Physicists Murray Gell-Mann and George Zweig independently discovered that the disturbingly large number of so-called elementary atomic particles could be effectively organized by assuming that they were composed of a smaller group of particles that came to be known as quarks.
Quarks were postulated in the early 1960’s in an attempt to simplify the field of elementary particle physics by identifying the smallest, and thus truly elementary, building blocks of matter. At the end of the nineteenth century, the most elementary particles were thought to be atoms, out of which the macroscopic world of molecules was composed. During the early decades of the twentieth century, physicists discovered that the atom was not truly elementary but rather was composed of even smaller particles, which became known as protons, electrons, and neutrons. These subatomic particles were believed to be truly elementary and tremendous simplification resulted from the recognition that all the elements on the periodic table were constructed from combinations of these three “elementary” particles.
Further developments in physics, however, especially the study of cosmic rays and collisions in particle accelerators, began to reveal many more supposedly elementary particles, and the number of such particles climbed from the familiar three in 1930 to more than one hundred by the 1980’s. This large number was disconcerting to the physics community and became known as the “particle zoo.” Physicists believed that all of these particles could not be elementary.
Quarks were postulated in 1964 as a possible way to restore the lost simplicity to the burgeoning particle zoo. It was suggested that many of the supposedly elementary particles were not elementary at all but were made up of even simpler units which came to be known as quarks. Perhaps the quarks could do for the particle zoo what the proton, electron, and neutron had done for the periodic table, the atomic zoo.
As is often the case in science, this great breakthrough was accomplished simultaneously by two scientists working independently: Murray Gell-Mann of the California Institute of Technology and George Zweig of CERN, a famous center for European nuclear physics located in Zurich, Switzerland. Gell-Mann was led to the idea of quarks, a word that he got from James Joyce’s Finnegans Wake (1939), by his analysis of mathematical and symmetrical relationships among some of the apparent groupings of the members of the particle zoo.
Gell-Mann, together with Yuval Ne’eman, had previously developed a way of organizing many of these particles into groups using a scheme called the “eightfold way.”
The “eightfold way” had been demonstrated to work effectively in simplifying the particle zoo. It was widely accepted, having been vindicated by its prediction of new particles needed to “fill up” some of the groups, particles which were subsequently discovered. One way to explain the remarkable, but mysterious, success of the “eightfold way” was to postulate that there was a trio of particles underlying the whole system, out of which many of these groups were constructed. When this was suggested to Gell-Mann in 1963 by Robert Serber, a theoretical physicist from Columbia University, he began to work out the details of what is now the widely accepted theory of quarks.
While Gell-Mann was developing the quark theory by analyzing the deep mathematical symmetries among the elementary particles, Zweig was led to the same ideas while trying to explain an experimental result. Zweig noticed that when a certain “elementary” particle known as the pi-meson disintegrated into other particles, it did not choose the most straightforward path to disintegration. (Normally, when such particles disintegrated, much more of the energy of the original particle showed up as kinetic energy, or energy of motion, of the new particles. In other words, the new particles, into which the pi-meson had disintegrated, should have been moving away from the disintegration location faster than they did.)
To explain this unexpected path to disintegration, Zweig suggested that the pi-meson was composed of two constituents, whose individual properties (known as the “strangeness”) were transmitted separately to the decay components. To make this scheme work theoretically, Zweig found it necessary to postulate that many of the particles were constructed from an underlying triplet of particles which he called “aces.” It was determined later that Zweig’s aces were the same as Gell-Mann’s quarks, and the idea began to spread among the particle physics community, with “quarks” becoming the accepted term for the new fundamental triplet.
Initially, there were three quarks and three associated antiquarks, the former of which Gell-Mann labeled u, d, and s, for “up,” “down,” and “strange.” They had fractional electric charges: u had +⅔, d had -⅓, and s had -⅓. A proton was composed of two up quarks and a down quark, a combination that gives the known electrical charge of +1 to the proton. A neutron was composed of one up and two down quarks, which yield zero total charge.
The nature of the individual quarks, however, was very controversial. Gell-Mann believed that they might be purely mathematical entities that would never be observed in the same sense that particles, like electrons, are observed by the trail of bubbles that they leave in the specially designed chambers used to chart their paths. Zweig, on the other hand, believed that the quarks should be physically observable. Quarks also had fractional charges, which flew in the face of a half century of particle physics that dealt only with integrally charged particles.
To resolve this dilemma, experimentalists began searching for individual quarks. They searched in accelerators, in cosmic rays, in chunks of normal matter, even in oysters, but to no avail. Quarks were nowhere to be found, suggesting that Gell-Mann had been correct. In 1968, however, experiments at the new Stanford Linear Accelerator Center (SLAC) showed that electrons bouncing off protons were recoiling in a way that suggested they were hitting something hard and small inside the proton. Further experimental evidence accumulated to the point where the quark hypothesis became widely accepted. Subsequent theoretical developments showed the quark theory to have remarkable explanatory power; additional quarks were postulated and then verified, and new properties of the existing quarks were uncovered. Nevertheless, no quarks had been observed directly, and no fractionally charged particles had been detected, despite much effort. Physicists began to develop a theory to explain this curious phenomenon called “quark confinement.”
