Answers are grounded in this paper and cite their sources.
A method is suggested (and applied to the Yang-Mills and gravitational fields) for the construction of the generating functional (S matrix) for fields possessing an invariance group. The unitarity and gauge independence of the S matrix on the mass shell are seen explicitly.
No takes yet. Share an insight, caveat, or question.
Fradkin et al. (1970) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: