quantum electrodynamics (QED)
/ QED = cue-ee-dee /
Quantum electrodynamics is the quantum theory of light and electrically charged matter — above all, electrons and photons. It answers a deceptively simple question: when charged particles push and pull on each other, or when light is emitted, absorbed, and scattered, what is really going on at the deepest level? QED says the answer is built from one tiny event repeated over and over: a charged particle emitting or absorbing a particle of light.
In practice, QED is a recipe for calculating the probability of any process involving electrons, positrons, and photons. You list every way a process can happen as a series of basic steps, assign each step a number, and add up all the possibilities. Because the basic interaction is weak — the strength is set by the fine-structure constant, about 1/137 — the simplest pictures dominate and small corrections can be added in order. This makes QED a perturbation theory: an answer built as a main term plus successively smaller refinements.
QED matters because it was the first quantum field theory to fully work, and it remains the most precisely tested theory in all of physics. Its prediction for the electron's magnetic moment agrees with experiment to about twelve decimal places — like measuring the distance from New York to Los Angeles to the width of a human hair. Every other force theory in the Standard Model, including the strong and weak forces, was built by copying QED's blueprint, so understanding QED is the gateway to understanding all of particle physics.
When two electrons repel each other, QED does not picture an invisible spring between them. Instead it pictures one electron tossing a photon to the other, like two ice skaters pushing apart by throwing a ball back and forth. Sum over every such toss and you recover the familiar electric force — but now derived from a single quantum rule.
QED rebuilds the electric force out of countless tiny photon exchanges.
The infinite-looking sums in QED only work because the coupling is small; this is why QED is so successful while the strong force, with a large coupling, needs different methods.