Supersymmetry: Unveiling the Ultimate Laws of Nature


"A fascinating account of the theoretical ideas behind supersymmetry...told by someone who has contributed deeply to the development of the field." -NatureFor most of human history, man has been trying to discover just how the universe works. In this groundbreaking work, renowned physicist Gordon Kane first gives us the basics of the Standard Model, which describes the fundamental constituents and forces of nature. He then explains the next great leap in understanding: the theory of supersymmetry, which implies ...

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"A fascinating account of the theoretical ideas behind supersymmetry...told by someone who has contributed deeply to the development of the field." -NatureFor most of human history, man has been trying to discover just how the universe works. In this groundbreaking work, renowned physicist Gordon Kane first gives us the basics of the Standard Model, which describes the fundamental constituents and forces of nature. He then explains the next great leap in understanding: the theory of supersymmetry, which implies that each of the fundamental particles has a "superpartner" that can be detected at energies and intensities only now being achieved in the giant accelerators. If Kane and his colleagues are correct, these superpartners will also help solve many of the puzzles of modern physics-such as the existence of the Higgs boson-as well as one of the biggest mysteries is cosmology: the notorious "dark matter" of the universe.

Just as supersymmetry will revolutionize our understanding of space and time, Gordon Kane's book will revolutionize our understanding of supersymmetry.

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Editorial Reviews

From Barnes & Noble
Our Review
Although string theory seems to be getting all the attention these days, the next concrete step for physics is confirmation of the theory of supersymmetry, which would encompass and extend the current Standard Model of particle physics. The Standard Model is the accepted framework for grouping particles and their relationships to one another. Particle physicist Gordon Kane gave a wonderful nontechnical explanation of the Standard Model in his previous book, The Particle Garden. In his new book, Kane makes a careful distinction between areas of science that are well tested and those that are "research in progress" (or RIP) and also between theories that describe the way the world is and those that explain why the world is the way it is.

The Standard Model is well tested and describes the way the world is, indicating, for example that there are three families of particles; but it does not explain why there are three families. A Supersymmetric Standard Model would answer some loose questions and would represent significant progress toward an eventual primary theory that would explain the whys. Above all, Supersymmetry is a window into how physicists formulate and test theories. Kane guides the reader step-by-step through the tools physicists use, such as the Feynman diagrams developed by Richard Feynman, and gives a detailed rundown of colliders and what scientists can hope will be discovered in the next few years.

Specifically, physicists are looking for evidence of the predicted "superpartners." Supersymmetry requires that every particle known must have a corresponding superpartner (somewhat analogous to matter/antimatter). Finding one is not a simple matter. Kane shows the reader simulated collider "events" that could qualify and explains just how these events are analyzed. Anyone interested in what can be expected from physics in the near future, or who has read The Elegant Universe or Hyperspace and wants an in-depth treatment of supersymmetry, should consider this book.

--Laura Wood, Science & Nature Editor

A fascinating account of the theoretical ideas behind supersymmetry...told by someone who has contributed deeply to the development of the field.
Samuel C. C. Ting
An excellent book on one of the most important advances in modern physics.
Science Books and Films
A good introduction to our search for the ultimate laws of the structure of matter.
Publishers Weekly - Publisher's Weekly
Physicists have, for years, used something called the "standard model" to explain the behavior of elementary particles and of the basic forces that connect them--to generally give "a complete description of how our physical world works." But the model also creates questions it can't answer: why are we made of matter and not antimatter? And why is there more gravity in the universe than all the objects we know about can produce? Kane, who teaches physics at the University of Michigan, explained the standard model in his first book for nonscientists, The Particle Garden; in this very readable follow-up, he shows how something else--"supersymmetry"--might answer the questions the standard model can't. He begins his careful map of difficult territory with an explanation of very basic terms like "particle," "equation," "structure" and "symmetry." Then he surveys what supersymmetry does: it interacts intriguingly, for example, with the recent, also speculative--but better publicized--superstring theory, and it's just now becoming testable in the newest, snazziest particle accelerators. Kane also devotes one chapter to "Testing Supersymmetry Experimentally," and another to its implications for questions about the cosmos: "Can We Really Understand the Origin of the Universe?" Equipped with his remarkable gifts for turning abstruse concepts and hard math into good English prose, he's careful to differentiate between accepted theories, currently testable hypotheses and speculations. A compact glossary gives easy access to quick definitions: many readers will need it. The same readers will probably be grateful for Kane's sophisticated, accessible guide to one of the frontiers of physics. Line illus. throughout. (May) Copyright 2000 Cahners Business Information.|
Internet Bookwatch
Supersymmetry covers the quest to uncover a grand unified theory of how the universe works - something Einstein failed to achieve. There have been numerous books on the subject but this is the first to describe what research is involved in creating and testing the theory. Different perspectives, arguments and challenges in developing a supersymmetry theory are contrasted.
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Product Details

