When a single electron collides with another single electron, quantum field theory predicts the scattering probability distribution as a sum of Feynman diagrams

When a single electron collides with another single electron, quantum field theory predicts the scattering probability distribution as a sum of Feynman diagrams. All of them must have two incoming and two outgoing legs, so the only difference between them is the number of internal loops. The basic ones, having no internal loops, is called the "tree level diagrams" (there may be one or more of these). The rest, having internal loops, can be thought of as higher order corrections to the results you get from just the tree-level diagrams. Now, these internal loops are best thought of as vacuum fluctuations that pop out and interact with the two scattering particles. Say two electrons scatter: the tree-level diagram is two electrons approaching, interacting via a virtual photon (the force carrier for electromagnetism) and then going away from each other again. (There is also another tree-level diagram for this.) And then there are higher-order contributions, e.g. from the case where the virtual photon fluctuates into an electron-positron pair, which then decays back into a photon, in between the two electrons. This is good for more detail (but a long series): http://www.quantumdiaries.org/2010/02/14/lets-draw-feynman-diagams/ http://www.quantumdiaries.org/2010/02/14/lets-draw-feynman-d...