the fabric of the cosmos - space, time, and the texture of reality (brian greene)

THIS IS .4 BORZOi BOOK PUBLISHED BY ALFRED A. KNOPF Copyright O 2004 by Brlan R. Greene All rights resented under International and PanAmerican Copyright Conventions. Published In the Unlted States by Alfred A. Knopf, a divmon of Random House, Inc., New York, and In Canada by Random Souse of Canada Limited, Toronto. Distributed by Random House, Inc., New York. awv.aaknopf.com Knopf, Borzo~ Books, and the colophon are registered trademarks of Random House, Inc. Library of Congress Catalog~ng-in-Publication Data Greene, B. (Brlan). The fabr~c of the cosmos . space, tlme, and the texture of reality 1 Brran Greene. p. cm. Includes bibliographical references (pp. 543-44). ISBY 0-375-41288-3 1. Cosmology-Popular works. I. Title. QB982.G742004 523.1-dci2 2003058918 To Tracy Manufactured In the United States of Amer~ca Fmt Edlt~on Contents Preface Part I REALITY'S ARENA 1. Roads to Reality Space, Time, and Why Thmgs Are as They Are 2. The Universe and the Bucket Is Space a Human .%bstractton or a Physlcal Enttfy? 3 Relativity and the Absolute Is Spacetzme an Einsteznian Abstraction or a Physical Entzt) ? 4. Entangling Space '\\'hat Does It Mean to Be Separate zn a Quantum Unwerse! Part 11 TIME AND EXPERIENCE 5. The Frozen River Does Time Flow! 6. Chance and the Arrow Does Time Have a Direction? 7. Time and the Quantum Insights into Time's Nature fion2 the Quantum Realm viii Contents Part Ill SPACETIME AND COSMOLOGY 8. Of Snowflakes and Spacetime Symmetry and the Evolution of the Cosmos 9. Vaporizing the Vacuum Heat, Nothzngness, and Unificatzon 10. Deconstructing the Bang What Banged? 11. Quanta in the Sky with Diamonds Inflation, Quantum Jitters, and the L4rrow ofTime Part IV ORIGINS AND UNIFICATION 12. The World on a String The Fabnc Accordmg to String Theory 13. The Universe on a Brane Speculatzons on Space and Time zn M-Theov Part V REALITY AND IMAGINATION 14. Up in the Heavens and Down in the Earth Experimenting wth Space and Time 15. Teleporters and Time Machines Traveling Through Space and Time 16. The Future of an Allusion Prospects for Space and Time Notes Glossary Sz~ggestions for Further Reading Index 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 cos- mos. 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 inexorabiy from past to future, as common ex- perience would indicate? Can we manipulate space and time? In this book, we follow three hundred years of passionate sc~entific 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 ques- tion, as encompassing as it is elusive: What is reali~? We humans only have access to the internal experiences of perception and thought, so how can we be sure they truly reflect an externai world? Philosophers have long recognized this problem. Filmmakers have popularized it through story lines involving artificial worlds, generated by finely tuned neurolog- ical stimulation that exist solely within the minds of their protagonists. And physicists such as myself are acuteiy 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 obser- vations are all we have, we take them seriously. We choose hard data and the framework of mathematics as our guides, not unrestrained imagina- tion or unrelenting skepticism, and seek the simplest yet most wide-reach- ing 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, x Preface 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 b!; scientists in the centuries that followed. For example, on the perpiexing question of whether completely empty space is, like a blank canvas, a real entity or merely an abstract idea, we follow the penduium 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 hventieth-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 uni- form 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 gen- eral relativity, illustrating well the fascinating and tangled web of scien- tific development. In our own era we encounter inflationary cosmology's gratifying insights into time's arrorv, string theory's rich assortment of extra spatial dimensions, hI-theory's radical suggestion that the space we inhabit may be but a sliver floating in a grander cosn~os, and the current wild specula- tion that the universe we see may be nothing more than a cosmic holo- gram. We don't yet know if the more recent of these theoretical proposals are right. But outrageous as they sound, we take them seriously 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 findings of modern physics. Preface x 1 The Fabric ojthe 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 con~plex and challenging concepts. As in my first book, The Elegant Universe, I've stayed close to the core scientific ideas throughout, bvhile 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 profes- sionals should also find much of interest in the book. Although the initial chapters cover the necessary but standard background material in relativ- ity 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 experi- ments, quantum