  • ISBN-13: 9780738204895
  • Publisher: Basic Books
  • Publication date: 7/28/2001
  • Edition description: Reprint
  • Pages: 224
  • Sales rank: 1,352,070
  • Product dimensions: 6.07 (w) x 9.25 (h) x 0.66 (d)

Meet the Author

Gordon Kane is an internationally acclaimed particle physicist at the University of Michigan in Ann Arbor. He is a popular public lecturer and the author of The Particle Garden.

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Read an Excerpt

Excerpt from Chapter 2:

The Standard Model of Particle Physics

To better understand the reasons why we think supersymmetry will be discovered, and how it extends our present description of nature, we need to know a little about the main achievements of over two centuries of physics. Past study has led to the establishment of the Standard Model of particle physics, a complete description of the basic particles and forces that shape our world. In this chapter we will consider first the five forces and then the particles. The world we see is built entirely of three particles: the electron and two particles similar to the electron called quarks. There are more particles-antiparticles, neutrinos, more quarks and more particles like the electron, and Higgs bosons.

We understand why some of the additional particles exist, but not others. Then we will learn a little about Feynman diagrams, a way of picturing how particles interact (for practitioners it is also a way to calculate the behavior of particles). Next we consider a property of all particles called spin and how it leads to categorizing particles into two groups, bosons and fermions. We will see later that at a deeper level, supersymmetry merges these two categories. Last we look at reasons why the Standard Model is not expected to be the final stage of particle physics, even though it successfully describes phenomena and experiments.

When Gauguin painted "Where do we come from? What are we? Where are we going?" we knew only of the electrical, magnetic, and gravitational forces. There was controversy about whether atoms existed. The first particle to be discovered, the electron, had just been found. Radioactive decays of nuclei ("radioactivity") had just been noticed. These decays could not be explained by the known interactions, so physicists realized that another force was needed to describe nature's behavior. It was called the weak force, because its effects were rare and essentially never occurred when two interacting objects were separated by distances larger than an atom. In 1911 atoms were found to have a nucleus, and it was discovered that heavier nuclei have a number of protons in them. Because physicists knew that the repulsive electrical force pushed the protons apart, it became clear that yet another force-the nuclear force-must exist to bind the protons together into a stable nucleus. The effects of the nuclear force also can be felt only at tiny distances, no larger than a nucleus. Although Newton had correctly foreseen three centuries ago that there could be forces that had effects only at small distances, finding evidence for them took over two centuries.

These five forces (the electrical, magnetic, gravitational, weak, and nuclear forces) account for all we observe in nature. The interactions of particles with a "Higgs field" (more about this later, especially in Chapter 7) can be thought of as giving mass to the particles, so one can think of the Higgs interaction as an additional force. There may in a sense be other forces, but they are not relevant for the behavior of particles or for how particles combine to make up the world around us. (For completeness, let me mention two at this point. One is a force that recent evidence suggests causes the universe to expand more rapidly than it would if only the gravitational force affected its expansion. This force does not in any way affect the behavior of individual particles. For historical reasons it is called the cosmological constant. The other possible forces have effects only at extremely tiny distances far smaller than the size of a proton.) We will return to possible additional forces later briefly; the five known forces are the important ones for our purposes. Any others do not affect how our world works, though understanding them may be essential to achieve a complete picture of the laws of nature.