Quark confinement grew out of the theory of quantum chromodynamics
It is unlikely that the theory of quarks will ever have any practical value, in the sense that a “product” will someday be based on the idea. The significance of the quark idea lies in the central role that it plays in the theories developed by the physics community to explain the various possible interactions in nature.
One of the deepest mysteries in the physics of the twentieth century has been the nature of the nuclear force that holds the positively charged protons so tightly packed in the nucleus. Since the protons are all positively charged, they experience a powerful repulsive force that should force them apart. Yet, there is some other force that holds them together, a “strong” force as it became known. The source of the strong force was a mystery until the development of the quark theory.
The quantum chromodynamics theory states that the quarks possess a mysterious property, whimsically named “color,” that is analogous to the familiar charge that is the source of the electromagnetic force. Particles with color attract one another by exchanging a force carrier called the gluon, which is analogous to the familiar photon, the carrier of the electromagnetic force. QCD explains the strong force as simply the exchange of gluons between quarks in different protons. So, even though the electrical force might try to separate the protons, the stronger color force holds them together.
In addition to providing the long-sought explanation for the strong force, the quark theory provided a foundation for much of elementary particle physics. By showing that many of the esoteric particles that had been showing up in the various experiments were only combinations of quarks, scientists were able to develop much simpler explanations for the observed phenomena. As the explanation of the “eightfold way,” the quark theory showed that the apparent groupings within the particle zoo were based on deep underlying symmetries among the particles in each family and not on coincidental similarities. Without the quark theory, particle physics would have no strong theoretical foundation.
Perhaps the most significant accomplishment of the quark theory will be its role in the development of a unified field theory. This is the name of the theory that will, hopefully, unify all the possible interactions in nature under a single theoretical umbrella. Some unity has been achieved already via the various (and competing) grand unified theories (GUTs) by showing that the electromagnetic, weak, and strong forces are similar in that they all have a “force carrier” called, respectively, the photon, the intermediate bosons, and the gluon. Physicists are searching for the graviton, the postulated carrier of the gravitational force.
Developments uniting cosmology with particle physics show that these forces may have all emerged from a single force during the first few moments of the big bang. If gravity can be merged with one of the GUTs, then scientists will have shown how all the forces are merely four different manifestations of a single original force. The quark theory is an indispensable part of this grand search.
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citation-type="booksimple" xlink:type="simple">Carrigan, Richard A., and W. Peter Trower, eds. Particle Physics in the Cosmos. New York: W. H. Freeman, 1989. This book is a collection of articles reprinted from Scientific American, some of which discuss the important role that quarks play in theories of cosmology. In particular, “A Unified Theory of Elementary Particles and Forces” by Howard Georgi (from Scientific American, April, 1981) and “Gauge Theories of the Forces Between Elementary Particles” by Gerard’t Hooft (from Scientific American, June, 1980) are excellent. -
citation-type="booksimple" xlink:type="simple">_______. Particles and Forces: At the Heart of the Matter. New York: W. H. Freeman, 1990. This book is a collection of articles reprinted from Scientific American, many of which are excellent introductions to quarks. In particular, “Elementary Particles and Forces” by Chris Quigg (from Scientific American, April, 1985) and “Quarks with Color and Flavor” by Sheldon Glashow (from Scientific American, October, 1975) are superb. There are several other relevant articles as well. -
citation-type="booksimple" xlink:type="simple">Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth Century Physics. New York: Macmillan, 1986. This book is an extraordinary, massive presentation of particle physics in the twentieth century. The authors interviewed many of the scientists involved and have produced a colorful introduction to the theory that is accessible to the nonscientist. Highly recommended. -
citation-type="booksimple" xlink:type="simple">Glashow, Sheldon, with Ben Bova. Interactions: A Journey Through the Mind of a Particle Physicist and the Matter of This World. New York: Warner Books, 1988. This highly personal account of particle physics is written by a Harvard professor and Nobel laureate. Glashow was instrumental in developing some of the later ideas that led to the present version of the quark theory. The book is humorous, enjoyable to read, and presents the theoretical developments from the perspective of one of its most important contributors. -
citation-type="booksimple" xlink:type="simple">Ne՚eman, Yuval, and Yoram Kirsh. The Particle Hunters. New York: Cambridge University Press, 1986. At a slightly more technical level, this book presents another “insider’s” view of the discovery of quarks. Very well written, with many figures and diagrams. -
citation-type="booksimple" xlink:type="simple">Riordan, Michael. The Hunting of the Quark: A True Story of Modern Physics. New York: Simon & Schuster, 1987. This book is written by a physicist who was involved in many of the experiments that helped establish the quark idea. He knows all the major scientists personally and has provided an interesting “insider’s” view. -
citation-type="booksimple" xlink:type="simple">Smith, Timothy Paul. Hidden Worlds: Hunting for Quarks in Ordinary Matter. Princeton, N.J.: Princeton University Press, 2003. Introductory overview of quark theory and its origins. Explains the case for quarks from the perspectives both of historical research and of logical analysis of the field of subatomic physics. Index.
Physicists Develop the First Synchrocyclotron
Lamb and Retherford Discover the Lamb Shift
Hofstadter Discovers That Protons and Neutrons Have Structure
Esaki Demonstrates Electron Tunneling in Semiconductors
Friedman, Kendall, and Taylor Discover Quarks