Gell-Mann Formulates the Theory of Quantum Chromodynamics

Date September, 1972

Physicist Murray Gell-Mann developed the theory of quantum chromodynamics to describe the characteristics of elementary atomic particles called quarks.

Locale Pasadena, California

Key Figures

• Murray Gell-Mann (1929–2019), American physicist
• Harald Fritzsch (b. 1943), German physicist
• William Bardeen (b. 1941), American physicist

Summary of Event

From antiquity, it was speculated by many philosophers and scientists that everything is made up of successively smaller particles, but that beyond a certain size, the particles become no smaller. These fundamental particles, of which everything was made, the Greeks called atomos, meaning indivisible. The English version of atomos became "atom." That idea held for thousands of years, moving from mere philosophical speculation to confirmed scientific detail by the beginning of the nineteenth century.

In the seventeenth century, Robert Boyle was instrumental in bridging the gap between the Greek philosophy and actual scientific manipulation of elementary atomic theory. John Dalton expanded on the idea and forged atomic theory into a full-blown experimental science based on elemental activity. It was not until the beginning of the twentieth century that it was discovered that atoms were not, in fact, indivisible, but actually made up of even smaller parts.

In 1904, the first suggestion was made that the atom incorporated tiny subparticles called electrons, which orbited a central core. In 1910, Ernest Rutherford, an English physicist, discovered the atom's core, which is called the nucleus. Three years later, one of Rutherford's students, Niels Bohr, a Danish physicist, qualified the nature of the electron's orbit around the nucleus. By 1927, the problem of determining the atom's structure had been largely solved. A new science called quantum mechanics defined the atom's internal structure as consisting of tiny electrons orbiting a nucleus that contained an assortment of relatively heavy protons and neutrons. By 1930, a concentrated effort had been launched by a newly emerging branch of physics—particle physics —to probe the atom's secrets. Much evidence existed that there were smaller particles yet to be discovered within the atom's core.

The first particle accelerator (atom smasher) was put into experimental use in 1932. The purpose of such an instrument is to cause atoms to collide with one another at extremely high speeds and break up into their elementary parts. Physicists then record the particles as they fly off in the collision. It was during a series of these particle accelerator experiments in the early 1960s that a physicist at the California Institute of Technology, Murray Gell-Mann, developed a series of brilliant postulations regarding the results of these particle accelerator experiments.

By late 1963, Gell-Mann had enough evidence to publish his theory that the nucleus of protons and neutrons was made up of even smaller particles. In reference to a passage in James Joyce's book Finnegans Wake (1939), Gell-Mann called these small pieces of protons and neutrons "quarks." He said he chose this name as "a gag . . . a reaction against pretentious scientific language." He published the first discussion of quarks in February, 1964. In 1969, he was awarded the Nobel Prize in Physics for his subatomic classification schemes.

Gell-Mann postulated six different kinds, or "flavors," of quarks (up, down, bottom, top, strange, and charm), each of which comes in three "colors" (red, green, or blue). The assignment of "colors" to quarks gave rise to a whole new branch of quantum physics—quantum chromodynamics (QCD).

There are no actual flavors in quantum mechanics (much less flavors defined as up, down, and so on), and likewise, at the subatomic level, there are no actual colors. These terms—"flavors" and "colors"—define the specific quantum characteristic of the elementary particle. Through classification and subclassification in the quantum chromodynamic nomenclature, the particles can be classed according to their characteristics and behavior.

Gell-Mann and his colleagues Harald Fritzsch and William Bardeen united the color concepts with the other quark ideas into a single formulation that united all the aspects of nuclear particles. Gell-Mann presented this QCD theory in September, 1972. In the theory of QCD, the multicolored quarks are held together by a binding force called a gluon. This binding force is not only critical to any discussion of QCD but also fundamental to all of nature; it still drives the community of particle physics. Gluons make up what the "strong force," one of the four forces of nature. These four forces are the gravitational force, which works over astronomical distances and is comparatively very weak; the electromagnetic force, which exists between atoms and molecules; the weak force, which is responsible for all radioactivity; and the strong force, which exists in the form of gluons that tie quarks together.

Significance

QCD clarified a mixture of perplexing observations that had been compiled from numerous accelerator experiments. It enabled a clear understanding of some previously undefined observations. Furthermore, it so completely described the workings within the atomic nucleus that physicists were able to predict certain events before experiments were conducted—the ultimate validation of any theory.

QCD involves exceptionally difficult mathematics that strings together probabilistic mathematical events in a bewildering fashion. Because of this degree of difficulty, it becomes an intricate and enigmatic task to relate the data streaming in from particle accelerators to the field theory itself. Supercomputers have been employed to handle such processing, and all the final possible results from QCD have yet to be compiled.

Quantum chromodynamics is a scientific achievement that stands as a benchmark hypothesis on the landscape of physical theory. It fulfills the long-term dream of physicists of formulating a complete theory of the strong nuclear force (one of the four fundamental forces of nature) and the way in which it interacts with elementary particles at the atomic core. The final goal is to unify all four field theories of the forces of nature into a single grand unified theory of nature.

Bibliography

Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick, N.J.: Rutgers University Press, 1996. Excellent work follows the development of twentieth century physics from its nineteenth century roots to the most enigmatic mysteries of the late 1980's. Examines characters and personalities as well as the issues of physics. Discusses in detail Gell-Mann's approach to QCD and how it fits into the other experimental questions of its time.

Hawking, Stephen W. A Brief History of Time. Rev. ed. New York: Bantam Books, 1998. Prominent physicist examines the universe, from his view of creation to the late twentieth century. Discusses the far-flung reaches of space and time, from black holes to the interior of the atom and discusses the elementary particles of the atomic nucleus. Written for a wide audience; illustrated.

Johnson, George. Strange Beauty: Murray Gell-Mann and the Revolution in Twentieth-Century Physics. New York: Alfred A. Knopf, 1999. Colloquial biography of the Nobel Prize-winning scientist.

Pagels, Heinz R. The Cosmic Code. New York: Simon & Schuster, 1982. Describes quantum physics as "the language of nature." Pagels, a physicist, embarks on a literary quest to explain some of the most profoundly difficult topics in quantum physics in a clear manner for the general reader. Illustrated.

Pagels, Heinz R. Perfect Symmetry. New York: Simon & Schuster, 1985. Classic of the genre delves deeply into quantum physics, providing a layperson's perspective. Presents a lively discussion of the grand unified field theories and touches on the frontiers of atomic physics. Very readable.

Sutton, Christine. The Particle Connection. New York: Simon & Schuster, 1984. A physicist turned reporter explains the particle accelerator. Discusses how the machine is used and the nature of the particle chase at CERN, the European particle accelerator laboratory. Written so that readers with a reasonable grounding in science can grasp its message. Illustrated.

Trefil, James S. The Unexpected Vista. New York: Charles Scribner's Sons, 1983. Seeks to explain some of the most exciting concepts of physics to the general reader. Wide-ranging discussion of contemporary ideas presents explanations of concepts from magnets to the ultimate theory of grand unification. For a wide audience; illustrated.

Watson, Andrew. The Quantum Quark. New York: Cambridge University Press, 2004. Complete account of the theory of quantum chromodynamics, accessible to readers without extensive mathematical or science background.