I understand (vaguely) how momentum is exchanged via virtual photons in electromagnetic interactions. But how do the changes in kinetic energy occur? For instance, a proton and electron are at rest separated by a distance x. The proton emits a virtual photon and recoils in the direction of the electron with momentum p. The electron absorbs the virtual photon and recoils in the direction of the proton with momentum -p. But the kinetic energy of the both the proton and the electron have increased. From whence this naughty energy appear? If my example doesn't work, please replace with any other where a force acting on a charge changes its energy.