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Brian Greene

The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos Hardcover – January 25, 2011

by Brian Greene (Author)

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From the best-selling author of The Elegant Universe and The Fabric of the Cosmos comes his most expansive and accessible book to date—a book that takes on the grandest question: Is ours the only universe?

There was a time when “universe” meant all there is. Everything. Yet, in recent years discoveries in physics and cosmology have led a number of scientists to conclude that our universe may be one among many. With crystal-clear prose and inspired use of analogy, Brian Greene shows how a range of different “multiverse” proposals emerges from theories developed to explain the most refined observations of both subatomic particles and the dark depths of space: a multiverse in which you have an infinite number of doppelgängers, each reading this sentence in a distant universe; a multiverse comprising a vast ocean of bubble universes, of which ours is but one; a multiverse that endlessly cycles through time, or one that might be hovering millimeters away yet remains invisible; another in which every possibility allowed by quantum physics is brought to life. Or, perhaps strangest of all, a multiverse made purely of math.

Greene, one of our foremost physicists and science writers, takes us on a captivating exploration of these parallel worlds and reveals how much of reality’s true nature may be deeply hidden within them. And, with his unrivaled ability to make the most challenging of material accessible and entertaining, Greene tackles the core question: How can fundamental science progress if great swaths of reality lie beyond our reach?

Sparked by Greene’s trademark wit and precision, The Hidden Reality is at once a far-reaching survey of cutting-edge physics and a remarkable journey to the very edge of reality—a journey grounded firmly in science and limited only by our imagination.

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384 pages
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English
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Knopf
Publication date

January 25, 2011
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6.56 x 1.36 x 9.64 inches
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0307265633
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978-0307265630
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Editorial Reviews

Amazon.com Review

Amazon Best Books of the Month, January 2011: Take any of physics' major theories of the fundamental nature of the universe, extrapolate its math to the logical extreme, and you get some version of a (so far unobservable) parallel universe. And who better to navigate these hypothetical versions of the "multiverse" than Brian Greene? Normally an unflinching apologist for string theory, the bestselling author of The Elegant Universe and The Fabric of the Cosmos here treats all viable alternate realities to a laudably fair shake. For a book exploring the most far-reaching implications of bleeding-edge mathematics, The Hidden Reality is surprisingly light on math, written as it is "for a broad audience … its only prerequisite the will to persevere." Such perseverance pays off with a motley cast of potential universes featuring doppelgängers, strings, branes, quantum probabilities, holographs, and simulated worlds. The result is that rare accomplishment in science writing for a popular audience: a volume that explains the science and its consequences while stimulating the imagination of even the uninitiated.

Oliver Sacks on The Hidden Reality

Oliver Sacks was born in London and educated in London, Oxford, California, and New York. He is professor of neurology and psychiatry at Columbia University, and Columbia's first University Artist. He is the author of many books, including Awakenings, The Man Who Mistook His Wife for a Hat, and Musicophilia. His newest book, The Mind's Eye, was published in October, 2010.

Brian Greene is not only a profound cosmological thinker--a pioneer of string theory--but a writer of exceptional clarity and charm. His books--The Elegant Universe and The Fabric of the Cosmos among them--take one ever deeper into a universe stranger and more wonderful than anyone could have conceived a generation ago. The Hidden Reality takes us deeper still, and it has a special personal quality and warmth that is evident from the opening of the book, when Greene recollects how, as a boy, he was fascinated by the multiple reflections in parallel mirrors. He has never lost this childlike wonder at the world of physics, but he brings it now to examining theories of multiple universes, of the continual birth of universes, starting long before our own. . . and destined to continue, perhaps, to the end of time.

In the 1930s, as a boy myself, I read The Mysterious Universe by James Jeans. Jeans was, like Greene, a brilliant theoretical astronomer and equally mesmerizing writer. I thought Jeans's book was the most exciting, revelatory book I had ever read, and now, seventy years later, I feel the same excitement reading Brian Greene's new book, where every chapter opens level after level of previously unimaginable, mind-expanding realities.

