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quantum electrodynamics (QED)





A quantum field theory that describes the interactions of electrically charged particles, such as electrons, through the electromagnetic field. Developed in the 1940s and '50s, quantum electrodynamics (QED) a mathematical description of the interaction of electromagnetic radiation, including light, with matter.


Discovery of QED

The key breakthrough which eventually led to QED came in 1927 when Paul Dirac successfully blended quantum theory and special relativity in his theory of the electron. A big surprise of his work (not least to Dirac) was the prediction of the first antiparticle – a positively-charged electron called the positron, which was found six years later by the American Carl Anderson. Armed with Dirac's electron theory, scientists hoped quickly to inject quantum theory into the whole of electromagnetism. But this didn't prove so straightforward. Whenever attempts were made to solve the quantized versions of Maxwell's equations, the results blew up in the faces of the frustrated theorists. Instead of getting sensible values for quantities such as the mass and charge of the electron, what popped up was the silly answer of infinity. Finally, in the late 1940s, a trio of physicists – Richard Feynman at Cornell, Julian Schwinger at Harvard, and Shin'ichiro Tomonaga in Japan – pulled a mathematical rabbit out of the hat called renormalization that saved the day. The "War of Infinities," as it became known, fought in the shadow of a far more momentous conflict, had been won, its upshot: quantum electrodynamics. So successful was this new theory, that it enabled the properties of the electron, and its heavier cousin the muon, to be calculated correctly to an astonishing ten significant figures after the decimal point.


Exchange particles

QED also gives us a physical insight into how a force is transmitted between two matter particles, such as electrons. Newton had problems with the idea of action-at-a-distance, as did Maxwell. Quantum mechanics, however, profoundly alters classical field theory. In the classical scheme of things, energy and momentum are continuous quantities somehow (though one's never quite sure how) carried by the field; in quantum mechanics, however, energy and momentum are fragmented into tiny discrete units – quanta – which show up as particles. These field particles act like little messengers, conveying the force by traveling between the interacting particles of matter, rather as a baseball pitcher transfers energy and momentum to the catcher when he hurls a ball. In QED, the messenger or "exchange" particles are none other than photons – particles of light. In other words, photons are the quanta of the electromagnetic field.

Each of the forces is mediated by its own kind of exchange particle, which has a definite rest mass (zero in the case of photons) and an intrinsic spin, or angular momentum, that can take integral values, such as 0, 1 or 2. Such particles are known as bosons. Particles of matter, on the other hand, have half-integral spin, such as 1/2 or 3/2, and belong to the family known as fermions. The exchange particles, or bosons, produce forces with distance ranges that vary as one divided by the particles' mass. Consequently, forces mediated by massive particles, such as the weak force, act over only a limited range, whereas forces mediated by massless particles, such as electromagnetism and gravity, have an apparently infinite range with the force diminishing in strength inversely as the square of the distance between the interacting particles.


Beyond QED

So amazingly effective did QED prove that researchers began looking for ways to extend and adapt its mathematical framework to the other forces in nature. A lot of effort went especially into trying to describe the weak force with equations similar in form to those of quantum electrodynamics. This work eventually led to the electroweak theory, which allows electromagnetism and the weak force to be described within a unified theoretical scheme.


Related entry

   • field theory


Related category

   • PARTICLE PHYSICS