The Fabric of the Cosmos: Space, Time, and the Texture of Reality

by Brian Greene

  The Fabric of the Cosmos

  Brian Greene

  To Tracy

  Praise for Brian Greene's THE FABRIC OF THE COSMOS

  "As pure intellectual adventure, this is about as good as it gets. . . . Even compared with A Brief History of Time, Greene's book stands out for its sweeping ambition ... stripping down the mystery from difficult concepts without watering down the science." — Newsday

  "Greene is as elegant as ever, cutting through the fog of complexity with insight and clarity. Space and time, you might even say, become putty in his hands." — Los Angeles Times

  "Highly informed, lucid and witty. . . . There is simply no better introduction to the strange wonders of general relativity and quantum mechanics, the fields of knowledge essential for any real understanding of space and time." —Discover

  "The author's informed curiosity is inspiring and his enthusiasm infectious." — Kansas City Star

  "Mind-bending. . . . [Greene] is both a gifted theoretical physicist and a graceful popularizer [with] virtuoso explanatory skills." — The Oregonian

  "Brian Greene is the new Hawking, only better." — The Times (London)

  "Greene's gravitational pull rivals a black hole's." — Newsweek

  "Greene is an excellent teacher, humorous and quick. . . . Read [to your friends] the passages of this book that boggle your mind. (You may find yourself reading them every single paragraph.)" — The Boston Globe

  "Inexhaustibly witty . . . a must-read for the huge constituency of lay readers enticed by the mysteries of cosmology." —The Sunday Times

  "Forbidding formulas no longer stand between general readers and the latest breakthroughs in physics: the imaginative gifts of one of the pioneers making these breakthroughs has now translated mathematical science into accessible analogies drawn from everyday life and popular culture. . . . Nonspecialists will relish this exhilarating foray into the alien terrain that is our own universe." — Booklist (starred review)

  "Holds out the promise that we may one day explain how space and time have come to exist." —Nature

  "Greene takes us to the limits of space and time." — The Guardian

  "Exciting stuff. . . . Introduces the reader to the mind-boggling landscape of cutting-edge theoretical physics, where mathematics rules supreme."— The News & Observer

  "One of the most entertaining and thought-provoking popular science books to have emerged in the last few years. The Elegant Universe was a Pulitzer Prize finalist. The Fabric of the Cosmos deserves to win it."— Physics World

  "In the space of 500 readable pages, Greene has brought us to the brink of twenty-first-century physics with the minimum of fuss." — The Herald

  "If anyone can popularize tough science, it's Greene."— Entertainment Weekly

  "Greene is a marvelously talented exponent of physics. . . . A pleasure to read." —Economist

  "Magnificent ... sends shivers down the spine." — Financial Times

  "This is popular science writing of the highest order. . . . Greene [has an] unparalleled ability to translate higher mathematics into everyday language and images, through the adept use of metaphor and analogy, and crisp, witty prose. . . . He not only makes concepts clear, but explains why they matter." — Publishers Weekly (starred review)

  Preface

  Space and time capture the imagination like no other scientific subject. For good reason. They form the arena of reality, the very fabric of the cosmos. Our entire existence—everything we do, think, and experience— takes place in some region of space during some interval of time. Yet science is still struggling to understand what space and time actually are. Are they real physical entities or simply useful ideas? If they're real, are they fundamental, or do they emerge from more basic constituents? What does it mean for space to be empty? Does time have a beginning? Does it have an arrow, flowing inexorably from past to future, as common experience would indicate? Can we manipulate space and time? In this book, we follow three hundred years of passionate scientific investigation seeking answers, or at least glimpses of answers, to such basic but deep questions about the nature of the universe.

