Once the first digital computers appeared in the 1940s it was only natural to ask what constraints govern the physical process of computing. For example, is a minimum amount of energy required per logic step, how large must the computing device be, how long must it take to perform a particular computation, etc. In 1949 John Von Neumann gave a lecture in which he identified kT (Boltzman's constant times temeperature) as a minimum energy required "per elementary act of information". In 1950 Leon Brillouin of IBM supported Von Neumann's ideas with more detailed analysis. About 1960 Rolf Landauer and John Swanson of IBM advanced the notion of logical reversibility.