When scientists found out about the atomic nucleus, they questioned why the protons, which have positive charge should remain so close without repelling. The scientists realized that there were new forces at work and the secrets must lie within the nucleus. They knew that the force which holds the protons together must be much stronger than the electromagnetic force and that the force must act over very small distances (otherwise they would have seen this force in interactions between the nucleus and the outer electrons).
In 1932, Werner Heisenberg concluded that charged particles bounce photons of light back and forth between them. This exchange of photons is the way that electromagnetic forces act between the particles. The theory is that a proton shoots a photon at the electron and the electron shoots a photon back at the proton. These photon exchanges go on all the time, very rapidly. However, because they are never seen, Heisenberg called these exchange particles, virtual photons. (Virtual meaning, not exactly ‘real’.)