measurement, accelerated expansion, the possibilib of producing black holes in the next generation of particle accelerators, fan- ciful worn~hole 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 con- sensus, 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 reference for some of the more specialized sci- entific 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 ~ ~ x!i Preface who feel their own work or favorite area of study is not given adequate attention. While writing The Fabric ofthe 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 Goidstein, Michael Gordin, Joshua Greene, Arthur Greenspoon, Gavin Guerra, Sandra Kauffman, Edward Kastenmeier, Robert Krulwich, Andrei Linde, Shani Offen, Maulik Parikh, Michael Popowits, Mariin Scully, John Stachel, and Lars Straeter read all or part of the manuscript, and their comments were extremeiy useful. I benefited from conversations with Andreas Albrecht, Michael Bassett, Sean Carrol, Andrea Cross, Rita Greene, Alan Guth, Mark Jackson, Daniel Kabat, Will Kinney, Justin Khoury, Iiiranya Peiris, Saul Perimutter, Koenraad Schalm, Paul Stein- hardt, Leonard Susskind, Neil Turok, Henry Tye, William V7armus, and Eiick 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 qualit). of the presen- tation. During the course of my career, my scientific research has been funded by the Department of Energy, the Nationai Science Foundation, and the Alfred P. Sloan Foundation. I gratefully acknowledge their sup- port. Roads to Reality SPACE. TIME, AND WHY THINGS ARE AS THEY ARE N one of the books in my father's dusty oid bookcase were forbidden. Yet while I mas growlng up, I never saw anyone take one down. Most were massive tomes-a comprehensive history of civiliza- tion, 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 high- est 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 Brobding- nagians. In hindsight, I'm not quite sure why I waited so long before tak- ing a iook. Perhaps, as the years went by, the books seemed less like material you read and more like family heirlooms you admire from afar. Ultimateiy, 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 bvere, to say the least, startling. "There is but one truly philosophicai 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, "conies 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 ofSisyphus and was written by the Algerian-born philosopher and Nobel laureate Albert Camus. After a moment, the ici- ness 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 con- 4 THE FABRIC OF THE COSMOS \mce you that life is worth living. That's what it all comes domm to. Every- thing else is detail. My chance encounter with Camus' book must have occurred during an especially impressionable phase because, more than anj~thing eise I'd read, his words stayed with me. Time and again I'd imagine hou. 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 asser- tion-regarding the role of scientific progress-that, for me, proved par- ticularly challenging. Camus acknowledged value In understanding the structure of the universe, but as far as 1 could tell, he rejected the possibil- ity that such understanding could make any difference to our assessment of life's worth. Now, certainly, my teenage reading of existential philoso- phy was about as sophisticated as Bart Simpson's reading of Romantic poetry, but even so, Camus' conciusion 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 thlnking 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 dur~ng early childhood so our emotional and anaiyti- cal 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 ordi- narily 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 philo- sophical 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 prii? to the defining, broad-brush strokes gracing nature's canvas. Surely, as Camus intimated, progress in physics, such as understanding the num- ber of space dimensions; or progress in neuropsycholog)., such as under- standing all the organizational structures in the brain; or, for that matter, Roads to Realitv progress in any number of other scientific undertaklngs may fill in impor- tant details, but their impact on our evaluation of life and reality would be minimal. Sureip, 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 expe- rience 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. Foi- lowers of the occult, devotees of astroloa., 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 thoroughl~. unlike what anyone ever expected. These developments are anything but details. Breakthroughs in physics have forced, and continue to force, dramatic revisions to our con- ception of the cosmos. I remain as convinced now as I did decades ago t'hat Camus rightly chose iife'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, show- ing 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 t'he arena and provides the illumination for grap- piing 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. The centrai concern of this book is to explain some