Subsidiary Condition and Longitudinal Field
In this chapter we shall discuss some of the peculiarities of the radiation field which have their origin in the vanishing of the photon mass. We begin with a covariant treatment of the Coulomb interaction and the elimination of the longitudinal and zero components of the field (Section 6-1). The construction of the state vector which satisfies the subsidiary condition requires special precautions (Section 6-2). Alternative solutions of this problem are the method of Gupta (Section 6-3), Valatin’s gaugeinvariant interaction (Section 6-4), and the radiation field with small finite mass (Section 6-5).
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- G. Wentzel, Quantum Theory of Fields, Interscience Publishers, New York, 1949Google Scholar