Herbert Callen

Herbert Bernard Callen (July 1, 1919 – May 22, 1993) was an American physicist specializing in thermodynamics and statistical mechanics. He is considered one of the founders of the modern theory of irreversible thermodynamics, and is the author of the classic textbook Thermodynamics and an Introduction to Thermostatistics, published in two editions. During World War II, his services were invoked in the theoretical division of the Manhattan Project. A native of Philadelphia, Herbert Callen received his Bachelor of Science degree from Temple University. His graduate studies were interrupted by the Manhattan Project. He also worked on a U.S. Navy project concerning guided missiles (Project Bumblebee) at Princeton University in 1945. Callen subsequently completed his PhD in physics at the Massachusetts Institute of Technology (MIT) in 1947. He was supervised by the physicist László Tisza. His doctoral dissertation concerns the Kelvin thermoelectric and thermomagnetic relations, and Onsager's reciprocal relations; it was titled On the Theory of Irreversible Processes. Upon receiving his degree, Callen spent a year at the MIT Laboratory for Insulation Research and developed his theory of electrical breakdown for insulators. In 1948, Callen joined the faculty of the department of physics at the University of Pennsylvania and became a professor in 1956. Specialists consider his most lasting contribution to physics to be the paper co-written with Theodore A. Welton presenting a proof of the fluctuation-dissipation theorem, an extremely general result describing how a system's response to perturbations relates to its behavior at equilibrium. This crucial result became the basis for the statistical theory of irreversible processes and explains how fluctuations dissipate energy into heat in general and the phenomenon of Nyquist noise in particular. Callen then pioneered the thermodynamic Green's functions for magnetism. With his students, he studied many-body problems involving spin operators.