The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next

by Lee Smolin

  Contents

  * * *

  Title Page

  Contents

  Dedication

  Copyright

  Introduction

  THE UNFINISHED REVOLUTION

  The Five Great Problems in Theoretical Physics

  The Beauty Myth

  The World As Geometry

  Unification Becomes a Science

  From Unification to Superunification

  Quantum Gravity: The Fork in the Road

  A BRIEF HISTORY OF STRING THEORY

  Preparing for a Revolution

  The First Superstring Revolution

  Revolution Number Two

  A Theory of Anything

  The Anthropic Solution

  What String Theory Explains

  BEYOND STRING THEORY

  Surprises from the Real World

  Building on Einstein

  Physics After String Theory

  LEARNING FROM EXPERIENCE

  How Do You Fight Sociology?

  What Is Science?

  Seers and Craftspeople

  How Science Really Works

  What We Can Do for Science

  Notes

  Acknowledgments

  Index

  Sample Chapter from TIME REBORN

  Buy the Book

  About the Author

  Footnotes

  To Kai

  First Mariner Books edition 2007

  Copyright © 2006 by Spin Networks, Ltd.

  ALL RIGHTS RESERVED

  For information about permission to reproduce selections from this book, write to Permissions, Houghton Mifflin Harcourt Publishing Company, 215 Park Avenue South, New York, New York 10003.

  www.hmhco.com

  The Library of Congress has cataloged the print edition as follows:

  Smolin, Lee, date.

  The trouble with physics : the rise of string theory, the fall of a science, and what comes next / Lee Smolin.

  p. cm.

  Includes bibliographical references and index.

  ISBN-13: 978-0-618-55105-7

  ISBN-10: 0-618-55105-0

  1. Physics—Methodology—History—20th century. 2. String models. 1. Title.

  QC6.S6535 2006

  530.14—dc22 2006007235

  ISBN-13: 978-0-618-91868-3 (pbk.)

  ISBN-10: 0-618-91868-x (pbk.)

  eISBN 978-0-547-34848-3

  v6.0615

  Introduction

  There may or may not be a God. Or gods. Yet there is something ennobling about our search for the divine. And also something humanizing, which is reflected in each of the paths people have discovered to take us to deeper levels of truth. Some seek transcendence in meditation or prayer; others seek it in service to their fellow human beings; still others, the ones lucky enough to have the talent, seek transcendence in the practice of an art.

  Another way of engaging life’s deepest questions is science. Not that every scientist is a seeker; most are not. But within every scientific discipline, there are those driven by a passion to know what is most essentially true about their subject. If they are mathematicians, they want to know what numbers are, or what kind of truth mathematics describes. If they are biologists, they want to know what life is, and how it started. If they are physicists, they want to know about space and time, and what brought the world into existence. These fundamental questions are the hardest to answer and progress is seldom direct. Only a handful of scientists have the patience for this work. It is the riskiest kind of work, but the most rewarding: When someone answers a question about the foundations of a subject, it can change everything we know.

  Because it is their job to add to our growing store of knowledge, scientists spend their days confronting what they don’t understand. And those scientists who work on the foundations of any given field are fully aware that the building blocks are never as solid as their colleagues tend to believe.

  This is the story of a quest to understand nature at its deepest level. Its protagonists are the scientists who are laboring to extend our knowledge of the basic laws of physics. The period of time I will address—roughly since 1975—is the span of my own professional career as a theoretical physicist. It may also be the strangest and most frustrating period in the history of physics since Kepler and Galileo began the practice of our craft four hundred years ago.

  The story I will tell could be read by some as a tragedy. To put it bluntly—and to give away the punch line—we have failed. We inherited a science, physics, that had been progressing so fast for so long that it was often taken as the model for how other kinds of science should be done. For more than two centuries, until the present period, our understanding of the laws of nature expanded rapidly. But today, despite our best efforts, what we know for certain about these laws is no more than what we knew back in the 1970s.

  How unusual is it for three decades to pass without major progress in fundamental physics? Even if we look back more than two hundred years, to a time when science was the concern mostly of wealthy amateurs, it is unprecedented. Since at least the late eighteenth century, significant progress has been made on crucial questions every quarter century.

  By 1780, when Antoine Lavoisier’s quantitative chemistry experiments were showing that matter is conserved, Isaac Newton’s laws of motion and gravity had been in place for almost a hundred years. But while Newton gave us a framework for understanding all of nature, the frontier was wide open. People were just beginning to learn the basic facts about matter, light, and heat, and mysterious phenomena like electricity and magnetism were being elucidated.

  Over the next twenty-five years, major discoveries were made in each of these areas. We began to understand that light is a wave. We discovered the law that governs the force between electrically charged particles. And we made huge leaps in our understanding of matter with John Dalton’s atomic theory. The notion of energy was introduced; interference and diffraction were explained in terms of the wave theory of light; electrical resistance and the relationship between electricity and magnetism were explored.