of the most prominent and pivotal of these revisions to our picture of reality, ~vith an 6 THE FABRIC OF THE COSMOS intense focus on those that affect our species' long-term project to under- stand space and time. From Aristotle to Einstein, from the astrolabe to the Hubble Space Telescope, from the pyramids to mountaintop obsewato- ries, space and time have framed thinking since thinking began. With the advent of the modern scientific age, their importance has been tremen- dously heightened. Over the last three centuries, developn~ents in physics have revealed space and time as the most baffling and most con~pelling concepts, and as those most instrumental in our scientific analysis of the universe. Such developments have also shown that space and time top the list of age-old scientific constructs that are being fantastically revised by cutting-edge research. To Isaac Newton, space and time simply were-they formed an inert, universal cosmic stage on which the events of the universe played them- sel\.es out. To his contemporary and frequent rival Gottfried Wilhelm von Leibniz, "space" and "time" were merely the vocabulary of relations between where objects were and when events took place. Nothing more. But to Albert Einstem, space and time were the raw material underlying realib. Through his theories of relativity, Einstem jolted our thinking about space and time and revealed the principai part they play in the evo- lution ofthe universe. Ever since, space and time have been the sparkling jewels of phys~cs. They are at once familiar and mystifying; fully under- standing space and time has become physics' most daunting challenge and sought-after prize. The developments we'll cover in this book interweave the fabr~c of space and time in various ways. Some ideas will challenge features of space and time so bas~c that for centuries, if not millennia, they've seemed beyond questioning. Others will seek the link between our theo- retical understanding of space and time and the traits we commonly expe- rience. Yet others will ralse questions unfathomable within the limited confines of ordinary perceptions. K7e will speak only minimally of philosophy (and not at all about sui- cide and the meaning of life). But in our scientific quest to solve the mys- teries of space and time, we will be resolutely unrestrained. From the universe's smallest speck and earliest moments to its farthest reaches and most distant future, we will examine space and time in environments familiar and far-flung, with an unflinching eye seeking their true nature. As the story of space and time has yet to be fully written, we won't arrive at any final assessments. But we will encounter a series of developments- some intensely strange, some deeply satisfying, some experimentally ven- Roads to Reality 7 fied, some thoroughly speculative-that will show how close we've come to wrapping our minds around the fabric of the cosmos and touching the true texture of reality. Classical Reality Historians differ on exactly when the modern scientific age began, but certainly by the time Galileo Galilei, RenC Descartes, and Isaac Newton had had their say, it was briskly under may. In those days, the new men- tific mind-set was being steadily forged, as patterns found in terrestrial and astronomicai data made it increasingly clear that there is an order to all the comings and goings of the cosmos, an order accessible to careful rea- soning and mathematical analysis. These early pioneers of modern scien- tific thought argued that, when looked at the right way, the happenings In the universe not only are explicable but predictable. The power of science to foretell aspects of the future-consistently and quantitatively-had been revealed. Early scientific study focused on the kinds of things one might see or experience in everyday life. Galileo dropped welghts from a leaning tower (or SO legend has it) and watched balls rolling down inclined surfaces; Newton studied falling apples (or so legend has it) and the orbit of the moon. The goal of these investigations was to attune the nascent scientific ear to nature's harmonies. To be sure, physical reality ivas the stuff of expe- rience, but the challenge was to hear the rhyme and reason behmd the rhythm and regularity. Many sung and unsung heroes contributed to the rapid and impressive progress that was made, but Newton stole the show. With a handful of mathematical equations, he synthesized everything known about motion on earth and in the heavens, and in so doing, com- posed the score for what has come to be known as classical physics. In the decades following Newton's work, his equations were devel- oped into an elaborate mathematical structure that significantly extended both their reach and their practical utility. Classical physics gradually became a sophisticated and mature scientific discipline. But shining clearly through all these advances was the beacon of Newton's original insights. Even today, more than three hundred years later, you can see Newton's equations scrawled on introductory-physics chalkboards world- wide, printed on NASA flight computing spacecraft trajectories, and embedded within the complex calculations of forefront research. 