= Radiation-gauge photon propagator
{title2=$\widetilde D^F_{ij}(k)=i(\delta_{ij}-k_ik_j/|\mathbf k|^2)/(k^2+i0)$}
Time-ordering the <canonical transverse photon field> gives a propagator with numerator equal to the spatial transverse projector and denominator $k^2+i0$. Its two energy poles encode forward propagation of the positive-frequency term and backward propagation of the negative-frequency term. This is the reduced free-field propagator; it is distinct from the covariant four-potential propagator.
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