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In page Paul Dirac:

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In the 1950s in his search for a better QED, Paul Dirac developed the Hamiltonian theory of constraints[3][4] based[citation needed] on lectures that he delivered at the 1949 International Mathematical Congress in Canada. Dirac had also solved the problem of putting the Schwinger–Tomonaga equation into the Schrödinger representation[5] and given explicit expressions for the scalar meson field (spin zero pion or pseudoscalar meson), the vector meson field (spin one rho meson), and the electromagnetic field (spin one massless boson, photon).