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This Idea Must Die: Scientific Theories That Are Blocking ProgressJohn Brockman
The bestselling editor of This Explains Everything brings together 175 of the world's most brilliant minds to tackle Edge.org's 2014 question: What scientific idea has become a relic blocking human progress?
Each year, John Brockman, publisher of Edge.org—"The world's smartest website" (The Guardian)—challenges some of the world's greatest scientists, artists, and philosophers to answer a provocative question crucial to our time. In 2014 he asked 175 brilliant minds to ponder: What scientific idea needs to be put aside in order to make room for new ideas to advance? The answers are as surprising as they are illuminating. In :
- Steven Pinker dismantles the working theory of human behavior
- Richard Dawkins renounces essentialism
- Sherry Turkle reevaluates our expectations of artificial intelligence
- Geoffrey West challenges the concept of a "Theory of Everything"
- Andrei Linde suggests that our universe and its laws may not be as unique as we think
- Martin Rees explains why scientific understanding is a limitless goal
- Nina Jablonski argues to rid ourselves of the concept of race
- Alan Guth rethinks the origins of the universe
- Hans Ulrich Obrist warns against glorifying unlimited economic growth
- and much more.
Profound, engaging, thoughtful, and groundbreaking, This Idea Must Die will change your perceptions and understanding of our world today . . . and tomorrow.
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DEDICATION To Richard Dawkins, Daniel C. Dennnet, Jared Diamond, and Steven Pinker Pioneers of the Third Culture CONTENTS DEDICATION ACKNOWLEDGMENTS PREFACE: THE 2014 EDGE QUESTION GEOFFREY WEST The Theory of Everything MARCELO GLEISER Unification A. C. GRAYLING Simplicity SETH LLOYD The Universe SCOTT ATRAN IQ LEO M. CHALUPA Brain Plasticity HOWARD GARDNER Changing the Brain VICTORIA WYATT "The Rocket Scientist" NIGEL GOLDENFELD Indivi-duality NICHOLAS HUMPHREY The Bigger an Animal's Brain, the Greater Its Intelligence LEE SMOLIN The Big Bang Was the First Moment of Time ALAN GUTH The Universe Began in a State of Extraordinarily Low Entropy BRUCE PARKER Entropy ANDREI LINDE The Uniformity and Uniqueness of the Universe MAX TEGMARK Infinity LAWRENCE M. KRAUSS The Laws of Physics Are Predetermined PAUL STEINHARDT Theories of Anything ERIC R. WEINSTEIN M-theory/String Theory Is the Only Game in Town FRANK TIPLER String Theory GORDON KANE Our World Has Only Three Space Dimensions PETER WOIT The "Naturalness" Argument FREEMAN DYSON The Collapse of the Wave Function DAVID DEUTSCH Quantum Jumps W. DANIEL HILLIS Cause and Effect NINA JABLONSKI Race RICHARD DAWKINS Essentialism PETER RICHERSON Human Nature JULIA CLARKE The Urvogel KURT GRAY Numbering Nature MICHAEL SHERMER Hardwired = Permanent DOUGLAS RUSHKOFF The Atheism Prerequisite ROGER HIGHFIELD Evolution Is "True" ANTON ZEILINGER There Is No Reality in the Quantum World STEVE GIDDINGS Spacetime AMANDA GEFTER The Universe HAIM HARARI The Higgs Particle Closes a Chapter in Particle Physics SARAH DEMERS Aesthetic Motivation MARIA SPIROPULU Naturalness, Hierarchy, and Spacetime ED REGIS Scientists Ought to Know Everything Scientifically Knowable SEAN CARROLL Falsifiability NICHOLAS G. CARR Anti-anecdotalism REBECCA NEWBERGER GOLDSTEIN Science Makes Philosophy Obsolete IAN BOGOST "Science" SAM HARRIS Our Narrow Definition of "Science" DANIEL C. DENNETT The Hard Problem SUSAN BLACKMORE The Neural Correlates of Consciousness TODD C. SACKTOR Long-Term Memory Is Immutable BRUCE HOOD The Self THOMAS METZINGER Cognitive Agency JERRY COYNE Free Will ROBERT PROVINE Common Sense JONATHAN GOTTSCHALL There Can Be No Science of Art GEORGE DYSON Science and Technology ALAN ALDA Things Are Either True or False GAVIN SCHMIDT Simple Answers MARTIN REES We'll Never Hit Barriers to Scientific Understanding SEIRIAN SUMNER Life Evolves Via a Shared Genetic Toolkit KEVIN KELLY Fully Random Mutations ERIC J. TOPOL One Genome per Individual TIMO HANNAY Nature Versus Nurture ROBERT SAPOLSKY The Particularist Use of "a" Gene-Environment Interaction ATHENA VOULOUMANOS Natural Selection Is the Only Engine of Evolution STEVEN PINKER Behavior = Genes + Environment ALISON GOPNIK Innateness KILEY HAMLIN Moral Blank-Slateism OLIVER SCOTT CURRY Associationism SIMON BARON-COHEN Radical Behaviorism DANIEL L. EVERETT "Instinct" and "Innate" TOR NØRRETRANDERS Altruism JAMIL ZAKI The Altruism Hierarch ADAM WAYTZ Humans Are by Nature Social Animals GARY KLEIN Evidence-Based Medicine DEAN ORNISH Large Randomized Controlled Trials RICHARD NISBETT Multiple Regression as a Means of Discovering Causality AZRA RAZA Mouse Models PAUL DAVIES The Somatic Mutation Theory of Cancer STEWART BRAND The Linear No-Threshold (LNT) Radiation Dose Hypothesis BENJAMIN K. BERGEN Universal Grammar N. J. ENFIELD A Science of Language Should Deal Only With "Competence" JOHN MCWHORTER Languages Condition Worldviews DAN SPERBER The Standard Approach to Meaning KAI KRAUSE The Uncertainty Principle IAN MCEWAN Beware of Arrogance! Retire Nothing! GARY MARCUS Big Data CHRISTINE FINN The Stratigraphic Column DIMITAR D. SASSELOV The Habitable-Zone Concept SHERRY TURKLE Robot Companions ROGER SCHANK "Artificial Intelligence" TANIA LOMBROZO The Mind Is Just the Brain FRANK WILCZEK Mind Versus Matter ALEXANDER WISSNER-GROSS Intelligence as a Property DAVID GELERNTER The Grand Analogy TERRENCE J. SEJNOWSKI Grandmother Cells PATRICIA S. CHURCHLAND Brain Modules TOM GRIFFITHS Bias Is Always Bad ROBERT KURZBAN Cartesian Hydraulicism RODNEY A. BROOKS The Computational Metaphor SARAH-JAYNE BLAKEMORE Left-Brain/Right-Brain STEPHEN M. KOSSLYN Left-Brain/Right-Brain ANDRIAN KREYE Moore's Law ERNST PÖPPEL The Continuity of Time ANDY CLARK The Input-Output Model of Perception and Action LAURIE R. SANTOS & TAMAR GENDLER Knowing Is Half the Battle JAY ROSEN Information Overload ALEX (SANDY) PENTLAND The Rational Individual MARGARET LEVI Homo Economicus RICHARD H. THALER Don't Discard Wrong Theories, Just Don't Treat Them as True SUSAN FISKE Rational Actor Models: The Competence Corollary MATT RIDLEY Malthusianism CESAR HIDALGO Economic Growth HANS ULRICH OBRIST Unlimited and Eternal Growth LUCA DE BIASE The Tragedy of the Commons MICHAEL I. NORTON Markets Are Bad; Markets Are Good GIULIO BOCCALETTI Stationarity LAURENCE C. SMITH Stationarity DANIEL GOLEMAN The Carbon Footprint STUART PIMM Unbridled Scientific and Technological Optimism BUDDHINI SAMARASINGHE Scientists Should Stick to Science SCOTT SAMPSON Nature = Objects EDWARD SLINGERLAND Scientific Morality ALEX HOLCOMBE Science Is Self-Correcting ADAM ALTER Replication as a Safety Net BRIAN CHRISTIAN Scientific Knowledge Structured as "Literature" KATHRYN CLANCY The Way We Produce and Advance Science AUBREY DE GREY Allocating Funds via Peer Review ROSS ANDERSON Some Questions Are Too Hard for Young Scientists to Tackle KATE MILLS Only Scientists Can Do Science MELANIE SWAN The Scientific Method FIERY CUSHMAN Big Effects Have Big Explanations SAMUEL ARBESMAN Science = Big Science JUNE GRUBER Sadness Is Always Bad, Happiness Is Always Good ELDAR SHAFIR Opposites Can't Both Be Right DAVID BERREBY People Are Sheep DAVID M. BUSS Beauty Is in the Eyes of the Beholder HELEN FISHER Romantic Love and Addiction BRIAN KNUTSON Emotion Is Peripheral PAUL BLOOM Science Can Maximize Our Happiness PASCAL BOYER Culture LAURA BETZIG Culture JOHN TOOBY Learning and Culture STEPHEN STICH "Our" Intuitions ALUN ANDERSON We're Stone Age Thinkers MARTIN NOWAK Inclusive Fitness MICHAEL McCULLOUGH Human Evolutionary Exceptionalism KATE JEFFERY Animal Mindlessness IRENE PEPPERBERG Humaniqueness STEVE FULLER Human Being = Homo sapiens SATYAJIT DAS Anthropocentricity DONALD D. HOFFMAN Truer Perceptions Are Fitter Perceptions GREGORY BENFORD The Intrinsic Beauty and Elegance of Mathematics Allows It to Describe Nature CARLO ROVELLI Geometry ANDREW LIH Calculus NEIL GERSHENFELD Computer Science SAMUEL BARONDES Science Advances by Funerals HUGO MERCIER Planck's Cynical View of Scientific Change JARED DIAMOND New Ideas Triumph by Replacing Old Ones MIHALY CSIKSZENTMIHALYI Max Planck's Faith MARY CATHERINE BATESON The Illusion of Certainty JONATHAN HAIDT The Pursuit of Parsimony GERALD SMALLBERG The Clinician's Law of Parsimony LISA BARRETT Essentialist Views of the Mind ABIGAIL MARSH The Distinction Between Antisociality and Mental Illness DAVID G. MYERS Repression JOEL GOLD & IAN GOLD Mental Illness Is Nothing but Brain Illness BEATRICE GOLOMB Psychogenic Illness EDUARDO SALCEDO-ALBARÁN Crime Entails Only the Actions of Criminals CHARLES SEIFE Statistical Significance GERD GIGERENZER Scientific Inference via Statistical Rituals EMANUEL DERMAN The Power of Statistics VICTORIA STODDEN Reproducibility NICHOLAS A. CHRISTAKIS The Average NASSIM NICHOLAS TALEB Standard Deviation BART KOSKO Statistical Independence RICHARD SAUL WURMAN Certainty. Absolute Truth. Exactitude. PAUL SAFFO The Illusion of Scientific Progress NOTES INDEX ABOUT THE AUTHOR ALSO BY JOHN BROCKMAN BACK ADS CREDITS COPYRIGHT ABOUT THE PUBLISHER ACKNOWLEDGMENTS My thanks to Laurie Santos for suggesting this year's Edge Question and to Paul Bloom and Jonathan Haidt for their refinements. As always, thanks to Stewart Brand, Kevin Kelly, George Dyson, and Steven Pinker, for their continued support. I also wish to thank Peter Hubbard of HarperCollins for his encouragement. I am also indebted to my agent, Max Brockman, who saw the potential for this book, and, as always, to Sara Lippincott for her thoughtful and meticulous editing. JOHN BROCKMAN Publisher & Editor, Edge THE 2014 EDGE QUESTION Science advances by discovering new things and developing new ideas. Few truly new ideas are developed without abandoning old ones first. As theoretical physicist Max Planck (1858-1947) noted, "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." In other words, science advances by a series of funerals. Why wait that long? WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT? Ideas change, and the times we live in change. Perhaps the biggest change today is the rate of change. What established scientific idea is ready to be moved aside so that science can advance? THE THEORY OF EVERYTHING GEOFFREY WEST Theoretical physicist; Distinguished Professor and past president, Santa Fe Institute Everything? Well, wait a minute. Questioning a Theory of Everything may be beating a dead horse, since I'm certainly not the first to be bothered by its implicit hyperbole, but let's face it, referring to one's field of study as "the Theory of Everything" smacks of arrogance and naïveté. Although it's been around for only a relatively short period and may already be dying a natural death, the phrase (though certainly not the endeavor) should be retired from serious scientific literature and discourse. Let me elaborate. The search for grand syntheses, for commonalities, regularities, ideas, and concepts transcending the narrow confines of specific problems or disciplines is one of the great inspirational drivers of science and scientists. Arguably, it is also a defining characteristic of Homo sapiens sapiens. Perhaps the binomial form of sapiens is some distorted poetic recognition of this. Like the invention of gods and God, the concept of a Theory of Everything connotes the grandest vision of all, the inspiration of all inspirations: namely, that we can encapsulate and understand the entirety of the universe in a small set of precepts—in this case, a concise set of mathematical equations. Like the concept of God, however, it's potentially misleading and intellectually dangerous. Among the classic grand syntheses in science are Newton's laws, which taught us that heavenly laws were no different than the earthly; Maxwell's unification of electricity and magnetism, which brought the ephemeral aether into our lives; Darwin's theory of natural selection, which reminded us that we're just animals and plants after all; and the laws of thermodynamics, which suggest we can't go on forever. Each of these has had profound consequences—not only in changing the way we think about the world but also in laying the foundations for technological advancements that have led to the standard of living many of us are privileged to enjoy. Nevertheless, they're all, to varying degrees, incomplete. Indeed, understanding the boundaries of their applicability and the limits to their predictive power, and the ongoing search for exceptions, violations, and failures, have provoked even deeper questions and challenges, stimulating the continued progress of science and