Curtis Gove Callan Jr., the James S. McDonnell Distinguished University Professor of Physics and a theoretical physicist with a record of distinguished service to the University, transferred to emeritus status on July 1, 2024. Curtis arrived in Princeton in the fall of 1961 to pursue a Ph.D. in physics. He was eighteen years old, fresh from his undergraduate degree at Haverford College. Since then, his connection to Princeton has been almost continuous. He leaves a legacy of achievement in theoretical physics marked by elegance and originality.

Curtis’s foundational work in quantum field theory provided the mathematical language in which many profound discoveries have been made. His most famous contribution is the Callan-Symanzik equation, which describes how our description of physical systems evolves as we change the scale on which we look. This “renormalization group flow” is at the heart of our understanding of problems ranging from the strong force that binds quarks into protons and neutrons to the remarkable behavior of gases and liquids near critical values of temperature and pressure. But this is just one of many ways in which Curtis has shaped our understanding of the world.

Born in North Adams, Massachusetts, in 1942, Curtis later moved to Staten Island, New York, where he finished high school. Curtis’s time at Haverford was enriched by interactions with faculty at the University of Pennsylvania, and on arriving at Princeton he was advised by Sam Treiman and Robert Dicke. 

After defending his Ph.D. in 1964, he stayed on as a postdoctoral fellow and instructor in physics. Back then, the community’s understanding of elementary particles was in a period of upheaval. What would become known as the Callan-Treiman relation, which describes the decays of K mesons, helped chart the way forward.

During his two years as a junior faculty member at Harvard University, Curtis worked with David Gross to show how measurements on the scattering of elementary particles could be analyzed to reveal that quarks—then still quite hypothetical objects—have the same spin as the more familiar electron. Curtis’s collaboration with Sidney Coleman produced elegant and influential results on the rate at which field theories escape from metastable states, a problem which arises in contexts as varied as the boiling of water and the early evolution of the universe. The pair, with Julius Wess and Bruno Zumino, systematized the exploration of theories for interactions among mesons, introducing methods that reappear in the work of many others.

After Harvard and a short stay at the California Institute of Technology, Curtis moved to the Institute for Advanced Study, where in 1970 he produced his foundational work on renormalization. Discovered independently by Kurt Symanzik, the beautiful and compact Callan-Symanzik equation involves derivatives with respect to the particle mass, or in massless theories with respect to a fiducial sliding momentum scale. Curtis stressed the appearance of an additional term, generated by quantum effects, which includes the all-important “beta function” that dictates the evolution of interaction strength with scale. This approach has proved to have very broad applicability. It is an invaluable tool for studying the theory of the strong forces acting inside the nucleus, quantum chromodynamics (QCD), and was crucial for the discovery of “asymptotic freedom.” The Callan-Symanzik equation also plays a central role in the theoretical exploration of critical phenomena and in the development of string theory. In later work with Emil Martinec, Malcolm Perry, and Dan Friedan, and then with Perry and his graduate student Igor Klebanov, Curtis derived the beta functions of the two-dimensional sigma model, which describes the propagation of strings in curved spacetimes. The vanishing of the beta functions in this model was shown to provide string theoretic generalizations of the Einstein equations in general relativity. 

Returning to Princeton as a full professor in 1972, Curtis continued his collaboration with Gross to show that the asymptotic freedom of QCD was uniquely connected to the observed behavior of elementary particles in scattering experiments. This result, derived from the Callan-Symanzik equation, was central to establishing QCD as the correct theory of the strong interaction, and thus one of the key components in our understanding of all matter. 

This period also produced landmark papers on the dynamics of quark confinement (with Gross and Roger Dashen) and on the ability of hypothetical magnetic monopoles to catalyze the decay of protons, now known as the Callan-Rubakov effect.

Collaboration with Steven Giddings, Jeffrey Harvey, and Andrew Strominger resulted in an influential quantum model of two-dimensional black holes. With Frank Wilczek, Curtis explored ideas of geometric entropy in quantum field theories that presaged an explosion of work on entanglement, with implications reaching from black holes to quantum computing. Curtis’s seminal paper with Harvey on anomaly inflow laid the foundations for the lattice realizations of chiral gauge theories, which would receive renewed attention with the discovery of materials that behave as topological insulators. Curtis and collaborators also found fascinating relations between the quantum behavior of dissipative systems and two-dimensional conformal field theory with boundary interactions.

Around 2000, Curtis shifted his interest from elementary particles to the physics of biological systems. Early steps included a theory/experiment collaboration with Edward Cox on the specificity of interactions between proteins and DNA, as well as theoretical work with William Bialek on the flow of information through these regulatory interactions. He then turned to understanding the physical processes that generate the diversity of antibodies, and the implications of this diversity for immune function. These problems continue to occupy him and to inspire a new generation.

Characteristically, Curtis’s move to biological physics engaged a number of Ph.D. students and postdoctoral fellows, including Justin Kinney, *08, Andreas Mayer, Thierry Mora, Gasper Tkacik *07, and Aleksandra Walczak, all now emerging leaders in the field. From his long career in elementary particle physics and quantum field theory, Curtis’s Ph.D. students include close colleagues Igor R. Klebanov *86, the Eugene Higgins Professor of Physics at Princeton, and Juan Maldacena *96, the Carl P. Feinberg Professor of Natural Sciences at the Institute for Advanced Study. Sadly, another of his prominent students, William Caswell *75, died on September 11, 2001, aboard the flight that crashed into the Pentagon. Caswell’s thesis research continued the use of the Callan-Symanzik equation to explore QCD, providing one of the true theoretical breakthroughs produced by Princeton graduate students. In 2022, Curtis’s remarkable work with graduate students was recognized by a Princeton Graduate Mentoring Award. 

Curtis served twice as chair of Princeton’s Department of Physics and was the founding director of the Princeton Center for Theoretical Sciences. As physics chair, he was on the planning committee for the Lewis-Sigler Institute. Partnership between the physics department and the Institute is part of what made possible the growth of a strong biological physics community at Princeton. He has served the broader community as president of the American Physical Society and as a member of advisory boards for institutions around the world, and he has provided advice to the U.S. government over decades as a member of the science advisory group JASON.

Curtis is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. He received the Sakurai Prize of the American Physical Society and the Dirac Medal of the International Center for Theoretical Physics, among many other honors. He is among the longest serving and most distinguished of Princetonians.

Written by members of the Department of Physics faculty.