Ezechiel Godert David “Eddie” Cohen passed away on 24 September 2017. He was a professor at the Rockefeller University from 1963 until his retirement in 1993, but he remained scientifically active until a few months before his death. With H. Eugene Stanley, Eddie was awarded a 2004 Boltzmann Medal by the International Union of Pure and Applied Physics; Eddie’s was given “for his fundamental contributions to nonequilibrium statistical mechanics, including the development of a theory of transport phenomena in dense gases, and the characterization of measures and fluctuations in nonequilibrium stationary states.”

Ezechiel Godert David Cohen

Ezechiel Godert David Cohen

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Eddie was born into a Jewish family in Amsterdam on 16 January 1923. The family was forced to go into hiding and was separated in 1942 during the German occupation of the Netherlands. Eddie escaped the fate of his parents, who were betrayed and perished in Auschwitz. He received his undergraduate degree in 1952 and his PhD in theoretical physics, under the direction of Jan de Boer, in 1957, both from the University of Amsterdam. After postdoctoral appointments at the University of Michigan with George Uhlenbeck and at Johns Hopkins University with Theodore Berlin, he joined the faculty of theoretical physics at the University of Amsterdam. In 1963 he accepted a professorship at the Rockefeller University, joining a group that included Uhlenbeck, Mark Kac, and Abraham Pais.

Eddie’s early scientific accomplishments included, with J. M. J. van Leeuwen, one of the first studies of the phase separations in helium isotope mixtures at low temperatures, important for helium dilution refrigerators. With one of us (Dorfman), he discovered divergences in the virial expansion of transport coefficients; that discovery caused a revolution in nonequilibrium statistical physics and provided a kinetic-theory explanation of the algebraic time decays that Berni Alder and Thomas Wainwright found in the Green–Kubo time-correlation functions of moderately dense gases.

Working with numerous students, postdocs, and visiting scientists, Eddie made many important contributions to nonequilibrium statistical physics. Among them was a deep study of the long- and short-wavelength behavior of microscopic hydrodynamic modes in dense hard-sphere fluids, important for understanding phenomena seen in neutron-scattering experiments and in studies of time-correlation functions in dense fluids.

Eddie’s research on fluids maintained in a nonequilibrium stationary state (NESS) produced some striking results. In particular, he and another of us (Kirkpatrick), then his graduate student, showed that the intensity of the central peak in the spectrum of light scattered by a fluid with a stationary temperature gradient is orders of magnitude larger than the corresponding intensity in an identical fluid in equilibrium. That prediction has been verified experimentally by the other of us (Sengers) and coworkers. With Denis Evans and Gary Morriss, Eddie studied the Lyapunov exponents for a system of particles kept in a NESS with a thermostat that maintains a constant internal energy. Analyzing such systems led him, in collaboration with Giovanni Gallavotti, to develop what is now known as the Gallavotti–Cohen fluctuation theorem.

Eddie’s contributions to science went far beyond his research and publications. He was devoted to physics and was passionate about everything he encountered both in his own work and in that of others. That passion made him an excellent mentor to scientists in the early stages of their careers. Those of us who worked with him—colleagues, students, and postdocs—can still hear Eddie’s voice commenting on our taste in the problems we chose, the need for careful and critical thinking, and the necessity of questioning the correctness and even the relevance of every phase of our research.

Eddie was invaluable at lectures, conferences, and meetings. His questions, sometimes numerous, were insightful and penetrating. Speakers would often learn something beneficial to their work, such as suggestions for new ideas and directions to consider, when questioned by Eddie.

The community of statistical physicists was greatly enriched by Eddie’s service to it. He organized a summer school that focused on fundamental problems in statistical physics, and he edited the proceedings into an ongoing series of books. The school, which Eddie started in the Netherlands in 1961, was later expanded across Europe and is still held every few years, most recently in Italy in 2017. His papers and articles surveying various aspects of research and problems in kinetic theory and nonequilibrium statistical mechanics were clearly written and widely read.

Together with his other students, postdocs, and collaborators, we are grateful for the scientific mentorship we received from Eddie Cohen.