Quantum Electrodynamics

I

Introduction

Quantum Electrodynamics or QED (QED), in physics, a theory that describes how electrically charged particles and electromagnetic fields interact. An electromagnetic field is an area of space containing vibrating electric and magnetic fields. These fields exert a force on charged particles in their vicinity (see Field (Physics)). QED describes both the particles and the electromagnetic field in quantum-mechanical terms (see Quantum Theory), that is, in terms of tiny, individual units of energy. Quantum mechanics is a theory and set of rules that describes both light (electromagnetic radiation) and particles as having characteristics of both waves and particles. QED also uses the special theory of relativity (see Relativity). German-born American physicist Albert Einstein developed the special theory of relativity to describe how matter behaves as its velocity approaches the speed of light. The special theory of relativity also describes situations in which matter is converted to energy, and vice versa. Quantum electrodynamics provided physicists with new insights into the behavior of matter and energy. For instance, the particle description of light explains much about electricity and magnetism that physicists could not explain before QED.

II

Background of QED

The need for a quantum-mechanical explanation of light arose when scientists observed phenomena that they could not explain with the classical theory of electromagnetic radiation. British physicist James Clerk Maxwell developed the classical theory of electromagnetic radiation in the 1860s. Maxwell was the first scientist to successfully predict the behavior of light (a form of electromagnetic radiation) by describing light as waves of vibrating electric and magnetic fields.

Maxwell’s theory broke down, however, in 1887 when German physicist Heinrich Hertz noticed that light shining on a piece of metal produced an electric current. This phenomenon is now known as the photoelectric effect. An electric current in metal is made up of electrons (tiny, negatively charged particles) from the metal atoms moving through the metal. Maxwell’s classical theory of light predicted that the energy of the electrons in the current produced by the photoelectric effect should depend on the intensity of the light. This would mean that as you smoothly increased the amount of light, it would smoothly increase the amount of energy in the electrons. Hertz found, however, that this was not the case. Instead, the energy of the electrons was only dependent on the wavelength of the light, which is related to the amount of energy carried by each wave of light. In 1900 German physicist Max Planck did further work on light that suggested that light may come in tiny packets, or quanta, of energy. Einstein suggested in 1905 that light could be described as being composed of particles called photons.

Most prominent physicists were skeptical about Einstein’s description of light until the discovery of a phenomenon, called the Compton effect, by American physicist Arthur Compton in 1923. Compton examined interactions between an electron and a tiny amount of light energy, the smallest amount he could produce. He discovered that a collision between an electron and light knocked the electron off course and changed the wavelength of the light. The fact that light could change the path of the electron suggested that light has momentum, a property normally associated with particles, not electromagnetic waves. The amount of the deflection of the electron depended on the wavelength of the light, suggesting that a photon’s wavelength and momentum are related. Maxwell’s theory did not explain either of those results, and physicists embarked on an effort to describe light and its interaction with particles.

---

---

While physicists were reevaluating the ways in which they described light, they were also developing an important new description of matter, called quantum mechanics. Quantum mechanics describes both light and matter in the terms of both waves and particles. Many of the same physicists worked on both problems. The first elements of quantum mechanics were put forth by Planck, Einstein, Danish physicist Niels Bohr, German physicist Arnold Sommerfeld, and others from 1900 to 1922.

III

Development of the Theory

German physicists Max Born, Werner Heisenberg, and Ernst Pascual Jordan published the first QED theory in 1926. They found that the energy and momentum of the electric and magnetic fields in a light ray come in bundles—the particle-like photons that Planck and Einstein had suggested earlier. In 1927 British physicist Paul A. M. Dirac applied the new rules of the quantum theory to electromagnetic radiation. This resulted in a theory that explained how atoms emit and absorb photons of electromagnetic radiation. In 1928 Dirac further advanced QED by constructing a description of electrons that was consistent with both quantum mechanics and the special theory of relativity. Reconciling quantum mechanics and relativity is important to QED because quantum mechanics provides the most accurate description of matter, but any description involving photons and velocities near the speed of light must involve special relativity. Dirac’s equation predicted the existence of antimatter, or a substance composed of particles that are opposite in electric and magnetic properties to the particles that make up ordinary matter.

The QED of the 1920s and early 1930s provided results that nearly matched what physicists had observed experimentally about the interaction of light and particles. The results were only approximate, however. In the 1930s, physicists such as Heisenberg, Austrian-born Swiss physicist Wolfgang Pauli, and American physicist J. Robert Oppenheimer added more corrections to the theory of QED to make the theory produce more accurate answers, but their corrections introduced some troubling infinite terms in the equations of QED. Infinite terms are parts of the equation that have infinitely large or infinitely small solutions. They are undesirable because they make the equations meaningless unless physicists ignore the terms or find ways around them, both solutions that are philosophically unpleasant. Physicists struggled for almost two decades to remove the infinite terms from QED equations and keep the theory consistent with increasingly accurate experimental measurements.

Finally, in the late 1940s, American physicists Willis Lamb and Robert Retherford made an important breakthrough while studying the energy state of a hydrogen atom. The energy state of an atom depends on the arrangement of its outer electrons. Electrons can jump from one orbit to another, and the farther they move from the nucleus, the higher the energy state of the atom. Lamb and Retherford showed that an energy state of the hydrogen atom that was previously believed to have only one value actually had two. The difference between the values of the energy state is called the Lamb shift. The Lamb shift showed that the interaction between light and electrons was more complicated than physicists had believed. The discovery and measurement of the Lamb shift caused QED physicists such as Oppenheimer, Austrian-born American physicist Victor Weisskopf, and German-born American physicist Hans Bethe to redefine QED’s description of the electron. This redefinition (called renormalization) made QED equations match the new experimental results and removed the infinite terms that had been plaguing the theory.

The changes that physicists made to QED to account for the Lamb shift resulted in a set of equations that required the addition of a series of terms. Each of the individual terms in this series violated the special theory of relativity, but collectively, they produced a final result that was consistent with relativity. This violation troubled some physicists, and they continued to search for a set of equations that would be consistent with relativity at each step. In the early 1950s American physicists Richard Feynman and Julian Schwinger and Japanese physicist Tomonaga Shin’ichirō, all working individually, developed versions of QED that were consistent with the special theory of relativity. Schwinger and Tomonaga used mathematical, relativistic descriptions of electromagnetic fields. Feynman used descriptions of particle paths through space-time, or the combination of the three dimensions of space and the fourth dimension of time. Feynman’s method allows physicists to represent particle interactions with simple diagrams, called Feynman diagrams. Later in the 1950s, British-born American physicist Freeman Dyson showed that the two approaches produced the same results, and that Feynman’s approach could be derived from the equations of Schwinger and Tomonaga. Feynman, Schwinger, and Tomonaga won the 1965 Nobel Prize in physics for their work with QED. QED has been one of the most successful theories of modern physics—showing remarkable agreement with experimental results.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain