I've been thinking about this for a while. I believe I have some misconceptions about light. Let's say we have a photon that is emitted from some atom as it de-exictes. It has definite momentum since it is of a specific energy. Where is it located? Let's ignore the issue of trying to pin down a photon without using a photon. Since the uncertainty of momentum is 0, the position is infinitely uncertain. Okay. One explanation I thought up for this is the photon's postion is a superposition of postion states from x=0 to x = ct, where t is the time since the photon was emitted. I can't think of how exactly one would write down a wavefunction for a photon though. Maybe that means the particle interpretation of a photon fails for this problem and I should interpret it as a wave instead. Maybe a standing wave in a "cavity" of length ct. Even if my little guess is right, it isn't very satisfying because the photon has to be somewhere definitely, I think. For example, if we intend to have it incident on a detector 1 light-seconds away. After a second passes, the detector will go off, so it was obviously at the detector. How how can we know it was at the detector and know its momentum? I suppose the argument could be made that we didn't actually mesure the momentum, we just "guessed" the right one because we knew about the energy levels of the atom beforehand. Can the momentum (wavelength, energy) of a photon be know exactly? If this is impossible, I'll easily accept that we can know where it is.