the unfolding of new ideas, techniques, and concepts. One of the great scientific challenges is the search for a Grand Unified Theory of the elementary particles and their interactions, including its extension to understanding the cosmos and even the origin of spacetime itself. Such a theory would be based on a parsimonious set of underlying mathematizable universal principles that integrate and explain all the fundamental forces of nature, from gravity and electromagnetism to the weak and strong nuclear forces, incorporating Newton's laws, quantum mechanics, and general relativity. Fundamental quantities like the speed of light, the dimensionality of spacetime, and the masses of the elementary particles would all be predicted, and the equations governing the origin and evolution of the universe through to the formation of galaxies and beyond would be derived—and so on. This constitutes the Theory of Everything. It's a truly remarkable and enormously ambitious quest, which has occupied thousands of researchers for over fifty years at a cost of billions of dollars. Measured by almost any metric, this quest, which is still far from its ultimate goal, has been enormously successful, leading, for example, to the discovery of quarks and the Higgs boson, to black holes and the Big Bang, to quantum chromodynamics and string theory . . . and to many Nobel Prizes. But "Everything"? Well, hardly. Where's life, where are animals and cells, brains and consciousness, cities and corporations, love and hate, etc., etc.? How does the extraordinary diversity and complexity seen here on Earth arise? The simplistic answer is that these are inevitable outcomes of the interactions and dynamics encapsulated in the Theory. Time evolves from the geometry and dynamics of strings, the universe expands and cools, and the hierarchy—from quarks to nucleons, to atoms and molecules, to cells, brains, and emotions and all the rest—comes tumbling out, a sort of deus ex machina, a result of "just" turning the crank of increasingly complicated equations and computations presumed, in principle, to be soluble to any sufficient degree of accuracy. Qualitatively, this extreme version of reductionism may have some validity, but Something is missing. The "Something" includes concepts like information, emergence, accidents, historical contingency, adaptation, and selection—all characteristics of complex adaptive systems, whether organisms, societies, ecosystems, or economies. These are composed of myriad individual constituents or agents, which take on collective characteristics generally unpredictable (certainly in detail) from their underlying components, even if the interactive dynamics are known. Unlike the Newtonian paradigm on which the Theory of Everything is based, the complete dynamics and structure of complex adaptive systems cannot be encoded in a small number of equations. Indeed, in most cases, probably not even in an infinite number! Furthermore, predictions to arbitrary degrees of accuracy are not possible, even in principle. Perhaps, then, the most surprising consequence of a visionary Theory of Everything is that it implies that on the grand scale the universe—including its origins and evolution—although extremely complicated, is not complex but in fact surprisingly simple, since it can be encoded in a limited number of equations. Conceivably, only one. This is in stark contrast to what we encounter here on Earth, where we're integral to some of the most diverse, complex, and messy phenomena occurring anywhere in the universe, and which require additional, possibly nonmathematizable, concepts to understand. So, while applauding and admiring the search for a Grand Unified Theory of all the basic forces of nature, let's drop the implication that it can in principle explain and predict Everything. Let us instead incorporate a parallel quest for a Grand Unified Theory of Complexity. The challenge of developing a quantitative, analytic, principled, predictive framework for understanding complex adaptive systems is surely a grand challenge for the 21st century. Like all grand syntheses, it will inevitably remain incomplete, but nevertheless it will undoubtedly inspire significant, possibly revolutionary new ideas, concepts, and techniques. UNIFICATION MARCELO GLEISER Theoretical physicist, Dartmouth College; author, The Island of Knowledge; The Limits of Science and the Search for Meaning There! I said it! The venerable notion of Unification needs to go. I don't mean the smaller unifications we scientists search for all the time, connecting as few principles with as many natural phenomena as possible. This sort of scientific economy is a major foundational stone for what we do: We search and we simplify. Over the centuries, scientists have done wonders following this motto. Newton's law of universal gravity, the laws of thermodynamics, electromagnetism, universal behavior in phase transitions. . . . The trouble starts when we take this idea too far and search for the Über-unification, the Theory of Everything, the arch-reductionist notion that all forces of nature are merely manifestations of a single force. This is the idea that needs to go. And I say this with a heavy heart; my early career aspirations and formative years were very much fueled by the impulse to unify it all. The idea of unification is quite old, as old as Western philosophy. Thales, the first pre–Socratic philosopher, posited that "All is water," thus dreaming up a single material principle to describe all of nature. Plato proposed elusive geometrical forms as the archetypal structures behind all there is. Math became equated with beauty and beauty with truth. From there, the highest of post–Plato aspirations was to erect a purely mathematical explanation for all there is: the all-encompassing cosmic blueprint, the masterwork of a supreme intelligence. Needless to say, the whole thing was always about our intelligence, even if often attributed to some foggy "mind of God" metaphor. We explain the world the way we think about it. There's no way out of our minds. The impulse to unify it all runs deep in the souls of mathematicians and theoretical physicists, from the Langlands program to superstring theory. But here's the rub: Pure mathematics isn't physics. The power of mathematics comes precisely from its detachment from physical reality. A mathematician can create any universe she wants and play all sorts of games with it. A physicist can't; his job is to describe nature as we perceive it. Nevertheless, the unification game has been an integral part of physics since Galileo and has produced what it should: approximate unifications. Yes, even the most sacred of our unifications are only approximations. Take, for example, electromagnetism. The equations describing electricity and magnetism are perfectly symmetric only in the absence of any sources of charge or magnetism—that is, in empty space. Or take the famous (and beautiful) Standard Model of particle physics, based on the "unification" of electromagnetism and the weak nuclear force. Here again, we don't have a real unification, since the theory retains two forces all along. (In more technical jargon, there are two coupling constants and two gauge groups.) A real unification, such as the conjectured Grand Unification between the strong, the weak, and the electromagnetic forces, proposed forty years ago, remains unfulfilled. So, what's going on? Why do so many insist in finding the One in Nature while Nature keeps telling us that it's really about the Many? For one thing, the scientific impulse to unify is crypto-religious. The West has bathed in monotheism for thousands of years, and even in polytheistic cultures there's always an alpha God in charge (Zeus, Ra, Para-Brahman). For another, there's something deeply appealing in equating all of nature to a single creative principle: To decipher the "mind of God" is to be special, is to answer to a higher calling. Pure mathematicians who believe in the reality of mathematical truths are monks of a secret order, open only to the initiated. In the case of high energy physics, all unification theories rely on sophisticated mathematics related to pure geometric structures: The belief is that nature's ultimate code exists in the ethereal world of mathematical truths and that we can decipher it. Recent experimental data has been devastating to such belief—no trace of supersymmetric particles, of extra dimensions, or of dark matter of any sort, all long-awaited signatures of unification physics. Maybe something will come up; to find, we must search. The trouble with unification in high energy physics is that you can always push it beyond the experimental range. "The Large Hadron Collider got to 7 TeV and found nothing? No problem! Who said nature should opt for the simplest versions of unification? Maybe it's all happening at much higher energies, well beyond our reach." There's nothing wrong with this kind of position. You can believe it until you die, and die happy. Or you can conclude that what we do best is construct approximate models of how nature works and that the symmetries we find are only descriptions of what really goes on. Perfection is too hard a burden to impose on nature. People often see this kind of argument as defeatist, as coming from someone who got frustrated and gave up. (As in "He lost his faith.") Big mistake. To search for simplicity is essential to what scientists do. It's what I do. There are essential organizing principles in nature, and the laws we find are excellent ways to describe them. But the laws are many, not one. We're successful pattern-seeking rational mammals. That alone is cause for celebration. However, let's not confuse our descriptions and models with reality. We may hold perfection in our mind's eye as a sort of ethereal muse. Meanwhile nature is out there doing its thing. That we manage to catch a glimpse of its inner workings is nothing short of wonderful. And that should be good enough. SIMPLICITY A. C. GRAYLING Philosopher; founder and master, New College of the Humanities, London; supernumerary fellow, St. Anne's College, Oxford; author, The Good Argument: The Case Against Religion and for Humanism When two hypotheses are equally adequate to the data, and equal in predictive power, extratheoretical criteria for choosing between them might come into play. They include not just questions about best fit with other hypotheses or theories already predicated to inquiry but also the aesthetic qualities of the competing hypotheses themselves—which is more pleasing, more elegant, more beautiful?—and of course the question of which of them is simpler. Simplicity is a desideratum in science, and the quest for it is a driver in the task of effecting reductions of complex phenomena to their components. It lies behind the assumption that there must be a single force in nature, of which the gravitational, electroweak, and strong nuclear forces are merely manifestations; and this assumption in turn is an instance of the general view that there might ultimately be a single kind of thing (or stuff or field or as-yet-undreamt-of phenomenon) out of which variety springs by means of principles themselves fundamental and simple. Compelling as the idea of simplicity is, there's no guarantee that nature itself has as much interest in simplicity as those attempting to describe it. If the idea of emergent properties still has purchase, biological entities cannot be fully explained except in terms of them, which means in their full complexity, even though considerations of structure and composition are indispensable. Two measures of complexity are the length of the message required to describe a given phenomenon and the length of the evolutionary history of that phenomenon. On a certain view, that makes a Jackson Pollock painting complex by the first measure, simple by the second, whereas a smooth pebble on a beach is simple by the first and complex by the second. The simplicity sought in science might be thought to be what is achieved by reducing the length of the descriptive message—encapsulation in an equation, for example. But could there be an inverse relationship between the degree of simplicity achieved and the degree of approximation that results? Of course it would be nice if everything in the end turned out to be simple, or could be made amenable to simple description. But some things might be better or more adequately explained in their complexity—biological systems again come to mind. Resisting too dissipative a form of reductionism might ward off those silly kinds of criticism claiming that science aims to see nothing in the pearl but the