Herbert Callen

Herbert Bernard Callen (1 July 1919 to 22 May 1993) was an American physicist who specialized in thermodynamics and statistical mechanics and is considered one of the founders of the modern theory of irreversible thermodynamics. He wrote the classic textbook Thermodynamics and an Introduction to Thermostatistics, which appeared in two editions, and he is known for developing the fluctuation-dissipation theorem. During the Second World War his services were invoked in the theoretical division of the Manhattan Project. A native of Philadelphia, he took his bachelor's and master's degrees in physics at Temple University and completed a PhD at the Massachusetts Institute of Technology in 1947 under Laszlo Tisza. In 1948 he joined the physics faculty at the University of Pennsylvania, becoming a professor in 1956, and his most lasting contribution is considered to be the proof of the fluctuation-dissipation theorem that he wrote with Theodore A. Welton.