Walter Dittrich 
The Development of the Action Principle [PDF ebook] 
A Didactic History from Euler-Lagrange to Schwinger

Short Historical Introduction.-  Curva Elastica.- The Curva Elastica, a Curve of Least Energy.- From Euler to Lagrange.- Laplace and the Capillary — 1807.- A Final Application in Elasticity with Jacobi Elliptic Functions.- Short List of Jacobi Elliptic Functions and Constants Used in Chapter 5.- Variational Methods for Periodic Motions; Mathieu Functions.-  Lagrangian for Isentropic Irrotational Flow.- Action Principle in Classical Electrodynamics.- The Two Giants in Gravity: Einstein and Hilbert.- The Quantum Action Principle.- The Action Principle in Quantum Field Theory.- Quantum Field Theory on Space-Like Hypersurfaces.- Lagrangian Formulation of Gauge Theories.- Effective Actions (Lagrangians) in Quantum Field Theory.-  Modified Photon Propagation Function, Source Theory.

Prof. Dr. Walter Dittrich was head of the quantum electrodynamics group at the University of Tübingen until his retirement in 2001 and is still actively publishing papers and books in classical and quantum physics. He received his doctorate under Prof. Heinz Mitter at Heisenberg’s institute in Munich and continued pre- and postdoc work at Brown University, Harvard and MIT. He profited immensely from lectures by and discussions with Profs. Herb Fried, Ken Johnson, Steve Weinberg, Julian Schwinger and, later on, at the Institute for Advanced Study (IAS) in Princeton, Steve Adler and David Gross at Princeton University. He started his work on gauge theories and QED in collaboration with Schwinger in the late 1960s. He was visiting professor at UCLA, Berkeley, Stanford and the IAS. He has over 30 years of teaching experience and is one of the key scientists in developing the theoretical framework of quantum electrodynamics.