Quantum Action Principle

Abstract

Abstract Starting from the earlier notions of stationary action principles, we show how Julian Schwinger’s Quantum Action Principle descended from Dirac’s formulation, which independently led Feynman to his path-integral formulation of quantum mechanics. The connection between the two is brought out, and applications are discussed. The Keldysh-Schwinger timecycle method of extracting matrix elements in nonequilibrium situations is described. The variational formulation of quanum field theory and the development of source theory constitute the latter part of this work. In this document, derived from Schwinger’s lectures over four decades, the continuity of concepts, such as that of Green’s functions, becomes apparent