Quantum Electrodynamics 213
in the Chlorine’s shell acts like a positive charge since this gap attracts
electrons to it and explains the intense chemical activity of Chlorine.
Dirac initially identified the holes in the negative energy sea with the
proton. He thought that he had shown the mass of the hole was
approximately 2000 times the mass of the electron. Pauli quickly pointed
out that if the proton were a hole in the negative energy sea of electrons
that the electron in the hydrogen atom would quickly jump into it and
annihilate the hydrogen atom leaving only two photons behind to carry
away the energy. The mystery of the identification of he hole in the
negative energy sea of electrons was resolved experimentally a short
time later by the discovery by Anderson of particles which had the same
mass as the electron but opposite charge. The new particles, called
positrons, or antielectrons, were detected in an experiment in which
cosmic rays were observed by a cloud chamber.
Cosmic rays are high-energy particles, produced in outer space, which
enter the earth’s atmosphere. The cloud chamber is a device used to
detect the charged particles, which leave tracks of ionization as they pass
through the vapor of the chamber. By immersing the entire cloud
chamber in a magnetic field and measuring the amount and direction of
the curvature of the charged particle’s tracks, one can determine the mass
and the charge of the particle from one’s knowledge of the magnetic
force. Anderson observed tracks in his cloud chamber with equal but
opposite curvature of the electron. These tracks could only be made by a
particle the same mass of the electron but with the opposite charge. The
positron was quickly identified with a hole in Dirac’s negative energy
sea of electrons.
If Dirac’s theory about the negative energy sea of electrons was
correct, then one would expect that the electrons should jump into the
holes in the negative energy sea and emit radiation. In this process, both
the electron and the positron would be annihilated since the electron
disappears by jumping into the hole and the positron or the hole
disappears by being filled. One would, therefore, expect to observe the
reaction: e+ + e– → γ + γ where e+, e– and γ represent the positron,
electron and photon, respectively. This is precisely what is observed to
happen experimentally.
Once a positron encounters an electron the two particles annihilate
each other leaving two quantas of light energy. The electron has jumped
into the empty negative energy state releasing the energy gained by
this transition as light energy. The reason two photons are created is