Ever since Charles-Augustin de Coulomb showed, over two hundred years ago, that the electrical and gravitational forces depend in the same way on the distance between interacting objects, and that therefore the formulas describing them have the same form, physicists have tried to unify our understanding of them. In the second half of the nineteenth century, using the work of Michael Faraday and others, James Clerk Maxwell succeeded in relating the electrical and magnetic forces, in the sense that electrical forces that vary in space or time generate magnetic forces, and vice versa. By the 1960s we knew of the five -forces, the electrical and magnetic ones being unified into electromagnetism. There was no theory at all of the weak and nuclear forces, only a few known regularities of their behavior. By the 1980s, however, the Standard Model had emerged and had been well tested. It was a complete description of the weak, electromagnetic, and strong (nuclear) forces, fully consistent with quantum theory and special relativity. The progress-over two decades was spectacular-in that short period, we went from a crude awareness of the weak and strong forces to their comprehensive description.

The picture of the electromagnetic force that emerges is that electrons, and any particles that have electric charge, interact by exchanging photons. The photons can carry energy between the electrons; two electrons can scatter off one another by exchanging a photon; and an electron and a proton bind by exchanging many photons, which provide an attractive force that keeps the electron and proton connected in a stable object, a hydrogen atom. All the forces work in a similar way. The gravitational force arises from the exchange of gravitons. The analogous particles for the weak interactions are called W and Z bosons, and for the strong force they are called gluons. In all these cases, we speak of the photon, W and Z bosons, and gravitons as "mediating" the forces. There is one more subtlety in connection with my use of the name strong force here. Because of the history, I cheated a little when I listed the nuclear force above. It turns out that the nuclear force between protons and neutrons that binds them into nuclei is not the fundamental force. Rather, the basic force is the strong force between quarks, and the nuclear force is a kind of residual effect after quarks are bound into protons and neutrons. We'll return to it after we describe the quarks.


Two frequently used words about particles that can be confusing are mass and decays. For our purposes, mass essentially means weight, and we don't need a more precise definition. Some particles, such as photons, don't have any mass. The masses of particles are measured mostly by bouncing particles off each other, since how much they bounce is related to how heavy they are.

Nearly all particles are unstable and decay into others. The word decay has a technical meaning in physics-one particle disappears, typically turning into two or three others. A major difference between the way decay is used in physics and its use in everyday life or biology is that the particles that characterize the final state are not in any sense already in the decaying particle. The initial particle really disappears, and the final particles appear. The photons that make up the light we see provide an example: The photons emitted from a light bulb when it is turned on are not particles that were in the bulb just waiting to come out, and photons that enter our eyes after bouncing off an object are absorbed by the molecules in our eyes and disappear. All particles can be created or absorbed in interactions with other particles. Most particles were created during the Big Bang, and particles can be created in collisions, e.g., at colliding-beam accelerators. Which particles can appear or disappear in any interaction is not arbitrary but is, rather, fully determined by the interactions described by the Standard Model (and its extensions such as supersymmetry).

Two constraints keep a few particles from decaying. First, the total amount of energy in any process must not change-we say energy is conserved. Imagine a decaying particle at rest; its energy is just its mass. That mass is divided into the sum of masses of the particles created in its decay, plus some energy of motion of those particles. Since energy can't appear from nothing, the particles created in the decay must be lighter than the decaying particle. Second, most particles carry some charge (electric charge and some similar charges associated with other forces). Charges are also conserved-the total amount of charge can't change in a process. Because of these two constraints, several of the lighter particles (electrons, for example) do not decay. Whenever basic particles are able to decay, the decay is typically rapid, occurring in a tiny fraction of a second.

Electromagnetic waves carry energy from antennas to our radios and from light bulbs to our eyes. One of the things quantum theory has taught us is that the energy is carried in little chunks (or quanta), photons. Any electric charge sets up an electric field around itself, and when that charge oscillates (say, in an antenna), it radiates the electromagnetic wave or the photons. We have also learned that there is a gravitational field associated with mass, and there are other fields associated with matter-with electrons and other particles. All the particles can be thought of as the quanta of the fields. In this book we won't make any technical use of such ideas, but every time we speak of some kind of field, we will associate quanta and therefore particles with it, and vice versa.


