Feynman Least Action Thesis
The Principle of Least Action in Quantum Mechanics was Feynman's 1942 PhD thesis.
This thesis introduced the ideas that came to be known as the [https://en.m.wikipedia.org/wiki/Wheeler%E2%80%93Feynman_absorber_theory Wheeler–Feynman absorber theory (also called the Wheeler–Feynman time-symmetric theory) ]derived from the assumption that the solutions of the electromagnetic field equations must be invariant under time-reversal transformation, as are the field equations themselves. Indeed, there is no apparent reason for the time-reversal symmetry breaking, Maxwell's equations and the equations for electromagnetic waves have, in general, two possible solutions: a retarded (delayed) solution and an advanced one. This symmetry can be viewed as the primary distinction between the laws of physics versus the common sense laws or time as we experience it.
- Richard P. Feynman, The Principle of Least Action in Quantum Mechanics, Princeton University 1942, reprinted in Feynman's Thesis, Ed Laurie M. Brown, 2005 ISBN 9789812563804