  The nonlinear propagation of intense incoherent photons in a photon gas is
considered. The photon-photon interactions are governed by a pair of equations
comprising a wave-kinetic equation for the incoherent photons in the presence
of the slowly varying energy density perturbations of sound-like waves, and an
equation for the latter waves in a background where the photon coupling is
caused by quantum electrodynamical effects. The coupled equations are used to
derive a dispersion relation, which admits new classes of modulational
instabilities of incoherent photons. The present instabilities can lead to
fragmentation of broadband short photon pulses in astrophysical and laboratory
settings.