When viewed from the particle side, the Standard Model is remarkable. What are we made of? What happens if we keep cutting an apple, or a person, into smaller and smaller pieces? Do we reach a smallest piece? What could it be? What are the stars made of? Everyone has wondered about such questions. The answer is that everything we see in the universe, from the smallest cell to flowers to people to stars, is made of three kinds of matter particles, bound together by gluons and photons. When we get to big objects such as planets and stars, gravity binds too. The three matter particles are the familiar electron and two particles similar to the electron called quarks, the up quark and the down quark. Up and down do have a technical meaning, but it's not important here. All of the basic particles carry various amounts of charges, electric charge and weak charge and strong charge. The weak and strong charges are somewhat like electric charge, but because they have no effects outside atoms, they are not familiar to us in everyday life. The main difference between electrons and quarks is that the quarks carry the strong charge, so they interact via exchanging gluons, whereas electrons do not interact with gluons at all. Electrons and quarks all have electric charges and masses (weights) in different amounts. Sometimes electrons are denoted by e, up quarks by u, and down quarks by d.

How do these seeds form our world? The quarks bind together to make neutrons and protons, neutrons having an up quark and two down quarks, protons having two up quarks and one down quark. The quarks interact via the strong force, mediated by gluon exchange. The edges of protons and neutrons are not sharp-gluons range outward a little before they are brought back by the attractive force of the quarks. Gluons that are a little outside the proton and neutron in turn exert an attractive force felt by other protons and neutrons; this residual strong force is the nuclear force that binds the protons and neutrons into nuclei. The nuclear force is strong enough to hold together many nuclei, with from 1 to 92 protons and varying numbers of neutrons. The electrical repulsion felt by one proton (due to all the other protons in the nucleus) increases as the number of protons increases, and with more than 92 protons the nucleus becomes unstable. This is why there are only 92 naturally occurring chemical elements. The electromagnetic force, mediated by photons, binds electrons to nuclei to make atoms, the atoms of the chemical elements. Outside an atom there is a residual electromagnetic force (analogous to the residual strong force outside protons and neutrons described earlier in this paragraph) that binds atoms into molecules. Atoms and molecules build up rocks, cells, and all of the world around us. All of the marvelous complexity and color and structure of our world arises from these simple foundations.

Always in the past when objects were studied at smaller distances, they turned out to have structure. The atoms of the 92 chemical elements were not the atoms invented by the Greeks as the basic indestructible units of matter, because they turned out to have a nucleus surrounded by electrons. The nucleus turned out to be made of protons and neutrons. Protons and neutrons themselves turned out to be made of quarks bound by gluons. Why, then, do we think that the electrons and quarks are the true Greek "atoms" and that despite a history of smaller and smaller units, the progression stops with them? There are three kinds of reasons that lead most particle theorists to think we have finally reached the end of the line.

The first reason, significant but less important than the others, is that investigators have tried by many means to determine whether electrons, quarks, W bosons, and gluons show any evidence of structure, and they have not found any. These experiments have probed perhaps 10,000 times further than it took to see structure in the past, but electrons and quarks continue to behave as point-like objects with no parts.

The second reason is that always before, the prevailing theory (that of atoms or nuclei or protons) did not agree with experiment or even make sense unless these objects had structure at smaller distances, but the Standard Model is different. Because the Standard Model is a quantum theory, it is possible to ask how the forces behave at smaller and smaller distances and to do calculations to answer the question. Suppose that the basic particles were indeed point-like. How strong would the forces be if we could examine them not just down to almost 10-" meter, where experiments can presently be done, but at a million million times smaller distances? The calculations show that in the Standard Model, all the interactions become weaker at smaller distances. The Standard Model does not lead to...

