= Conservative Thomson scattering does not thermalize radiation
Coherent <Thomson scattering> by cold <electrons> changes a <photon>'s direction without changing its <frequency> in the electron rest frame. A monochromatic incident radiation field therefore remains monochromatic after arbitrarily many such scatterings. Large scattering <optical depth> can isotropize radiation but does not imply a <Planck photon distribution>. True absorption and emission, or suitable additional inelastic processes and photon production, are needed to establish a <blackbody> spectrum independently of the initial radiation field.
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