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QED: the theory of light and electrons
1948· Theory
ProvenTested many times; part of working physics.
Quantum electrodynamics: the quantum theory of how light and charged particles like electrons interact.
See it on the Science map →How it works
QED says electrons and other charged particles interact by exchanging photons, particles of light. Every electric, magnetic and chemical effect, from a lightning bolt to the color of copper, follows from it. Early versions gave infinite nonsense answers. In 1947-49, Feynman, Julian Schwinger, Sin-Itiro Tomonaga and Freeman Dyson found a consistent way to remove the infinities, called renormalization, leaving precise finite predictions.
Why it matters
It explains all of chemistry and light at the deepest level, and became the template for the rest of particle physics.
How we know
Its prediction of the electron's magnetic strength matches measurement to better than one part in a billion, among the best agreements in science. Nobel Prize 1965 to Feynman, Schwinger and Tomonaga.
Connections (4)
▸Electromagnetism (Maxwell's equations) was made quantum in QED: the theory of light and electronsProven
The relationship
Maxwell's smooth electromagnetic field was rewritten under quantum rules: its energy comes only in lumps, which are photons. Charged particles interact by emitting and absorbing photons. In everyday conditions, with huge numbers of photons, QED reproduces Maxwell's equations exactly, the way a crowd's average behavior hides individual people.
How the link was found
Dirac began quantizing the electromagnetic field in 1927. The working theory came in 1947-49 from Feynman, Schwinger, Tomonaga and Dyson.
Why it's proven
QED reproduces all of Maxwell's classical results and adds quantum corrections, like the Lamb shift (1947), measured to extreme precision.
Open questions and mysteries
Why electric charge comes in exactly the units it does, and why the strength of electromagnetism (about 1/137) has its value, are unexplained.
▸Richard Feynman co-created QED: the theory of light and electronsProven
The relationship
Feynman drew each possible particle interaction as a simple diagram: lines for electrons and photons, joints where they meet. Each diagram stands for a term in the calculation, so adding up diagrams gives the answer. Combined with renormalization, this turned QED from an infinity-plagued theory into a practical, precise tool.
How the link was found
Feynman, at Cornell, presented the method in 1948 and published it in 1949. Freeman Dyson showed in 1949 it was equivalent to the methods of Schwinger and Tomonaga.
Why it's proven
Calculations using his diagrams match the electron's magnetism to better than a part in a billion. Nobel Prize 1965.
Open questions and mysteries
Why renormalization works so well is now understood better, but the sum of all diagrams may not strictly converge.
▸Quantum field theory had its first triumph in QED: the theory of light and electronsProven
The relationship
QED was the first quantum field theory that produced precise, finite answers. It applied the framework to just two fields, electrons and light, and the predictions matched experiment to many decimal places. That proved the framework worked and became the template used for the strong and weak forces.
How the link was found
Feynman, Schwinger, Tomonaga and Dyson, 1947-49, prompted by Willis Lamb's 1947 measurement of a tiny shift in hydrogen's energy levels.
Why it's proven
Schwinger's 1948 calculation of the electron's extra magnetism matched Polykarp Kusch's measurement. Agreement has since been pushed to better than a part in a billion.
Open questions and mysteries
QED probably fails at extremely high energies, where its strength grows; that is likely where it merges into a deeper theory.
▸QED: the theory of light and electrons is one part of The Standard ModelProven
The relationship
QED, the theory of light and electrons, became part of a larger 'electroweak' theory. At everyday energies, electromagnetism and the weak force look separate. At energies above about 100 times a proton's mass, they behave as one. QED is the low-energy face of that combined theory, which is one pillar of the Standard Model.
How the link was found
Sheldon Glashow (1961), Steven Weinberg (1967) and Abdus Salam (1968) built the electroweak theory; Nobel 1979.
Why it's proven
Predicted 'neutral current' interactions were seen at CERN in 1973, and the W and Z particles in 1983, at the predicted masses.
Open questions and mysteries
Why the electroweak force split into two at a particular energy is described, but not deeply explained.