QFT states that all forces are due to the exchange of virtual particles, I'm interested in photons and thier role in the EM force. I was wondering what the maximum time allowed for existance of an exchange photon is when it's emmitted from a proton in the nucleus of an atom. I'm thinkin' that the number of protons in the nucleus (possibly neutrons too), and number of electrons in the cloud enveloping the nucleus has a relation to this also. My question is, what is the average time of existance of a virtual photon in these conditions, be it a function of Z and or #e or not? Do I have to give more constraints like the atom is at rest, in zero gravity (flat spacetime), not in the vicinity of any other atoms? Can someone direct me towards a website that has some tables of virtual photon lifetimes for various situations like the above mentioned ones? Does the energy of the emmitting particle have anything to do with the energy of the photon emmitted?