  Our journey also brings us repeatedly to another, tightly related question, as encompassing as it is elusive: What is reality? We humans only have access to the internal experiences of perception and thought, so how can we be sure they truly reflect an external world? Philosophers have long recognized this problem. Filmmakers have popularized it through story lines involving artificial worlds, generated by finely tuned neurological stimulation that exist solely within the minds of their protagonists. And physicists such as myself are acutely aware that the reality we observe—matter evolving on the stage of space and time—may have little to do with the reality, if any, that's out there. Nevertheless, because observations are all we have, we take them seriously. We choose hard data and the framework of mathematics as our guides, not unrestrained imagination or unrelenting skepticism, and seek the simplest yet most wide-reaching theories capable of explaining and predicting the outcome of today's and future experiments. This severely restricts the theories we pursue. (In this book, for example, we won't find a hint that I'm floating in a tank, connected to thousands of brain-stimulating wires, making me merely think that I'm now writing this text.) But during the last hundred years, discoveries in physics have suggested revisions to our everyday sense of reality that are as dramatic, as mind-bending, and as paradigm-shaking as the most imaginative science fiction. These revolutionary upheavals will frame our passage through the pages that follow.

  Many of the questions we explore are the same ones that, in various guises, furrowed the brows of Aristotle, Galileo, Newton, Einstein, and countless others through the ages. And because this book seeks to convey science in the making, we follow these questions as they've been declared answered by one generation, overturned by their successors, and refined and reinterpreted by scientists in the centuries that followed.

  For example, on the perplexing question of whether completely empty space is, like a blank canvas, a real entity or merely an abstract idea, we follow the pendulum of scientific opinion as it swings between Isaac Newton's seventeenth-century declaration that space is real, Ernst Mach's conclusion in the nineteenth century that it isn't, and Einstein's twentieth-century dramatic reformulation of the question itself, in which he merged space and time, and largely refuted Mach. We then encounter subsequent discoveries that transformed the question once again by redefining the meaning of "empty," envisioning that space is unavoidably suffused with what are called quantum fields and possibly a diffuse uniform energy called a cosmological constant—modern echoes of the old and discredited notion of a space-filling aether. What's more, we then describe how upcoming space-based experiments may confirm particular features of Mach's conclusions that happen to agree with Einstein's general relativity, illustrating well the fascinating and tangled web of scientific development.

  In our own era we encounter inflationary cosmology's gratifying insights into time's arrow, string theory's rich assortment of extra spatial dimensions, M-theory's radical suggestion that the space we inhabit may be but a sliver floating in a grander cosmos, and the current wild speculation that the universe we see may be nothing more than a cosmic hologram. We don't yet know if the more recent of these theoretical proposals are right. But outrageous as they sound, we investigate them thoroughly because they are where our dogged search for the deepest laws of the universe leads. Not only can a strange and unfamiliar reality arise from the fertile imagination of science fiction, but one may also emerge from the cutting-edge fin
dings of modern physics.

  The Fabric of the Cosmos is intended primarily for the general reader who has little or no formal training in the sciences but whose desire to understand the workings of the universe provides incentive to grapple with a number of complex and challenging concepts. As in my first book, The Elegant Universe, I've stayed close to the core scientific ideas throughout, while stripping away the mathematical details in favor of metaphors, analogies, stories, and illustrations. When we reach the book's most difficult sections, I forewarn the reader and provide brief summaries for those who decide to skip or skim these more involved discussions. In this way, the reader should be able to walk the path of discovery and gain not just knowledge of physics' current worldview, but an understanding of how and why that worldview has gained prominence.

  Students, avid readers of general-level science, teachers, and professionals should also find much of interest in the book. Although the initial chapters cover the necessary but standard background material in relativity and quantum mechanics, the focus on the corporeality of space and time is somewhat unconventional in its approach. Subsequent chapters cover a wide range of topics—Bell's theorem, delayed choice experiments, quantum measurement, accelerated expansion, the possibility of producing black holes in the next generation of particle accelerators, fanciful wormhole time machines, to name a few—and so will bring such readers up to date on a number of the most tantalizing and debated advances.

