1) Is a photon elastic? 2)Is it possible to prove gravity of photon along its line of motion? IMO no, because this would causse the photon to travel at either more or less than c, or change its frequency. Perhaps diffraction can be taken as a cause of gravity of photon perpendicular to its motion, it is observed "when" the photon passes through the slit, not before or after. We have also learnt the force required to circulate a photon in a grav.field.