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Feynman diagrams

These were developed by the American physicist Richard Feynman as a way of showing the interaction between two sub-nuclear particles. They are a way of representing what is happening between the two particles during an interaction. (In the following Feynman diagrams time goes from bottom to top). Each point where lines come together is called a vortex and at each vortex charge, baryon number and lepton number must be conserved.


An example of a Feynman diagram for an electromagnetic interaction is shown in Figure 1. It shows the interaction between two electrons. In classical physics the electrons, both with a negative charge would repel each other. The Feynman diagram shows that this repulsion occurs because of the interchange of photons. Each electron emits a photon which is then absorbed by the other electron. The photons in the interaction are known as virtual photons because they are emitted and absorbed in a time so short that the uncertainty principle is not violated. (To simplify the diagram only one of the virtual photons is shown).
A line which begins and ends in the diagram (as with the wavy line in Figure 1) represents a virtual particle.

Figure 2 shows the interaction between an up quark and a down quark due to the strong force between them. Here the interaction is due to exchange of gluons.
(As before to simplify the diagram only one of the virtual gluons is shown)

Figure 3 shows the interaction between a proton and a neutron. Here the interaction is mediated by the exchange of pions.

The weak force interaction of Figure 4 shows the Feynman diagram for beta decay.

© Keith Gibbs 2010