Time-Ordered Green's Functions and Electromagnetic Interactions

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Abstract
In the present paper we study various aspects of the Ward-Takahashi equations. In perturbation theory the equivalence between this set of equations and the requirement of gauge invariance is shown. It is then shown that these equations are valid for composite particles as well as for elementary particles. Based on our new formulation the definition of composite particles is given, and then we show with the aid of the Ward-Takahashi equations that the photon is an elementary particle.