Ok I have no deep knowledge in QM but I have this thought experiment... which I believe is a quite known one... I don't know. But well here goes: Imagine I have a photon gun that can fire one photon at a time. I put this photon gun about half a meter away from a wall(wall A) with two small holes on it. The photon gun is pointing to the place between the two holes (which are quite close together). Now, the photon gun is not very accurate so it can shoot the photon directly toward a hole or even farther away from the middle of the two holes. At the other side of wall A I place a second wall (wall B) to record where the photon hits. So, I shoot ONLY one photon and look at wall B. What do I see? A spot? If something was not clear please let me know, I am SO interested in this. Thanks.