I am reading Feynman's book QED: The Strange Theory of Light and Matter". What I am getting out of this is that a photon is like a tiny harmonic oscillator traveling through space. Each harmonic oscillator has frequency and phase. If two photons have the same source and start out nearly in phase (and have same frequency) and take nearly the same path(in terms of time) then they will still be nearly in phase when they reach their terminal point and they will constructively interfere.? As I think about it, they do not have to start at the same point and do not have to be in phase when they start. All they have to do is strike the same point on the detector screen and travel paths, such that, they are in phase when they strike the detector screen.