Feynman diagram

A Feynman diagram is a bookkeeping device for performing calculations in quantum field theory, invented by American physicist Richard Feynman. They are also (rarely) referred to as Stückelberg diagrams or (for a subset of special cases) penguin diagrams.

Related Topics:
Quantum field theory - Physicist - Richard Feynman

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The interaction between two particles is quantified by the cross section corresponding to their collision. If this interaction is not too large, i.e. if it can be tackled via perturbation theory, this cross section (or more precisely the corresponding time evolution operator, propagator or S matrix) can be expressed as a sum of terms (the Dyson series) which can be described as a short story in time sounding like:

Related Topics:
Interaction - Cross section - Perturbation theory - Time evolution operator - Propagator - S matrix - Dyson series

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• (once upon a time) two particle were moving freely with some relative speed (one draws two lines --edges-- going upwards),
• they met each other (the two lines meet at a first point -- vertex),
• took a stroll together on a common path (the lines merge in one vertical line)
• and, then separated again (second vertex)
• but they realized their speed had changed and they were not really the same anymore (two lines are drawn upwards coming from the last vertex -- maybe in a different style for symbolizing the change experienced by the particles).

And this nice story can be drawn as a diagram (where the evolving time is the upwards direction) which is much easier to remember than the corresponding mathematical formula in the Dyson series. These diagrams are called Feynman diagrams. They are of course meaningful only if the Dyson series converges fast. Their easy story telling character and the similarity with the early bubble chamber experiments have made the Feynman diagrams very popular.

Related Topics:
Dyson series - Bubble chamber

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Feynman diagram: Motivation and history ►