ABSTRACT
This chapter provides a description of electromagnetism. After the successes of the chiral SU(2)×SU(2) symmetry in the mid-1960s, it was natural to suppose that the strong interactions also respect an approximate SU(3) × SU(3) symmetry, which like SU(2) × SU(2) is spontaneously broken to its diagonal subgroup, the Gell-Mann SU(3). Returning to the usual notation, by the mid-1960s, it was understood that the weak interaction processes of hadrons with each other and with leptons are well described at low energy by the effective Lagrangian. The real point of the Lagrangian formalism is that it provides a natural framework for the quantum mechanical implementation of symmetry principles. The dynamical equations in the Lagrangian formalism take the form of a variational principle, the principle of stationary action.
