ON THE GROUP-THEORY OF THE POLARIZATION STATES OF A MASSLESS FIELD

It is shown that the theory of representations of the Poincare group a pplied to the vector potentials of a massless field yields in a simple and direct way (without assuming that the field is a gauge field, and without detailed assumption on the form of the equation of motion), t he structure of the polarizations, the Gupta-Bleuler condition, and ga uge invariance of the theory. The method is shown to apply to the Maxw ell field and to a five-dimensional generalization of the Maxwell fiel d (which properly contains the Maxwell theory) associated with manifes tly covariant dynamics.