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Quantum electrodynamics (also called QED for short) is the quantum theory of electricity and magnetism. QED describes how electric and magnetic forces work on the scale of things smaller than atoms. QED also describes how single photons of light behave. QED is one of the most well-tested theories in physics.
Hans Bethe
Feynman (center) and Oppenheimer (right) at Los Alamos.
Feynman replaces complex numbers with spinning arrows, which start at emission and end at detection of a particle. The sum of all resulting arrows represents the total probability of the event. In this diagram, light emitted by the source S bounces off a few segments of the mirror (in blue) before reaching the detector at P. The sum of all paths must be taken into account. The graph below depicts the total time spent to traverse each of the paths above.
Multiplication of probability amplitudes as complex numbers
