Hello! I have this doubt with a variation of the double slit experiment: Let us suppose that we are sending electrons to the screen. When both slits are open, the wave function at a given point x of the screen is the sum of the functions phi1 and phi2, respectively corresponding to reach x through slit 1 or slit 2. If we put a photon source near one of the slits, if one electron crosses the slit it will scatter one photon. If we place also a photodetector in a proper position, every time that we detect a photon we can say that the electron crossed that slit, and according to the theory, the wave function collapses to phi1. As a result, no interference pattern is observed. Here is my problem: if we place the photodetector very far, so that the scattered photon is detected AFTER the electron reaches the screen, the wave function does not collapse before the electron reaches the screen. Then, in this case I think that we would find interference pattern, and also we would detect later with the photodetector which slit had been used by the electron. Is this right? Thank you very much for your help!