  Some of the material I cover is controversial. For those issues that remain up in the air, I've discussed the leading viewpoints in the main text. For the points of contention that I feel have achieved more of a consensus, I've relegated differing viewpoints to the notes. Some scientists, especially those holding minority views, may take exception to some of my judgments, but through the main text and the notes, I've striven for a balanced treatment. In the notes, the particularly diligent reader will also find more complete explanations, clarifications, and caveats relevant to points I've simplified, as well as (for those so inclined) brief mathematical counterparts to the equation-free approach taken in the main text. A short glossary provides a quick reference for some of the more specialized scientific terms.

  Even a book of this length can't exhaust the vast subject of space and time. I've focused on those features I find both exciting and essential to forming a full picture of the reality painted by modern science. No doubt, many of these choices reflect personal taste, and so I apologize to those who feel their own work or favorite area of study is not given adequate attention.

  While writing The Fabric of the Cosmos, I've been fortunate to receive valuable feedback from a number of dedicated readers. Raphael Kasper, Lubos Motl, David Steinhardt, and Ken Vineberg read various versions of the entire manuscript, sometimes repeatedly, and offered numerous, detailed, and insightful suggestions that substantially enhanced both the clarity and the accuracy of the presentation. I offer them heartfelt thanks. David Albert, Ted Baltz, Nicholas Boles, Tracy Day, Peter Demchuk, Richard Easther, Anna Hall, Keith Goldsmith, Shelley Goldstein, Michael Gordin, Joshua Greene, Arthur Greenspoon, Gavin Guerra, Sandra Kauffman, Edward Kastenmeier, Robert Krulwich, Andrei Linde, Shani Offen, Maulik Parikh, Michael Popowits, Marlin Scully, John Stachel, and Lars Straeter read all or part of the manuscript, and their comments were extremely useful. I benefited from conversations with Andreas Albrecht, Michael Bassett, Sean Carrol, Andrea Cross, Rita Greene, Wendy Greene, Susan Greene, Alan Guth, Mark Jackson, Daniel Kabat, Will Kinney, Justin Khoury, Hiranya Peiris, Saul Perlmutter, Koenraad Schalm, Paul Steinhardt, Leonard Susskind, Neil Turok, Henry Tye, William Warmus, and Erick Weinberg. I owe special thanks to Raphael Gunner, whose keen sense of the genuine argument and whose willingness to critique various of my attempts proved invaluable. Eric Martinez provided critical and tireless assistance in the production phase of the book, and Jason Severs did a stellar job of creating the illustrations. I thank my agents, Katinka Matson and John Brockman. And I owe a great debt of gratitude to my editor, Marty Asher, for providing a wellspring of encouragement, advice, and sharp insight that substantially improved the quality of the presentation.

  During the course of my career, my scientific research has been funded by the Department of Energy, the National Science Foundation, and the Alfred P. Sloan Foundation. I gratefully acknowledge their support.

  I - REALITY'S ARENA

  1 - Roads to Reality

  SPACE, TIME, AND WHY THINGS ARE AS THEY ARE

  None of the books in my father's dusty old bookcase were forbidden. Yet while I was growing up, I never saw anyone take one down. Most were massive tomes—a comprehensive history of civilization, matching volumes of the great works of western literature, numerous others I can no longer recall—that seemed almost fused to shelves that bowed slightly from decades of steadfast support. But way up on the highest shelf was a thin little text that, every now and then, would catch my eye because it seemed so out of place, like Gulliver among the Brobdingnagians. In hindsight, I'm not quite sure why I waited so long before taking a look. Perhaps, as the years went by, the books seemed less like material you read and more like family heirlooms you admire from afar. Ultimately, such reverence gave way to teenage brashness. I reached up for the little text, dusted it off, and opened to page one. The first few lines were, to say the least, startling.

  "There is but one truly philosophical problem, and that is suicide," the text began. I winced. "Whether or not the world has three dimensions or the mind nine or twelve categories," it continued, "comes afterward"; such questions, the text explained, were part of the game humanity played, but they deserved attention only after the one true issue had been settled. The book was The Myth of Sisyphus and was written by the Algerian-born philosopher and Nobel laureate Albert Camus. After a moment, the iciness of his words melted under the light of comprehension. Yes, of course, I thought. You can ponder this or analyze that till the cows come home, but the real question is whether all your ponderings and analyses will convince you that life is worth living. That's what it all comes down to. Everything else is detail.

