Feynman 1947 letter on path integral for the Dirac equation

Abstract

In 1947, four months before the famous Shelter Island conference, Richard Feynman wrote a lengthy letter to his former MIT classmate Theodore Welton, reporting on his efforts to develop a path integral describing the propagation of a Dirac particle. While these efforts never came to fruition, and were shortly abandoned in favor of a very different method of dealing with the electron propagator appearing in in QED, the letter is interesting both from the historical viewpoint of revealing what Feynman was thinking about during that period just before the development of QED, and for its scientific ideas. It also contains at the end some philosophical remarks, which Feynman wraps up with the comment, “Well enough for the baloney.” In this article I present a transcription of the letter along with editorial notes, and a facsimile of the original handwritten document. I also briefly comment on Feynman’s efforts and discuss their relation to some later work.