Kendall, Henry

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KENDALL, HENRY

Henry Way Kendall shared the 1990 Nobel Prize in Physics with Jerome Friedman and Richard Taylor for their pioneering studies of the scattering of electrons from protons, studies that produced the first solid evidence that quarks exist and are the basic constituents of neutrons and protons.

Kendall was born on December 9, 1926, in Boston, Massachusetts, the oldest son of one of the wealthiest families in New England. As he wrote in his autobiography, written upon receiving the Nobel Prize, until he went to college, he considered himself a poor student "more interested in nonacademics matters and bored with school work." Encouraged and supported by his father, Kendall devoted most of his time to exploring "things mechanical, chemical and electrical." Near the end of the World War II, he entered the U.S. Merchant Marine Academy, spending a winter on a troop transport in the North Atlantic. Older and more experienced, he was a serious student interested in many disciplines when he enrolled at Amherst College. Kendall majored in mathematics but was so interested in other fields that he could also have majored in English, history, biology, or physics. He spent the summers learning to be an expert underwater diver and photographer. These efforts resulted in two successful books, written with a schoolboy friend, on shallow-water diving and underwater photography, both of which became Kendall's life-long hobbies.

By the end of Kendall's undergraduate years, physics began to dominate his studies. He chose to do his senior thesis in physics, and, with the support of his father who understood that his son wanted a life in science rather than business, he chose to study physics in graduate school. At that time, he also made the decision to become self-supporting, without monetary assistance from his family.

Kendall described the years from 1950 to 1954 that he spent as a graduate student in the physics department of MIT as "a continuing delight—the first sustained immersion in science at a full professional level." His mentor was Martin Deutsch, the discoverer of positronium, the bound state of a positron and an electron, the simplest possible atom. Kendall's attempt to measure the Lamb shift in positronium was unsuccessful, but his interest in electromagnetic interactions continued throughout his professional career.

After a two-year National Science Foundation Postdoctoral Fellowship at MIT, Kendall accepted an appointment at Stanford University in 1956. He joined the group of Robert Hofstadter who was deep

into his Nobel Prize–winning studies of the scattering of electrons from protons. These experiments showed that the proton was not a point but had an extended structure, with no hint, however, that the structure was anything but uniform.

Kendall remained at Stanford for five years. It was there that his fascination with mountain climbing and mountain photography began. Over the years he indulged this passion with climbs all over the world, most notably the Andés and Himalayas. It was also at Stanford that he commenced his lifelong collaboration with Jerome Friedman.

Kendall left Stanford's faculty in 1961 to take an assistant professorship at MIT. In doing so, he again joined Friedman, who had joined the faculty a year earlier. They formed a research group to continue experiments at Stanford, most notably, the construction of the world's most powerful electron accelerator, a 2-mile linear machine that produced intense beams of 20-GeV electrons. This was to be a national facility for use by physicists of every nationality. It was called the Stanford Linear Accelerator Center (SLAC).

The electrons in Hofstadter's experiments had energies of a few hundred MeV. When considered as wave packets, the electrons had wavelengths of about 10^-13 cm, sufficient to determine the size of the proton but insufficient to probe its internal structure. The new SLAC accelerator was expected to probe more than an order of magnitude more deeply.

In concept, the deep-inelastic experiment was simple: use an electron spectrometer to precisely measure the energy and angular distributions of the electrons scattered from a hydrogen target and make deductions from the analysis of the electrons alone. The experiment was fundamental; it needed to be done. But a meaningful deep-inelastic collision scattering result was definitely a long shot. Physicists were not eager to tackle it for two reasons. It was generally accepted that the results would be difficult, perhaps impossible, to interpret, and even if the roadblocks to interpretation could be overcome, the results were likely to be not very interesting since it was widely assumed that there was no structure to be found inside the proton. The MIT group under the leadership of Kendall and Friedman, in collaboration with a group led by Richard Taylor of SLAC, took on the challenge.

The daunting problem in interpreting the results was how to account for the electromagnetic radiation that would inevitably be produced in the scattering of electrons, obscuring the effects due to nuclear structure. The experiments would have to be carried out at unexplored energies. The radiative corrections would be large and increasingly important the deeper the electron probed the proton. Kendall and Friedman spent several years studying this problem until they had confidence that the uncertainties in the radiative corrections would be no greater than approximately 10 percent.

The experiments began in the fall of 1967. Results from the very first runs are shown in Figure 1, taken from Kendall's Nobel lecture of 1990. The measured cross section was expected to drop precipitously,

FIGURE 1

following the downward slope of the inelastic cross section, shown as a dashed line. Instead, the cross sections stayed unexpectedly high. (The cross sections, in units of inverse energy, are presented as a ratio to the idealized cross section expected if the entire charge were concentrated in a point. They are plotted as a function of the square of the momentum transferred to the proton, a convenient measure of the ability of the electron to probe the structure.) A rapidly falling cross section was expected if the proton charge were uniform throughout the proton's volume since, as the electron traveled deeper inside the proton, there would be less and less proton material to scatter from. Clearly, this was not the case. The cross sections were 10 to 100 times greater than could be accounted for by a uniform structure. Electrons, as they probe to a tenth of the proton size, were still scattering copiously. The radiative issues that Kendall and Friedman had tried so hard to understand had turned out to be negligible compared to the observed enhancements.

The phenomenon was reminiscent of Ernest Rutherford's revelation in 1911 that the backward scattering of alpha particles from gold nuclei meant that the atomic mass was concentrated in a central "point" and could not be spread uniformly over the atomic volume. The results showing deep-inelastic scattering clearly indicated that there were hard, pointlike entities inside the proton. James Bjørken, whose theoretical guidance was important to the SLAC experiments, made correct predictions of the deep-inelastic results from a model based on the possible particlelike constituents in the proton. Richard Feynman, on seeing the early SLAC data, identified the entities with his pointlike partons that he conjectured were the building blocks of nucleons. The specifics of the structure of the proton could not be resolved by deep-inelastic studies alone, but these experiments were the foundation for the next wave of new discoveries and theoretical insights that culminated in quantum chromodynamics (QCD), one of the pillars of the Standard Model.

It is worth remarking on another parallel between the deep-inelastic scattering experiments and the alpha-particle-scattering experiments a half-century earlier. The Rutherford atom could not be reconciled with electrodynamics, which demanded that orbiting electrons radiate energy, leading to the collapse of the atom. It was not until Niels Bohr's introduction of the quantum concepts into atomic physics that the Rutherford atom was accepted. The quark model, which could explain the deep-inelastic results and so much more, faced the dilemma that experimenters could find no evidence of particles with fractional charge, despite diligent and ingenious efforts. The reality of quarks did not become generally accepted until the believable theory of asymptotic freedom convinced the physics community that quarks would not be found individually but would remain bound in hadron structures.

In 1969, even as Kendall continued his central role in the studies of deep-inelastic scattering, he entered a phase of his career that would propel him into the public arena. In that year the Union of Concerned Scientists (UCS) was founded by faculty members of MIT, Kendall among them, to educate the public about the science and technology issues that impacted society. Kendall became the chairman of UCS in 1974 and proceeded to transform it into one of the nation's most effective venues for public awareness of science. Educating the public and policy makers on science issues, especially those that threatened the environment, became an increasingly important focus for Kendall. His work, which continued until his untimely death in 1999, was recognized with several international awards.