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.
- 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