The Search for Superstrings, Symmetry, and the Theory of Everything

Praise for John Gribbin's

The Search for Superstrings, Symmetry, and the Theory of Everything

"Ever since Einstein came up with the General Theory of Relativity, the Holy Grail of physics has been a 'Theory of Everything' that would explain the behavior of all the particles and forces in nature in one set of equations. Popular science writer Gribbin tackles this quest in a thorough yet palatable primer geared to the serious reader.… Gribbin helps us get our bearings in a universe of eleven dimensions.… Diligent readers without any specialized knowledge of physics or mathematics will come away with a flavor of the latest ideas theorists are grappling with.… Overall, this is an exciting tour de force."

—Publishers Weekly

"Gribbin's enthusiasm is unmistakable, and his voice is friendly and reassuring as he guides us through this exotic material."

—John Ashkenas, San Francisco Chronicle

"A clear, comprehensive popular treatment of the cutting edge of physics."

—Kirkus Reviews

"From the author of In Search of Schrödinger's Cat and In Search of the Big Bang comes yet another enthusiastic exploration on and lucid explanation of scientific theory.… Gribbin's straightforward approach leads the layman through the maze of scientific babble and ideas without either complicating or oversimplifying matters."

—Katrina Dixon, The Scotsman (U.K.)

"Writing in his clear prose style, Gribbin introduces the general reader to the mysterious world of high-energy physics—a formidable task because of the complex theories involved; nevertheless, he translates these ideas into a readable, enjoyable narrative. His extensive historical treatment of physics research from the foundation work done in the 19th century to the latest concepts of superstrings is remarkable. Gribbin takes the reader to a world of multidimensions—a fictionlike picture—where scientists are trying to merge the forces of the universe in a grand unified theory called supersymmetry."

—Nestor Osorio, Library Journal

"One of England's best-known science writers… Gribbin is an optimist, believing that we are ever closer to another 'Deep Truth' about the universe.… For those who want the story up to now, this is a fine place to begin."

—David Williams, Seattle Times

"A useful summary of the particle zoo. I think that people who have some knowledge of physics, and have encountered particle physics, will appreciate and enjoy this book."

—Helen J. Walker, The Observatory (U.K.)

"Unless one holds a physics doctorate and can tolerate 26 dimensions, the world of the 'supersmall' seems unapproachably alien. Once again, the guide to the frontier is Gribbin, prolific popularizer of physics and physicists.… Here he introduces the 'quarky' zoo of subatomic particles and their mediating forces, Gribbin himself mediating for generalists the theories advanced to explain and unify them.… In these mind-bending realms, Gribbin's seasoned skills wonderfully simplify matters (and forces) without 'dumbifying' them."

—Gilbert Taylor, Booklist

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BY THE SAME AUTHOR

In Search of the Edge of Time

Hothouse Earth

Being Human

In Search of the Big Bang

In Search of Schrödinger's Cat

The Hole in the Sky

Stephen Hawking: A Life in Science

(WITH MICHAEL WHITE)

Albert Einstein: A Life in Science

The Matter Myth

(WITH PAUL DAVIES)

In the Beginning

Schrödinger's Kittens and the Search for Reality

Companion to the Cosmos

The Case of the Missing Neutrinos

Almost Everyone's Guide to Science

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Thanks to Benjamin Gribbin

for editorial assistance

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Copyright

Copyright © 1998 by John and Mary Gribbin

Illustrations copyright © 1998 by John Gribbin

All rights reserved. No part of this book may be reproduced in any form or by any electronic or mechanical means, including information storage and retrieval systems, without permission in writing from the publisher, except by a reviewer who may quote brief passages in a review.

First North American edition published by Little, Brown and Company, January 1999

First eBook Edition: November 2009

Portions of this book first appeared in In Search of the Big Bang.

Hachette Book Group

237 Park Avenue

New York, NY 10017

Visit our website at www.HachetteBookGroup.com

www.twitter.com/littlebrown

ISBN: 978-0-316-09292-0

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Preface

When I revised my book In Search of the Big Bang to bring it up to date and tell the entire story of the life and prospective death of the Universe, something had to go to make way for the new material. That 'something' was mainly the detailed discussion of the world of sub-atomic particles, which was slightly tangential to the story of the Big Bang. No sooner had I done so, however, than various friends and colleagues that I discussed the project with bemoaned the loss, telling me that the kind of historical overview that the material had provided was all too rare in popular accounts of particle physics, or even in books aimed at students taking physics courses.

I took a second look, and felt that they might be right. So here, updated to the late-1990s, is the story of the particle world, from the discovery of the electron to the search for a supersymmetric theory explaining all of the forces and particles of nature in one mathematical package. It draws on materials from the original version of my Big Bang book, but does not overlap with the revised version of In Search of the Big Bang. The story isn't complete, because the mathematical physicists haven't yet found the ultimate theory of everything that they seek. But it will, I hope, shed some light on why they are looking where they are looking for the ultimate theory.

