Quantum Electrodynamics

quantum electrodynamics (QED), quantum field theory that describes the properties of electromagnetic radiation and its interaction with electrically charged matter in the framework of quantum theory. QED deals with processes involving the creation of elementary particles from electromagnetic energy, and with the reverse processes in which a particle and its antiparticle annihilate each other and produce energy. The fundamental equations of QED apply to the emission and absorption of light by atoms and the basic interactions of light with electrons and other elementary particles. Charged particles interact by emitting and absorbing photons, the particles of light that transmit electromagnetic forces. For this reason, QED is also known as the quantum theory of light.

QED is based on the elements of quantum mechanics laid down by such physicists as P. A. M. Dirac, W. Heisenberg, and W. Pauli during the 1920s, when photons were first postulated. In 1928 Dirac discovered an equation describing the motion of electrons that incorporated both the requirements of quantum theory and the theory of special relativity. During the 1930s, however, it became clear that QED as it was then postulated gave the wrong answers for some relatively elementary problems. For example, although QED correctly described the magnetic properties of the electron and its antiparticle, the positron, it proved difficult to calculate specific physical quantities such as the mass and charge of the particles. It was not until the late 1940s, when experiments conducted during World War II that had used microwave techniques stimulated further work, that these difficulties were resolved. Proceeding independently, Freeman J. Dyson, Richard P. Feynman and Julian S. Schwinger in the United States and Shinichiro Tomonaga in Japan refined and fully developed QED. They showed that two charged particles can interact in a series of processes of increasing complexity, and that each of these processes can be represented graphically through a diagramming technique developed by Feynman. Not only do these diagrams provide an intuitive picture of the process but they show how to precisely calculate the variables involved. The mathematical structures of QED later were adapted to the study of the strong interactions between quarks, which is called quantum chromodynamics.

Bibliography

See R. P. Feynman, QED (1985); P. W. Milonni, The Quantum Vacuum: An Introduction to Quantum Electrodynamics (1994); S. S. Schweber, QED and the Men Who Made It: Dyson, Feynman, Schwinger, and Tomonaga (1994); G. Scharf, Finite Quantum Electrodynamics: The Causal Approach (1995).

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quantum electrodynamics (QED)

Quantum theory of the interactions of charged particles with the electromagnetic field. It describes the interactions of light with matter as well as those of charged particles with each other. Its foundations were laid by P. A. M. Dirac when he discovered an equation describing the motion and spin of electrons that incorporated both quantum mechanics and the theory of special relativity. The theory, as refined and developed in the late 1940s, rests on the idea that charged particles interact by emitting and absorbing photons. It has become a model for other quantum field theories.

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quantum electrodynamics [′kwän·təm i¦lek·trō·dī′nam·iks]

(quantum mechanics)

The quantum theory of electromagnetic radiation, synthesizing the wave and corpuscular pictures, and of the interaction of radiation with electrically charged matter, in particular with atoms and their constituent electrons. Also known as quantum theory of light; quantum theory of radiation.

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Quantum electrodynamics

The field of physics that studies the interaction of electromagnetic radiation with electrically charged matter within the framework of relativity and quantum mechanics. It is the fundamental theory underlying all disciplines of science concerned with electromagnetism, such as atomic physics, chemistry, biology, the theory of bulk matter, and electromagnetic radiation.

Efforts to formulate quantum electrodynamics (QED) were initiated by P. A. M. Dirac, W. Heisenberg, and W. Pauli soon after quantum mechanics was established. The first step was to remedy the obvious shortcoming of quantum mechanics: that it applies only to the case where particle speeds are small compared with that of light, c. This led to Dirac's discovery of a relativistic wave equation, in which the wave function has four components and is multiplied by certain 4 × 4 matrices. His equation incorporates in a natural manner the observed electron-spin angular momentum, which implies that the electron is a tiny magnet. The strength of this magnet (magnetic moment) was predicted by Dirac and agreed with observation. A detailed prediction of the hydrogen spectrum was also in good agreement with experiment. See Atomic structure and spectra, Electron spin

