Herbert Bernard Callen (1919 – May 22, 1993) was an American physicist best known as the author of the textbook Thermodynamics and an Introduction to Thermostatistics, the most frequently cited thermodynamic reference in physics research literature. During World War II he was also called upon to undertake theoretical studies of the principles underpinning the effort to create the atom bomb.
In 1948, Callen joined the faculty of the University of Pennsylvania Department of Physics and, in 1984, received the Elliott Cresson Medal from the Franklin Institute. He was also the recipient of a Guggenheim Fellowship for the academic year 1972/1973, and his distinguished career in physics was capped by induction into the National Academy of Sciences in 1990. Specialists consider that his most lasting contribution to physics remains the proof of the fluctuation-dissipation theorem, an extremely general result describing how a system's response to perturbations relates to its behavior at equilibrium; as well as his classic text on Thermodynamics, which was published in two editions, translated into many languages, and continues to be used at the graduate and undergraduate level in Thermodynamics courses around the world.
After battling Alzheimer's disease for eleven years, Herbert Callen died in the Philadelphia suburb of Merion at the age of 73. He and his wife, Sara Smith, had two children, Jed and Jill.