startling was among the first. Maybe, that early suggestion went, the familiar notion that any given experiment has one and only one outcome is flawed. The mathematics underlying quantum mechanics—or at least, one perspective on the math— suggests that all possible outcomes happen, each inhabiting its own separate universe. If a quantum calculation predicts that a particle might be here, or it might be there, then in one universe it is here, and in another it is there. And in each such universe, there’s a copy of you witnessing one or the other outcome, thinking—incorrectly—that your reality is the only reality. When you realize that quantum mechanics underlies all physical processes, from the fusing of atoms in the sun to the neural firing that constitutes the stuff of thought, the far-reaching implications of the proposal become apparent. It says that there’s no such thing as a road untraveled. Yet each such road— each reality—is hidden from all others.

This tantalizing Many Worlds approach to quantum mechanics has attracted much interest in recent decades. But investigations have shown that it’s a subtle and thorny framework (as we will discuss in Chapter 8); so, even today, after more than half a century of vetting, the proposal remains controversial. Some quantum practitioners argue that it has already been proven correct, while others claim just as assuredly that the mathematical underpinnings don’t hold together.

What is beyond doubt is that this early version of parallel universes resonated with themes of separate lands or alternative histories that were being explored in literature, television, and film, creative forays that continue today. (My favorites since childhood include The Wizard of Oz, It’s a Wonderful Life, the Star Trek episode “The City on the Edge of Forever,” and, more recently, Sliding Doors and Run Lola Run). Collectively, these and many other works of popular culture have helped integrate the concept of parallel realities into the zeitgeist and are responsible for fueling much public fascination with the topic. But the mathematics of quantum mechanics is only one of numerous ways that a conception of parallel universes emerges from modern physics. In fact, it won’t be the first I’ll discuss.

Instead, in Chapter 2, I’ll begin with a different route to parallel universes, perhaps the simplest route of all. We’ll see that if space extends infinitely far—a proposition that is consistent with all observations and that is part of the cosmological model favored by many physicists and astronomers—then there must be realms out there (likely way out there) where copies of you and me and everything else are enjoying alternate versions of the reality we experience here. Chapter 3 will journey deeper into cosmology: the inflationary theory, an approach that posits an enormous burst of superfast spatial expansion during the universe’s earliest moments, generates its own version of parallel worlds. If inflation is correct, as the most refined astronomical observations suggest, the burst that created our region of space may not have been unique. Instead, right now, inflationary expansion in distant realms may be spawning universe upon universe and may continue to do so for all eternity. What’s more, each of these ballooning universes has its own infinite spatial expanse, and hence contains infinitely many of the parallel worlds explored in Chapter 2.

In Chapter 4, our trek turns to string theory. After a brief review of the basics, I’ll provide a status report on this approach to unifying all of nature’s laws. With that overview, in Chapters 5 and 6 we’ll explore recent developments in string theory that suggest three new kinds of parallel universes. One is string theory’s braneworld scenario, which posits that our universe is one of potentially numerous “slabs” floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf. If we’re lucky, it’s an approach that may provide an observable signature at the Large Hadron Collider in Geneva, Switzerland, in the not too distant future. A second variety involves braneworlds that slam into one another, wiping away all they contain and initiating a new, fiery big-bang-like beginning in each. As if two giant hands were clapping, this could happen over and over—branes might collide, bounce apart, attract each other gravitationally, and then collide again, a cyclic process generating universes that are parallel not in space but in time. The third scenario is the string theory “landscape,” founded on the enormous number of possible shapes and sizes for the theory’s required extra spatial dimensions. We’ll see that, when joined with the Inflationary Multiverse, the string landscape suggests a vast collection of universes in which every possible form for the extra dimensions is realized.

In Chapter 6, we’ll focus on how these considerations illuminate one of the most surprising observational results of the last century: space appears to be filled with a uniform diffuse energy, which may well be a version of Einstein’s infamous cosmological constant. Indeed, this observation has inspired much of the recent research on parallel universes, and it’s responsible for one of the most heated debates in decades on the nature of acceptable scientific explanations. Chapter 7 extends this theme by asking, more generally, whether consideration of hidden universes beyond our own can be rightly understood as a branch of science. Can we test these ideas? If we invoke them to solve outstanding problems, have we made progress, or have we merely swept the problems under a conveniently inaccessible cosmic rug? I’ve sought to lay bare the essentials of the clashing perspectives, while ultimately emphasizing my own view that, under certain specific conditions, parallel universes fall unequivocally within the purview of science.

