= Coulomb-gauge photon propagator
{c}
{title2=$D_{00}=i/|\mathbf q|^2$}
For $q=(q^0,\mathbf q)$ with $\mathbf q\ne0$, the <Coulomb gauge> propagator has
$$
D_{00}(q)=\frac{i}{|\mathbf q|^2},\qquad D_{0i}=D_{i0}=0,\qquad D_{ij}(q)=\frac{i}{q^2+i0}\left(\delta_{ij}-\frac{q_iq_j}{|\mathbf q|^2}\right).
$$
The temporal component is instantaneous; the spatial part projects onto transverse photon polarizations. Although these components are not individually Lorentz covariant, their contraction with conserved external currents equals the covariant answer. Zero-spatial-momentum limits should be taken after combining the current contractions.
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