Herbert Callen

Herbert Bernard Callen 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. He is also known for developing the fluctuation–dissipation theorem. During World War II, his services were invoked in the theoretical division of the Manhattan Project.