= Photon number current
{title2=$J^\mu=\int\frac{d^3p}{E}f p^\mu$}
In an <orthonormal tetrad>, $J^{\hat0}=\int f\,d^3p$ is the <photon number density> and $J^{\hat i}=\int f e^i\,d^3p$ is its number flux. Integrating the <Boltzmann equation> gives $\nabla_\mu J^\mu=\int C[f]\,d^3p/E$ when momentum-boundary terms vanish. Collisionless propagation and photon-number-conserving scattering give zero <divergence>. <Photon emission>, <photon absorption>, <electron-positron annihilation in cosmology>, <bremsstrahlung> and <double Compton scattering> generally do not. This is a number current, not an electric charge current: <photons> are neutral.
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