disease of the oyster. THE UNIVERSE SETH LLOYD Professor of quantum mechanical engineering, MIT; author, Programming the Universe I know. The universe has been around for 13.8 billion years and is likely to survive for another 100 billion years or more. Plus, where would the universe retire to? Florida isn't big enough. But it's time to retire the 2,500-year-old scientific idea of the universe as the single volume of space and time that contains everything. Twenty-first-century cosmology strongly suggests that what we see in the cosmos—stars, galaxies, space and time since the Big Bang—does not encompass all of reality. Cosmos, buy the condo. What is the universe, anyway? To test your knowledge of the universe, please complete the following sentence. The universe (a)consists of all things visible and invisible—what is, has been, and will be. (b)began 13.8 billion years ago in a giant explosion called the Big Bang and encompasses all planets, stars, galaxies, space, and time. (c)was licked out of the salty rim of the primordial fiery pit by the tongue of a giant cow. (d)All of the above. (Correct answer below.) The idea of the universe as an observed and measured thing has persisted for thousands of years. Those observations and measurements have been so successful that today we know more about the origin of the universe than we do about the origin of life on Earth. But the success of observational cosmology has brought us to a point where it's no longer possible to identify the universe—in the sense of answer (a) above—with the observed cosmos—answer (b). The same observations that establish the detailed history of the universe imply that the observed cosmos is a vanishingly small fraction of an infinite universe. The finite amount of time since the Big Bang means that our observations extend only a little more than 10 billion light-years from Earth. Beyond the horizon of our observation lies more of the same—space filled with galaxies stretching on forever. No matter how long the universe exists, we will have access to only a finite part, while an infinite amount of universe remains beyond our ken. All but an infinitesimal fraction of the universe is unknowable. That's a blow. The scientific concept universe = observable universe has thrown in the towel. Perhaps that's OK. What's not to like about a universe encompassing infinite unknowable space? But the hits keep coming. As cosmologists delve deeper into the past, they find more and more clues that, for better or worse, there's more out there than just the infinite space beyond our horizon. Extrapolating backward in time to the Big Bang, cosmologists have identified an epoch called inflation, in which the universe doubled in size many times over a tiny fraction of a second. The vast majority of spacetime consists of this rapidly expanding stuff. Our own universe, infinite as it is, is just a "bubble" that has nucleated in this inflationary sea. It gets worse. The inflationary sea contains an infinity of other bubbles, each an infinite universe in its own right. In different bubbles, the laws of physics can take different forms. Somewhere out there in another bubble universe, the electron has a different mass. In another bubble, electrons don't exist. Because it consists not of one cosmos but of many, the multi-bubble universe is often called a multiverse. The promiscuous nature of the multiverse may be unappealing (William James, who coined the word, called the multiverse a "harlot"), but it's hard to eliminate. As a final insult to unity, the laws of quantum mechanics indicate that the universe is continually splitting into multiple histories, or "many worlds," out of which the world we experience is only one. The other worlds contain the events that didn't happen in our world. After a two-millenium run, the universe as observable cosmos is kaput. Beyond what we can see, an infinite array of galaxies exists. Beyond that infinite array, an infinite number of bubble universes bounce and pop in the inflationary sea. Closer by, but utterly inaccessible, the many worlds of quantum mechanics branch and propagate. MIT cosmologist Max Tegmark calls these three kinds of proliferating realities the type I, type II, and type III multiverses. Where will it all end? Somehow, a single, accessible universe seemed more dignified. There's hope, however. Multiplicity itself represents a kind of unity. We now know that the universe contains more things than we can ever see, hear, or touch. Rather than regarding the multiplicity of physical realities as a problem, let's take it as an opportunity. Suppose that everything that could exist does exist. The multiverse is not a bug but a feature. We have to be careful: The set of everything that could exist belongs to the realm of metaphysics rather than physics. Tegmark and I have shown that with a minor restriction, however, we can pull back from the metaphysical edge. Suppose that the physical multiverse contains all things that are locally finite, in the sense that any finite piece of the thing can be described by a finite amount of information. The set of locally finite things is mathematically well defined: It consists of things whose behavior can be simulated on a computer (more specifically, on a quantum computer). Because they're locally finite, the universe we observe and the various other universes are all contained within this computational universe. As is, somewhere, a giant cow. Answer to quiz: (c) IQ SCOTT ATRAN Anthropologist, Centre Nationale de la Recherche Scientifique, Paris; author, Talking to the Enemy: Religion, Brotherhood, and the (Un)Making of Terrorists There's no reason to believe, and much reason not to believe, that the measure of a so-called "Intelligence Quotient" in any way reflects some basic cognitive capacity or "natural kind" of the human mind. The domain-general measure of IQ isn't motivated by any recent discovery of cognitive or developmental psychology. It thoroughly confounds domain-specific abilities—distinct mental capacities for, say, geometrical and spatial reasoning about shapes and positions, mechanical reasoning about mass and motion, taxonomic reasoning about biological kinds, social reasoning about other people's beliefs and desires, and so on—which are the only sorts of cognitive abilities for which an evolutionary account seems plausible in terms of natural selection for task-specific competencies. Nowhere in the animal or plant kingdoms does there ever appear to have been natural selection for a task-general adaptation. An overall measure of intelligence or mental competence is akin to an overall measure for "the body," taking no special account of the various and specific bodily organs and functions, such as hearts, lungs, stomach, circulation, respiration, digestion, and so on. A doctor or biologist presented with a single measure for "Body Quotient" (BQ) wouldn't be able to make much of it. IQ is a general measure of socially acceptable categorization and reasoning skills. IQ tests were designed in behaviorism's heyday, when there was little interest in cognitive structure. The scoring system was tooled to generate a normal distribution of scores with a mean of 100 and a standard deviation of 15. In other societies, a normal distribution of some general measure of social intelligence might look very different; some "normal" members of our society could well produce a score that's a standard deviation from "normal" members of another society on that other society's test. For example, in forced-choice tasks, East Asian students (Chinese, Koreans, Japanese) tend to favor field-dependent perception over object-salient perception, thematic reasoning over taxonomic reasoning, and exemplar-based categorization over rule-based categorization. American students generally prefer the opposite. On tests measuring these various categorization and reasoning skills, East Asians average higher on their preferences and Americans average higher on theirs. There's nothing particularly revealing about these different distributions other than that they reflect underlying sociocultural differences. There's a long history of acrimonious debate over which, if any, aspects of IQ are heritable. The most compelling studies concern twins raised apart and adoptions. Twin studies rarely have large sample populations; moreover, they often involve twins separated at birth because a parent dies or cannot afford to support both and one is given over to be raised by relatives, friends, or neighbors. This disallows ruling out the effects of social environment and upbringing in producing convergence among the twins. The chief problem with adoption studies is that the mere fact of adoption reliably increases IQ, regardless of any correlation between the IQs of the children and those of their biological parents. Nobody has the slightest causal account of how or why genes, singly or in combination, might affect IQ. I don't think it's because the problem is too hard but because IQ is a specious rather than a natural kind. BRAIN PLASTICITY LEO M. CHALUPA Vice president of research, George Washington University Brain plasticity refers to the ability of neurons to change their structural and functional properties with experience. That hardly seems surprising, since every part of the body changes with age. What's special about brain plasticity (but not unique to this organ) is that the changes are mediated by events that are in some sense adaptive. The field of brain plasticity primarily derives from the pioneering studies of Torsten Wiesel and David Hubel, who showed that depriving one eye of normal visual input during early development resulted in a loss of functional connections of that eye with the visual cortex, while the connections of the eye not deprived of visual input expanded. These studies convincingly demonstrated that early brain connections aren't hardwired but can be modified by early experience; hence they're plastic. For this work and related studies done in the 1960s, Wiesel and Hubel received the 1981 Nobel Prize in physiology or medicine. Since that time, there have been thousands of studies showing a wide diversity of neuronal changes in virtually every region of the brain, ranging from the molecular to the systems level, in young, adult, and elderly subjects. As a result, by the end of the 20th century our view of the brain had evolved from the hardwired to the seemingly ever changeable. Today, "plasticity" is one of the most commonly used words in the neuroscience literature. Indeed, I've employed this term many times in my own research articles and used it in the titles of some of my edited books. So what's wrong with that, you may ask? For one thing, the widespread application of "brain plasticity" to virtually every type of change in neuronal structure and function has rendered the term largely meaningless. When almost any change in neurons is characterized as plasticity, the term encompasses so much that it no longer conveys any useful information. Moreover, many studies invoke brain plasticity as the underlying cause of modified behavioral states without having any direct evidence for neuronal changes. Particularly egregious are the studies showing improvement in performance on some particular task with practice. The fact that practice improves performance was noted before anything was known about the brain. Does it really add anything to state that improvements in function demonstrate a remarkable degree of brain plasticity. The word "remarkable" is often used to denote practice effects in seniors, as if those old enough to receive Social Security are incapable of showing enhanced performance with training. Studies of this type have led to the launch of the brain-training industry. Many of these programs are focused on the very young. Particularly popular in past years was the "Mozart effect," which led parents without any interest in classical music themselves to continually play pieces by Mozart to their infants. This movement seems to have abated, replaced by a plethora of games supposed to improve the brains of children of all ages. But the largest growth in the brain-plasticity industry has focused on the aging brain. Given the concerns most of us have about memory loss and decreasing cognitive abilities with age, this is understandable. There are large profits to be made, as is evident from the number of companies that have proliferated in this sector in recent years. There is of course nothing wrong in having children or seniors engage in activities that challenge their cognitive functions. In fact, there may be some genuine benefits in doing so. Certainly, undergoing such training