Quantum mechanics, with its Many Worlds version of parallel universes, is the subject of Chapter 8. I’ll briefly remind you of the essential features of quantum mechanics, then focus on the formidable problem just referred to: how to extract definite outcomes from a theory whose basic paradigm allows for mutually contradictory realities to coexist in an amorphous, but mathematically precise, probabilistic haze. I’ll carefully lead you through the reasoning that, in seeking an answer, proposes anchoring quantum reality in its own profusion of parallel worlds.

Chapter 9 takes us yet further into quantum reality, leading to what I consider the strangest version of all parallel universes proposals. It’s a proposal that emerged gradually over thirty years of theoretical studies spearheaded by luminaries including Stephen Hawking, Jacob Bekenstein, Gerardt Hooft, and Leonard Susskind on the quantum properties of black holes. The work culminated in the last decade, with a stunning result from string theory, and it suggests, remarkably, that all we experience is nothing but a holographic projection of processes taking place on some distant surface that surrounds us. You can pinch yourself, and what you feel will be real, but it mirrors a parallel process taking place in a different, distant reality.

Finally, in Chapter 10 the yet more fanciful possibility of artificial

universes takes center stage. The question of whether the laws of physics give us the capacity to create new universes will be our first order of

business. We’ll then turn to universes created not with hardware but

with software—universes that might be simulated on a superadvanced computer—and investigate whether we can be confident that we’re not now living in someone or something else’s simulation. This will lead to the most unrestrained parallel universe proposal, originating in the philosophical community: that every possible reality is realized somewhere in what’s surely the grandest of all multiverses. The discussion naturally unfolds into an inquiry about the role mathematics has in unraveling the mysteries of science and, ultimately, our ability, or lack thereof, to gain an ever-deeper understanding of the expanse of reality.

The Cosmic Order

The subject of parallel universes is highly speculative. No experiment or observation has established that any version of the idea is realized in nature. So my point in writing this book is not to convince you that we’re part of a multiverse. I’m not convinced—and, speaking generally, no one should be convinced—of anything not supported by hard data. That said, I find it both curious and compelling that numerous developments in physics, if followed sufficiently far, bump into some variation on the parallel universe theme. Of particular note, it’s not that physicists are standing ready, multiverse nets in their hands, seeking to snare any passing theory that might be slotted, however awkwardly, into a parallel- universe paradigm. Rather, all of the parallel-universe proposals that we will take seriously emerge unbidden from the mathematics of theories developed to explain conventional data and observations.

My intention, then, is to lay out clearly and concisely the intellectual steps and the chain of theoretical insights that have led physicists, from a range of perspectives, to consider the possibility that ours is one of many universes. I want you to get a sense of how modern scientific investigations— not untethered fantasies like the catoptric musings of my boyhood— naturally suggest this astounding possibility. I want to show you how certain otherwise confounding observations can become eminently understandable within one or another parallel-universe framework; at the same time, I’ll describe the critical unresolved questions that have, as yet, kept this explanatory approach from being fully realized. My aim is that when you leave this book, your sense of what might be— your perspective on how the boundaries of reality may one day be redrawn by scientific developments now under way— will be far more rich and vivid.

Some people recoil at the notion of parallel worlds; as they see it, if we are part of a multiverse, our place and importance in the cosmos are marginalized. My take is different. I don’t find merit in measuring significance by our relative abundance. Rather, what’s gratifying about being human, what’s exciting about being part of the scientific enterprise, is our ability to use analytical thought to bridge vast distances, journeying to outer and inner space and, if some of the ideas we’ll encounter in this book prove correct, perhaps even beyond our universe. For me, it is the depth of our understanding, acquired from our lonely vantage point in the inky black stillness of a cold and forbidding cosmos, that reverberates across the expanse of reality and marks our arrival.

What People are Saying About This

From the Publisher

“Brian Greene has a gift for elucidating big ideas. . . Captures and engages the imagination. . . . It’s exciting and rewarding to read him.” —The New York Times

“A wonderful way to coax your brain into a host of strange and unfamiliar domains.” —The Boston Globe

“Exciting physics, wrapped up in effortless prose. . . . Greene has done it again.” —New Scientist

