I have heard a virtual phton as used in QED defined as being forced carriers between two fermions which last for very short periods of time. A couple of questions about this: 1) how do we know these are photons and not some other force carrier if we cannot detect them directly? 2) can a frequency or wavelength be calculated or measured for these photons? Does it depend on, for example, how fast two electrons are approaching each other and therefore the energy required to push them apart? 3) If we put a detector between two magnets, or two charged particles, can there be any chance of catching these photon? For example, if they are interacting in the x-ray range, would putting an x-ray film between two magnets catch some of the four times going back-and-forth between these two magnets? Thanks.