  A normal-diffusion theory for heat transfer in many-body systems via carriers
of thermal photons is developed. The thermal conductivity tensor is rigorously
derived from fluctuational electrodynamics as a coefficient of diffusion term
for the first time. In addition, a convection-like heat transfer behavior is
revealed in systems of asymmetric distribution of particles, indicating
violation of Fourier's law for such system. Considering the central role of
thermal conductivity in heat transfer, this work paves a way for understanding,
analysis and manipulation of heat transfer in nanoparticle system via thermal
photons with many-body interactions.
