214 The Poetry of Physics and The Physics of Poetry
so that momentum will be conserved. This explains why the two photons
go off in opposite directions. The amount of energy created by the
annihilation is just equal to the total kinetic energy of the electron-
positron pair plus the rest mass energy of the pair, which equals 2mec^2.
Electron-positron pair annihilation not only confirmed Dirac’s theory but
also demonstrated the validity of Einstein’s relation between mass and
energy, namely, E = mc^2.
Dirac’s theory of the electron also explains the creation of an
electron-positron pair from light energy. When high-energy photons pass
close to a nucleus they are frequently converted into electron-positron
pairs. In the cloud chamber this observation appears as the spontaneous
appearance of a pair of tracks with equal and opposite curvature. The
photon does not leave a track because it carries no charge. From the
initial direction of the electron and positron tracks, however, it is
possible to determine the direction from which the photon came, and
hence, the source which produced the photon. The energy of the photon,
which creates the electron-positron pair, must be at least 2mec^2 in order
to provide enough energy to produce the rest mass for the two particles.
The presence of the nucleus is only necessary to conserve momentum
and energy.
The process of pair creation can be envisaged using Dirac’s image of
the negative energy sea. One may consider the photon as providing
energy to an electron in one of the negative energy states and lifting it to
a positive energy state. Since the electron in the negative energy sea is
unobservable, it suddenly becomes visible when it is elevated to a
positive energy state. The hole in the negative energy sea of electrons
created by the absence of this electron also becomes visible as a positron.
Pair creation is just the opposite process of annihilation. In pair
annihilation an electron falls from positive energy to negative energy
radiating light whereas in pair creation an electron absorbs a photon and
jumps from a state of negative energy to one of positive energy.
The key concept Dirac used in interpreting his equation was the idea
of the unobservable negative energy sea of electrons. Although this
notion was essential in Dirac’s development of his ideas, it is not
absolutely necessary to the formulation of Dirac’s theory. Having once
gained an insight into how the electron behaves it is now possible to
discard the concept of the negative energy sea and to view the Dirac
equation as describing a charged spin one half particle and its oppositely
charged spin one half antiparticle for which the processes of annihilation