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Table of Contents

Foreword by Edward Witten ..... xi
Preface ..... xv
1: Where Do We Come From? What Are We? Where Are We Going?
To understand nature we need to know about particles, forces, and rules
Research in progress (RIP) Equations?
Prediction, postdiction, and testing Where are the superpartners?
The boundaries of science have moved
2: The Standard Model of Particle Physics ..... 16 The forces
Mass, decays, and quanta
The particles: Do we really know the fundamental constituents of matter? • Particles and fields
There are more particles
New ideas and remarkable predictions of the Standard Model Experimental foundations of the Standard Model Picturing Standard Model processes: Feynman diagrams Spin, fermions, and bosons
Beyond the Standard Model
3: Why Physics is the Easiest Science-Effective Theories ..... 40 Organizing effective theories by distance scales Supersymmetry is an effective theory too The physics of the Planck scale
Effective theories replace renormalization
The human scales
4: Supersymmetry and Sparticles ..... 53 What is Supersymmetry?
Some mysteries Supersymmetry would solve
The superpartners supersymmetry as a spacetime symmetry: superspace Hidden or "broken" Supersymmetry
5: Testing Supersymmetry Experimentally ..... 72 Detectors and colliders
Recognizing superpartners Sparticles: their personalities, backgrounds, and signatures at LEP and Fermilab
Visit Fermilab
Future colliders Can we do the experiments we need to do?
6: What is the Universe Made of? ..... 98 What particles are there in the universe? Is the lightest superpartner the cold dark matter of the universe?
7: Higgs Physics ..... 108 Finding Higgs bosons
Current evidence
LEP, Fermilab, and LHC
Studying Higgs bosons at Fermilab
8: Some Additional Help from Supersymmetry and some Challenges ..... 117 Matter and antimatter asymmetry
Proton decay? 41 Rare decays
CP violation
Perspectives and concerns
9: Supersymmetry STRING THEORY, AND THE PRIMARY THEORY ..... 130 String theory and M-theory
Broken or hidden Supersymmetry
The role of data
Effective theories and the primary theory
10: CAN WE REALLY UNDERSTAND THE ORIGIN OF THE UNIVERSE AND ITS NATURAL LAW(S)? ..... 136 Testing string theory and the primary theory
Practical limits?
Anthropic questions and Supersymmetry
The end of science?
Appendices ..... 149
A. The Standard Model Higgs mechanism ..... 149
B. The Supersymmetry explanation of the Higgs mechanism ..... 153
C. Charginos and neutralinos ..... 157
D. Extra dimensions-large extra dimensions? ..... 159
Some Recommended Reading ..... 163
Glossary ..... 165
Symbols ..... 165
Acronyms and Abbreviations ..... 66
Terms ..... 166
Index ..... 195
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If you take a little trouble, you will attain to a thorough understanding of these truths. For one thing will be illuminated by another, and eyeless night will not rob you of your road till you have looked into the heart of nature's darkest mysteries. So surely will facts throw light upon facts.
-Lucretius, On the Nature of the Universe (Translated by R. E. Latham, Penguin Books)

Most people realize that anyone who is interested in how an old-fashioned watch works can get a good idea of what is happening inside. Few people, however, realize that physicists now have a similarly clear image of the mechanisms of the subatomic universe-the stuff that makes the world run. That image is formulated in what we call the Standard Model of particle physics. It is a description of the underlying structure of the universe. Someone who probes and studies a watch not only can describe the workings of that watch but also can say why the watch works-why this cog moving that one at a given ratio mimics the progress of time. Physicists, too, are increasingly able to peer into the workings of the universe and say why the ingredients they study are able to create and sustain what we know as nature.

A nice way to learn more about a watch is to see a watchmaker disassemble and reassemble one, and a nice way to learn more about nature is to go for a walk with a naturalist. This book is meant as a leisurely walk that can be enjoyed by anyone with the curiosity to come along and observe the particles and their behavior. We will stroll not only in the known territory of the Standard Model but also along the frontier of topics where breakthroughs into even more remote regions may soon occur. For various practical and theoretical reasons, many particle physicists think that the next major discovery will be direct evidence for the property called supersymmetry.

Those reasons, and the implications if direct evidence of supersymmetry is indeed observed, are much of what this book is about.

