I know this is not possible but I want to know why. Why cannot a directional photon (or electron) detector placed on the screen of a 2 slit experiment detect which slit an individual photon comes from without destroying the fringes? Fire the photons one at a time. They can be detected building up on the screen over time into the interferance fringe pattern. Why cannot I build a directional detector into the screen positioned at one of the bright fringes aimed at one of the slits so I can tell at a particular firing of a photon which slit it came through? After all, the photons land individually on the screen anyway. I can see where they land. All I am doing is putting a directional detector set either on or behind the (semi-transparent?) screen at one spot. I am imagining the detector as a long tube with a small hole at the front and the photomultiplier tube at the back, aimed so that the field of view is one of the slits only.