[3] A native of Philadelphia, Herbert Callen received his Bachelor of Science degree from Temple University.
[6] 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.
[3] This crucial result became the basis for the statistical theory of irreversible processes and explains how fluctuations dissipate energy into heat in general[6] and the phenomenon of Nyquist noise in particular.
[3] In it, he presents a rigorous axiomatic treatment of thermodynamics in which the state functions are the fundamental entities and the processes are their differentials.
[9] After battling Alzheimer's disease for eleven years, Callen died in the Philadelphia suburb of Merion in 1993.
[3] Commenting on his own approach to science, Callen noted the importance of "inspired insight guided by faith in the simplicity of nature.