Abstract
Gauge-invariance of theS-matrix is formulated in terms of a functional derivative condition with respect to a vanishing classical 4-vector field depending on macroscopic space-time co-ordinates (or a family of such fields for the case of non-Abelian gauge groups). This condition can be used to derive identities of the Ward-Takahashi type for the coefficient functions of expansions of theS-matrix in terms of asymptotic field operators. The connection of gauge theories (especially electrodynamics) with macroscopic space-time and with the asymptotic momentum-space behaviour of scattering amplitudes is briefly discussed.
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