Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwells equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we unify different theoretical descriptions of these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and SiO2 nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems. We validate our calculations by comparison to the exact solution for a one-dimensional particle chain, and we demonstrate that the dipolar approximation significantly underpredicts the exact results at separation distances less than the particle radius.
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