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

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

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by John Gribbin

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No one is more successful than this author when it comes to making the cutting edge of physics more accessible to a broad lay audience. In Schrodinger's Kittens, he took readers to the eerie world of subatomic particles&waves. Now, he explores the most exciting area of research in physics today: string theory. Following a series of major breakthroughs in the 1990s


No one is more successful than this author when it comes to making the cutting edge of physics more accessible to a broad lay audience. In Schrodinger's Kittens, he took readers to the eerie world of subatomic particles&waves. Now, he explores the most exciting area of research in physics today: string theory. Following a series of major breakthroughs in the 1990s, physicists are putting together a clearer picture of how subatomic particles work. By hypothesizing particles as a single loop of vibrating "string," they are on the brink of discovering a way to explain all of nature's forces in a single theory. Grandly named "superstrings,"&incorporating the ideas of "supersymmetry," these models are the prime candidate for the long sought-for "Theory of Everything." Written in clear&accessible language. The Search for Superstrings, Symmetry,&the Theory of Everything brings to life the remarkable scientific research that is on the cusp of radically altering our conception of the universe.

Editorial Reviews

Library Journal
Writing in his clear prose style, prolific science writer Gribbin (Q Is for Quantum, LJ 1/99) 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, a topic he also covered in In Search of the Big Bang (1986). For popular physics collections. [Previewed in Prepub Alert, LJ 9/15/98.]--Nestor Osorio, Univ. Libs, Northern Illinois Univ., DeKalb
Kirkus Reviews
Physics changes so rapidly that a new survey of its landmarks is necessary every few years; here's an update from a popular British science writer. The focus in this book (a spinoff from the author's earlier The Search for Black Holes) is the nature of matter. Our modern conception of matter dates from the realization roughly a century ago that the atom, in theory the tiniest particle of matter, was itself a compound entity. The discovery of radioactivity opened up a window to peer inside atoms, revealing some of the structural units (electrons, the nucleus) of which these "indivisible" atoms are made. Ernest Rutherford's "classical" picture of the atom as a small but solid nucleus surrounded by a whirling cloud of electrons, had taken shape by the 1920s. But this easily understood model was rapidly modified, as quantum theory began to blur the intuitive divisions between matter and energy. The number of subatomic particles also underwent a population explosion, leading to quantum chromodynamics, in which the ultimate divisions of matter are invisible quarks and gluons, from which other particles are built. But even this "standard model" fails to unite the various forces and particles into a coherent structure; thus the search for a Grand Unified Theory, the holy grail of modern physics. This takes us into the rarefied territory of supersymmetry, superstrings, and gauge theory in a ten-dimensional matrix. Gribbin gives the reader a good overview of the progress to date of this research, describing its key experiments and noting the contributions of various scientists, and making the theory itself as clear as possible for readers not prepared to tackle serious math. Perhaps in tenyears' time the key questions will be answered-always assuming that some new discovery doesn't send everyone back to the starting line again. A clear and comprehensive popular treatment of the cutting edge of physics. .

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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


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.


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