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$R_\xi$ gauge and degrees of freedom counting
In the standard classical Maxwell theory, we use the following arguments to claim that there are only two propagating degrees of freedom
$A_\mu$ has 4 components
$A_0$ is non-dynamical (-1)
$\...
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Counting degrees of freedom without fixing the gauge?
In electrodynamics, the current-current interaction in the momentum space is described by
$$p^2 A_\mu J^\mu = J_\mu J^\mu \, ,$$
where $J$ denotes an arbitrary external current. Since photon-...
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