Quantum Electrodynamics
During the 1910s and 1920s, the power of quantum theory became obvious to most physicists. Its success was demonstrated most notably in the explanation of atomic phenomena, as in the Bohr model of the atom and its subsequent refinements. The natural next step was to apply quantum theory to the field of electromagnetic phenomena. The development of quantum electrodynamics (QED) was begun in the late 1920s by Paul Dirac and extended by a number of theorists, including Werner Heisenberg, Wolfgang Pauli, and Enrico Fermi.
QED theory attempts to use quantum principles to explain the properties of an electromagnetic field and its interaction with electrically charged particles. For example, it tries to describe what happens when an electron travels through an electromagnetic field.
The simplest electromagnetic phenomena yield rather easily to quantum analysis. For example, in describing an electromagnetic field, Dirac pictured the field as consisting of a number of harmonic oscillators in various states of excitation. Each oscillator contributes to the state of the field by absorbing and emitting photons. As specified by quantum theory, each oscillator can take on only discrete values that are some multiple of .
Other electromagnetic phenomena present more difficult problems. For example, Dirac was able to derive an equation that describes the motion of an electron in quantum mechanical terms. However, the solution to that equation has four components, two of which appear to represent states of negative energy. Dirac was able to explain this apparently non-physical result by introducing the concept of an antielectron, or positron. Carl David Anderson's discovery of the positron in 1932 provided valuable confirmation for Dirac's theory.
Another problem in QED arises with efforts to describe the interaction of a charged particle with an electromagnetic field. Straight-forward application of quantum theory to this problem originally resulted in equations that sometimes have infinite answers. For two decades, this problem remained unsolved since physicists had no idea as to how these results should be interpreted.
During the 1940s, the problem was resolved independently and nearly simultaneously by Sin-itiro Tomonaga in Japan and Julian Schwinger and Richard Feynman in the United States. The three investigators suggested that infinite results were obtained because existing equations made use of the "bare" mass and charge of an electron, that is, the mass and charge they would have at rest in isolation from an electromagnetic field. They proposed using instead the actual mass and charge of an electron as measured within the field.
This process is called renormalization. Essentially it involve subtracting out a number of terms in QED equations that lead to infinite results. The renormalized equations can then be solved to give finite answers which can, in turn, be compared to experimental results.
In its modern form, QED is an enormously successful theory. It can be used to predict with a high degree of precision such phenomena as the magnetic momentum of the electron and other particles, the detailed structure of spectroscopic lines, the properties of cosmic ray showers, superconductivity in metals, and the superfluidity of helium.
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