is preferable to watching television for hours each day. It's also the case that any and all changes in performance reflect some underlying changes in the brain. How could it be otherwise, since the brain controls all behaviors? But as yet we don't know what occurs in the brain when performance on a specific video game improves, nor do we understand how to make such changes long-lasting and generalizable to diverse cognitive states. Terming such efforts "brain training" or "enhanced brain plasticity" is often just hype intended to sell a product. This doesn't mean the so-called brain exercises should be abandoned; they're unlikely to cause harm and may even do some good. But please refrain from invoking brain plasticity, remarkable or otherwise, to explain the resulting improvements. CHANGING THE BRAIN HOWARD GARDNER Hobbs Professor of Cognition and Education, Harvard Graduate School of Education; author, Truth, Beauty, and Goodness Reframed When I speak to students or lay audiences about any kind of digital innovation, the feedback from my listeners goes, roughly, "Do smartphones change the brain?" or "We can't let infants play with pads, because it might affect their brains." I try to explain that everything we do affects the nervous system, and that their comments are therefore either meaningless or need to be unpacked. Here is one such unpacking: "Does this experience affect the nervous system significantly and perhaps even permanently?" Another: "Do you mean 'affect the mind' or 'affect the brain'?" When the questioner looks blank, I sense that he or she needs a refresher course in philosophy, psychology, and neuroscience. "THE ROCKET SCIENTIST" VICTORIA WYATT Associate professor of indigenous arts of North America, University of Victoria It's time to retire "the rocket scientist" of cliché fame: "It doesn't take a rocket scientist to. . . ." "The rocket scientist" is a personage rather than a principle, and a fictitious personage at that. He (or she) was constructed by popular usage, not by scientists. Still, the cliché perpetuates outdated public perceptions of scientific principles, and that's critical. "The rocket scientist" needs a good retirement party. I'll start with a disclaimer. My dreams of that retirement gala may appear tinged with professional envy. I've never heard anyone say, "It doesn't take an ethnohistorian to. . . ." I never will. So, yes, the cliché does slight the humanities, but that's not my concern. Rather, "the rocket scientist," as popularly conceived, dangerously slights the sciences. Our Earth cannot afford that. "The rocket scientist" stands outside society, frozen on a higher plane. Widely embraced and often repeated, the cliché reflects the public's comfort with divorcing science from personal experience. The cliché imposes a boundary (of brilliance) between the scientist and everyone else. This makes for popular movies and television shows, but it's insidious. Artificially constructed boundaries isolate. They focus attention on differences and distinctions. In contrast, the exploration of relationships and process feeds rapid scientific developments—systems biology, epigenetics, neurology and brain research, astronomy, medicine, quantum physics. Complex relationships also shape the urgent challenges we face: global epidemics, climate change, species extinction, finite resources—these all comprise integral interconnections. Approaching such problems demands an appreciation of diversity, complexity, relationships, and process. Popular understanding of contemporary science demands the same. We can address urgent global issues only when policy makers see science clearly—when they view diversity, complexity, relationships, and process as essential to understanding rather than as obstacles to it. At present, though, constructed boundaries pervade not just our clichés but also our institutions and policy structures. Examples abound. Universities segment researchers and students into disciplinary compartments with discrete budgetary line items, competing for scarce resources. ("Interdisciplinary" makes a good buzzword, but the paradigm on which our institutions rest militates against it.) The model of nations negotiating as autonomous entities has failed abysmally to address climate change. In my provincial government's bureaucracy, separate divisions oversee oceans and forests, as if a fatal barrier sliced the ecosystem at the tideline. Time suffers, too. Past gets alienated from present, and present from future, as our society zooms in on short-term fiscal and political deadlines. Fragmented time informs all other challenges and makes them all the more dire. So much of our society still operates on a paradigm of simplification, compartmentalization, and boundaries, when we need a paradigm of diversity, complexity, relationships, and process. Our societal structures fundamentally conflict with the messages of contemporary science. How can policy makers address crucial global issues while ignoring contemporary scientific principles? The real world plays out as a video. The relationships between frames make the story comprehensible. In contrast, "the rocket scientist" stands fictitiously yet firmly alone, on a lofty pinnacle, apart from society, not a part of it. Yes, it's just a cliché, but language matters and jokes instruct. It's time for "the rocket scientist" to retire. I'll close with another disclaimer: I mean no offense to real rocket scientists. (Some of my best friends have been rocket scientists.) Real rocket scientists exist. They inhabit the real world, with all the attending interconnections, relationships, and complexities. "The rocket scientist" embodies the opposite. We'd all be well served by that retirement. INDIVI-DUALITY NIGEL GOLDENFELD Center for Advanced Study Professor in Physics, director, Institute for Universal Biology, University of Illinois, Urbana-Champaign In physics, we use the convention that the suffix -on denotes a quantized unit of something. For example, in classical physics there are electromagnetic waves. But in the quantum version of the theory, originating with Einstein's 1905 Nobel Prize–winning work, we know that under certain circumstances it's more precise to regard electromagnetic radiant energy as being distributed in particles called photons. This wave-particle duality is the underpinning of modern physics—not just photons but a zoo of what were once called elementary particles that include protons, neutrons, pions, mesons, and of course the Higgs boson. (Neutrino? It's a long story. . . .) And what about you? You're a person. Are you a quantum of something too? Well, clearly there are no fractional humans and we are trivially quantized. But elementary particles, or units, are useful conceptually because they can be considered in isolation, devoid of interactions—like point particles in an ideal gas. You would certainly not fulfill that description, networked, online, and cultured as you undoubtedly are. Your strong interaction with other humans means that your individuality is complicated by the fact that you're part of a society and can function properly only in such a milieu. We could go further and say you're a quantum of a spatially distributed field, one that describes the density around each point in space of humans, rather than the electromagnetic field intensity. This description turns out to be technically powerful for describing the behavior of ecosystems in space and time, particularly for describing extinction, where discontinuous change is important. It seems apt to invoke here the strangely oxymoronic term indivi-duality, a counterpart to wave-particle duality. The notion of "individual" has several other connotations. It can mean discrete or single, but its etymology is also reminiscent of "indivisible." Clearly we're not indivisible but constituted from cells, which are themselves constituted of cytoplasm, nucleic acids, proteins, etc., themselves constituted of atoms, which contain neutrons, protons, electrons, all the way down to the elementary particles now believed to be products of string theory, itself now known not to be a final description of matter. In other words, it's "turtles all the way down," and there are no indivisible units of matter, no meaning to the notion of "elementary particle," no place to stop. Everything is made of something, and so on, ad infinitum. However, this doesn't mean that everything is simply the sum of its parts. Take the proton, for example—which is made up of three quarks. It has a type of intrinsic angular momentum called spin, which was initially expected to be the sum of that of its constituent quarks. Yet experiments carried out over the last twenty or thirty years have shown that this is not the case: The spin arises out of some shared collective aspect of the quarks and short-lived fluctuating particles called gluons. The notion of individual quarks is not useful when the collective behavior is so strong. The proton is made of something, but its properties aren't found by adding up the properties of its parts. When we try to identify the something, we discover that, as some say of Los Angeles, there's no "there" there. You probably already knew that naïve reductionism is often too simplistic. However, there's another point. It's not just that you're a composite, something you already knew, but you're in some senses not even human. You have perhaps 100 trillion bacterial cells in your body, numbering 10 times more than your human cells and containing 100 times as many genes as your human cells. These bacteria aren't just passive occupants of the zoo that is you. They self-organize into communities within your mouth, guts, and elsewhere, and these communities—microbiomes—are maintained by varied, dynamic patterns of competition and cooperation between the various bacteria, which allows us to live. In the last few years, genomics has given us a tool to explore the microbiome by identifying microbes by their DNA sequences. The story emerging from these studies is not yet complete, but it has already led to fascinating insights. Thanks to its microbes, a baby can better digest its mother's milk. And your ability to digest carbohydrates relies to a significant extent on enzymes that can be made only by genes present not in you but in your microbiome. Your microbiome can be disrupted—for example, due to treatment by antibiotics—and in extreme cases can be invaded by dangerous monocultures such as Clostridium difficile, leading to your death. Perhaps the most remarkable finding is the gut/brain axis: Your gastrointestinal microbiome can generate small molecules that may be able to pass through the blood-brain barrier and affect the state of your brain. Although the precise mechanism isn't yet clear, there's growing evidence that your microbiome may be a significant factor in mental states such as depression and autism spectrum conditions. In short, you may be a collective property arising from the close interactions of your constitutents. Now, maybe it's true that you're not an individual in one sense of the word, but how about your microbes? Well, it turns out that your microbes are a strongly interacting system, too: They form dense colonies within you and not only exchange chemicals for metabolism but communicate by emitting molecules. They can even transfer genes between themselves, and in some cases do that in response to signals emitted by a hopeful recipient—a bacterial cry for help! A single microbe in isolation doesn't do these things, thus these complex behaviors are a property of the collective and not the individual microbes. Even microbes that seem to be from the same nominal species can have genomes differing in content by as much as 60 percent of their genes. So much for the intuitive notion of "species"! That's another too-anthropomorphic scientific idea that doesn't apply to most of life. Up to now, I've talked about connections in space. But there are also connections in time. If the stuff that makes the universe is strongly connected in space and not usefully thought of as the aggregate sum of its parts, then attributing a cause of an event to a specific component may not be meaningful either. Just as you can't attribute the spin of a proton to any one of its constituents, you can't attribute an event in time to a single earlier cause. Complex systems have neither a useful notion of individuality nor a proper notion of causality. THE BIGGER AN ANIMAL'S BRAIN, THE GREATER ITS INTELLIGENCE NICHOLAS HUMPHREY Psychologist; Darwin College, Cambridge; author, Soul Dust: The Magic of Consciousness The bigger an animal's brain, the greater its intelligence. You