In electrostatics, charged particles attract or repel via exchange of virtual photons. But when charges wiggle (or more generally accelerate), they generate electromagnetic waves which are often called "real" photons. For such a photon to be a real particle, its dispersion relation must sit on-shell. This requires the precise measurement of its momentum and energy. However, uncertainty principle says we can't get precise measurement unless the field is indefinite in time and space.
In my head, a massless spin-1 photon field exists indefinitely without any beginning or end. On the other hand, the EM field generated by a wiggling charge has a beginning so that it can't be infinite in time and space. So, why is this EM field called a "real" photon? Or is it an asymptotically real photon but indeed virtual? Or am I misunderstanding something?
I read the Wikipedia article about virtual particle. According to the article, the Casimir effect is due to virtual photons interacting with metal plates. I don't understand why the photon here is virtual. Can anybody explain it, too?
For more clarification, the notion of real particle conflicts in my head. I think it is on-shell particle and it has either a beginning or end or none of these in a Feynman diagram. However these two concepts are conflicting because for the particle newly annihilated or created, the energy and momentum relation must remain vague due to uncertainty principle. I can say a particle or field is on-shell only when it exists indefinitely like a planewave. Can anybody resolve it?