So this is basic question but the more I read the more I am confusing myself! I was assuming that the wavelength of a photon was the same wavelength as the associated probability amplitude (although a complex number). So to make constructive interference it means one path takes say ten wavelengths more to arrive at the same point in the configuration space....but doesn't that mean path has taken more time than the other? As the photon travels at the speed of light I assume (again probably incorrectly) that if the time was measured it would be seen as taking the shortest path (I am postulating minimum action)....but if that is the case the other longer path hasn't even had enough time to arrive there so it can't interfere with it yet! Sure - I probably have mix up a few things....some clarity and good further reading suggestions would great.... Thanks.