The half-life of knowledge is the amount of time that has to elapse before half of the knowledge in a particular area is superseded or shown to be untrue. The concept is attributed to Fritz Machlup (1962). For example, Donald Hebb estimated the half-life of psychology to be five years.
The half-life of knowledge differs from the concept of half-life in physics in that there is no guarantee that the truth of knowledge in a particular area of study is declining exponentially. In addition, knowledge can not be quantified and falsification of a doctrine is hardly comparable to exponential decay process that atomic nuclei go through.