From Publishers Weekly

Starred Review. "There was a time when ÿuniverse' meant ÿall there is,' " writes Greene, but soon we may have to redefine that word, along with our own meager understanding of the cosmos. A theoretical physicist and celebrated author, Greene offers intrepid readers another in-depth yet marvelously accessible look inside the perplexing world of modern theoretical physics and cosmology. Greene's book The Elegant Universe explained late 20th-century efforts to find a unified theory of everything, culminating with string theory. But string theory opened up a new can of worms, hinting at the possible existence of multiple universes and other strange entities. The possibility of other universes existing alongside our own like holes in "a gigantic block of Swiss cheese" seems more likely every day. Beginning with relativity theory, the Big Bang, and our expanding universe, Greene introduces first the mind-blowing multiplicity of forms those parallel universes might take, from patchwork quilts or stretchy "branes" to landscapes and holograms riddled with black holes. With his inspired analogies starring everyone from South Park's Eric Cartman to Ms. Pac-Man and a can of Pringles, Greene presents a lucid, intriguing, and triumphantly understandable state-of-the-art look at the universe. Illus. (Feb.)
(c) Copyright PWxyz, LLC. All rights reserved.

From Booklist

That our cosmos is but one of many has inspired a recent spate of theoretical physicist-authors�Lisa Randall, Michio Kaku, Leonard Susskind�to advance their particular idea of the multiverse in general-interest works. Here their peer, Greene, comprehensively covers major variations of the multiverse concept. Numbering nine, they emerge from mathematical substrates that have yet to connect multiverse speculation with physical reality. Theory as arbiter of experimental and observational proof looms over each multiverse Greene discusses, some of which might become detectable with contemporary or some fantastic future technology, and some of which in principle could never be perceived. Greene explains that more accessible versions of the multiverse arise from investigations into general relativity and quantum mechanics. Puzzling over gravity, the wave function of electrons, and values of physical constants, physicists have devised daughter universes budding off our own or hovering nearby in higher dimensions postulated by string theory. Greene�s exceptional clarity shows what evidence would hint at such parallel universes emerging from well-established theory and presents mind-benders suggesting that all universes are holograms projected from the surfaces of black holes. As bizarre as multiverses appear, the credence that scientists accord them is wonderfully transmitted to a lay readership by Greene. HIGH-DEMAND BACKSTORY: Greene�s as big a name as science produces: he hosted a PBS Nova television series based on his The Elegant Universe (1999); his encore, The Fabric of Reality (2004), loitered for 10 weeks on the New York Times best-seller list. All-out marketing from book tour to tweets portend demand equal to the publisher�s celebrity-scale first printing of 300,000 copies. --Gilbert Taylor

Review

“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.”
—Timothy Ferris, The New York Times Book Review“Few living writers write so lucidly about such complicated stuff. In

Greene’s prose, cutting-edge cosmology and particle physics become something a plucky and well-rested reader can apprehend. . . Greene might be the best intermediary I’ve found between the sparkling, absolute zero world of mathematics and the warm, clumsy world of human language.”
—Anthony Doerr, Boston Globe

“Mr. Greene has a gift for elucidating big ideas . . . Exciting and rewarding . . . [The Hidden Reality] captures and engages the imagination.”
—Janet Maslin, The New York Times

“It's impossible to summarize every step of Greene's balletic footwork, by which, like some multi-limbed Asian deity, he dances into being each different theoretical framework that could support multiple universes. . . His arguments are constructed like classical cathedrals, with intricate arches and buttresses that all uphold the central spire. Sometimes you think he's lost in the details of some sculpted gargoyle, only to realize how essential to the whole structure this particular feature is.”
—Paul di Filippo, Barnes and Noble Review

“[Greene] leads the general reader on an excursion to the farthest and most mind-bending reaches of speculative physics . . . An exhilarating—if sometimes vertigo-inducing—journey.”
—Alden Mudge, Bookpage

“An in-depth yet marvelously accessible look inside the perplexing world of modern theoretical physics and cosmology . . . Greene presents a lucid, intriguing, and triumphantly understandable state-of-the-art look at the universe.”
—Publishers Weekly (Starred review)

About the Author

Brian Greene received his undergraduate degree from Harvard University and his doctorate from Oxford University, where he was a Rhodes Scholar. He joined the physics faculty of Cornell University in 1990, was appointed to a full professorship in 1995, and in 1996 joined Columbia University, where he is professor of physics and mathematics. He has lectured at both a general and a technical level in more than thirty countries, and on all seven continents, and is widely regarded for a number of groundbreaking discoveries in superstring theory. His first book, The Elegant Universe, was a national best seller and a finalist for the Pulitzer Prize. His most recent book, The Fabric of the Cosmos, was also a best seller. He lives in Andes, New York, and New York City.

Excerpt. © Reprinted by permission. All rights reserved.

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.