JOHN GRIBBIN

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Introduction

The Material World

During the nineteenth century, chemists developed the idea, which dated back to the time of Democritus, in the fourth century BC, that everything in the material world is made up of tiny, indivisible particles called atoms. Atoms were thought of as being like tiny billiard balls, so small that it would take a hundred million of them, side by side, to stretch along a line I cm long. Atoms of a particular element each had the same mass, but the atoms of different elements, such as carbon, oxygen or iron, had different masses from one another, and the properties of the atoms, it was realized, determine the gross properties of larger quantities of the elements. When elements combine (for example, when carbon burns in air), it is because individual atoms of each element combine to make molecules (in this example, each atom of carbon combines with two atoms of oxygen to make carbon dioxide).

But just as the idea of atoms was becoming firmly established, in 1897 the English physicist J.J. Thomson, working at the Cavendish Laboratory in Cambridge, found a way to study bits that had been broken off atoms. The bits he broke off were much smaller and lighter than atoms, and carried negative electric charge; they were called electrons. They left behind 'atoms' with a residual positive charge, now known as ions. Thomson's experiments in the 1890s showed that although atoms of different elements are different from each other, they all contain electrons, and that the electrons broken off from any atom are the same as the electrons broken off from any other atom.

While physicists were still coming to terms with the idea that bits could be chipped off from the 'indivisible' atoms, the discovery of radioactivity was both giving them a new tool with which to probe the structure of atoms themselves and (although it was not realized at first) demonstrating that particles much larger than electrons could break off from atoms. At the beginning of the twentieth century, the New Zealander Ernest Rutherford, working at McGill University in Montreal with Frederick Soddy, showed that radioactivity involves the transformation of atoms of one element into atoms of another element. In the process, the atoms emit one or both of two types of radiation, named (by Rutherford) alpha and beta rays. Beta rays, it turned out, were simply fast-moving electrons. The alpha 'rays' also turned out to be fast-moving particles, but much more massive—particles each with a mass about four times that of an atom of hydrogen (the lightest element), and carrying two units of positive charge. They were, in fact, identical (apart from the speed with which they moved) to atoms of helium (the second lightest element) from which two electrons had been removed—helium ions. And their combination of relatively large mass (compared with an electron) and high speed gave Rutherford the tool he needed to probe the structure of atoms.

Soon Rutherford (by now working at the University of Manchester in England) and his colleagues were using alpha particles, produced by naturally radioactive atoms, as tiny bullets with which to shoot at the atoms in a crystal, or in a thin foil of metal. They found that most often alpha particles went right through a thin metal foil target, but that occasionally a particle would be bounced back almost the way it came. Rutherford came up with an explanation of this behaviour in 1911, and gave us the basic model of the atom that we learn about in school today.

Rutherford realized that most of the material of an atom must be concentrated in a tiny inner core, which he called the nucleus, surrounded by a cloud of electrons. Alpha particles, which come from radioactive atoms, are actually fragments of the atomic nucleus from which they are emitted (and are, in fact, nuclei of helium). When such a particle hits the electron cloud of an atom, it brushes its way through almost unaffected. But electrons carry negative charge, while atoms as a whole are electrically neutral. So the positive charge of an atom must be concentrated, like its mass, in the nucleus. Alpha particles too are positively charged. And when an alpha particle hits an atomic nucleus head on, the repulsion between like electric charges halts it in its tracks and then pushes it back from where it came.

Later experiments confirmed the broad accuracy of Rutherford's picture of the atom. Most of the mass and all of the positive charge is concentrated in a nucleus about one hundred thousandth of the size of the atom. The rest of the space is occupied by a tenuous cloud of very light electrons that carry negative charge. In round numbers, a nucleus is about 10⁻¹³ cm across, ¹ while an atom is about 10⁻⁸ cm across. Very roughly, the proportion is like a grain of sand at the centre of Carnegie Hall. The empty hall is the 'atom'; the grain of sand is the 'nucleus'.

The particle that carries the positive charge in the nucleus is called the proton. It has a charge exactly the same as the charge on the electron, but with opposite sign. Each proton is about 2,000 times as massive as each electron. In the simplest version of Rutherford's model of the atom, there was nothing but electrons and protons, in equal numbers but with the protons confined to the nucleus, in spite of them all having the same charge, which ought to make them repel one another. (Like charges behave in the same way as like magnetic poles do in this respect.) As we shall see, there must therefore be another force, which only operates at very short ranges, that overcomes the electric force and glues the nucleus together. But over the twenty years following Rutherford's proposal of this model of the atom, a suspicion grew up among physicists that there ought to be another particle—a counterpart of the proton with much the same mass but electrically neutral. Among other things, the presence of such particles in the nucleus would provide something for the positively charged protons to hold on to without being electrically repulsed. And the presence of neutrons, as they were soon called, could explain why some atoms could have identical chemical properties to one another but slightly different mass.