8 THE FABRIC OF THE CCS~IOS Newton brought a wealth of physical phenomena within a single theoretl- cal framework. But while formulating his iaws of motion, Newton encountered a crit- ical stumbling block, one that is of particular importance to our story (Chapter 2). Everyone knew that things could move, but what about the arena within urhich the motion took place? Well, that's space, we'd all ansn3er. But, Newton would reply, what is space? Is space a real physical entity or is it an abstract Idea born of the human struggle to comprehend the cosn~os? Newton realized that this key question had to be answered, because without taking a stand on the meaning of space and time, his equations describing motion would prove meaningless. Understanding requlres context; insight must be anchored. And so, with a fen. brief sentences in his Principia Mathematzca, Newton articulated a conception of space and time, declaring them absolute and immutable entities that provided the universe with a rigid, unchangeable arena. '4ccording to Newton, space and time supplied an invisible scaffolding that gave the universe shape and structure. Not everyone agreed. Some argued persuasively that it made little sense to ascribe existence to something you can't feel, grasp, or affect. But the explanatory and predictive power of Newton's equations quieted the critics. For the next two hundred years, his absolute conception of space and time was dogma. Relativistic Reality The class~cal Newtonian worldview was pleasing. Not only did ~t describe natural phenomena m.ith striking accuracy, but the details of the descrip- tion-the mathematics-aligned tightly with experience. If you push something, it speeds up. The harder you throw a ball, the more impact ~t has when it smacks ~nto a wall. If you press against something, you feel it pressing back against you. The more massive something is, the stronger its gravitational pull. These are among the most bas~c properties of the nat- ural world, and ~vhen you learn Newton's framework, you see them repre- sented in his equations, clear as day. Unlike a crystal ball's ~nscrutable hocus-pocus, the workings of Newton's laws were on display for all with minimal mathematical training to take in fully. Classical physics provided a rigorous grounding for human intuition. Newton had included the force of gravity in his equations, but it was Roads to Reality 9 not until the 1860s that the Scottish scientist James Clerk Maxwell extended the framework of classicai physics to take account of electrical and magnetic forces. Maxwell needed additional equations to do so and the mathematics he employed required a higher level of training to grasp fully. But his new equations were every bit as successful at explaining electrical and magnetic phenomena as Newton's were at describing motion. By the late 1800s, it was evident that the universe's secrets were proving no match for the power of human intellectual might. Indeed, with the successful incorporation of electricity and magnet- ism, there was a growing sense that theoretical physics would soon be complete. Physics, some suggested, was rapidly becoming a finished sub- ject and its laws would shortly be chiseled in stone. In 1894, the renowned experimental Albert Michelson remarked that "most of the grand underlying principles have been firmly established" and he quoted an "eminent scientistn-most believe it was the Br~tish physicist Lord Kelv~n-as saylng that all that remained were details of determining some numbers to a greater number of decimal places.' In 1900, Kelvin himself did note that "two clouds" were hovering on the horizon, one to do with properties of light's motion and the other with aspects of the radiation objects emit when heated,' but there was a general feeling that these Lvere mere details, which, no doubt, would soon be addressed. Within a decade, everything changed. As ant~cipated, the two prob- lems Kelvin had raised were promptly addressed, but they proved any- thing but minor. Each ignited a revolution, and each required a fundamental rewriting of nature's laws. The classical conceptions of space, time, and reality- the ones that for hundreds of years had not only worked but also concisely expressed our intuitive sense of the world- were overthrown. The relatiwty revolution, which addressed the first of Kelvin's "clouds," dates from i905 and 1915, when Albert Einstein completed his special and general theories of relativity (Chapter 3). While struggling with puzzles involving electricity, magnetism, and light's motion, Ein- stein realized that Newton's conception of space and time, the corner- stone of classical physics, was flawed. Over the course of a few intense weeks in the spring of 1905, he determmed that space and time are not independent and absolute, as Newton had thought, but are enmeshed and relative in a manner that flies in the face of common experience. Some ten years later, Einstein hammered a final nail in the Newtonian coffin by rewriting the laws of gravitational physics. This time, not only ! 