may think the connection is obvious. Just look at the evolutionary lineage of human beings. Humans have bigger brains, and are cleverer, than chimpanzees; and chimpanzees have bigger brains, and are cleverer, than monkeys. Or, as an analogy, look at the history of computing machines in the 20th century. The bigger the machines, the greater their number-crunching powers. In the 1970s, the new computer at my university department took up a whole room. From the phrenology of the 19th century to the brain-scan sciences of the 21st, it has indeed been widely assumed that brain volume determines cognitive capacity. In particular, you'll find the idea repeated in every modern textbook—that the brain size of different primate species is causally related to their social intelligence. I admit I'm partly responsible for this, having championed the idea back in the 1970s. Yet for a good many years now, I've had a hunch that the idea is wrong. There are too many awkward facts that don't fit in. For a start, we know that modern humans can be born with only two-thirds the normal volume of brain tissue and show next to no cognitive deficit as adults. We know that during normal human brain development, the brain actually shrinks as cognitive performance improves (a notable example being changes in the "social brain" during adolescence, where the cortical gray matter decreases in volume by about 15 percent between ages ten and twenty). And most surprising of all, we know that there are nonhuman animals, such as honeybees or parrots, that can emulate many feats of human intelligence with brains only a millionth (bee) or a thousandth (parrot) the size of a human's. The key, of course, is programming: What really matters to cognitive performance isn't so much the brain's hardware as its onboard software. And smarter software doesn't need a bigger hardware base (in fact, as the shrinkage of the cortex during adolescence shows, it may actually need a smaller—tidier—one). It's true that programs to deliver superior performance may require a lot of designing, either by natural selection or learning. But once they've been invented, they'll likely make fewer demands on hardware than the older versions. To take the special case of social intelligence, I'd say it's quite possible that the algorithm for solving "theory of mind" problems could be written on the back of a postcard and implemented on an iPhone. In which case, the widely touted suggestion that the human brain had to double in size for humans to be capable of "second-order mind-reading" makes little sense. Then why did the human brain double in size? Why is it much bigger than you might think it needs to be to underpin our level of intelligence? There's no question that big brains are costly to build and maintain. So if we're to retire the "obvious theory," what can we put in its place? The answer, I'd suggest, lies in the advantage of having a large amount of cognitive reserve. Big brains have spare capacity that can be called on if and when working parts get damaged or wear out. From adulthood onward, humans—like other mammals—begin to lose a significant amount of brain tissue to accidents, hemorrhages, and degeneration. But because humans can draw on this reserve, the loss doesn't have to show. This means humans can retain their mental powers into relative old age, long after their smaller-brained ancestors would have become incapacitated. (And as a matter of fact, the unfortunate individual born with an unusually small brain is much more likely to succumb to senile dementia in his forties.) True, many of us die for other reasons, with unused brain power to spare. But some of us live considerably longer than we might have done if our brains were half the size. So, what evolutionary advantage does longevity bring, even the post-reproductive longevity typical of humans? The answer surely is that humans can benefit—as no other species could do—from the presence of mentally sound grandparents and great-grandparents, whose role in caretaking and teaching has been key to the success of human culture. THE BIG BANG WAS THE FIRST MOMENT OF TIME LEE SMOLIN Physicist, Perimeter Institute, Waterloo, Ontario; author, Time Reborn In my field of fundamental physics and cosmology, the idea most ready for retirement is that the Big Bang was the first moment of time. In popular parlance, the Big Bang has two meanings. First, Big Bang cosmology is the hypothesis that our universe has been expanding for 13.8 billion years from an extremely hot and dense primordial state more extreme than the center of a star—or, indeed, than anywhere now existing. This I have no quarrel with; it's established scientific fact, which has been elaborated into a detailed story narrating the expansion of the universe from a uniform and dense hot plasma to the beautifully varied and complex world that is our home. We have detailed theories that pass numerous observational tests and explain the origins of all the structures we see, from the elements to galaxies, stars, planets, and the molecular building blocks of life. As in any good scientific theory, there are questions still to be answered, such as the precise nature of the dark matter and dark energy, which are prominent actors in the story, or the very interesting question of whether or not there was an early phase of inflationary exponential expansion. But these don't suggest that the basic picture is wrong. What concerns me is the other meaning of Big Bang, which is the further hypothesis that the ultimate origin of our universe was a first moment of time, at which our universe was launched from a state of infinite density and temperature. According to this idea, nothing that exists is older than 13.8 billion years. It makes no sense to ask what was before that, because before that there wasn't even time. The main problem with this second meaning of Big Bang is that it's not very successful as a scientific hypothesis, because it leaves big questions about the universe unanswered. It turns out that our universe had to have started in an extraordinarily special state to have evolved into anything like our universe. The hypothesis that there was a first moment of time is remarkably generic and unconstraining, as it's consistent with an infinite number of possible states in which the universe might have begun. This follows from a theorem proved by Stephen Hawking and Roger Penrose—that almost any expanding universe described by general relativity has such a first moment of time. Compared to almost all of these, our own early universe was extraordinarily homogeneous and symmetric. Why? If the Big Bang was the first moment of time, there can be no scientific answer, because there was no "before" on which to base an explanation. At this point, theologians see their opening and, indeed, have been lining up at the gates of science to impose their kind of explanation: that God made the universe and made it so. Similarly, if the Big Bang was the first moment of time, there can be no scientific answer to the question of what chose the laws of nature. This leaves the field open to explanations such as the anthropic multiverse, which are unscientific because they call on unobservable collections of other universes and make no predictions by which their hypotheses might be tested and falsified. There is, however, a chance for science to answer these questions, and that's if the Big Bang was not the first moment of time but a transition from an earlier era of the universe—an era that can be investigated scientifically, because processes acting then would have given rise to our world. For there to have been a time before the Big Bang, the Hawking-Penrose theorem must fail. But there's a simple reason to think it must: General relativity is incomplete as a description of nature, because it leaves out quantum phenomena. Unifying quantum physics with general relativity has been a major challenge for fundamental physics, one on which there has been much progress in the last thirty years. In spite of the absence of a definitive solution to the problem, there's robust evidence from quantum cosmology models that the infinite singularities forcing time to stop in general relativity are eliminated, turning the Big Bang—in the sense of a first moment of time—into a Big Bounce, which allows time to continue to exist before the Big Bang, deep into the past. Detailed models of quantum universes show a prior era ending with a collapse, where the density increases to very high values, but before the universe becomes infinitely dense, quantum processes take over which bounce the collapse into an expansion, launching a new era that could be our expanding universe. There are currently several scenarios under study for what happened in the era before the Big Bang and how it transitioned to our expanding universe. Two of them hypothesize a quantum bounce and go under the name of "loop quantum cosmology" and "geometrogenesis." Two others—due respectively to Roger Penrose and to Paul Steinhardt and Neil Turok—describe cyclic scenarios in which universes die giving rise to new universes. A fifth posits that new universes are launched when quantum effects bounce black-hole singularities. These scenarios offer insights as to how the laws of nature governing our universe might have been chosen, and they may also explain how the initial conditions of our universe evolved from the previous universe. The important thing is that each of these hypotheses makes predictions for real, doable observations by which they might be falsified and distinguished from one another. During the 20th century, we learned a great deal about the first three minutes (in Steven Weinberg's phrase) of our expanding universe. During this century, we can look forward to gaining scientific evidence of the last three minutes of the era before ours and learning how physics before the Big Bang gave rise to the birth of our universe. THE UNIVERSE BEGAN IN A STATE OF EXTRAORDINARILY LOW ENTROPY ALAN GUTH Cosmologist; Victor F. Weisskopf Professor of Physics, MIT; inaugural winner, Milner Foundation's Fundamental Physics Prize; author, The Inflationary Universe The roots of this issue go back at least to 1865, when Rudolf Clausius coined the term "entropy" and stated that the entropy of the universe tends to a maximum. This idea is now known as the second law of thermodynamics, which is most often stated thus: The entropy of an isolated system always increases or stays constant but never decreases. Isolated systems tend to evolve toward the state of maximum entropy—the state of thermodynamic equilibrium. Even though entropy will play a crucial role in this discussion, it will suffice to use a fairly crude definition: Entropy is a measure of the disorder of a physical system. In the underlying quantum description, entropy is a measure of the number of quantum states corresponding to a given description in terms of macroscopic variables such as temperature, volume, and density. The classic example is a gas in a closed box. If we start with all the gas molecules in a corner of the box, we can imagine watching what happens next. The gas molecules will fill the box, increasing the entropy to the maximum. But it never goes the other way: If the gas molecules fill the box, we'll never see them spontaneously collect into one corner. This behavior seems natural but is hard to reconcile with our understanding of the underlying laws of physics. The gas makes a huge distinction between the past and the future, always evolving toward larger entropy in the future. This one-way behavior of matter in bulk is called the "arrow of time." Nonetheless, the microscopic laws that describe collisions of molecules are time-symmetric, making no distinction between past and future. Any movie of a collision could be played backward and it would also show a valid picture of the collision. (To account for some very rare events discovered by particle physicists, the movie is guaranteed to be valid only if it is also reflected in a mirror and has every particle relabeled as the corresponding antiparticle. But these complications don't change the key issue.) There's an important problem, therefore, which is over a century old: to understand how the arrow of time could possibly arise from time-symmetric laws of evolution. The arrow-of-time mystery has driven physicists to seek possible causes within the laws of physics we observe, but in vain. The laws make no distinction between the past and the future. Physicists have understood, however, that a low entropy state is always likely to evolve into a higher entropy state simply because there are many more states of higher