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http://peeterjoot.com/2018/12/17/phy2403h-quantum-field-theory-lecture-22-dirac-sea-charges-angular-momentum-spin-u1-symmetries-electrons-and-positrons-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 22: Dirac sea, charges, angular momentum, spin, U(1) symmetries, electrons and positrons. Taught by Prof. Erich Poppitz