The origins of Schwinger׳s Euclidean Green׳s functions

•Shows Schwinger׳s path to his Euclidean Green׳s functions.•Identifies the central physical motivation for the Euclidean formalism.•Traces the origin of the central objects of constructive field theory.

This paper places Julian Schwinger׳s development of the Euclidean Green׳s function formalism for quantum field theory in historical context. It traces the techniques employed in the formalism back to Schwinger׳s work on waveguides during World War II, and his subsequent formulation of the Minkowski space Green׳s function formalism for quantum field theory in 1951. Particular attention is dedicated to understanding Schwinger׳s physical motivation for pursuing the Euclidean extension of this formalism in 1958.