Hi, Carter; Cool! Even Feynman confessed that he didn't Really understand that (and that no one did, does or will?) But, regarding the ~duality~ (and, hence, implicitly, your "where is/was/will be the photon, as it propagates thru spacetime" query). Here's my take on it, which sorta eliminates the ~paradox~ and has other coolish aspects: NOTICE that the ONLY TWO times the photon manifests its ~light particle~ aspect is when it is Emitted and when it is Absorbed. More generally, only when it Manifests interaction with other particles. For example, when we see the little bitsy flash ~Right There~ on the phosphor screen... a (bunch of) photons were born There and Then, in space-time. And, ever so slightly later, some of them found their way to your retina There' and Then' in space-time. Now HERE's a cool extended note regarding those two events: Unto Themselves, Because they were [identically, in the rigerous sense] traveling from there to here ~at the speed of light/information~ Unto themselves, it took Zero Time to make the trip. They were born and died instantaneously/simultaneously. So far, so good. NOW... during the trip, it is ONLY the EM field that ~exists~ [Here, note that the EM field is nothing more or less than the space-time Probability Density Distribution Function associated with the Photon(s)] Feynman, most notably, noticed that one can and should sum ALL POSSIBLE contributors over ALL space-time. That's his ~sum over ALL possible histories~ (see where the word probability and possible mesh?!) So... where "The Photon" was as it made the trip is, best stated... as "Everywhere and Everywhen" as given by Feynman's (infinite number of) possible histories. This is Most Simply manifested in a Twyman Green Interferometer. Each and every photon ~travels both arms~. That is not an analogy... it is exactly what happens! And, this is so easily demonstrated by blocking one, other, both or neither arms. ALL four cases instantly obey The inclusion and exclusion of possible histories. It's as simple as that! Sorry for being so long-winded. I hope that helps. I, and friends, have "argued" over this stuff for decades. PS I was an aerospace scientist, specializing in optical metrology. We did a lot of complex wavefront shaping stuff for Uncle Sam, so were heavily immersed in this stuff. The EM field is complex (amplitude and phase) throughout space-time. You might say it happily carries the essence of (all) photons as they interact throughout the universe. Ummm... if this is for some homework question, just say you discussed with others. Your teacher might be looking for a much simpler answer, or one that disagrees with mine! Tom
