Virtual photon exchange, running coupling, and loop diagrams
Quantum electrodynamics describes how light and charged matter interact, and it is the most precisely tested theory in science. Here are its main ideas, one demo at a time.
Watch two electrons swap a virtual photon and push each other apart. Turn up the coupling strength and see the deflection grow.
Quantum Electrodynamics (QED) describes how light and matter interact. It is the most precisely tested theory in all of science — its predictions for the electron's magnetic moment agree with experiment to 12 decimal places. QED is the prototype for all modern quantum field theories.
Electromagnetic forces arise from the exchange of virtual photons. Two electrons repel each other by tossing a photon back and forth. The Feynman diagram and the physical scattering are two views of the same process.
Move the cursor across the plot to read α at different energies. Notice that it climbs from 1/137 as the energy rises.
The fine structure constant α ≈ 1/137 is not actually constant. Due to vacuum polarization (virtual e⁺e⁻ pairs screening the charge), α increases at higher energies. At the Z boson mass, α ≈ 1/128.
Step through with Next to see the loop appear, its integral blow up, and renormalization turn the infinity into a finite, measured mass.
An electron can emit and reabsorb a virtual photon, creating a loop in its propagator. This loop integral diverges — but through the miracle of renormalization, we absorb the infinity into the definition of the electron's mass and charge, leaving finite, measurable predictions.
Slide from Dirac to QED and watch the 2S and 2P levels of hydrogen split apart.
In 1947, Willis Lamb measured a tiny energy splitting between the 2S₁/₂ and 2P₁/₂ levels of hydrogen — levels that Dirac theory predicted should be degenerate. This ~1058 MHz shift was the first experimental proof of QED radiative corrections.
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