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The Search for Superstrings, Symmetry, and the Theory of Everything
By John R. Gribbin
Back Bay Books Copyright © 2000 John R. Gribbin
All right reserved. ISBN: 0316326143
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 1 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.
Excerpted from The Search for Superstrings, Symmetry, and the Theory of Everything by John R. Gribbin Copyright © 2000 by John R. Gribbin.
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