Product details

Publisher ‏ : ‎ Knopf; First Edition (January 25, 2011)
Language ‏ : ‎ English
Hardcover ‏ : ‎ 384 pages
ISBN-10 ‏ : ‎ 0307265633
ISBN-13 ‏ : ‎ 978-0307265630
Item Weight ‏ : ‎ 1.65 pounds
Dimensions ‏ : ‎ 6.56 x 1.36 x 9.64 inches

Best Sellers Rank: #284,506 in Books (See Top 100 in Books)
- #220 in Quantum Theory (Books)
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- #481 in Astrophysics & Space Science (Books)

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Brian Greene

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Brian Greene received his undergraduate degree from Harvard University and his doctorate from Oxford University, where he was a Rhodes scholar. He is a professor of physics and mathematics at Columbia University and lives in New York City.

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Alan S. Glassman

Science now tells us we must seriously consider we live in a multiverse

Reviewed in the United States 🇺🇸 on December 30, 2012

Verified Purchase

Following up his two previous bestsellers, The Elegant Universe and The Fabric of the Cosmos, Columbia University Professor of Physics and Mathematics Brian Greene has carried on in his attempt to enlighten the lay reader to leading edge developments in scientific research into the deepest mysteries of both the micro-world and the macro-world of not only our own universe, but other universes that may exist beyond our current ability to contact them.

Including 30 pages of notes, mostly for the more technically inclined, an extensive index, and using metaphor, analogy, historical anecdotes, and a touch of humour, Professor Greene looks at the latest theoretical thinking and experimental analyses to give, as he says in the Preface, "...a broadly accessible account of some of the strangest and, should they prove correct, most revealing insights of modern physics. Many of the concepts require the reader to abandon comfortable modes of thought and to embrace unanticipated realms of reality."

What spurred Greene to give us an up-to-date account of developments in fundamental theoretical physics is that they have led investigators to the serious consideration of different types of parallel universes. In the book, he identifies 9 varieties of what are called "multiverses." And, what is so amazing to Greene and to his colleagues around the world is that "...all of the parallel-universe proposals that we will take seriously (in the book) emerge unbidden from the mathematics of theories developed to explain conventional data and observation."

Proceeding like a class in the conceptual, non-technical overview of contemporary physics, displaying his knack for making difficult concepts easy to understand by relating them to common, everyday examples from life, and ascribing nomenclature generally used in the field of cosmology, Dr. Greene first examines what is known as the Quilted Multiverse. He begins the discussion with a review of the Big Bang and Einsteinian relativity to illustrate "...that basic physical principles establish that if the cosmos is infinitely large, it is home to infinitely many parallel worlds - some identical to ours, some differing from ours, many bearing no resemblance to our world at all."

Because astronomers have calculated that we can only see out from earth about 41 billion light-years (called the cosmic horizon - an enormous distance, but certainly less than infinite), if the universe itself is infinite, there must be an infinite amount of other regions of space that have their own cosmic horizons. If those regions are sufficiently distant from each other, such an array would look like a patchwork quilt of an infinite number of finite regions that are individual universes themselves. Such a scenario is called a Patchwork or Quilted Multiverse. In such a multiverse, there would be endless doppelgangers - exact, repetitive reproductions of everything we experience, even ourselves.

Greene's second type of multiverse is called the Inflationary Multiverse. This one is based on the continual expansion of our universe that would eternally produce bubble universes, and only one of those would be the one we see. We can think of the bubble universes in the Inflationary Multiverse as the holes in an ever-expanding Swiss cheese cosmos.

Professor Greene's third and fourth types of multiverses derive from String theory and from the braneworlds of M-theory. In Chapter 4, he reiterates from his previous books a clear explanation of various aspects of quantum mechanics resulting in String theory, extra spatial dimensions, singularities, and black holes that lead us into Chapter 5's discussion of the Brane Multiverse and the Cyclic Multiverse. The former consists of three-dimensional branes (don't worry - Greene explains what branes are) that float in higher dimensions with other branes, and the latter derives from the collisions of those braneworlds that result in new universes with their own big bangs.

We are next introduced to something called the Landscape Multiverse. It derives from a combination of the Inflationary Multiverse and String theory.

In 1998, two separate teams of astronomers measured a positive but tiny number for Einstein's cosmological constant - a value that gives us the amount of dark, invisible energy thought to be existing uniformly throughout space. Dark energy governs the repulsive gravitational force that drives our universe's inflation. Contrary to what we would expect - that after the Big Bang the inflation of the universe would gradually slow down - the measurements indicated that for approximately the past half of our universe's life its rate of expansion has been accelerating.

