Among the most convincing phenomena that demonstrate the quantum nature of light are the following. As the intensity of light is dimmed further and further, one can see individual quanta being registered in light detectors. If the eyes were about 10 times more sensitive, one would perceive the individual light pulses of fainter and fainter light sources as fewer and fewer flashes of equal intensity. Moreover, a movie has been made of the buildup of a two-slit interference pattern by individual photons, such as shown in Figure 9. Photons are particles, but they behave differently from ordinary particles like billiard balls. The rules of their behaviour and their interaction with electrons and other charged particles, as well as the interactions of charged particles with one another, constitute QED.

Photons are created by perturbations in the motions of electrons and other charged particles; in reverse, photons can disappear and thereby create a pair of oppositely charged particles, usually a particle and its antiparticle (e.g., an electron and a positron). A description of this intimate interaction between charged particles and electromagnetic radiation requires a theory that includes both quantum mechanics and special relativity. The foundations of such a theory, known as relativistic quantum mechanics, were laid beginning in 1929 by Paul A.M. Dirac, Heisenberg, and Wolfgang Pauli.

The discussion that follows explains in brief the principal conceptual elements of QED. Further information on the subject can be found in subatomic particle: The development of modern theory and quantum mechanics.

Many phenomena in nature do not depend on the reference scale of scientific measurements. For instance, in electromagnetism the difference in electrical potentials is relevant but not its absolute magnitude. During the 1920s, even before the emergence of quantum mechanics, the German physicist Hermann Weyl discussed the problem of constructing physical theories that are independent of certain reference bases or absolute magnitudes of certain parameters not only locally but everywhere in space. He called this property “Eichinvarianz,” which is the conceptual origin of the term “gauge invariance” that plays a crucial role in all modern quantum field theories.

In quantum mechanics all observable quantities are calculated from so-called wave functions, which are complex mathematical functions that include a phase factor. The absolute magnitude of this phase is irrelevant for the observable quantities calculated from these wave functions; hence, the theory describing, for example, the motion of an electron should be the same when the phase of its wave function is changed everywhere in space. This requirement of phase invariance, or gauge invariance, is equivalent to demanding that the total charge is conserved and does not disappear in physical processes or interactions. Experimentally one does indeed observe that charge is conserved in nature.

It turns out that a relativistic quantum theory of charged particles can be made gauge invariant if the interaction is mediated by a massless and chargeless entity which has all the properties of photons. Coulomb’s law of the force between charged particles can be derived from this theory, and the photon can be viewed as a “messenger” particle that conveys the electromagnetic force between charged particles of matter. In this theory, Maxwell’s equations for electric and magnetic fields are quantized.

The range of a force produced by a particle with nonzero mass is its Compton wavelength h/mc, which for electrons is about 2 × 10⁻¹⁰ cm. Since this length is large compared with distances over which stronger nuclear forces act, QED is a very precise theory for electrons.

Despite the conceptual elegance of the QED theory, it proved difficult to calculate the outcome of specific physical situations through its application. Richard P. Feynman and, independently, Julian S. Schwinger and Freeman Dyson of the United States and Tomonaga Shin’ichirō of Japan showed in 1948 that one could calculate the effects of the interactions as a power series in which the coupling constant is called the fine structure constant and has a value close to 1/137. A serious practical difficulty arose when each term in the series, which had to be summed to obtain the value of an observed quantity, turned out to be infinitely large. In short, the results of the calculations were meaningless. It was eventually found, however, that these divergences could be avoided by introducing “renormalized” couplings and particle masses, an idea conceived by the Dutch physicist Hendrik A. Kramers. Just as a ship moving through water has an enhanced mass due to the fluid that it drags along, so will an electron dragging along and interacting with its own field have a different mass and charge than it would without it. By adding appropriate electromagnetic components to the bare mass and charge—that is, by using renormalized quantities—the disturbing infinities could be removed from the theory. Using this method of renormalization and the perturbation theory, Feynman developed an elegant form for calculating the likelihood of observing processes that are related to the interaction of electromagnetic radiation with matter to any desired degree of accuracy. For example, the passage of an electron or a photon through the double slit illustrated in Figure 9 will, in this QED formalism, produce the observed interference pattern on a photographic plate because of the superposition of all the possible paths these particles can take through the slits.

The success of unifying electricity, magnetism, and light into one theory of electromagnetism and then with the interaction of charged particles into the theory of quantum electrodynamics suggests the possibility of understanding all the forces in nature (gravitational, electromagnetic, weak nuclear, and strong nuclear) as manifestations of a grand unified theory (GUT). The first step in this direction was taken during the 1960s by Abdus Salam, Steven Weinberg, and Sheldon Glashow, who formulated the electroweak theory, which combines the electromagnetic force and the weak nuclear force. This theory predicted that the weak nuclear force is transmitted between particles of matter by three messenger particles designated W⁺, W⁻, and Z, much in the way that the electromagnetic force is conveyed by photons. The three new particles were discovered in 1983 during experiments at the European Organization for Nuclear Research (CERN), a large accelerator laboratory near Geneva. This triumph for the electroweak theory represented another stepping stone toward a deeper understanding of the forces and interactions that yield the multitude of physical phenomena in the universe.