reason, we compute in Appendix A some components of field-strength two-point function in four-dimensional Minkowski spacetime using spherical polar coordinates. We find that they agree with the standard results obtained in Cartesian coordinates, as they should.

The physical modes in the spherical Coulomb gauge {#sec:Coulomb}
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As we have seen, the modified Feynman gauge defined by Eq. (\[Kchoice\]) is useful in finding explicit mode functions. We also noted that this gauge results in the same physical modes as in the $A_0=0$ gauge. There is another convenient gauge for finding physical modes, i.e., the spherical Coulomb gauge, $\tilde{\nabla}^i A_i = 0$. (One can readily show that any vector $A_\mu$ is gauge-equivalent to a vector satisfying this gauge condition.) Let us discuss the relation of the physical modes obtained in this gauge and those found in the previous section. [^4]

It is clear that the physical modes $A^{(2n;\,\omega lm)}_\mu$ in the previous section are also in the spherical Coulomb gauge. The other independent physical modes must have $A_i = 0$ in this gauge. First we note that if $A_t$ and $A_r$ are assumed to be spherically symmetric, then we find from the equations $\nabla^\mu F_{\mu t} = \nabla^\mu F_{\mu r} = 0$ 