Ezechiel Godert David Cohen

Ezechiel Godert David (Eddie) Cohen 1923-2017

Eddie Cohen passed away on September 24, 2017 at the age of 94. He was a professor at the Rockefeller University since 1963 until his retirement in 1993 but remained scientifically active until a few months before his death. Together with Eugene Stanley, Eddie was awarded in 2004 the Boltzmann Prize by IUPAP. The citation reads:

“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.”

Eddie was born in Amsterdam on January 16, 1923, to a Jewish family and was forced to go into hiding during the occupation of the Netherlands by German forces. He was hidden near the town of Twisk and escaped the fate of his parents who were betrayed while in hiding and perished in Auschwitz. Eddie received his Ph. D. at the University of Amsterdam in 1957 under the direction of Jan de Boer. After postdoctoral appointments at the University of Michigan with George Uhlenbeck and at The Johns Hopkins University with Theodore Berlin, he joined the faculty of theoretical physics at the University of Amsterdam. In 1963 Eddie joined the faculty at the Rockefeller University in the group that included George Uhlenbeck, Marc Kac, and Abraham Pais. Eddie’s early scientific accomplishments include, 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, and, with J. R. Dorfman. the discovery of divergences in the virial expansion of transport coefficients, causing a revolution in nonequilibrium statistical physics. It also led to a kinetic-theory explanation of the algebraic time decays of Green-Kubo time correlation functions for moderately dense gases found by B. Alder and T. Wainwright.

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

Eddie’s work on fluids maintained in nonequilibrium stationary states (NESS) has produced a number of very striking results. In particular the work done with his then graduate student, T. R. Kirkpatrick, showing 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. This prediction has been verified experimentally by light scattering studies carried out by J. V. Sengers and co-workers. Eddie continued his work on nonequilibrium systems by studying, with D. Evans and G. Morriss, the Lyapunov exponents for a system of N particles maintained in a thermostatted NESS. The work on thermostatted systems also led to a well-known result, obtained in collaboration with G. Gallavotti, known as the Gallavotti-Cohen Fluctuation Theorem.

Eddie’s contributions to science went far beyond his research and published work. He was passionately devoted to physics and took everything he encountered, either in his own work or that of others, very seriously. That passion made him an excellent mentor for scientists in the early stages of their careers. He taught his young co-workers, students and post-docs, how to carry out research. Those of us who worked with him carry in our minds, even after many years of not working with him, Eddie’s voice commenting on our taste in the problems we chose to work on, the need for careful and critical thinking, and the need to question the correctness and even the relevance of every phase of our work. At the same time, he was a very noticeable and valuable person to have at lectures, conferences and meetings. His questions, sometimes many of them, were extremely insightful and penetrating. Everyone who encountered Eddie while giving a talk learned something valuable that affected their work, usually suggestions for new ideas and directions to consider.

Finally we must mention his service to the community of statistical physicists. He took the initiative of organizing summer schools with a focus on Fundamental Problems in Statistical Physics and edited the proceedings as a series of volumes with that title. These summer schools still continue to be active in Europe. His papers and articles surveying various aspects of research and remaining problems in kinetic theory and non-equilibrium statistical mechanics were clearly written and widely read.

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