Hi to everyone! I'm searching information about evidences of photons existence. It seems like the photoelectric effect isn't for itself a proof of photons existence. Some people tried a semi-classical discussion of this effect (Lamb - "The Photoelectric effect without photons"). I'm looking for an experiment that shows photons existence not only in the process of emitting or absorbing them, also in the electromagnetic waves while they travel in space. I heard about an experiment where researchers made an atom of a crystal emit a high energy photon. The atoms break away from the crytal, showing that photons have momentum, because of the conservation of this physical mangnitude. This would be a proof of the fact that photons are emited in a single direction, and that their energy is not distributed in a spherical wave; otherwise the atom will rest in his original place. I searched a lot in the web, but couldn't find any reference to this experiment. If you know any other evidences os the existence of single photons in waves, not only when they ara emitted or absorbed I would be glad to read about them. Thank you very much for your help and time. Have a nice day!