String theory tells us that each of the ever-increasing number of bubble universes in an Inflationary Multiverse contains a different configuration of extra dimensions "...providing a cosmological framework that realises all possibilities." It also tells us that different values of the cosmological constant in each bubble universe give rise to "...bubbles inside of bubbles inside of bubbles..." When combined with accelerating expansion, this bubble tunnelling process provides an entire "landscape" of different universes. Hence, the totality is called a Landscape Multiverse.

50-page long, Chapter 8 describes what's called the Quantum Multiverse - a multiverse that emerges directly from quantum mechanics. Greene reminds us of the double-slit experiment and its consequential interference pattern in order to guide us through his explanations of a particle's probability wave and Niels Bohr's Copenhagen Interpretation which dictate that the act of measurement/observation results in the collapse of the wave function locating only one position for a given particle - one definite, observed reality outcome.

Professor Greene then takes us to the realm of the Holographic Multiverse where reality takes place on a universe's distant boundary surface and projects its information into the 3D world we know and experience as a kind of holographic movie. We can think of this as we would think of the information in an architect's blueprints being translated into the actual physical realisation of a building. In other words, the boundary surface of a universe can be thought of as a physically equivalent parallel universe.

Greene adds, "That familiar reality may be mirrored, or perhaps even produced, by phenomena taking place on a faraway, lower-dimensional surface ranks among the most unexpected developments in all of theoretical physics.... Looking to the future, I suspect that the holographic principle will be a beacon for physicists well into the twenty-first century."

The 8th and 9th multiverses identified in the book involve both actual and computer simulations. The 8th variety is called a Simulated Multiverse. Here, Greene takes a bold step in contemplating universe creation by future humans in the possession of very advanced technologies. There are two types of these we can think of: (1) usual, physical universes, and (2) virtual, computer-generated universes. The first involves artificially producing a white hole that spews out matter. The second is akin to the conceptual presentation in movies like The Matrix, The Thirteenth Floor, and Vanilla Sky.

The 9th and final multiverse discussed is what Professor Greene calls the Ultimate Multiverse. It is his own rationalisation for the existence of a multiverse, independent of being a by-product of quantum mechanics, inflationary cosmology, String theory, or any other such applications that led indirectly to the previous 8 types of multiverses. He surmises, "Maybe math is more than just a description of reality. Maybe math is reality." Perhaps, "Different collections of mathematical equations are different universes. The Ultimate Multiverse is thus the by-product of this perspective on mathematics." (M.I.T.'s Max Tegmark calls this the Mathematical Universe Hypothesis.)

Greene further posits, "Mathematical existence is synonymous with physical existence. And since this would be true for any and all math, this provides another road leading us to the Ultimate Multiverse." It's another way of saying that every possible universe we can imagine, and therefore describe with a mathematical equation, is, somewhere and at some time, a real universe.

Greene ends with questions like, "Can scientific theories that invoke a multiverse be tested?" And, "Should we believe mathematics?" In fact, he admits that math is central to all he discusses. The multiverse theories examined in his book "rely on a belief that mathematics is tightly stitched into the fabric of reality."

He adds in conclusion, "It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality."

- This review first appeared in New Dawn magazine issue #132

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A. Menon

A guide through the multiverse possibilities that emerge as consequences of theoretical physics

Reviewed in the United States 🇺🇸 on May 16, 2011

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Brian Greene is truly an excellent science writer. Though there are many ideas that cant be communicated to the lay reader and specifics almost never can, Brian Greene manages to illustrate the general ideas behind what many physicists are thinking and working on and why. The Hidden Reality is an overview of where modern day physics has taken us in terms of the potential nature of reality. In particular he explores the potential initial configurations of the universe and as well the subsequent potential mechanics and what these potentials mean for the nature of reality. He also discusses the nature of reality from the perspective of general relativity, string theory and quantum mechanics. I realize this is vague, but its hard to give a specific overview of what is talked about as the author is almost as concise as he can be to convey the ideas he attempts to. Nonetheless i'll give it a try.

