| In
1930, Paul Dirac developed the first description of the electron that
was consistent with both quantum mechanics and special relativity.
One of the remarkable predictions of this theory was that an anti-particle
of the electron should exist. This antielectron would be expected
to have the same mass as the electron, but opposite electric charge
and magnetic moment. In 1932, Carl Anderson, was examining tracks
produced by cosmic rays in a cloud chamber. One particle made a track
like an electron, but the curvature of its path in the magnetic field
showed that it was positively charged. He named this positive electron
a positron. We know that the particle Anderson detected was the anti-electron
predicted by Dirac. In the 1950s, physicists at the Lawrence Radiation
Laboratory used the Bevatron accelerator to produce the anti-proton,
that is a particle with the same mass and spin as the proton, but
with negative charge and opposite magnetic moment to that of the proton.
In order to create the anti-proton, protons were accelerated to very
high energy and then smashed into a target containing other protons.
Occasionally,
the energy brought into the collision would produce a proton-antiproton
pair in addition to the original two protons. This result gave credibility
to the idea that for every particle there is a corresponding antiparticle.
A particle and its antimatter particle annihilate when they meet:
they disappear and their kinetic plus rest-mass energy is converted
into other particles (E = mc^2). For example, when an electron and
a positron annihilate at rest, two gamma rays, each with energy
511 keV, are produced. These gamma rays go off in opposite directions
because both energy and momentum must be conserved. The annihilation
of positrons and electrons is the basis of Positron Emission Tomography
(PET). When a proton and an antiproton annihilate at rest, other
particles are usually produced, but the total kinetic plus rest
mass energies of these products adds up to twice the rest mass energy
of the proton (2 x 938 MeV).
Although
from a distance matter and antimatter would look essentially identical,
there appears to be very little antimatter in our universe. This
conclusion is partly based on the low observed abundance of antimatter
in the cosmic rays, which are particles that constantly rain down
on us from outer space. All of the antimatter present in the cosmic
rays can be accounted for by radioactive decays or by nuclear reactions
involving ordinary matter like those described above. We also do
not see the signatures of electron-positron annihilation, or proton-proton
annihilation coming from the edges of galaxies, or from places where
two galaxies are near each other. As a result, we believe that essentially
all of the objects we see in the universe are made of matter not
antimatter.
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