Robert B. Leighton (1919–1997)

Robert Benjamin Leighton died 9 March 1997 after a decade-long illness, gracefully endured. His survivors include his wife Marge Leighton, sons Alan and Ralph, and two grandchildren.

Leighton was born in Detroit on 10 September 1919. He began his undergraduate career at Los Angeles City College, but transferred to the California Institute of Technology as a junior, receiving a BS in electrical engineering in 1941. He remained at Caltech the rest of his life, earning MS and PhD degrees in physics in 1944 and 1947 and the titles of Research Fellow (1947-49), Assistant to Associate to Full Professor of Physics (1949-53, 1953-59, 1959-84), William L. Valentine Professor of Physics (1984-85), and Valentine Professor Emeritus (1984-97). He was, in addition, a staff member of the Hale Observatories (1963-80), a principal or co-investigator on Mariner 4, 6, 7, and 9, and Chairman of the Division of Physics, Mathematics, and Astronomy at Caltech (1970-75)

Leighton's thesis work and first published paper concerned the specific heat of face-centered cubic crystals, and the tale of how he did a complicated triple numerical integral for the project by machining the shape out of metal and weighing it has come down to his students. During World War II, Bob worked with a group headed by Carl Anderson to develop and test solid propellant rockets to be mounted under the wings of aircraft. Another assignment was to design the burning surface of a solid propellant charge in such a way that gas generated by combustion would provide a nearly constant thrust to the projectile as it flowed through the exhaust nozzle. He was a key figure in the theoretical calculations, static testing in Eaton Canyon, and dynamic (free flight) testing at Inyokern in the Mojave Desert.

Leighton moved definitively into Anderson's cosmic ray group soon after completing his thesis. He was instrumental in the 1949 demonstration that the products of mu-meson decay were two neutrinos and an electron, and he made the first measurements of the energy spectrum of the decay electron (at the time, low statistics experiments suggested that only one neutrino was involved). In 1950, he made the first observation of strange particle decays after the initial discovery of two cases in England in 1947. Over the next seven years, he elucidated many of the properties (mass, lifetime, decay modes, and energies) of several of the new strange particles, particularly the lambda, the xi, and what were then called theta particles (K mesons).

About 1956, Leighton became interested in the physics of the outer layers of the sun. As Mount Wilson Observatory management pondered the future of its solar program, Bob obtained permission to use the early morning hours of good seeing at the 60 foot solar tower. His study of solar granulation was the definitive work up to that time. He added a beam splitter and polarizers to the Mt. Wilson spectroheliograph and obtained photospheric magnetograms of unprecedented spatial resolution. These observations showed that strong magnetic fields exist outside sunspots and coincide with the network of intense chromospheric emission.

Bob further modified the instrument so that it could be used to obtain Doppler images of the sun, and, in one glorious month in 1960, discovered both the 5-minute oscillation and the supergranulation. He soon realized that solar magnetoconvection cells would lead to an effective diffusion of flux on the solar surface (now called Leighton diffusion), and he included it in a dynamo model of the solar cycle.

In the mid-1960's, the Caltech astronomy department recognized the importance of this solar work by creating a professorship of solar astronomy. The first (and only) holder was Harold Zirin, who joined with Bob and others in a solar site survey that ultimately led to the creation of Big Bear Solar Observatory (now the property of New Jersey Institute of Technology). The oscillations that Leighton discovered opened a whole new field of solar seismology, now pursued at Caltech by Kenneth Libbrecht. Caltech graduate students who worked with Bob on the various solar observing projects included Bob Noyes, George Simon, Neil Sheeley, Alan Title, Phil Roberts, Bob Smithson, and Jim Mosher.

In the early 1960's, Leighton's interests shifted again, to infrared instrumentation and observing. He had the idea of making nearly optical quality mirrors by spinning liquid epoxy, then a new material, and letting it harden in the parabolic shape resulting from the spin. He used an air bearing like the one he and Vic Neher had used to demonstrate frictionless motion to freshman students (which won that year's prize of the American Association of Physics Teachers for best experiment). Leighton, his student Gerry Neugebauer, and others built a telescope using one of these spun mirrors in the machine shops of Bridge Lab at Caltech. It was truly Bob's telescope. He had built an amateur telescope at home, and used every trick he had learned to design a novel, inexpensive 62-inch infrared telescope. Chopping between source and sky was done by shaking the whole telescope back and forth at 20 Hz, and Leighton led a group of students (including Neugebauer and Jerry Nelson) in constructing a roll-off building to house the telescope on Mt. Wilson. Astronomy pundits had expected that they would detect at most tens of sources. They found thousands. The very first night that Leighton and Neugebauer turned the telescope to the sky, they found the reddest, most extreme source of the three year survey. It was typical of Bob's good, and hard-earned, luck. That Neugebauer is the first author of the paper was typical of his generosity. The telescope is now in the Air and Space Museum in Washington.

Leighton's contributions to the Mariner project included an important role in JPL's development of the first digital television system for use in deep space and the early efforts at image processing and enhancement techniques that first suggested and then confirmed the importance of dry ice in the polar caps, and did not confirm the presence of canals. His final shift of disciplines in the 1970's was to millimeter and submillimeter astronomy and the development of suitable, inexpensive dish antennas. "Leighton dishes" are now in use at the Owens Valley Radio Observatory in California and at the Caltech Sub-millimeter Observatory on Mauna Kea.

In addition to guiding many graduate students, Leighton was active in undergraduate teaching. A three-term physics course for senior majors led to the classic textbook Principles of Modem Physics, published in 1959 (his own students were expected to work all the problems!), and in the early 1960's he directed a program to revise the freshman and sophomore physics courses. This led to The Feynman Lectures, which he co-edited into their printed form, as well as a set of problems to accompany the Lectures, co-authored with Robbie Vogt. Leighton was also a pioneer, with Feynman, in abolishing the requirement that jackets and ties be worn at lunchtime in the Caltech faculty club (the Athenaeum).

Leighton's scientific papers are archived at the California Institute of Technology. Oral history material from 1977 is available at the Center for the History of Physics of the American Institute of Physics. Because Leighton was a member of the National Academy of Sciences, an extended obituary should eventually appear in their Biographical Memoirs.

Photo (available in PDF version) courtesy California Institute of Technology.