My layman's intuition tells me that wave packets normally spread out in space and disperse, except in special circumstances. Photons don't behave like that. In The Principles of Quantum Mechanics, pp 124-125, Dirac discusses the equations of motions of a photon wave packet. He says: Thus Dirac doesn't show that the packets don't disperse, he imposes it as a condition on the solution. I presume that the thing he left unsaid was "because that agrees with experiment," which is a compelling argument. However, on the very next page he seems to waffle: "the wave packet undergoing a spreading" So what's the deal? Photon wave packets do or do not spread?