So I've read that at sufficiently high energies, photons will interact with each other rather than passing through each other in the usual ghostly ways. So in these situations, is constructive/destructive interference still possible? What is the threshold where it might stop, for practical purposes? (ie. what frequency or photonic energy level?) Is there some energy where 2 photons would collide like hadrons would? Is this type of coupling purely due to EM potential, or is there anything analogous to Pauli Exclusion Principle that could happen?