In Peskin & Schröder's book, they wrote that there is no propagator for an Abelian field due to gauge invariance of the action, and then proposed a gauge fixing condition: $$\partial_\mu A^\mu = \omega(x)\tag{9.55}$$ in order to count each physical configuration once. Where $\omega(x)$ can be any scalar function.

I don't really understand what this equation means intuitively and how it actually helps counting each physical configuration only once?


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