Coalescent Photons in Blackbody Radiation

Within the Bose-Einstein statistics it is shown that the blackbody radiation contains coalescent photons along with single photons. Coalescent photons were first observed in the famous Hong-Ou-Mandel experiment of 1987. For the sake of convenience, $N$ coalescent photons are referred to as the $N$-photon cluster. In this work, statistics of photon clusters and probability that a photon cluster contains $N$ photons are found versus radiation frequency and temperature. Spectra of photon-cluster radiation are calculated as functions of blackbody temperature for different cluster ranks. Derivation of the Planck's radiation law is discussed in view of the existence of photon clusters in blackbody radiation.