In order to go beyond this initial success and calculate higher-order effects, however, the interaction of charge and electromagnetic field had to be treated dynamically. To begin with, a good theoretical framework had to be found for describing the wave-particle duality of light, that is, the experimentally well-established fact that light behaves like a particle (photon) in some cases but like a wave in others. Similarly, the electron manifests wave-particle duality, another observed fact. Once this problem was settled, the next question was how to deal with the interaction of charge and electromagnetic field. It is here that the theory ran into severe difficulties. Its predictions often diverged when attempts were made to calculate beyond lowest-order approximations. This inhibited the further development of the theory for nearly 20 years. Stimulated by spectroscopic experiments vastly refined by microwave technology developed during World War II, however, S. Tomonaga, R. P. Feynman, and J. Schwinger discovered that the difficulties disappear if all observable quantities are expressed in terms of the experimentally measured charge and mass of the electron. With the discovery of this procedure, called renormalization, quantum electrodynamics became a theory in which all higher-order corrections are finite and well defined. See Photon, Quantum mechanics, Relativistic quantum theory, Relativity, Wave mechanics

Quantum electrodynamics is the first physical theory ever developed that has no obvious intrinsic limitation and describes physical quantities from first principles. Nature accommodates forces other than the electromagnetic force, such as those responsible for radioactive disintegration of heavy nuclei (called the weak force) and the force that binds the nucleus together (called the strong force). A theory called the standard model has been developed which unifies the three forces and accounts for all experimental data from very low to extremely high energies. This does not mean, however, that quantum electrodynamics fails at high energies. It simply means that the real world has forces other than electromagnetism.

High-precision tests have provided excellent confirmation for the validity of the renormalization theory of quantum electrodynamics. In the high-energy regime, tests using electron-positron colliding-beam facilities at various high-energy physics laboratories have confirmed the predictions of quantum electrodynamics at center-of-mass energies up to 1.8 × 10¹¹ electronvolts (180 GeV). The uncertainty principle implies that this is equivalent to saying that quantum electrodynamics is valid down to about 10^-17 meter, a distance 100 times shorter than the radius of the proton.

High-precision tests of quantum electrodynamics have also been carried out at low energies by using various simple atomic systems. The most accurate is that of the measurement of the magnetic moment of the electron, or the gyromagnetic ratio g, the ratio of spin and rotation frequencies, which is correctly predicted by quantum electrodynamics to 12 significant figures. This is the most precise confirmation of any theory ever carried out. See Quantum field theory

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Quantum Electrodynamics 

the quantum theory of electromagnetic processes; the most thoroughly studied area of quantum field theory. Classical electrodynamics takes into account only the continuous properties of an electromagnetic field. However, quantum electrodynamics is based on the concept of an electromagnetic field that also has some discontinuous (discrete) properties. The carriers of such properties are field quanta, or photons. Photons have zero rest mass, energy ε = hv, and momentum p = (h/2π)k, where h is Planck’s constant, v is the frequency of the electromagnetic wave, and k is a wave vector of magnitude k = 2πv/c (c is the speed of light), oriented in the direction of propagation of the wave. In quantum electrodynamics the interaction of electromagnetic radiation with charged particles is viewed as absorption and emission of photons by the particles.

Quantum electrodynamics quantitatively explains the effects of the interaction of radiation with matter (emission, absorption, and dispersion) and furnishes a consistent description of electromagnetic interactions among charged particles. Some of the highly important problems not explained by classical electrodynamics but successfully solved by quantum electrodynamics are the thermal radiation of bodies, X-ray scattering on free (or, more accurately, weakly bound) electrons (the Compton effect), emission and absorption of photons by atoms and more complex systems, and emission of photons during the scattering of fast electrons in external fields (bremsstrahlung). Quantum electronics describes these phenomena, as well as any other phenomena concerning the interaction of electromagnetic radiation with electrons and positrons, with a high degree of accuracy. The theory is less successful in interpreting other processes, since these processes are decisively influenced not only by electromagnetic interactions but also by interactions of other types (strong and weak interactions).

The gradual construction of quantum electrodynamics has led to a reexamination of classical concepts dealing with the motion of matter.

V. I. GRIGOR’EV