  Several basic concepts underlying modern physics emerged in the next quarter century, from 1830 to 1855. Michael Faraday introduced the notion that forces are conveyed by fields, an idea he used to greatly advance our understanding of electricity and magnetism. During the same period, the conservation of energy was proposed, as was the second law of thermodynamics.

  In the quarter century following that, Faraday’s pioneering ideas about fields were developed by James Clerk Maxwell into our modern theory of electromagnetism. Maxwell not only unified electricity and magnetism, he explained light as an electromagnetic wave. In 1867, he explained the behavior of gases in terms of the atomic theory. During the same period, Rudolf Clausius introduced the notion of entropy.

  The period from 1880 to 1905 saw the discoveries of electrons and X rays. The study of heat radiation was developed in several steps, leading to Max Planck’s discovery, in 1900, of the right formula to describe the thermal properties of radiation—a formula that would spark the quantum revolution.

  In 1905, Albert Einstein was twenty-six. He had failed to find an academic job in spite of the fact that his early work on the physics of heat radiation alone would come to be seen as a major contribution to science. But that was just a warm-up. He soon zeroed in on the fundamental questions of physics: First, how could the relativity of motion be reconciled with Maxwell’s laws of electricity and magnetism? He told us in his speci
al theory of relativity. Should we think of the chemical elements as Newtonian atoms? Einstein proved we must. How can we reconcile the theories of light with the existence of atoms? Einstein told us how, and in the process showed that light is both a wave and a particle. All in the year 1905, in time stolen from his work as a patent examiner.

  The working out of Einstein’s insights took the next quarter century. By 1930, we had his general theory of relativity, which makes the revolutionary claim that the geometry of space is not fixed but evolves in time. The wave-particle duality uncovered by Einstein in 1905 had become a fully realized quantum theory, which gave us a detailed understanding of atoms, chemistry, matter, and radiation. By 1930 we also knew that the universe contained huge numbers of galaxies like our own, and we knew they were moving away from one another. The implications were not yet clear, but we knew we lived in an expanding universe.

  With the establishment of quantum theory and general relativity as part of our understanding of the world, the first stage in the twentieth-century revolution in physics was over. Many physics professors, uncomfortable with revolutions in their areas of expertise, were relieved that we could go back to doing science the normal way, without having to question our basic assumptions at every turn. But their relief was premature.

  Einstein died at the end of the next quarter century, in 1955. By then we had learned how to consistently combine quantum theory with the special theory of relativity; this was the great accomplishment of the generation of Freeman Dyson and Richard Feynman. We had discovered the neutron and the neutrino and hundreds of other apparently elementary particles. We had also understood that the myriad phenomena in nature are governed by just four forces: electromagnetism, gravity, the strong nuclear force (which holds atomic nuclei together), and the weak nuclear force (responsible for radioactive decay).

  Another quarter century brings us to 1980. By then we had constructed a theory explaining the results of all our experiments on the elementary particles and forces to date—a theory called the standard model of elementary-particle physics. For example, the standard model told us precisely how protons and neutrons are made up of quarks, which are held together by gluons, the carriers of the strong nuclear force. For the first time in the history of fundamental physics, theory had caught up with experiment. No one has since done an experiment that was not consistent with this model or with general relativity.

  Going from the very small to the very large, our knowledge of physics now extended to the new science of cosmology, where the Big Bang theory had become the consensus view. We realized that our universe contains not only stars and galaxies but exotic objects such as neutron stars, quasars, supernovas, and black holes. By 1980, Stephen Hawking had already made the fantastic prediction that black holes radiate. Astronomers also had evidence that the universe contains a lot of dark matter—that is, matter in a form that neither emits nor reflects light.

  In 1981, the cosmologist Alan Guth proposed a scenario for the very early history of the universe called inflation. Roughly speaking, his theory asserts that the universe went through a spurt of enormous growth extremely early in its life, and it explains why the universe looks pretty much the same in every direction. The theory of inflation made predictions that seemed dubious, until the evidence began to swing toward them a decade ago. As of this writing, a few puzzles remain, but the bulk of the evidence supports the predictions of inflation.

  Thus, by 1981, physics had enjoyed two hundred years of explosive growth. Discovery after discovery deepened our understanding of nature, because in each case theory and experiment had marched hand in hand. New ideas were tested and confirmed and new experimental discoveries were explained in terms of theory. Then, in the early 1980s, things ground to a halt.

