Virtual photons are not "observed" because they have a deficit of energy. When they are emitted from particles momentum is conserved but energy is not conserved. The deficit of energy is accommodated with the uncertainty relation: delta_E * delta_t of the order of h_bar Now if delta_t is large , [tex]\Delta[/tex]E becomes very small and the uncertainty in energy disappears and we should have "true photons". The "infinite range" of electromagnetic interaction lends support to this assertion. But an electromagnetic interaction is never accompanied by the mediation of true photons even when we consider charges at macroscopic distances.