The first phase of the age-old search for understanding how the physical world works has been brought to a successful close in recent years with the development and testing of the Standard Model of particle physics. The Standard Model (summarized in Chapter 2) gives a comprehensive description of the basic particles and forces of nature and of how all of the physical phenomena we see can be described. It contains the underlying principles of all the behavior of protons, nuclei, atoms, molecules, condensed matter, stars, and more. The Standard Model has explained much that was not understood before; it has made hundreds of successful predictions, including many dramatic ones; and there are no phenomena in its domain (Chapter 3) that are not explained (though some calculations are too complicated to carry through). The Standard Model has loose ends (such as the "Higgs physics," Chapter 7), but they don't affect most of its explanations and tests.

If the Standard Model describes the world successfully, how can there be physics beyond it, such as supersymmetry? There are two reasons. First, the Standard Model does not explain aspects of the study of the large-scale universe, cosmology. For example, the Standard Model cannot explain why the universe is made of matter and not antimatter (Chapter 8), nor can it explain what the dark matter of the universe is (Chapter 6). Supersymmetry suggests explanations for both of these mysteries. Second, the boundaries of physics have been changing. Now scientists ask not only how the world works (a question the Standard Model answers) but why it works that way (a question the Standard Model cannot answer). Einstein asked "why" earlier in the century, but only in the past decade or so have the "why" questions become normal scientific research in particle physics, rather than philosophical afterthoughts. One ambitious approach to "why" is known as string theory (Chapter 9), which is formulated in an eleven-dimensional world. Work on string theory has proceeded so far by study of the theory itself, rather than via the historically fruitful interplay of experiment and theory. This approach has led to significant and exciting progress; if it succeeds we will all be delighted. As Edward Witten remarks in his Foreword to this book, string theory predicts that nature should be supersymmetric.

Supersymmetry is a surprising and subtle idea-the idea that the equations representing the basic laws of nature don't change if certain particles in the equations are interchanged with one another. Just as a square on a piece of paper looks the same if you rotate it by 90 degrees, the equations that physicists have found to describe nature often do not change when certain operations are performed on them. When that happens, the equations are said to have a symmetry. Supersymmetry is such a proposed symmetry-the "super" is included in its name because this symmetry (Chapter 4) is more surprising and more hidden from everyday view than previously discovered symmetries. It turns out that the idea has remarkable consequences for explaining aspects of the world that the Standard Model cannot explain, particularly the Higgs physics; they are described in Chapters 4-8. The most important implication may be that supersymmetry can provide a window that enables us to look at the minute world of string theory from our full-size world, so that experiment can provide guidance to help formulate string theory, and so that the predictions of string theory can be tested. Supersymmetry ushers in the second phase of the search for understanding.

Supersymmetry is still an idea as this book is being written (mid-1999). There is considerable indirect evidence that it is a property of the laws of nature, but the confirming direct evidence is not yet in place. That is not an argument against nature being supersymmetric; rather, the accelerator facilities that could confirm it are just beginning to cover the region where the signals could appear (Chapter 5).1 have tried to present the material in this book in such a way that it will remain valid and interesting after the superpartners and Higgs bosons predicted by supersymmetry are found. When we have positive signals, the focus can be sharpened, but the explanations that supersymmetry can provide, the way it can connect with string theory, and how we recognize and test it are likely to be very close to what is presented here.

If the world we live in does exhibit the property called supersymmetry, even though it has been hidden from our view until now, we will have a systematic way to peer at the most basic law(s) that govern nature and our universe. Without supersymmetry that may not be possible. Though there is considerable indirect evidence that the world is indeed supersymmetric, this is not yet certain. It is worth a lot of effort to find out.

A number of people have enriched this book. I am very grateful to my most relentless editor, my wife Lois, who contributed greatly to the book's intelligibility; to Jim Wells and Lisa Everett for many very helpful suggestions; and to Kate Logan for extensive assistance, particularly with the figures. I appreciate very much the encouragement I have received from Perseus Books, comments from Steve Mrenna, help from Judy Jackson in obtaining Fermilab photos, and help from Jane Nachtman, Andrei Nomerotski, Daniel Treille, Jianming Qian, and Saul Youssef in obtaining pictures of events.

