Leslie Lawrance Foldy, Institute Professor Emeritus of Physics at Case Western Reserve University, whose pioneering work elucidated the theory of the multiple scattering of waves and the nonrelativistic limit of the Dirac equation, died on 18 January 2001 in Cleveland, Ohio, after suffering a heart attack on the previous day.
Les was born in Sabinov, Czechoslovakia, on 26 October 1919, into a family with Hungarian roots. His parents named him Laszlo Földi. In the turbulent times following World War I, he immigrated with his parents to the US in 1921. His father changed the family’s last name and Les’s first name; Les later added his middle name, unaware of its more common spellings. He attended the Case School of Applied Science (which later was named the Case Institute of Technology, the forerunner, with Western Reserve University, of Case Western Reserve University), where he received his BS in physics in 1941. He took one year of graduate study at the University of Wisconsin-Madison with Leon Brillouin, and was awarded a physics master of philosophy degree in 1942.
That same year, Les joined Columbia University’s Underwater Sound Laboratory, where he participated in research associated with the war effort. It was there that he published, in 1945, his first, and perhaps most influential, paper on the behavior of waves in the presence of a disordered array of scatterers. Originally developed to treat the multiple scattering of sound from the cloud of cavitation bubbles in the wake of a submarine, this theory was found to have a wide application in areas as diverse as neutron scattering and the behavior of electrons in disordered alloys. Les’s fundamental insights were subsequently rediscovered many times by later workers who were unaware of his paper.
His time at Columbia was eventful, and is especially poignant now in light of the World Trade Center airplane disasters on 11 September 2001. Working one Saturday on the 64th floor of the Empire State Building, he was startled to hear the sound of a rapidly approaching airplane. A tremendous noise signaled that it had crashed into the building a few floors above him, resulting in some loss of life. Considerate, but also sparing with his words, Les sent a telegram to his wife, Roma, in Florida with this simple message: “Don’t worry. I’m all right!” She was unaware of the accident and, on receiving his telegram, worried!
In 1945, Les went to the University of California, Berkeley, to pursue his PhD in physics in the group of J. Robert Oppenheimer. After two years there, during which he also worked with Luis Alvarez and David Bohm, he followed Oppenheimer to the Institute for Advanced Study in Princeton, New Jersey, where he completed his doctoral degree in 1948. Les then returned to the Case Institute of Technology as an assistant professor of physics.
The paper for which Les is most widely recognized arose from a collaboration with another former student of Oppenheimer, Dutch physicist Siegfried Wouthuysen. During the summer of 1949 at the University of Rochester, where they worked with Robert Marshak, Les and Wouthuysen wrote a preprint on the nonrelativistic limit of the Dirac equation. At a summer school in Michigan, a French physicist vigorously criticized this work, claiming to have found an error in it. This criticism stimulated Les and Wouthuysen to reformulate their calculation in such a completely clear and elegant way that all objections crumbled. The approach they adopted was to make use of a canonical transformation that has now come to be known as the Foldy-Wouthuysen transformation. Before their work, there was some difficulty in understanding and gathering all the interaction terms of a given order, such as those for a Dirac particle immersed in an external electromagnetic field. With their procedure, the physical interpretation of the terms was clear, and it became possible to apply their work in a systematic way to a number of problems that had previously defied solution.
Les’s work in the four decades that followed covered an extraordinarily rich variety of topics, ranging over particle, nuclear, many-body, atomic, and solid-state physics, as well as acoustics, and scattering and accelerator theory. With Fred Milford, he developed, in 1950, the theory of nuclear magnetic moments in distorted nuclei. He was also the first (1952) to realize that the magnetic field of the neutron had to be considered in the theory of electron-neutron scattering; this realization led to what is now called the Foldy term. This idea came to him while he was brushing his teeth one night; he developed the formalism and sent off his paper to the Physical Review the very next day! On another occasion, in 1954, he gave a proof that there could exist no charged particle lighter than the electron, because if there had been such particles, they would have modified the Lamb shift.
Les was revered by his students and colleagues for more than his intellectual gifts. His gentle modesty concealed a firm core of moral principle that led him to refuse government support for his research during the Vietnam War. When four students were killed by the Ohio National Guard at the nearby campus of Kent State University, Les was an influential voice in his university’s decision to suspend classes.
Les will be most remembered for the generosity he showed in sharing his knowledge, insight, and wisdom to all who sought his advice. If one approached Les for scientific help because of his reputation as a fount of wisdom (and he indeed was so besieged by students and senior faculty alike), then one stayed because of his kindness. For a celebration of Les’s 80th birthday, Case Western Reserve University physics department received hundreds of enthusiastic letters and e-mail messages from well-wishers; the outpouring of affection and admiration was stunning. His death, coming so soon after this celebration of his life, will not diminish the memory of his impressive scientific achievements and his unique goodheartedness.