entropy. Thus, the entropy today is higher than the entropy yesterday, because yesterday the universe was in a low entropy state. And it was in a low entropy state yesterday because the day before that it was in an even lower entropy state. The traditional understanding follows this pattern back to the origin of the universe, attributing the arrow of time to some not-well-understood property of cosmic initial conditions, which created the universe in a special low entropy state. As Brian Greene wrote in The Fabric of the Cosmos: The ultimate source of order, of low entropy, must be the big bang itself. . . . The egg splatters rather than unsplatters because it is carrying forward the drive toward higher entropy that was initiated by the extraordinarily low entropy state with which the universe began. Based on an elaboration of a 2004 proposal by Sean Carroll and Jennifer Chen, there's a possibility of a new solution to the age-old problem of the arrow of time. This work, by Sean Carroll, Chien-Yao Tseng, and me, is still in the realm of speculation and hasn't yet been vetted by the scientific community. But it seems to provide an attractive alternative to the standard picture. The standard picture holds that the initial conditions for the universe must have produced a special low-entropy state because one is needed to explain the arrow of time. (No such assumption is applied to the final state, so the arrow of time is introduced through a time-asymmetric condition.) We argue, to the contrary, that the arrow of time can be explained without assuming a special initial state, so there's no longer any motivation for the hypothesis that the universe began in a state of extraordinarily low entropy. The most attractive feature of this idea is that there's no longer a need to introduce any assumptions that violate the time symmetry of the known laws of physics. The basic idea is simple: We don't really know if the maximum possible entropy for the universe is finite or infinite, so let's assume it's infinite. Then, no matter what entropy the universe started with, the entropy would have been low compared with its maximum. That's all that's needed to explain why the entropy has been rising ever since! The metaphor of gas in a box is replaced by a gas with no box. In the context of what physicists call a toy model—that is, one meant to illustrate a basic principle without trying to be otherwise realistic—we can imagine choosing, in a random and time-symmetric way, an initial state for a gas composed of some finite number of noninteracting particles. It's important here that any well-defined state will have a finite value for the entropy and a finite value for the maximum distance of any particle from the origin of our coordinate system. If such a system is followed into the future, the particles might move inward or outward for some finite time, but ultimately the inward-moving particles will pass the central region and will start moving outward. All particles will eventually be moving outward, and the gas will continue indefinitely to expand into the infinite space, with the entropy rising without limit. An arrow of time—the steady growth of entropy with time—has been generated, without introducing any time-asymmetric assumptions. An interesting feature of this picture is that the universe need not have a beginning but could be continued from where we started in both directions of time. Since the laws of evolution and the initial state are time-symmetric, the past will be statistically equivalent to the future. Observers in the deep past will see the arrow of time in the opposite direction from ours, but their experience will be no different from ours. ENTROPY BRUCE PARKER Oceanographer; visiting professor, Center for Maritime Systems, Stevens Institute of Technology; author, The Power of the Sea Could one really have the nerve to suggest "retiring" the idea of entropy? I don't believe we abandon old ideas before new ones are developed. Old ideas disappear, or are modified, only when new, better ideas are developed; they're never just retired. So, no, we shouldn't retire entropy, but perhaps we should treat it with a little less importance and recognize the paradox it creates. Entropy, the measure of the degree of disorder in a system, has held a lofty place in physics—being part of a law, no less, not just a theory. The second law of thermodynamics says that in any closed system, entropy always increases with time. Unless work is done to prevent it, a closed system will eventually reach maximum entropy and a state of thermal equilibrium. Max Planck believed that entropy (along with energy) was the most important property of physical systems. In The Nature of the Physical World (1927), Sir Arthur Eddington wrote: "The law that entropy always increases—the second law of thermodynamics—holds, I think, the supreme position among the laws of Nature." But as a young physics student in college, I must admit I never understood their excitement (and I was not the only student to be unimpressed). The second law seemed of minor importance compared with the first law of thermodynamics, the conservation of energy: Energy could be transformed into different forms, but it was always conserved. The first law had beautiful partial differential equations (as do all the conservation equations of physics), whose solutions accurately described and predicted so much of the world and literally changed our lives. The second law was not a conservation equation and had no beautiful partial differential equations. It wasn't even an equality. Has the idea of entropy and the second law had any major effect on science and engineering or changed the world? The second law was a statistical law, initially a generalization of conclusions reached when looking at the motion of molecules or particles. As students, we could easily understand the classic example of how hot (fast-moving) molecules on one side of a closed box mixed with cold (slowing-moving) molecules on the other side, and why they couldn't separate again once they were together and all at the same temperature. We understood why it was irreversible. And we understood the concept of the "arrow of time." Sure, the mathematics of the first law (and the other conservation equations of physics) worked in both directions of time—but with initial conditions and boundary conditions, we always knew which way things moved. It didn't seem to require another law. In fact, the second law (as applied now to all situations) seemed to be an assumption rather than a law. Especially when it was applied to the entire universe, which we understand so little about. Where the universe (whatever that entails, which may be more than our presently visible/observable universe) is concerned, the first law tells us that all the energy in it will be conserved, although perhaps converted into various forms. But the second law says that at some time in the future no more energy transformations can take place. The universe will reach some stage of maximum entropy and thermal equilibrium. The second law essentially says that the universe must have had a beginning and an end. That's very difficult to accept. The universe must be timeless, for if there was a beginning, what was there before this beginning? Something cannot come out of nothing (and by "nothing," I mean the lack of anything, even things we don't know about yet). Of course, the present Big Bang theory posits a beginning (of sorts), and our present form of the universe has apparently expanded from a singularity, but we don't know what came before that, and oscillating models of the universe are being proposed so that the universe is timeless. With such models, if entropy is very high at the end of our universe and was very low at the beginning of our universe, what process could essentially reset entropy to a low value? As relates to an oscillating universe, should entropy perhaps be conserved somehow? Could there be some type of energy conversion that doesn't require work (in our classical sense)? Could the universe actually be the one and only possible perpetual-motion machine (a contraption forbidden by the second law)? If existence is endless in time, it would seem so. The whole idea of entropy has always felt wrong or misplaced in other ways also. We talk about the universe going from order to disorder. Yet this supposed order is merely the compression of all the universe's matter in some tiny volume—a singularity—and when it expanded there was less order because the particles were more spread out. And yet order is being created all the time. The greatest result of our expanding and evolving universe is its ever increasing complexity: first, from gravity condensing matter, then from supernovae creating higher-number elements, then from chemical evolution, and then from biological evolution driven by natural selection, which culminated in self-reproducing life and eventually the incredible complexity of our brains. Complexity is synonymous with low entropy. The expanding universe has countless small (relative to the universe's size) pockets of extremely low entropy surrounded by vast areas of higher entropy (much of which resulted from the creation of these low-entropy areas). Are the higher orders of complexity (and thus lower orders of entropy) taken into account in attempts to balance the entropy of the universe? Many current scientific papers in cosmology try to sum up the universe's total entropy with formulas that may well be far too simple to account for all the as-yet-unknown physics going on in our strange universe. We cannot retire entropy, but should we maybe rethink it? THE UNIFORMITY AND UNIQUENESS OF THE UNIVERSE ANDREI LINDE Theoretical physicist, Stanford University; father of eternal chaotic inflation; inaugural winner, Milner Foundation's Fundamental Physics Prize, 2012 For most of the 20th century, scientific thought was dominated by the idea of the uniformity of the universe and the uniqueness of the laws of physics. Indeed, cosmological observations indicated that the universe on the largest possible scales is almost exactly uniform, with an accuracy better than 1 in 10,000. The situation is similar with respect to the uniqueness of the laws of physics. We knew, for example, that the electron mass is the same everywhere in the observable part of the universe, so the obvious assumption was that it took the same value everywhere—that it was just a constant of nature. For a long time, one of the great goals of physics was to find a single theory—a Theory of Everything—that would unify all fundamental interactions and provide an unambiguous explanation for all known parameters of particle physics. Some thirty years ago, a possible explanation arose for the uniformity of the universe. The main idea was that our part of the world emerged as a result of an exponentially rapid stretching of space called cosmic inflation. As all "wrinkles" and nonuniformities of space stretched out and disappeared, the universe became incredibly smooth. Add some quantum fluctuations, stretch them, and the uniformity became just a little bit less perfect, and galaxies emerged. At first, inflationary theory looked like the exotic product of a vivid imagination. But thanks to the enthusiastic work of thousands of scientists, its various predictions have been confirmed by many observations made by COBE, WMAP, Planck, and most recently by BICEP2. If, as I think, the theory is correct, we finally have a scientific explanation of why the world is so uniform. But inflation does not predict that this uniformity must extend beyond the observable part of the universe. To give an analogy: Suppose the universe is the surface of a big soccer ball consisting of black and white hexagons. If we inflate it, the size of each white or black part becomes exponentially large. If inflation is powerful enough, those who live in a black part of the universe will never see a white part. They will believe the whole universe is black, and they will try to scientifically explain why it cannot be any other color. Those who live in a white universe will never see the black parts and will therefore think the whole world must be white. But black and white parts alike can coexist in an inflationary universe without contradicting observations. Unlike the black/white analogy, in physics the number of different "colors"—that is, different states of matter—can be exponentially large. The best current candidate for a Theory of Everything is string theory, which can be successfully formulated in spacetime with ten dimensions (nine of space and one of time). But we live in a universe with three