Why did Schwinger dislike Feynman diagrams?
My objection to Feynman diagrams was not to their utility—they are certainly effective for calculations—but to their lack of mathematical rigor and their tendency to obscure the underlying principles. I preferred to work with operators and Green's functions derived from an action principle, which provides a systematic and unambiguous foundation. Diagrams, as I saw them, were a crutch that could lead to errors if one did not carefully track the algebraic structure. I once remarked, 'If you can't join them, beat them,' reflecting my commitment to developing source theory as a more principled alternative. The diagrams are a computational tool, not a physical explanation; the true physics lies in the variational symmetries and the causal propagation encoded in the Green's functions.
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