“If extraterrestrials landed tomorrow and demanded to know what the human mind is capable of accomplishing, we could do worse than to hand them a copy of this book.” —The New York Times Book Review
“The multiverse is an idea whose time has come. . . . The book serves well as an introduction . . . and will open up many people’s eyes.” —The Wall Street Journal
“Greene takes us down the rabbit hole yet again, this time setting a course for the terra incognita of parallel universes, hidden worlds, alternate realities, holographic projections, and multiverse simulations. Greene likes to drop you into the middle of the action first and then explain the backstory, but he has an elegant knack for anticipating questions and immediately dealing with any confusion or objections.” —The Daily Beast
“An accessible and surprisingly witty handbook to parallel universes…. Greene is immensely gifted at finding apt and colorful everyday analogies for the arcane byways of theoretical physics.” —The Toronto Star
“Mind-stretching. . . . [The Hidden Reality is] Greene’s impassioned argument ‘for the capacity of mathematics to reveal hidden truths about the workings of the world.’” —The New Yorker
“Like [Stephen] Hawking and [Roger] Penrose before him, [Greene] is an author who writes with the confidence and authority of one who . . . has seen the promised land of cosmic truth.” —Bookforum
“If you like your science explained rather than asserted, if you like your science writers articulate and intelligible, if you like popular science to make sense, even as it probes the heart of difficult theory, you are going to love The Hidden Reality and its author, Brian Greene.” —New York Journal of Books
“Greene’s forte is his amazing ability to give clear, everyday examples to illustrate complicated physical theories.” —The Globe and Mail
“Ambitious. . . . Entertaining and well-written. . . . Greene is a keen interpreter.” —The Christian Science Monitor
“A lucid, intriguing, and triumphantly understandable state-of-the-art look at the universe.” —Publishers Weekly
 “With a slew of clever analogies, Greene communicates with uncommon clarity, intuition, and honesty.” —The Oxonian Review
“Greene’s success at explaining the patently inexplicable lies in the way he delightfully melds the utterly bizarre and the utterly familiar.” —Providence Journal
“Exotic cosmic terrain through which Greene provides expert guidance.” —The Oregonian
“Mind-blowing.” —The Sunday Times (London)
“Highly rewarding.” —Scotland on Sunday
“[Greene] has something fresh and insightful to say about pretty much everything”—
“Vast, energetic and complex.” —The Easthampton Star
“The best guide available, in this universe at least.”—Science News
“Greene’s greatest achievement is that even as you grapple with these allusive concepts, you start falling in love with these mysteries.” —The Express Tribune

