As far as I know, forces are supposed to be mediated by virtual particles. Let's take the example of a magnetic field, mediated by photons. This seems to be a good idea, because it sort of eliminates the nasty concept of a field, which is just an abstract concept. This has been bothering me since childhood: how does a magnet know that there is another magnet nearby? :-) So they are supposed to be radiating photons in all directions. Sure, magnets have little currents in them, but let's take two electrons floating in the middle of nothing (maybe alone in the universe). They are also radiating virtual photons all the time? (There is nothing to excite them, though.) After all, they have to let each other know somehow they are there. So do these photons have a specific frequency related to some property of the electron? When one sends such a virtual photon to the other, is the other instantly repelled, or does it also send another photon back and they first agree that they are electrons and that they should move away from each other? What do we mean by exchange? How does it work between an electron and a proton? How is the virtual photon emitted by a proton any different from that of an electron?