0 THE FABRIC OF THE COSMOS did Einstein show t'hat space and time are part of a unified whole, he also showed that by warping and curving they participate in cosmic evolution. Far from being the rigid, unchanging structures envisioned by Newton, space and t~me in Einstein's reworking are flexible and dynamic. The two theories of relativity are among humankind's most precious achievements, and with them Einstein toppled Newton's conception of reality. Even though Newtonian physics seemed to capture mathemati- cally much of what we experience physically, the reality it describes turns out not to be the reality of our world. Ours is a relativistic reality. Yet, because the deviation between classical and relativistic reality is manifest only under extreme conditions (such as extremes of speed and gravit).), Newtonian phys~cs still provides an approximat~on that proves extremelj. accurate and useful in many circumstances. But utility and realib are ver). different standards. As LG will see, features of space and time that for many of us are second nature have turned out to be figments of a false Newtonian perspective. Quantum Reality The second anomaly to which Lord Kelvin referred led to the quantum revolution, one of the greatest upheavals to which modern human under- standing has ever been subjected. By the time the fires subsided and the smoke cleared, the veneer of classical physics had been singed off the newiy emerging framework of quantum reality. A core feature of classical physics is that if you know the positions and velocities of all objects at a particular moment, Newton's equations, together with their Maxwellian updating, can tell you their positions and velocities at any other moment, past or future. Without equivocation, classical physlcs declares that the past and future are etched mto the pres- ent. This feature 1s aiso shared by both special and general relativity. Although the relativistic concepts of past and future are subtler than their famiiiar classical counterparts (Chapters 3 and 5j, the equations of reia- tivity, together with a complete assessment ofthe present, determine them just as completely. By the 1930s, however, phps~cists were forced to introduce a whole new conceptual schema called quantum mechanics. Quite unexpectedly, they found that only quantum laws were capable of resolving a host of puzzles and explaining a variety of data newly acquired from the atomic Roads to Reality and subaton~ic realm. But according to the quantum laws, even if you make the most perfect measurements possible of how things are today, the best you can ever hope to do is predict the probability that things will be one way or another at some chosen time in the future, or that things were one way or another at some chosen time in the past. The universe, according to quantum mechanics, is not etched into the present; the uni- verse, according to quantum mechanics, participates in a game of chance. Although there is still controversy over precisely how these develop- ments should be interpreted, most physicists agree that probability is deeply woven into the fabric of quantum reality. Whereas human intu- ition, and its embodiment in classical physics, envision a reality in which things are always definitely one way or another, quantum mechanics describes a reality in which things sometimes hover in a haze of being partly one way and ~artly another. Things become definite only when a suitable observation forces them to relinquish quantum possibilities and settle on a specific outcome. The outcon~e that's realized, though, cannot be predicted-we can predict only the odds that things will turn out one way or another. This, plainiy speaking, is weird. We are unused to a reality that remains ambiguous until perceived. But the oddity of quantum mechan- ics does not stop here. At least as astounding is a feature that goes back to a paper Einstein wrote in 1935 with two younger colleagues, Nathan Rosen and Boris Podolsky, that was intended as an attack on quantum the- 01-y.~ With the ensuing twists of scientific progress, Einstein's paper can now be viewed as among the first to point out that quantum mechanics- if taken at face value-implies that something you do over here can be instantaneously linked to something happening over there, regardless of distance. Einstein considered such instantaneous connections ludicrous and interpreted their emergence from the mathematics of quantum the- ory as evidence that the theory was in need of much development before it \vould attain an acceptable form. But by the 19SOs, when both theoreti- cal and tech~~ological deveiopments brought experimental scrutmy to bear on these purported quantum absurdities, researchers confirmed that there can be an instantaneous bond between what happens at widely sep- arated locations. Under pristine iaboratory conditions, what Einstem thought absurd really happens [Chapter 4). The implications of these features of quantum mechanm for our pic- ture of reality are a subject of ongoing research, Many scient~sts, myself ~ncluded, view them as part of a radical quantum updating of the meaning [...]