  My chance encounter with Camus' book must have occurred during an especially impressionable phase because, more than anything else I'd read, his words stayed with me. Time and again I'd imagine how various people I'd met, or heard about, or had seen on television would answer this primary of all questions. In retrospect, though, it was his second assertion—regarding the role of scientific progress—that, for me, proved particularly challenging. Camus acknowledged value in understanding the structure of the universe, but as far as I could tell, he rejected the possibility that such understanding could make any difference to our assessment of life's worth. Now, certainly, my teenage reading of existential philosophy was about as sophisticated as Bart Simpson's reading of Romantic poetry, but even so, Camus' conclusion struck me as off the mark. To this aspiring physicist, it seemed that an informed appraisal of life absolutely required a full understanding of life's arena—the universe. I remember thinking that if our species dwelled in cavernous outcroppings buried deep underground and so had yet to discover the earth's surface, brilliant sunlight, an ocean breeze, and the stars that lie beyond, or if evolution had proceeded along a different pathway and we had yet to acquire any but the sense of touch, so everything we knew came only from our tactile impressions of our immediate environment, or if human mental faculties stopped developing during early childhood so our emotional and analytical skills never progressed beyond those of a five-year-old—in short, if our experiences painted but a paltry portrait of reality—our appraisal of life would be thoroughly compromised. When we finally found our way to earth's surface, or when we finally gained the ability to see, hear, smell, and taste, or when our minds were finally freed to develop as they ordinarily do, our collective view of life and the cosmos would, of necessity, change radically. Our previously compromised grasp of reality would have shed a very different light on that most fundamental of all philosophical questions.


  But, you might ask, what of it? Surely, any sober assessment would conclude that although we might not understand everything about the universe—every aspect of how matter behaves or life functions—we are privy to the defining, broad-brush strokes gracing nature's canvas. Surely, as Camus intimated, progress in physics, such as understanding the number of space dimensions; or progress in neuropsychology, such as understanding all the organizational structures in the brain; or, for that matter, progress in any number of other scientific undertakings may fill in important details, but their impact on our evaluation of life and reality would be minimal. Surely, reality is what we think it is; reality is revealed to us by our experiences.

  To one extent or another, this view of reality is one many of us hold, if only implicitly. I certainly find myself thinking this way in day-to-day life; it's easy to be seduced by the face nature reveals directly to our senses. Yet, in the decades since first encountering Camus' text, I've learned that modern science tells a very different story. The overarching lesson that has emerged from scientific inquiry over the last century is that human experience is often a misleading guide to the true nature of reality. Lying just beneath the surface of the everyday is a world we'd hardly recognize. Followers of the occult, devotees of astrology, and those who hold to religious principles that speak to a reality beyond experience have, from widely varying perspectives, long since arrived at a similar conclusion. But that's not what I have in mind. I'm referring to the work of ingenious innovators and tireless researchers—the men and women of science—who have peeled back layer after layer of the cosmic onion, enigma by enigma, and revealed a universe that is at once surprising, unfamiliar, exciting, elegant, and thoroughly unlike what anyone ever expected.

  These developments are anything but details. Breakthroughs in physics have forced, and continue to force, dramatic revisions to our conception of the cosmos. I remain as convinced now as I did decades ago that Camus rightly chose life's value as the ultimate question, but the insights of modern physics have persuaded me that assessing life through the lens of everyday experience is like gazing at a van Gogh through an empty Coke bottle. Modern science has spearheaded one assault after another on evidence gathered from our rudimentary perceptions, showing that they often yield a clouded conception of the world we inhabit. And so whereas Camus separated out physical questions and labeled them secondary, I've become convinced that they're primary. For me, physical reality both sets the arena and provides the illumination for grappling with Camus' question. Assessing existence while failing to embrace the insights of modern physics would be like wrestling in the dark with an unknown opponent. By deepening our understanding of the true nature of physical reality, we profoundly reconfigure our sense of ourselves and our experience of the universe.