The book starts out with the first example of how if we consider our planet and current aggregate configuration and history as unique, then the law of large numbers might cause some alarm in an infinite universe. In particular if we do thought experiment of assuming the universe is truly infinite (spatially with matter in all regions) then it implies that there must be copies of us out there. This results can be deduced from several vantage points and in the book, it is the fact that any negligbly small probability will be hit an infinite number of times if we have an infinite number of experiments. Though the "initial conditions" presented are just a hypothesis, this first chapter sets an eerie start which the book continues to build momentum on. The author then discusses inflationary universes and how they too give rise to multiverse scenarios in which parallel universes will remain unseen as they have inflated at distances that will never be reached. He also discusses what seem to be paradoxes of differing views of infinity, for example, those inside an expanding universe will consider it infinite space with finite time whereas outside they will consider it finite space with infinite time. One gets a glimpse of some of physicists insights but the ideas can be tough and i think to truly understand much of it deeply requires studying the math. The author then gives a brief outline of string theory and gets into the potential multiverse of branes that we might live in (i wont bother to try to paraphrase this part). Subsequent to this the author then introduces what I thought was the first model for parallel universes which is the quantum multiverse in which there is no collapse of the wave function and all possibilities are realized and the wave patterns we see are the parallel worlds interfering. The history of this is given as well as the philosophical misgivings of many scientists. The author then gets into the holographic universe which is pretty hard to grasp, (im pretty sure its hard to grasp even for the expert!) in which our sensations might all be a product of what happens on a lower dimensional space as there is a mathematical mapping between information of a surface with its higher dimensional body. This evolved from some blackhole information theory result... Clearly the book discusses things which are pretty out there but does a good job in trying to communicate what can be communicated to the reader. The book then ventures further into philosophy rather than science and discusses some ideas about what is the "reality" of a computed world and what is the "reality" that mathematical equations exist in.

Intertwined throughout the book is the authors discussion of what is science. How much of these interesting ideas about the universe is science rather than philosophy. Much theoretical physics, in particular string theory, has come under a lot of pressure for producing no real testable experiments to its validity (though the experiments being thought up are getting closer to testable) and the author discusses his ideas as to what is legitimate and what is not. The book is primarily on various multiverse possibilities for the universe and how basically all modern theories have multiverse interpretations. Embedded in the writing is also the authors philosophy to legitimize his views on science - he includes some fairly out there theories of computational universes (ie extraordinarily complex simlife type computations) and the multiverse of logically correct statements and equations to show where he draws the line. I pretty much think this is a 5 star book but it is not always consistent. For example, infinity is a complicated issue as there are levels of infinity, integers are countably infinite, the real line is not. In his first chapter, the author argues as to why a multiverse will have a copy of us, he invokes quantum mechanics to legitimize the discreteness of space and thus uses a limiting exercise in the countable sense to get to the conclusion. In the last chapter though he questions this implicitly (but doesnt discuss how it affects his previous arguments) by using the continuum of schrodingers equations values to say we cant escape an infinite configuration space. This is obviously subtle and in aggregate this is a very interesting read in which complicated arguments and phenomenon are well described and the nature of reality and ideas are tackled. I learned a lot and I think everyone can get something out of it.

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Philip M

Greene is gifted at describing complex mathematics and physics in human language.

Reviewed in the United Kingdom 🇬🇧 on February 22, 2021

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This subject can never be easy read, but Brian Greene is immensely gifted at offering understandable analogues for the arcane intricacies of theoretical physics. This book is no exception; it is entertaining, exciting and well written in a rewarding way that coaxes your brain into strange and unfamiliar territories.

In Greene’s own words, “There was a time when universe meant all there is, but soon we may have to redefine that word, along with our own meager understanding of the cosmos.” And Greene’s expert guidance uses masterly metaphor and analogy to create a lucid and accessible account of some of the strangest and most revealing insights of modern physics.

He encourages the reader to abandon comfortable ways of thinking and to embrace unexpected realms of reality. From string theory to quantum mechanics, Greene describes the mathematical rigors and intuitive insight into the big bang and parallel worlds and universes, guided and confirmed by experimentation and observation to establish that space, time, matter and energy behave in ways any of us have ever witnessed. And as a result, physicists are facing the next upheaval in understanding the possibility that our universe is not unique.

I found The Hidden Reality much easier to comprehend after reading Greene’s previous books, The Elegant Universe and The Fabric of the Cosmos. But nonetheless, The Hidden Reality is an accessible and surprisingly witty guide to parallel universes, a rare accomplishment in science writing for a popular audience.

Whilst Greene has the ability to translate the absolute zero of mathematics to intelligible human language, he also provides excellent footnotes and appendices for those with a mathematical inclination. This book is a perfect read for the casual but interested layman, and both scientists and mathematicians.