Chemical properties depend on the electron cloud of an atom, the visible 'face' that it shows to other atoms. Atoms with identical chemistry must have identical numbers of electrons, and therefore identical numbers of protons. But they could still have different numbers of neutrons and therefore different masses. Such close atomic cousins are now called isotopes.

The great variety of elements in the world are, we now know, all built on this simple scheme. Hydrogen, with a nucleus consisting of one proton, and with one electron outside it, is the simplest. The most common form of carbon, an atom that is the very basis of living things, including ourselves, has six protons and six neutrons in the nucleus of each atom, with six electrons in a cloud surrounding the nucleus. But there are nuclei which contain many more particles (more nucleons) than this. Iron has 26 protons in its nucleus and, in the most common isotope, 30 neutrons, making 56 nucleons in all, while uranium is one of the most massive naturally occurring elements, with 92 protons and no less than 143 neutrons in each nucleus of uranium-235, the radioactive isotope which is used as a source of nuclear energy.

Energy can be obtained from the fission of very heavy nuclei because the most stable state an atomic nucleus could possibly be in, with the least energy, is iron-56. In terms of energy, iron-56 lies at the bottom of a valley, with lighter nuclei, including those of oxygen, carbon, helium and hydrogen, up one side and heavier nuclei, including cobalt, nickel, uranium and plutonium, up the other side. Just as it is easier to kick a ball lying on the valley's sloping side down into the bottom of the valley than to kick it higher up the slope, so if heavy nuclei can be persuaded to split, they can, under the right circumstances, form more stable nuclei 'lower down the slope', with energy being released. Equally, if light nuclei can be persuaded to fuse together, then they too form a more stable configuration with energy being released. The fission process is what powers an atomic bomb. The fusion process is what provides the energy from a hydrogen (or fusion) bomb, or of a star, like the Sun; in both cases hydrogen nuclei are converted into helium nuclei. But all that still lay in the future in the 1920s. Although there was circumstantial evidence for the existence of neutrons in that decade, it was only in 1932 that James Chadwick, a former student of Rutherford who was working at the Cavendish Laboratory (where Rutherford was by then the Director), carried out experiments which proved that neutrons really existed.

So the picture which most educated people have of atoms as being made up of three basic types of particles—protons, neutrons and electrons—really only dates back just over sixty-five years, less than a human lifetime. In that lifetime, things first got a lot more complicated for the particle physicists, and then began to get simple again. Those complications, and the search for a simplifying principle to bring order to the particle world, are what this book is all about. Many physicists now believe that they are on the verge of explaining the way all the particles and forces of nature work within one set of equations—a 'theory of everything' involving a phenomenon known as supersymmetry, or SUSY. The story of the search for SUSY begins with the realization, early in the twentieth century, that subatomic particles such as electrons do not obey the laws of physics which apply, as Isaac Newton discovered three centuries ago, to the world of objects such as billiard balls, apples, and the Moon. Instead, they obey the laws of the world of quantum physics, where particles blur into waves, nothing is certain, and probability rules.

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Chapter One

Quantum Physics for Beginners

Before 1900, physicists thought of the material world as being composed of little, hard objects—atoms and molecules which interacted with one another to produce the variety of materials, living and non-living, that we see around us. They also had a very good theory of how light propagated, in the form of an electromagnetic wave, in many ways analogous to the ripples on a pond or to the sound waves which carry information in the form of vibrations in the air. Gravity was a little more mysterious. But, by and large, the division of the world into particles and waves seemed clearcut, and physics seemed to be on the threshold of dotting all the i's and crossing all the t's. In short, the end of theoretical physics and the solution of all the great puzzles seemed to be in sight.

Scarcely had physicists started to acknowledge this cosy possibility, however, than the house of cards they had so painstakingly constructed came tumbling down. It turned out that the behaviour of light could sometimes only be explained in terms of particles (photons) while the wave explanation, or model, remained the only valid one in other circumstances. A little later, physicists realized that, as if waves that sometimes behave as particles were not enough to worry about, particles could sometimes behave like waves. And meanwhile Albert Einstein was overturning established wisdom about the nature of space, time and gravity with his theories of relativity. When the dust began to settle at the end of the 1920s, physicists had a new picture of the world which was very different from the old one. This is still the basis of the picture we have today. It tells us that there are no pure particles or waves, but only, at the fundamental level, things best described as a mixture of wave and particle, occasionally referred to as 'wavicles'. It tells us that it is impossible to predict with absolute certainty the outcome of any atomic experiment, or indeed of any event in the Universe, and that our world is governed by probabilities. And it tells us that it is impossible to know simultaneously both the exact position of an object and its exact momentum (where it is going).

