>but yes, each "photon" really does interact with EVERY ELECTRON IN THE CRYSTAL at once.
Take a crystal from the cave...10m long..perhaps not 'really'... It is however helpful to think of a coherence volume of the photon in which it interacts with every atom. We had some discussions and estimates before, and starting from transition lifetime we ended up with an estimated 'single photon coherence length' of a several 1000 A to several 10um or so on synchrotron sources. These numbers at least seem reasonable and are not contradicting any practical observations. What 'really' happens depends on your definition of reality....I like the photon annihilation- creation picture, but in condensed multi-particle matter that also stretches at least my imagination... Best, BR
