11th Pancharatnam Lecture - by Professor Sidney R. Nagel

Basins, landscapes, and memory Every memory should be an inspiration for asking new questions. When we apply this dictum to memory in materials, it gives us a chance to examine the nature of far-from-equilibrium behavior in a new light and, in the best cases, allows for an innovative classification of material and dynamics. I will discuss one case where the memory formation suggests several possible interpretations of its cause. A cyclically sheared packing of particles remembers multiple values from a series of training inputs. The evolution takes place in a rugged energy landscape where scores of local energy minima can be visited during each cycle; nevertheless the jammed solid can readily find the periodic orbits. This seems at odds with our understanding of a glass as a highly complex, interacting system. Thinking about such memories inspires questions not only about how glassy ground states are selected but also about whether we should concentrate on local or global features. Prof. Sidney R. Nagel The University of Chicago Sidney Nagel received his Ph.D. degree from Princeton University in 1975. He joined the University of Chicago in 1976 where he is currently the Stein-Freiler Distinguished Service Professor in the Department of Physics and the James Franck and Enrico Fermi Institutes. His research has sought to understand the physics of disordered systems that are far from equilibrium. This has led in various directions – from studies that delve into the physics of why drops splash and form rings at their contact lines to the theory of jamming and studies of how materials store memories. Photographs from his research are in museum collections. Nagel received the Quantrell Undergraduate Teaching Award and the Ph.D. Mentoring Award from the University. He is a member of the U.S. National Academy of Sciences, and the American Philosophical Society, and a Fellow of American Academy of Arts and Sciences. He was awarded the 1999 Oliver Buckley Condensed Matter Prize and the 2023 Medal for Exceptional Achievement in Research from the American Physical Society.