5 people found this helpful

j

Disappointed

Reviewed in the United Kingdom 🇬🇧 on October 7, 2020

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Bought this book expecting an interesting outlook on the view of all things weird and wonderful. The author spends far to much time writing about the useless things such as where so and so went to school and waffling on rather than actually telling the story. If you bought this book cause you’re like me and wanted information on parallel universes etc then this is covered in about the last 10 pages of a 300 page book. Highly disappointed. The writing is also so small it gave me a headache reading more than a few pages so I am afraid I have had to give up and skip to the chapters I wanted to learn about to save my head and eyesight. Would not recommend.

3 people found this helpful

Tim Dumble

You too can write like this - in a quantum multiverse!

Reviewed in the United Kingdom 🇬🇧 on May 15, 2013

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The strength of this fine book lies in the meticulous manner in which the author systematically builds the evidence necessary to convince even the most sceptical reader to at least think seriously about the concept of parallel universes. Just as Copernicus overturned the anthropic solar-centric universe model, Greene draws upon the mathematics of the most recent developments in physics, all of which support the existence of a multiverse of which our own is far from unique.

In citing: quantum mechanics, string theory and M theory the reader is drawn ever closer to the probable conclusion that our universe is merely one of many. In so doing Greene brilliantly raises questions about the empirical basis of science and proposes a paradigm shift from the classical physics approach of using equations describing physical laws, knowledge of the constants of nature and initial conditions to predict future outcomes with certainty.

Perhaps the most compelling argument for parallel universes, besides the fact almost all the most recently developed theories support it, is the manner in which it can potentially reconcile that most thorny issue of contemporary physics- how to reconcile the inherent weakness of gravity and its relationship with the quantum world. The extra dimensions implicit in M theory and the possibilities of string theory offer an exciting future solution to the gravity problem which has so far stood in the way of the holy grail of a theory of everything. Indeed so powerful are the arguments proffered by Greene here, the multiverse hypothesis is set to make the pursuit of such a theory obsolete. If ours is just one of many universes, the need to understand its constants becomes less significant. In a multiverse all possible universes exist with all possible ranges of cosmological constant, Higgs field strengths and atomic particle masses.

Greene uses metaphor and illustrations to great effect and is forensic in the piecing together of evidence to support his theories. For example he convincingly reconciles quantum wave theory with string theory in support of the holographic multiverse arguably the most perplexing of all the multiverse hypotheses.

Memorable discussions about simulated universes and whether we are currently part of such a reality also feature- enough to entertain P.K.Dick officianados. Greene latterly also reflects upon the consequences of multiverse theories for science as an empirical investigative discipline based on observations. He also cogently explores the power of mathematics as the basis of reality and its role in multiverse theory.

25 people found this helpful

Chris

Wow!

Reviewed in the United Kingdom 🇬🇧 on April 25, 2019

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Plenty to take in but this author has it at his fingertips. If you are interested in the subject it is a must read. I think everyone should, at least, try to get a grasp of the big cosmic picture just to see how much we have progressed in theoretical and actual knowledge. It also shows how little we know about a finely tuned universe, the vastness of which is terrifying and throws up all sorts of possibilities to ponder upon.

7 people found this helpful

Nicolas Milne

Brilliant

Reviewed in the United Kingdom 🇬🇧 on July 7, 2013

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I consider this to the most readable and wide ranging book that Brian Greene has written and struggle to understand some of the comments slating it for being too theoretical and lacking empiricism. The book sells itself quite openly as a presentation of the breadth of theory, there is a clue in the title, so if you don't want that ,don't read it. However if you want a very clearly written account of what theories there are then this is it. For an example of clarity in prose read pages 32 and 33 on cosmic repetition.
The book ranges from Newton and classical theory,through the difficulties in getting the maths to reconcile themselves with a unifying theory to an extreme theoretical position which can best be described as" In the beginning was the computer and the computer was Maths."
Greene is open on his position and doubts and despite the fact I suspect he is maths driven he is also prepared to debate the proposition which I, as a non mathematician hold, that if we can't understand or prove much of the theory then altering our perceptions of logic just so the maths works is a double edged sword. We don't understand and we created the maths which is at odds with what we can test and substantiate and it is quite possible that in a few hundred years our thinking will have evolved way beyond this.

A brilliant read, even for a non mathematical archaeologist.

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