dimensions of space. Where are the six others? The answer is that they're compactified—squeezed so small that we cannot move in those directions—and so we perceive the world as if it were three-dimensional. From the early days of string theory, physicists knew there were exponentially many different ways to compactify the extra six dimensions, but we didn't know what kept the compactified dimensions from expanding. This problem was solved about ten years ago, and the solution validated the earlier expectation of the exponentially large number of possibilities. Some estimates are as large as 10500. And each of these options describes a part of the universe with a different vacuum energy and different types of matter. In the context of the inflationary theory, this means the world may well consist of 10500 huge universes with different types of matter. A pessimist would argue that since we don't see other parts of the universe, we cannot prove that this picture is correct. An optimist will counter that neither can we disprove it, because its main assumption is that the other universes are far away from us. And since we know that our best current theory allows 10500 different universes, anybody who argues that the universe must have the same properties everywhere has to prove that only one of those 10500 universes is possible. And there's something else: There are many strange coincidences in our world. The mass of the electron is 2,000 times smaller than the mass of the proton. Why? The only "reason" is that if it were even a little different, life as we know it would be impossible. The masses of the proton and neutron almost coincide. Why? If the mass of either were even a little different, life as we know it would be impossible. The energy of empty space in our part of the universe is not zero but a tiny number—more than a 100 orders of magnitude below the naïve theoretical expectations. Why? The only explanation we have is that we couldn't live in a world with a larger vacuum energy. The correlation between our properties and the properties of the world is called the anthropic principle. But if the universe came in only one copy, this correlation would not explain why. We'd need to speculate about a divine cause making the universe custom-built for humans. However, with a multiverse consisting of many different parts with different properties, the correlation between our properties and the properties of the part of the world we live in makes perfect sense. Can we return to the old picture of a single universe? Possibly. But in order to do so, we must (1) invent a better cosmological theory, (2) invent a better theory of fundamental interactions, and (3) propose an alternative explanation for the aforementioned miraculous coincidences. INFINITY MAX TEGMARK Physicist, cosmologist, MIT; scientific director, Foundational Questions Institute; author, Our Mathematical Universe I was seduced by infinity at an early age. Georg Cantor's diagonality proof that some infinities are bigger than others mesmerized me, and his infinite hierarchy of infinities blew my mind. The assumption that something truly infinite exists in nature underlies every physics course I've ever taught at MIT—and, indeed, all of modern physics. But it's an untested assumption, which begs the question: Is it actually true? There are in fact two separate assumptions: "infinitely big" and "infinitely small." By infinitely big, I mean that space can have infinite volume, that time can continue forever, and that there can be infinitely many physical objects. By infinitely small, I mean the continuum—the idea that even a liter of space contains an infinite number of points, that space can be stretched out indefinitely without anything bad happening, and that there are quantities in nature that can vary continuously. The two assumptions are closely related, because inflation, the most popular explanation of our Big Bang, can create an infinite volume by stretching continuous space indefinitely. The theory of inflation has been spectacularly successful and is a leading contender for a Nobel Prize. It explains how a subatomic speck of matter transformed into a massive Big Bang, creating a huge, flat, uniform universe, with tiny density fluctuations that eventually grew into today's galaxies and cosmic large-scale structure—all in beautiful agreement with precision measurements from experiments such as the Planck and the BICEP2 experiments. But by predicting that space isn't just big but truly infinite, inflation has also brought about the so-called measure problem, which I view as the greatest crisis facing modern physics. Physics is all about predicting the future from the past, but inflation seems to sabotage this. When we try to predict the probability that something particular will happen, inflation always gives the same useless answer: infinity divided by infinity. The problem is that whatever experiment you make, inflation predicts there will be infinitely many copies of you, far away in our infinite space, obtaining each physically possible outcome; and despite years of teeth-grinding in the cosmology community, no consensus has emerged on how to extract sensible answers from these infinities. So, strictly speaking, we physicists can no longer predict anything at all! This means that today's best theories need a major shakeup by retiring an incorrect assumption. Which one? Here's my prime suspect: ∞. A rubber band can't be stretched indefinitely, because although it seems smooth and continuous, that's merely a convenient approximation. It's really made of atoms, and if you stretch it too far, it snaps. If we similarly retire the idea that space itself is an infinitely stretchy continuum, then a big snap of sorts stops inflation from producing an infinitely big space and the measure problem goes away. Without the infinitely small, inflation can't make the infinitely big, so you get rid of both infinities in one fell swoop—together with many other problems plaguing modern physics, such as infinitely dense black-hole singularities and infinities popping up when we try to quantize gravity. In the past, many venerable mathematicians were skeptical of infinity and the continuum. The legendary Carl Friedrich Gauss denied that anything infinite really exists, saying "Infinity is merely a way of speaking" and "I protest against the use of infinite magnitude as something completed, which is never permissible in mathematics." In the past century, however, infinity has become mathematically mainstream, and most physicists and mathematicians have become so enamored with infinity that they rarely question it. Why? Basically, because infinity is an extremely convenient approximation for which we haven't discovered convenient alternatives. Consider, for example, the air in front of you. Keeping track of the positions and speeds of octillions of atoms would be hopelessly complicated. But if you ignore the fact that air is made of atoms and instead approximate it as a continuum—a smooth substance that has a density, pressure, and velocity at each point—you'll find that this idealized air obeys a beautifully simple equation explaining almost everything we care about: how to build airplanes, how we hear them with sound waves, how to make weather forecasts, and so forth. Yet despite all that convenience, air of course isn't truly continuous. I think it's the same way for space, time, and all the other building blocks of our physical word. Let's face it: Despite their seductive allure, we have no direct observational evidence for either the infinitely big or the infinitely small. We speak of infinite volumes with infinitely many planets, but our observable universe contains only about 1089 objects (mostly photons). If space is a true continuum, then to describe even something as simple as the distance between two points requires an infinite amount of information, specified by a number with infinitely many decimal places. In practice, we physicists have never managed to measure anything to more than about seventeen decimal places. Yet real numbers, with their infinitely many decimals, have infested almost every nook and cranny of physics, from the strengths of electromagnetic fields to the wave functions of quantum mechanics. We describe even a single bit of quantum information (qubit) using two real numbers involving infinitely many decimals. Not only do we lack evidence for the infinite but we don't need the infinite to do physics. Our best computer simulations, accurately describing everything from the formation of galaxies to tomorrow's weather to the masses of elementary particles, use only finite computer resources by treating everything as finite. So if we can do without infinity to figure out what happens next, surely nature can, too—in a way that's more deep and elegant than the hacks we use for our computer simulations. Our challenge as physicists is to discover this elegant way and the infinity-free equations describing it—the true laws of physics. To start this search in earnest, we need to question infinity. I'm betting that we also need to let go of it. THE LAWS OF PHYSICS ARE PREDETERMINED LAWRENCE M. KRAUSS Physicist, cosmologist; director, Origins Project, Arizona State University; author, A Universe from Nothing Einstein once said, "What really interests me is whether God had any choice in the creation of the universe." By "God," of course, he didn't mean God. What he was referring to was the question that has driven most scientists who, like me, are attempting to unravel the fundamental laws governing the cosmos at its most basic scale: Is there only one consistent set of physical laws? If we change one fundamental constant, one force law, would the whole edifice tumble? Most scientists of my generation, like Einstein before us, implicitly assumed that the answer to these questions was yes. We wanted to uncover the One True Theory, the mathematical formulation explaining why there had to be four forces in nature, why the proton is 2,000 times heavier than the electron, and so on. In recent memory, this effort reached its most audacious level in the 1980s, when string theorists argued that they had found the Theory of Everything—that using the postulates of string theory, one would be driven to a unique physical theory, with no wiggle room, that would ultimately explain at a fundamental level everything we see. Needless to say, that grand notion has had to be put aside for now, as string theory has failed—thus far, at least—to live up to such lofty promises. In the process, however, in part driven by string theory's lack of success, we've embraced the opposite alternative: The laws of nature we measure may be totally accidental, local to our environment (namely, our universe), not prescribed with robustness by any universal principle, and by no means generic or required. String theory, for example, suggests a host of new possible dimensions. To make contact with our observed four-dimensional universe, it requires these other dimensions to be invisible, by curling up on such small scales that they cannot be probed; or it requires the known forces and particles to be restricted to a four-dimensional "brane." But it appears that there are many, many different ways to hide the extra dimensions, and each one produces a different four-dimensional universe with different laws. It also appears that four dimensions themselves need not be universal; perhaps there are two-dimensional universes, or six-dimensional ones. You don't have to go to such speculative heights to reach the possible conclusion that the laws of our universe may have come into existence when our universe did. The theory of inflation, currently the best theory for how our universe obtained the characteristics it's measured to have on large scales, suggests that at very early times there was a runaway period of expansion. In different places, and perhaps different times, small regions will stop "inflating," as a cosmic phase transition occurs in those regions, changing the stable configuration of particles and fields. But in this picture, most of the "metaverse," if you will, is still inflating, and each region that departs from inflation, each universe, can settle into a different state with different laws, just as ice crystals on a window can form in different directions. All this strongly suggests there may be nothing fundamental whatsoever about the "fundamental" laws we measure in our universe. They could simply be accidental. Physics becomes, in this sense, an environmental science. Now, many people have picked up on this notion to suggest that somehow we can understand our laws because