... to Reality it shifts the puzzle to the realm of cosmology -the study of the origin and e.i,olution of the entire cosmos- and compels us to find out whether the universe actually had the highly ordered beginning that this expianation of time's arrotv requires Cosmology is among the oldest subjects to captivate our species And it's no wonder We're storytellers, and h a t story could be more grand than the. .. attention and he drew much inspiration from hIach's proposal while developing his own theory of gravity, the general theory of relativity When the dust of relativity had finally settled, the question of whether space is a something -of whether the absolutist or relationist view of space is correct-was transformed in a manner that shattered all previous ways of looking at the universe Relativity and the Absolute... want to ensure that clocks on the front and back of the train are perfectly synchronized Since they are still equidistant from the gunpowder, they come up with the following plan They agree to set thelr ciocks to noon just as they see the light from the faring gunpowder From their perspective, the light has to travel the same dis- 1 II 1 I I I I tance to reach either of them, and since light's speed is... regardless of how each chose to lay out the grid, there are some things thej, definitely still agree on For exampie, if in the interest of increasing worker efficiency during lunchtime, a trail is painted on the ground from the nuclear plant straight to the Kwik-E-Mart, hlarge and Lisa will not agree on the streets and avenues through which the trail passes, as you can see 60 6! THE FABRIC OF THE COSMOS. .. the universe's earliest moments (in this approach, the size of the universe increased by a factor larger than a million trillion trillion in less than a millionth of a trillionth of a trillionth of a second) As will become clear, this stupendous growth of the young universe goes a long way toward filling in the gaps ieft by the big bang model -of explaining the shape of space and the uniformity of the. .. electricit)., magnetism, and their intimate reiationship.' Maxwell developed these equations by carefully studying the work of the English physicist Michael Faraday, who in the early 1800s had carried out tens of thousands of experiments that exposed hitherto unknown features of electricity and magnetism Faraday's key breakthrough was the concept of the field Later expanded on by Maxwell and many others, this concept... aether as a down-to-earth stand-in for the divine spirit that H e n v More, Newton, and others had envisioned permeating absolute space, (Newton and others in hrs age had even used the term "aether" in their descriptions of absolute space.) But what actually is the aether? What is it made of? Where did it come from? Does it exist everywhere? These questions about the aether are the same ones that for centuries... hit, and so on) by giving the time when the event occurred -the page on which it appears -and the locatlon within the region of space depicted on the pages Here is the key polnt: Just as Lisa realized that there are different, equally valid ways to dice u p a region of space into streets and avenues, ii Figure 3.3 (c) Block of spacetime containmg the duel Pages, or "time slices," organize the events in the. .. to travel farther, rendering the duel unfair 56 Relativztv a n d the Absolute THE FABRIC O F THE COSXIOS IVhc 1s right, Xpu or Martin? Einstein's unexpected answer is that they both are Although the conclusions of our hvo referees differ, the observations and the reasoning of each are flawless Like the bat and the baseball, they simply have different perspectives on the same sequence of events T h... developing a so-called unzfied theov U n i f i e d Reality Over the past few centuries, physicists have sought to consolidate our understanding of the natural world by showing that d~verseand apparently distinct phenomena are actually governed by a single set of physical laws To Einstein, this goal of unification -of explaining the widest array 16 THE FABRIC OF THE C O S ~ ~ O S of phenomena with the fewest . (Brlan). The fabr~c of the cosmos . space, tlme, and the texture of reality 1 Brran Greene. p. cm. Includes bibliographical references (pp. 54 3-4 4). ISBY 0-3 7 5-4 128 8-3 1. Cosmology-Popular. and never the reverse, but it does not fullJ, solve the mystery of time's arrow. Instead, ! 4 THE FABRIC OF THE COSMOS it shifts the puzzle to the realm of cosmology -the study of. growth of the young universe goes a long way toward fill- ing in the gaps ieft by the big bang model -of explaining the shape of space and the uniformity of the microwave radiation, and also of