Particle physicists Jerome I. Friedman, Henry W. Kendall, and Richard E. Taylor discovered quarks, the subatomic building blocks of protons and neutrons, confirming the theory of Murray Gell-Mann that they must exist.
From some of the earliest recorded documents in ancient science, the early philosophers believed that all matter was made up of microscopically small particles containing yet smaller particles. The Greeks postulated that when the particles are finely subdivided, the smallest particles cannot be divided further. They are called atomos (meaning indivisible), the word from which “atom” is derived.
This thinking was little more than philosophical musing until modern times, since there was no way such a theory could be supported by evidence. In 1808, John Dalton
This version of the atom—a particle consisting of three kinds of subparticles—held for decades. Many were convinced that there was no evidence for any other kind of particles smaller than these. This idea was buttressed, in part, by the science of quantum physics, which demonstrated that at the subatomic level, the tiny electron, proton, and neutron could be either wavelike or particle-like, depending on the state and quality of observation. Albert Einstein’s theory of relativity stated further that mass was equivalent to energy and, under certain conditions, could be converted to energy in a proportion described by the equation E = mc2.
The subatomic structure was unique. Subatomic particle positions could not be nailed down; they could only be predicted statistically with a certain degree of uncertainty. The interchangeability with energy states and their adherence to quantum and relativistic laws made them difficult to study. The first particle to be found inside the atomic nucleus was called a pion, which is the binding force between the proton and neutron and was discovered via cosmic-ray tracings. Later, other particles would be discovered using particle accelerators.
By the end of World War II, huge devices called particle accelerators were perfected, which were used to accelerate subatomic particles toward atoms at great speeds. When the particles collided with an atom, the atom would break apart and create subparticles, which could be recorded and analyzed. It was determined that there were literally hundreds of particles. By the early 1950’s, theoretical physicists were trying to sort out the newly discovered particles. Classification schemes were set up to categorize all the particles, which were then classified in families.
Not only did such bombardment yield subparticles and families, but also a careful analysis indicated what the subatomic world looked like. Accelerator work by Stanford physicist Robert Hofstadter
In 1964, American physicist Murray Gell-Mann had examined the variety and classifications of subatomic forces and particles and had devised a classification scheme for the particles. In his scheme, Gell-Mann postulated that the proton and neutron were actually composed of smaller, discrete particles he called “quarks.” He theorized later that quarks were held together by “gluons.” Gell-Mann had actually carried out a classifications system for the array of subatomic particles and forces that had been harvested from the accelerators. Gell-Mann had no direct evidence of an actual physical entity he was calling a quark, but his classification scheme was highly accurate with respect to the information and mathematical models relating to the enigmatic subnuclear world that was nearly impossible to describe objectively.
One of the first experiments planned at SLAC was one in which electrons would be fired at protons at very high speeds—close to the speed of light. The purpose of this experiment was to see at what angle the electron would scatter off the proton. Although the proposal sounded easy enough, since the electrons would be traveling at speeds close to light speed, the problem became one involving relativistic physics. SLAC physicist James Bjorken began to analyze the complex mathematics. All the previous work had been accomplished using accelerators with much less energy and slower speeds so that conventional physics could be used to determine the results. With the power of the linear accelerator, the problem required relativistic solutions for the first time.
The first set of experiments at SLAC began and ended with predictable results—so predictable, in fact, that the initial group of experimenters departed Stanford. Experimental physicists Jerome I. Friedman, Henry W. Kendall of the Massachusetts Institute of Technology (MIT), and SLAC’s Richard E. Taylor were left behind to continue the experiments. The experiments had been predictable because the group was examining two kinds of collisions between the proton and the speeding electron. They knew well beforehand that from the impact there could be only an elastic collision or an inelastic one. The elastic collision meant that the electron would strike the proton and literally bounce off at some angle. The inelastic collision meant that the electron would collide with the proton dead center with such energy that it would break the proton into subparticles.
The early experiments were looking for the angle of the elastic collisions, because examining these angles would predict the same shapes Hofstadter had examined. It was the team’s hope that since the energy of SLAC was so much higher than what Hofstadter had used, a finer resolution could be obtained. Unfortunately, the results were roughly the same as Hofstadter’s.
