the mass of a photon? http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html ^ The phrase "no matter how intense" refers to intensities below approximately 10 13 W/cm 2 at which point perturbation theory begins to break down. In contrast, in the intense regime, which for visible light is above approximately 10 14 W/cm 2 , the classical wave