they're selected anthropically—that is, if they were any different, life wouldn't have developed in our universe. However, this idea is full of problems—not least because we have no idea what possibilities exist, and whether changing a few or a huge number of fundamental parameters could result in viable, habitable universes. We also have no idea whether or not we're typical life-forms. Most life that evolves or will evolve in our universe might be quite different. Focusing on anthropics misses the point in any case. The important fact is that we must be willing to give up the idea that the laws of physics in our universe reflect some underlying fundamental order, that the laws are somehow preordained by principles of beauty or symmetry. There's nothing new about this. It was myopic to assume that life on our planet was preordained. We now understand that accidents of natural selection and environmental traumas governed the history of life that led to our existence. It's equally myopic to assume we are somehow the pinnacles of evolution—that all roads lead to us, or that we won't lead to something completely different in the future. It's myopic to assume that the universe we now live in will always be this way. It won't be. As several of us have argued, it seems that in the far future all the galaxies we now see will disappear. But it may be much worse. It's myopic to assume our laws are universal in time and space, even in our universe. Current data on the Higgs particle suggests that the universe could yet again undergo a cosmic phase transition that would change the stable forces and particles, and we and everything we see might disappear. We've come to accept the notion that life is not preordained; we also need to give up the quaint notion that the laws of physics are. Cosmic accidents are everywhere—it's possible that our entire universe is just another accident. THEORIES OF ANYTHING PAUL STEINHARDT Albert Einstein Professor in Science, Departments of Physics and Astrophysical Sciences, Princeton University; coauthor (with Neil Turok), Endless Universe A pervasive idea in fundamental physics and cosmology that should be retired: the notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another. According to this view, the laws and properties within our observable universe cannot be explained or predicted, because they're set by chance. Different regions of space too distant to ever be observed have different laws and properties, according to this picture. Over the entire multiverse there are infinitely many distinct patches. Among these patches, in the words of Alan Guth, "anything that can happen will happen; in fact, it will happen an infinite number of times."a Hence, I refer to this concept as a Theory of Anything. Any observation or combination of observations is consistent with a Theory of Anything. No observation or combination of observations can disprove it. Proponents seem to revel in the fact that the theory cannot be falsified. The rest of the scientific community should be up in arms, since an unfalsifiable idea lies beyond the bounds of normal science. Yet, except for a few voices, there has been surprising complacency about (and in some cases grudging acceptance of) a Theory of Anything as a logical possibility. The scientific journals are full of papers treating the Theory of Anything seriously. What's going on? Have experiments revealed that our observable universe and the fundamental laws are too complicated to be explained by normal science? Absolutely not! Quite the opposite. On the macroscopic scale, the latest measurements show our observable universe to be remarkably simple, described by a very few parameters, obeying the same physical laws throughout, and exhibiting remarkably uniform structure in all directions. On the microscopic scale, the Large Hadron Collider at CERN has revealed the existence of the Higgs boson, in accord with what theorists predicted nearly fifty years ago based on sound scientific reasoning. A simple outcome calls for a simple explanation for why it had to be so. Why, then, consider a Theory of Anything that allows any possibility, including complicated ones? The motivation is the failure of two favorite theoretical ideas—inflationary cosmology and string theory. Both were thought to produce a unique outcome. Inflationary cosmology was invented to transform the entire cosmos into a smooth universe populated by a scale-invariant distribution of hot spots and cold spots, just as we observe it to be. String theory was supposed to explain why elementary particles could have only the precise masses and forces they do. After more than thirty years' investment in each of these ideas, theorists have found them unable to achieve those ambitious goals. Inflation, once started, runs eternally and produces a multiverse of "pockets," whose properties vary over every conceivable possibility—flat and nonflat, smooth and nonsmooth, scale-invariant and not scale-invariant, and so on. Despite laudable efforts by many theorists to save the theory, there's no solid reason known today why inflation should cause our observable universe to be in a pocket with the smoothness and other very simple properties we observe. A continuum of other conditions is equally possible. In string theory, a similar explosion of possibilities has occurred, driven by attempts to explain the 1998 discovery of the accelerated expansion of the universe. The acceleration is thought to be due to positive vacuum energy, an energy associated with empty space. Instead of predicting a unique possibility for the vacuum state of the universe and particles and fields that inhabit it, our current understanding of string theory is that there's a complex landscape of vacuum states corresponding to exponentially different kinds of particles and different physical laws. The set of vacuum space contains so many possibilities that surely, it is claimed, one will include the right amount of vacuum energy and the right kinds of particles and fields. Mix inflation and string theory and the unpredictability multiplies. Now every combination of macrophysical and microphysical possibilities can occur. I suspect the theories would never have gained the acceptance they have if these problems had been broadly recognized at the outset. Historically, if a theory failed to achieve its goals, it was improved or retired. In this case, though, the commitment to the theories has become so strong that some prominent proponents have seriously advocated moving the goalposts. They say we should be prepared to abandon the old-fashioned idea that scientific theories should give definite predictions and accept a Theory of Anything as the best that can ever be achieved. I draw the line there. Science is useful insofar as it explains and predicts why things are the way they are and not some other way. The worth of a scientific theory is gauged by the number of do-or-die experimental tests it passes. A Theory of Anything is useless, because it doesn't rule out any possibility, and worthless because it submits to no do-or-die tests. (Many papers discuss potential observable consequences, but these are only possibilities, not certainties, so the Theory of Anything is never really put at risk.) A priority for theorists today is to determine if inflation and string theory can be saved from devolving into a Theory of Anything—and if not, to seek new ideas to replace them. Because an unfalsifiable Theory of Anything creates unfair competition for real scientific theories, leaders in the field can play an important role by speaking out—making it clear that Anything is not acceptable—to encourage talented young scientists to rise to the challenge. The sooner we can retire the Theory of Anything, the sooner this important science can progress. M-THEORY/STRING THEORY IS THE ONLY GAME IN TOWN ERIC R. WEINSTEIN Mathematician and economist; managing director, Thiel Capital If one views science as an economist, it stands to reason that the first scientific theory to be retired would be the one offering the greatest opportunity for arbitrage in the marketplace of ideas. Thus it's not enough to look for ideas that are merely wrong; we should look instead for troubled scientific ideas that block progress by inspiring zeal, devotion, and what biologists politely term "interference competition" all out of proportion to their history of achievement. Here it's hard to find a better candidate for an intellectual bubble than that which has formed around the quest for a consistent Theory of Everything physical, reinterpreted as if it were synonymous with "quantum gravity." If Nature were trying to send a polite message that there's other work to be done before we quantize gravity, it's hard to see how she could send a clearer message than dashing the Nobel aspirations of two successive generations of Niels Bohr's brilliant descendants. To recall: Modern physics rests on a stool with three classical geometric legs, fashioned individually by Albert Einstein, James Clerk Maxwell, and P. A. M. Dirac. The last two of those legs can be together retrofitted to a quantum theory of force and matter known as the Standard Model, while the first stubbornly resists any such attempt at an upgrade, rendering the semi-quantum stool unstable and useless. It's from this that the children of Bohr have derived the need to convert the children of Einstein to the quantum religion at all costs, so that the stool can balance. But, to be fair to those who insist that Einstein must now be made to bow to Bohr, the most strident of those enthusiasts have offered a reasonable challenge. Quantum exceptionalists claim, despite an unparalleled history of non-success, that string theory (now rebranded as M-theory, variously standing for "matrix," "membrane," "mother," or "magic") remains the only game in town, because fundamental physics has gotten so hard that no one can think of a credible alternate unification program. If we're to dispel this as a canard, we must make a good-faith effort to answer the challenge by providing interesting alternatives, lest we be left with nothing at all. My reason for believing there's a better route to the truth is that out of what seems misplaced loyalty to our beloved Einstein, we've been too reverential to the exact form of general relativity. For example, if before retrofitting we look closely at the curvature and geometry of the three legs of the stool, we can see something striking: They're subtly incompatible at a classical geometric level before any notion of a quantum is introduced. Einstein's leg seems the sparest and sturdiest, clearly showing the attention to function found in the school of "intrinsic geometry" founded by the German Bernhard Riemann. The Maxwell and Dirac legs are somewhat more ornamented, exploring the freedom of form that's the raison d'être for a more whimsical school of "auxiliary geometry" pioneered by the Alsatian Charles Ehresmann. This leads us naturally to a very different question: What if the quantum incompatibility of the existing theories is really a red herring with respect to unification, and the real sticking point is a geometric conflict between the mathematicians Ehresmann and Riemann rather than an incompatibility between the physicists Einstein and Bohr? Even worse, it could be that none of the foundations are ready to be quantized. What if all three theories are subtly incomplete at a geometric level and the quantum will follow only when all three are retired and replaced with a unified geometry? If such an answer exists, it cannot be expected to be a generic geometric theory, because all three of the existing theories are in some sense the simplest possible in their respective domains. Such a unified approach might instead involve a new kind of mathematical toolkit, combining elements of the two major geometric schools, which would be relevant to physics only if the observed world can be shown to be of a very particular subtype. Happily, with the discoveries of neutrino mass, nontrivial dark energy, and dark matter, the world we see looks increasingly to be of the special class that could accommodate such a hybrid theory. I could go on in this way, but it isn't the only interesting line of thinking. Whereas ultimately there may be a single unified theory to summit, there are few such intellectual peaks that can be climbed from just one face. We need to return physics to its natural state of individualism, so that independent researchers need not fear large research communities that, in the quest for mindshare and resources, crowd out isolated rivals pursuing genuinely interesting inchoate ideas that head in