[Mentor's note: This thread was split out from another longer one because it is an interesting topic in its own right] Something that I've been confused about when it comes to interference between photons is the distinction between two different quantities that can interfere constructively or destructively: There is a quantum amplitude describing the occurrence of a photon at a particular location at a particular time. If there are multiple ways that a photon can arrive at that location, then the amplitudes add, and depending on the relative phases, produce constructive or destructive interference. Classically, light is a fluctuating electromagnetic field, and at every point and time, there is an associated vector quantity, the polarization, which describes how the electric and magnetic fields point. Light from different sources that combine at a point will interfere constructively if their electric and magnetic fields point in the same directions and destructively otherwise. Effect #1 only makes sense quantum-mechanically, and the interference involves a particle interfering with itself. Effect #2 is true classically, and it does not seem to require self-interference. Is there some sense in which effect #2 is also self-interference? That doesn't seem right to me, because you can produce interference effects from two different light sources, where it doesn't make sense (or at least not in any simple way) to think that it's the same photon interfering with itself.