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Looking back at century's end, it is stunning to think how our understanding of physics has changed in the last hundred years. The great insights of the early part of the century were of course Relativity Theory and Quantum Mechanics. We learned in Einstein's Special Relativity theory of the strange behavior of fast moving objects, and in his even more surprising General Relativity we learned to reinterpret gravity in terms of the curvature of space and time caused by matter. As for Quantum Mechanics, it taught us that fact is far more wondrous than fiction in the atomic world. Special Relativity and Quantum Mechanics were fused in Quantum Field Theory, whose most remarkable prediction-verified experimentally in cosmic rays around 1930-is the existence of "antimatter." Quantum Field Theory is a very difficult theory to understand even for specialists; trying to understand it has occupied the attention of many leading physicists for generations.

The last fifty years have been an amazing period of experimental discoveries and surprises, including "strange particles," the breaking of symmetry between left and right and between past and future, neutrinos, quarks, and more. Drawing on this material, theoretical physicists have been able, in the Quantum Field Theory framework, to construct the Standard Model of particle physics, which puts under one roof most of what we know about fundamental physics. It describes in one framework electricity and magnetism, the weak force responsible among other things for nuclear beta decay, and the nuclear force.

Is this journey of discovery nearing an end? Or will the next half century be a period of surprises and discoveries rivaling those of the past? The questions we can ask today are as exciting as any in the past, and at least some of the answers can be found in the coming period if we stay the course.

Just in the last few months, newspapers have been filled with exciting accounts of recent and forthcoming experiments testing the strange properties of neutrinos, and perhaps showing that the "little neutral particle" of Fermi does have a tiny but nonzero mass after all. Astronomers have unraveled new and challenging hints that General Relativity may need to be corrected by adding Einstein's "cosmological constant"-the energy of the vacuum. Novel and inventive dark matter searches are probing the invisible stuff of the universe. Satellite probes of fluctuations in the leftover radiation from the big bang are likely, in the next few years, to challenge our understanding of the, large scale structure of the universe.

But one of the biggest adventures of all is the search for "supersymmetry" Supersymmetry is the framework in which theoretical physicists have sought to answer some of the questions left open by the Standard Model of particle physics. The Standard Model, for example, does not explain the particle masses. If particles had the huge masses allowed by the Standard Model, the universe would be a completely different place. There would be no stars, planets, or people, since any collection of more than a handful of elementary particles would collapse into a Black Hole. Subtle mysteries of modern physics-like spacetime curvature, Black Holes, and quantum gravity-would be obvious in everyday life, except that there would be no everyday life.

Supersymmetry, if it holds in nature, is part of the quantum structure of space and time. In everyday life, we measure space and time by numbers, "It is now three o'clock, the elevation is two hundred meters above sea level," and so on. Numbers are classical concepts, known to humans since long before Quantum Mechanics was developed in the early twentieth century. The discovery of Quantum Mechanics changed our understanding of almost everything in physics, but our basic way of thinking about space and time has not yet been affected.

Showing that nature is supersymmetric would change that, by revealing a quantum dimension of space and time, not measurable by ordinary numbers. This quantum dimension would be manifested in the existence of new elementary particles, which would be produced in accelerators and whose behavior would be governed by supersymmetric laws. Experimental clues suggest that the energy required to produce the new particles is not much higher than that of present accelerators. If supersymmetry plays the role in physics that we suspect it does, then it is very likely to be discovered by the next generation of particle accelerators, either at Fermilab in Batavia, Illinois, or at CERN in Geneva, Switzerland.

When Einstein introduced Special Relativity in 1905 and then General Relativity in 1915, Quantum Mechanics was still largely in the future, and Einstein assumed that space and time can be measured by ordinary numbers. Einstein's conception of space and time has been adequate for discoveries made until the present, but discovery of supersymmetry would begin a reworking of Einstein's ideas in the light of Quantum Mechanics.

Discovery of supersymmetry would be one of the real milestones in physics, made even more exciting by its close links to still more ambitious theoretical ideas. Indeed, supersymmetry is one of the basic requirements of "string theory," which is the framework in which theoretical physicists have had some success in unifying gravity with the rest of the elementary particle forces. Discovery of supersymmetry would surely give string theory an enormous boost.

The search for supersymmetry is one of the great dramas in present-day physics. Hopefully, the present book will introduce a wider audience to this ongoing drama!

Edward Witten Princeton, New Jersey June 30, 1999

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