Quantum Electrodynamics 217
In addition to the relativistic equation Dirac developed for the
electron, he also developed an equation to describe the photon and its
interaction with electrons and other charged particles. Dirac’s equations
gave results in excellent agreement with experiment. They described all
of the basic interactions of electrons and photons and included the
processes of pair creation, pair annihilation, the emission of radiation and
the scattering of electrons and light. As experimental measurements of
energy levels of atoms improved, it was discovered that there were
extremely small discrepancies between Dirac’s theory and experiment.
Through the work of Tomonaga, Schwinger and Feynman in 1948, it was
discovered that these discrepancies could be accounted for by calculating
corrections to Dirac’s theory.
The quantum electrodynamics developed by these physicists and their
co-workers yielded results of unprecedented accuracy. They were able to
calculate corrections to the energy levels of the hydrogen atom to one
part in 10^6. They were able to explain the 0.1% discrepancy between the
experimental value of the electron’s magnetic moment and the one
predicted by Dirac theory. They predicted that the magnetic moment of
the electron, μ, would be equal to μ(theo) = eh/4πmec (1.001159655 ± 3)
whereas the measured value is μ(exp) = eh/4πmec (1.001159657 ± 4).
The two numbers agree to 1 part in 10^9. The calculation of the electron’s
magnetic moment by quantum electrodynamics represents the most
accurate determination of a number in atomic physics.
We could never come to understand the intricacy of quantum
electrodynamics without delving into the mathematics of the theory. We
can, however, gain an understanding of the physics behind the theory and
at the same time gain a deeper understanding of the nature of the static
electric force. Perhaps the most mysterious aspect of the electric force is
the concept of action at a distance. It is very hard to conceive how two
charged particles separated by a finite distance can interact with each
other with no medium between them. A hint of how this can take place,
however, is provided by the production and detection of light or photons.
Let us consider the process whereby we see the light produced by a lamp.
Light is produced within the filament of the light bulb by exciting the
atoms of the filament. The atoms are excited by collisions with the
electrons passing through the filament as an electric current. The photons
are actually emitted by electrons jumping from one orbit to another
within their respective atoms. The photons or light travels through empty
space as small packets or bundles of energy. The stream of photons