Bjorken, undaunted, confided in Friedman, Kendall, and Taylor that the messy and extraordinarily complex inelastic scattering results could be interpreted in such a way as to suggest resolution inside the proton. According to Bjorken, if there were hard substructures located inside the proton, they would be deflected after the inelastic scattering took place by an unmistakable signature. In 1968, the team, armed with Bjorken’s tools, proceeded to probe the proton. Using this method in an approach remarkably similar to the approach used by Rutherford in 1910, the proton was found to consist of smaller subparticles, Gell-Mann’s theoretical quarks, and to be held together by gluons.
The Royal Swedish Academy of Sciences compared the Friedman, Kendall, and Taylor discovery to the discovery of the nucleus by Rutherford both in method and in importance. It was the finest positive resolution on the smallest particle of matter that may ever be available. Said one American physicist, their work is “. . . one of the pivotal contributions to physics in this century.”
Gell-Mann’s postulation of quarks was completely theoretical—an attempt to make some logical order out of the avalanche of tiny particles that emanate from every subatomic collision. It was not at all certain that anyone would ever refine the resolution of the nucleus beyond the work done by Hofstadter on protons and neutrons. Without the actual physical confirmation by Friedman, Kendall, and Taylor, Gell-Mann’s theory would have remained merely a theory, and the ability of experimental physics to confirm its findings would have diminished considerably.
The importance of Bjorken’s contribution cannot be underestimated: His suggestion to evaluate a proton’s shape mathematically by working through the signatures of the proton’s breakup was remarkable. It was essentially a back-door solution of the inelastic scattering that all other experimenters had ignored as either too difficult to analyze or completely impossible to explain. Such creative insight, based on observed results, is the essence of experimental science. The team could have given up easily were it not for Bjorken’s extraordinary insight and confidence.
The 3.2-kilometer-long, powerful SLAC tool was used to delve into the very center of an atom and subdivide an object (the proton) no bigger than 10-13 centimeters across and resolve what was found to be inside. Such an accomplishment is unsurpassed in theoretical physics. The SLAC is not as powerful as those that have been built later and is of a unique design. Yet, SLAC was used successfully as an important theoretical tool to discover one of the most significant aspects of the atomic structure.
For centuries, humankind has sought the smallest particles of nature. The philosophers of ancient Greece accurately surmised that matter could be divided only to a point. It appears likely that the smallest size has been discovered. It will probably be the final assault on the atom. It is believed by many that the quark may be the smallest distinguishable subatomic particle. If this is so, then it will be Friedman, Kendall, and Taylor who discovered the true atomos.
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Crease, Robert P., and Charles C. Mann. The Second Creation. New York: Macmillan, 1986. Crease and Mann follow the making of twentieth century physics from its nineteenth century roots to the most enigmatic mysteries of the late 1980’s. Their book examines microscopically characters and personalities as well as the issues of physics. Friedman, Kendall, and Taylor’s approach is discussed, as is SLAC and its historic first experiments, in detail. -
Hawking, Stephen W. A Brief History of Time. New York: Bantam Books, 1988. One of the most prominent physicists of the twentieth century examines the universe from his view of creation to the late 1980’s. Hawking examines 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, including quarks. For a wide audience. Illustrated. -
Mackintosh, Ray, et al. Nucleus: A Trip into the Heart of Matter. Baltimore: Johns Hopkins University Press, 2001. Short but comprehensive popular work explaining both nuclear physics and its history for the engaged layperson. Bibliographic references and index. -
Pagels, Heinz R. The Cosmic Code. New York: Simon & Schuster, 1982. This book 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 to the general reader. Pagels succeeds and opens up the interior of the atom for a clear view of what is inside. Illustrated. Nontechnical. -
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. -
Sutton, Christine. The Particle Connection. New York: Simon & Schuster, 1984. Christine Sutton, former physicist turned reporter, discusses the particle accelerator. Describes how the machine is used, the nature of the particle chase at CERN—the European particle accelerator laboratory—as well as work at SLAC. Illustrated. Suitable for a student with a reasonable background in science. -
Waldrop, M. Mitchell. “Physics Nobel Honors the Discovery of Quarks.” Science 250 (October 26, 1990): 508-509. This article describes how the discovery of quarks was accomplished. Evaluates the significance of the discovery with specific reference to the work of Gell-Mann and Rutherford. Details the input of Bjorken and the interface of the MIT team with SLAC. Illustrated.
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
Gell-Mann and Zweig Advance Quark Theory