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The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos 3.9 out of 5 based on 0 ratings. 101 reviews.
LolPoop More than 1 year ago
I have been reading theoretical physics since i was 10 years old, and believe that I have the authority to say that this book is accurate, and a great read. However, the concepts outlined take a bit of getting used to. It is very abstract, and if you are new to the field, i suggest reading the elegant universe before this one, as it is a great introduction to theoretical physics.
Anonymous More than 1 year ago
I liked the book, but it was a little advanced for me.  Greene did a good job of *trying* to explain in a way that can be understood by anyone, but  it was still over my head.  I'll try it again some other time.
Anonymous More than 1 year ago
Good book good read
TessCallahan on LibraryThing More than 1 year ago
As in his previous books, The Elegant Universe and The Fabric of the Cosmos, Brian Greene uses The Hidden Reality to convey complex scientific ideas in a way that non-scientists can understand. His concept of the "multiverse," with it's ten potential manifestations, is mind blowing and fascinating. I listened to the audio version of this book, narrated by the author, and enjoyed every minute.
tonynetone on LibraryThing More than 1 year ago
Astrophysics, Recommended Reading, author of The Elegant Universe and The Fabric of the Cosmos comes his most expansive and accessible book about infinite a vast ocean of bubble universes Infinitely many worlds like our own,looking today we look back in time to understand what the book talking about, to understanding these multiverses we look around Astrophysics, but not regarded with great favour,In a nutshell, that string theory attempts to reconcile a mathematical conflict between. Brian Greene shows how a range of different ¿multiverse¿ proposal emerges.
steve.clason on LibraryThing More than 1 year ago
String Theorist (or string-theory apologist, depending on where you stand) Brian Greene believes the next expansion of the field physics will come when we prove, or collect evidence that argues in favor of, our universe being one of very many universes, collectively accumulated in what he calls "The Multiverse." In this book, Greene describes 9 ways these many universes could have come into being and what it means to us, both as physicists (a lot) and as regular humans (not so much) if the various theories turn out to be true and also whether or not there is any possible evidence that can be discovered that support them.So largely this is an exercise in imagination and a status-report on string theory, the mathematics of which give rise to several of the multiverse theories. I suspect it's also a brief for continued funding, but if so it's not flamboyant. Greene writes well and is able to provide metaphors and narratives that throw light on topis that are normally closed to those of us not proficient in high-level mathematics.
jaybeee on LibraryThing More than 1 year ago
Here¿s the thing. I am a science nerd. I very nearly went into theoretical physics at university, but instead opted for pure maths. So it is obvious why I could not resist the siren song of Brian Greene¿s latest The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos. (As an aside, I am so tired of lengthy subtitles for non-fiction books. I get that the publisher feels compelled to attract readers by giving them a hint of what the book¿s about, but you know, that¿s what the blurb on the inside jacket is for. I hope we can start having non-fiction books with just one title someday.) In some ways, Brian Greene is living my dream: a professor of both physics and mathematics doing groundbreaking research in superstring theory. (Of course, this is the dream of the me that plays well with others.) And his previous books were well-written, accessible explanations of some of that groundbreaking research with ample background given to ground the reader.I¿m sure that anyone who has read even five pages of science fiction has considered the idea of parallel universes before. It is such a science fiction thing: other worlds, parallel universes! (Explored to great effect, I might add, in Neal Stephenson¿s latest, Anathem.) But the thing that Greene does here is make the case for non-fictional parallel worlds. No, seriously. Take a minute to get your head around that. Greene offers up a theoretical framework from established physics and research-in-progress in which other universes are not only plausible, but maybe even necessary. Sadly, here is where science parts again from fiction because even if one of these proposals is correct, chances are we are not going to be able to interact with these universes.Saving the maths and tricky equations for the endnotes for the most part, Greene casually compares inflation fields to South Park, and turns Samuel Johnson into mathematics. His tone is conversational and he is kind to science-wary readers, offering up brief summaries of complicated ideas and encouraging those timid readers to skip ahead if they need to. And he offers up information in the right place at the right time, so the facts you need to make sense of the theories are fresh in your brain when you need them, something this forgetful brain was grateful for.What we are faced with is basically nine possible universes, or rather multiverses, since the ¿uni¿ in universe can be misleading. We¿ve spent a lot of time thinking of ¿universe¿ in the singular, but our universe is just one of many universes in this book, so the idea of a universe of universes comes in handy. Welcome to the multiverse! Greene gives these multiverses names like ¿Quilted Multiverse¿ and ¿Holographic Multiverse,¿ breaking them up into their own chapters and exploring various facets of relativity, quantum mechanics, quantum field theory, M-theory and so much more to make the case for the possibility of their existence. He follows all this up with a needed discussion on the limits of scientific inquiry as we know it, since at least one of these multiverses simply cannot be detected or measured by us ever. So is it scientific to even discuss its existence? This gets into the very nature of science itself, which is fascinating and got me thinking that I should study more about the philosophy of science.In the midst of all these fantastic and far-reaching ideas, the one thing I was sad to not see was any real appearance by the ladies. This book is a serious sausage fest. I¿m sure this was not deliberate on the part of Greene. He probably looked to the people whose work he was most familiar with, people who made the big developments in the fields that he was looking at. But in the further reading list at the back of the book, three and a half of the authors are of the lady persuasion (one was a co-author), and of the scientists that make an appearance in the text itself, only seven are woman, and three of these only made it into the endnotes. I didn¿t keep t
applemcg on LibraryThing More than 1 year ago
A disappointment. No, it's not that I "missed" the physics. Surely half a point deduction is due to my e-reader attempt. But a roster of (ten) possible multi-verses. File this one under the philosophy of science. Greene offers plausible means for verifying two or three of the candidates. It's interesting that we are on the verge of such experiments, but this feels like Greene is laying down a marker for the 23 (or later) century. It was fun speculating on "where" the electron may be: Grant's Tomb, or Central Park. The _one_ takeaway is to consider the observer herself must be "located" with quantum considerations as well.
jasonlf on LibraryThing More than 1 year ago
Brian Greene really is one of the best popular science writers. His books give you a real sense of being guided by someone who genuinely knows what they're talking about, who uses metaphors effectively, and who effectively weaves the traditional material in with the new points he is making. He also approaches science with curiosity untainted by dogmatism. He is very much open to speculation, but equally open to the speculation not panning out.This book is about different concepts of the Multiverse. Greene devotes a chapter to each of what he defines as the major types and then has one or two additional chapters on questions like whether these theories are testable and broader implications.The multiverse's he consider include the quilted multiverse (which is just our universe extending out infinitely, leaving the possibility of endless accidental repetition -- which follows from some cosmological theories that follow the big bang), the inflationary multiverse (a product of repeated episodes of inflationary expansion, which follows from the addition of inflation to the previous theories), three multiverses that come from different versions of string theory (brane, cyclic and landscape), a quantum multiverse (which is Everett's Many Worlds interpretation, and is more conceptual), a holographic multiverse (which comes from the study of black holes and string theory), and simulated and ultimate multiverses (the last two coming from computer simulations and a deeper mathematical world).In every case, Greene does a good job of describing the physical theories that lead, usually by accident, to the implication that there is a particular type of multiverse, discusses the scientific status of those theories, and addresses issues around testing them. In the end, Greene has some sympathy with Steven Weinberg's adage that the problem with physics is that we do not take our theories/equations seriously enough as a real description of the world. The example he cites is the Positron, which was a byproduct of Dirac's solution of a math problem that turned out to be real. Greene clearly leans towards the view that the same is true of the multiverse, but he doesn't do much to tip his hand about which one.
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