Obituary: Professor Henry W. Kendall

HENRY W. KENDALL, together with his colleagues Richard E. Taylor and Jerome I. Friedman, won the 1990 Nobel Prize in Physics for experiments which proved the existence of fundamental building bricks, called quarks, as constituents of the neutron and proton of everyday matter.

Born in Boston, Massachusetts, in 1926, Kendall took his degree in Mathematics at Amherst College and his PhD in Physics at Massachusetts Institute of Technology (MIT) in 1955. Later he was to move for a time to Stanford University, California, where he met his future collaborators, Taylor from Stanford, and Friedman, also from MIT. They embarked on a long series of experiments, scattering high-energy electrons from the protons in a target of liquid hydrogen, using a beam produced from a two-mile-long linear accelerator.

The first experiments concentrated on elastic scattering, in which the target proton recoils intact from the collision with the incident electron, just as in a billiard ball collision. These experiments seemed to show that the proton, although extremely small (about one million millionth of a millimetre across) behaved as a relatively diffuse lump of material.

Hit hard enough, it usually shattered with the creation of several new particles (mesons). For the most violent collisions at the largest angles of scatter, only about one in a million of the protons hit would "hold together" and recoil coherently as a single proton. However, careful study of the much more common and more complex inelastic collisions involving many secondary particles revealed by 1968 a surprising and unexpected result: adding up the energy and momentum of all the secondaries, it became clear that the electrons were being scattered elastically from mysterious hard, pointlike objects inside the proton.

Some years before, in 1964, Murray Gell-Mann and George Zweig had independently analysed somewhat vague symmetry patterns among various types of unstable particles produced in high-energy collisions, and had speculated that the symmetries might be understood if particles like the proton and neutron were built from more fundamental objects which Gell-Mann christened "quarks" ( a word taken from a passage in James Joyce's Finnegan's Wake). A proton for example was thought to consist of three quarks, of electric charges one-third and two-thirds that of the electron.

Quarks as free particles should have been easily detectable, but intensive searches at accelerators and in locations as diverse as oyster shells and moonrocks failed to find any trace. The conventional wisdom at the time was that quarks were either just mathematical fictions of the symmetry patterns, or were real but very massive and tightly bound in the proton and thus hard to pry loose.

The electron scattering results of Kendall and his colleagues in the Stanford/MIT experiments came therefore as a revelation. Their mysterious constituents were not only pointlike but light rather than heavy, and behaved as if almost free rather than tightly bound. In time, their intrinsic angular momentum (spin) and electric charges were measured - the latter when combined with neutrino scattering data from across the Atlantic at Cern, Geneva - and it became clear that Kendall's pointlike constituents had exactly the properties expected for the quarks. Why then had they not been seen as free particles?

It eventually became clear that, while the force between a pair of quarks at small distances was relatively weak, at larger distances the attraction became much stronger - about 14 tons weight in fact, and enough to confine quarks permanently in the quark composites we call neutrons and protons. In the words of the Nobel Prize panel, it was "a breakthrough in our understanding of matter".

Kendall was to continue his researches in the field of particle physics, first at Fermilab near Chicago and then again at Stanford. However he found, as did many physicists of his generation, that the inexorable increase in the scale, complexity and duration of experiments in this field, and the need for large international teams of hundreds of physicists to carry them out, was becoming less congenial. Perhaps this was one reason for his focusing his attentions more on the effects of scientific developments on the world at large. Indeed, for much of his life Kendall had shown increasing concern over the possible effects of scientific and technological advances on our environment and social structure. It is probably for his efforts in these areas that he is best known and will be best remembered.

As a professor at MIT, Kendall was, in 1969, a founder member, and later chairman, of the Union of Concerned Scientists. This body had been formed originally to oppose the nuclear arms race and the spread of nuclear power plants and the consequent possibility of radiation leaks. As he himself said, any lingering doubts he may have had "were put to rest at Three Mile Island" in 1979. He was also a strong critic of President Reagan's Strategic Defence Initiative - the anti-missile defence project known as Star Wars.

Kendall saw his reputation as a Nobel Laureate as a means of giving scientific authority to such concerns. During the last decade, his attention had turned more to the dangers for the ecosystem of global warming (he was an adviser to President Clinton on the subject), the problems raised by the use of fossil fuels as well as by the unrestricted growth of the world population, and the assurance of adequate food production for the Third World. Of course many scientists do worry about these things, but Henry Kendall was exceptional in that he devoted most of his available time and energy to actually doing something about them.

From his earliest years, Kendall had had instilled into him from his family a great love of the outdoors. Mountaineering and underwater photography were two of his principal outside activities. In his youth he had joined several expeditions to the Andes, the Arctic and the Himalayas. Tragically he was to lose his life in an accident while pursuing his interest in photographing the underwater world.

Henry Way Kendall, physicist: born Boston, Massachusetts 9 December 1926; National Science Foundation Postdoctoral Fellow, Brookhaven National Laboratory and Massachusetts Institute of Technology 1954-56; Research Associate, Lecturer, Assistant Professor, Physics Department, Stanford University 1956-61; Professor of Physics, Physics Department, MIT 1967- 91, J.A. Stratton Professor of Physics 1991-99; Director and founding member, Union for Concerned Scientists 1969-99, Chairman 1973-99; Nobel Prize in Physics (with Richard E. Taylor and Jerome I. Friedman) 1990; died Wakulla Springs, Florida 15 February 1999.