How and why physicists came to these startling conclusions I have described at length in my book In Search of Schrödinger's Cat. Here I intend only to present an outline of the new world picture, without going into the historical and experimental details on which it is founded. But that foundation is secure; quantum physics is as solidly based, and as thoroughly established by experiments and observations, as Einstein's General Theory of Relativity. Together they provide the best description we have of the Universe and everything in it, and there is real hope that the two pillars of twentieth-century physics may yet be combined in one unified theory.

Photons

The best place to pick up the story of quantum physics and the search for unification is with the work of the great Scottish physicist James Clerk Maxwell, in the third quarter of the nineteenth century. Maxwell, who was born in Edinburgh in 1831, made many contributions to physics, but his greatest work was undoubtedly his theory of electromagnetism. Like many of his contemporaries and immediate predecessors, Maxwell was fascinated by the fact that an electric current flowing in a wire produces a magnetic field, which in its fundamentals is exactly the same as the magnetic field of a magnet itself. The field around a wire carrying a current will, for example, deflect a small compass magnet placed nearby. But also, a moving magnet, passing by a wire, will cause a current to flow in the wire. Moving electricity, a current, produces magnetism, and moving magnets produce electric currents. Electric forces and magnetic forces, which had once seemed to be quite separate phenomena, now seemed to be different facets of some greater whole, the electromagnetic field.

Maxwell tried to write down a set of equations that would link together all of the electric and magnetic phenomena that physicists had observed and measured. There were four equations: one to describe the magnetic field produced by an electric current, a second to describe the electric field produced by a changing magnetic field, the third giving the electric field produced by an electric charge itself, and the fourth giving a description of the magnetic field itself, including the strange fact that magnetic poles always come in pairs (north matched with south). But when Maxwell examined the equations, he found that they were flawed mathematically. In order to correct the maths, he had to introduce another term into the first equation, a term equivalent to a description of how a magnetic field could be produced by a changing electric field without any current flowing.

At that time, nobody had observed such a phenomenon. But once Maxwell had reconstructed the equations in the most elegant mathematical form, the reason for this extra term soon became clear. Physicists knew about condensers (now called capacitors), which are flat metal plates separated by a short gap across which an electric potential difference can be applied. One plate may be connected to the positive pole of a battery and the other plate to the negative pole. In this case, one plate builds up a charge of positive electricity, and the other a negative charge. The gap in between the plates is a region with a strong electric field, but no current flows across the gap and there is no magnetic field. Maxwell's new mathematical term described, among other things, what happens between the plates of such a capacitor just as the battery is connected to the plates. While the electric charge on the plates is building up, there is a rapidly changing electric field in the gap between the plates, and according to the equations this produces a magnetic field. Maxwell was soon able to confirm that the equations were correct, simply by placing a little compass magnet in the gap between two metal plates, and watching how it was deflected when the plates were connected to a battery. Like all the best scientific theories, the new theory of electromagnetism had successfully predicted how an experiment would turn out.

But now came the really dramatic discovery. Maxwell realized that if the changing electric field could produce a changing magnetic field, and the changing magnetic field could produce a changing electric field, the two components of the single, unified electromagnetic field could get along quite nicely together without any need for electric currents or magnets at all. The equations said that a self-reinforcing electromagnetic field, with the electricity producing the magnetism and the magnetism producing the electricity, could set off quite happily through space on its own, once it was given a push to start it going. The continually changing electromagnetic field predicted by the equations was in the form of a wave moving at a certain speed—300,000 km/sec. This is exactly the speed of light. Maxwell's equations of electromagnetism had predicted the existence of electromagnetic waves moving at the speed of light, and it didn't take Maxwell long to realize that light must indeed be an electromagnetic wave.

Figure 1.1 A wave is defined by its amplitude and its wavelength.

There was already a well-established body of evidence that light was a form of wave motion, so Maxwell's discovery fitted right in to the mainstream of nineteenth century science, and was welcomed with open arms. The best evidence for the wave nature of light comes from the way it can be made to 'interfere' with itself, like the interference between two sets of ripples on a pond, producing patterns of shade and light that cannot be explained in any other way.

Thomas Young, a British physicist and physician who was born in Somerset in 1773, produced the crucial experimental evidence in the early 1800s, when he shone a beam of pure light of one colour (monochromatic light) through a pair of narrow slits in a screen, to produce two sets of 'ripples' and make a classic interference pattern on a second screen. This work effectively pulled the rug from under the old idea, going back to Newton, that light came in the form of tiny particles, or corpuscles.

The combination of Maxwell's and Young's work provided what seemed to be a thorough understanding of light. Interference experiments made it possible to measure the wavelength of light, the distance from the crest of one wave to the next crest, which turns out to be about one ten-millionth (10⁻⁷