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of the triptych that had begun at Shelter Island two years earlier. He published an extended set of papers—they would stretch over three years and one hundred thousand words—that defined the start of the modern era for the next generation of physicists. After his path-integrals paper came, in the Physical Review, “A Relativistic Cut-Off for Classical Electrodynamics,” “Relativistic Cut-Off for Quantum Electrodynamics,” “The Theory of Positrons,” “Space-Time Approach to Quantum Electrodynamics,” “Mathematical Formulation of the Quantum Theory of Electromagnetic Interaction,” and “An Operator Calculus Having Applications in Quantum Electrodynamics.” As they appeared, the younger theorists who devoured them realized that Dyson had given only a bare summary of Feynman’s vision. They felt invigorated by his images—beginning with the unforgettable bombardier metaphor in the positron paper—and by his way of insisting on the plainest statements of physical principles in physical language:

The rest mass particles have is simply the work done in separating them against their mutual attraction after they are created… .

How would such a path appear to someone whose future gradually became past through a moving present? He would first see …

No aspiring physicist could read these papers without thinking about what space was, what time was, what energy was. Feynman was helping physics live up to the special promise it made to its devotees: that this most fundamental of disciplines would bring them face to face with the primeval questions. Above all, however, to young physicists the diagrams spoke loudest.

Feynman had told Dyson, with a slight edge, that he had not bothered to read his papers. “Feynman and I really understand each other,” Dyson wrote home cheerily. “I know that he is the one person in the world who has nothing to learn from what I have written; and he doesn’t mind telling me so.” Feynman’s students, however, sometimes noticed what seemed to them an undercurrent of anger in the pointed comments he would make about Dyson. He had started hearing about Dyson’s graphs—irritating. Why graphs? he asked Dyson. Was that the mathematician in him, putting on airs?

Feynman’s space-time method had other antecedents besides Dyson’s graphs, as it happened. A 1943 German textbook by Gregor Wentzel contained a parallel depiction of a particle exchange process in beta decay. A Swiss student of Wentzel’s, Ernst Stückelberg, had developed a diagrammatic approach that even embraced the conception of time-reversed positrons; parts of this he published, in French, and parts were returned as unpublishable. (Wentzel himself was the unimpressed referee.) Their diagrams showed glimmerings of the style of visualization that Feynman now brought to fruition. His own full-dress version finally appeared in a paper he sent off in late spring 1949. “The fundamental interaction”—an image that would burn itself into the brains of the next generation of field theorists—showed two electrons interacting by exchanging a single photon.

A diagram from a little-known 1941 paper of Ernst Stuckelberg, showing aversion of time reversal in particle trajectories.

He drew electrons as solid lines with arrows. For photons he used wavy lines without arrows: no directionality needed because the photon’s anti-particle is itself. “The fundamental interaction” reinterpreted the basic textbook process of electromagnetic repulsion. Two negative charges, electrons, repel. A standard picture, showing lines of force or merely two balls pressing apart from each other, would beg the question of how an entity feels the force of another entity at a distance. It would imply that force can be transmitted instantly, when in truth, as Feynman’s diagrams automatically made explicit, whatever carries force can move only as fast as light. In the case of electromagnetism, it is light—in the form of fugitive “virtual” particles that flash into existence just long enough to help quantum theorists balance their books.

These were space-time diagrams, of course, representing time as