  I am a member of the first generation of physicists educated since the standard model of particle physics was established. When I meet old friends from college and graduate school, we sometimes ask each other, “What have we discovered that our generation can be proud of?” If we mean new fundamental discoveries, established by experiment and explained by theory—discoveries on the scale of those just mentioned—the answer, we have to admit, is “Nothing!” Mark Wise is a leading theorist working on particle physics beyond the standard model. At a recent seminar at the Perimeter Institute of Theoretical Physics, in Waterloo, Ontario, where I work, he talked about the problem of where the masses of the elementary particles come from. “We’ve been remarkably unsuccessful at solving that problem,” he said. “If I had to give a talk on the fermion-mass problem now, I’d probably end up talking about things I could have in the 1980s.”1 He went on to tell a story about when he and John Preskill, another leading theorist, arrived at Caltech in 1983, to join its faculty. “John Preskill and I were sitting together in his office, talking. . . . You know, the gods of physics were at Caltech, and now we were there! John said, ‘I’m not going to forget what is important to work on.’ So he took what was known about the quark and lepton masses, and he wrote it on a yellow sheet of paper and stuck it on his bulletin board . . . so as not to forget to work on them. Fifteen years later, I come into his office . . . and we’re talking about something, and I look up at his bulletin board and [notice that] that sheet of paper is still there but the sun has faded everything that was written on it. So the problems went away!”

  To be fair, we’ve made two experimental discoveries in the past few decades: that neutrinos have mass and that the universe is dominated by a mysterious dark energy that seems to be accelerating its expansion. But we have no idea why neutrinos (or any of the other particles) have mass or what explains their mass value. As for the dark energy, it’s not explained in terms of any existing theory. Its discovery cannot then be counted as a success, for it suggests that there is some major fact we are all missing. And except for the dark energy, no new particle has been discovered, no new force found, no new phenomenon encountered that was not known and understood twenty-five years ago.

  Don’t get me wrong. For the past twenty-five years we have certainly been very busy. There has been enormous progress in applying established theories to diverse subjects: the properties of materials, the molecular physics underlying biology, the dynamics of vast clusters of stars. But when it comes to extending our knowledge of the laws of nature, we have made no real headway. Many beautiful ideas have been explored, and there have been remarkable particle-accelerator experiments and cosmological observations, but these have mainly served to confirm existing theory. There have been few leaps forward, and none as definitive or important as those of the previous two hundred years. When something like this happens in sports or business, it’s called hitting the wall.

  Why is physics suddenly in trouble? And what can we do about it? These are the central questions of my book.

  I’m an optimist by nature, and for a long time I fought the conclusion that this period in physics—the period of my own career—has been an unusually fallow one. For me and many of my friends who entered science with the hope of making important contributions to what then was a rapidly moving field, there is a shocking fact we must come to terms with: Unlike any previous generation, we have not achieved anything that we can be confident will outlive us. This has given rise to personal crises. But, more important, it has produced a crisis in physics.

  The main challenge for theoretical particle physics over the last three decades has been to explain the standard model more deeply. Here there has been a lot of activity. New theories have been posited and explored, some in great detail, but none has been confirmed experimentally. And here’s the crux of the problem: In science, for a theory to be believed, it must make a new prediction—different from those made by previous theories—for an experiment not yet done. For the experiment to be meaningful, we must be able to get an answer that disagrees with that prediction. When this is the case, we say that a theory is falsifiable—vulnerable to being shown false. The theory also has to be confirmable; it must be possible to verify a new predi
ction that only this theory makes. Only when a theory has been tested and the results agree with the theory do we advance the theory to the ranks of true theories.

  The current crisis in particle physics springs from the fact that the theories that have gone beyond the standard model in the last thirty years fall into two categories. Some were falsifiable, and they were falsified. The rest are untested—either because they make no clean predictions or because the predictions they do make are not testable with current technology.

  Over the last three decades, theorists have proposed at least a dozen new approaches. Each approach is motivated by a compelling hypothesis, but none has so far succeeded. In the realm of particle physics, these include Technicolor, preon models, and supersymmetry. In the realm of spacetime, they include twistor theory, causal sets, supergravity, dynamical triangulations, and loop quantum gravity. Some of these ideas are as exotic as they sound.

  One theory has attracted more attention than all the others combined: string theory. The reasons for its popularity are not hard to understand. It purports to correctly describe the big and the small—both gravity and the elementary particles—and to do so, it makes the boldest hypotheses of all the theories: It posits that the world contains as yet unseen dimensions and many more particles than are presently known. At the same time, it proposes that all the elementary particles arise from the vibrations of a single entity—a string—that obeys simple and beautiful laws. It claims to be the one theory that unifies all the particles and all the forces in nature. As such, it promises to make clean and unambiguous predictions for any experiment that has ever been done or ever could be done. Much effort has been put into string theory in the last twenty years, but we still do not know whether it is true. Even after all this work, the theory makes no new predictions that are testable by current—or even currently conceivable—experiments. The few clean predictions it does make have already been made by other well-accepted theories.