In 1928, a British theoretical physicist called Paul Dirac proposed an equation of motion for the electron
that encompassed both quantum mechanics and special relativity. Not only did it contain all the physics
described by nonrelativistic quantum mechanics (invented a few years earlier), it also contained some
new physics. Of particular interest to us is the prediction of antimatter.
An important equation from special relativity relates the energy E, the momentum p and the
mass m of a particle:
E^{ 2 }
=
p^{ 2 }c^{ 2}
+
m^{ 2 }c^{ 4}
where c is the speed of light. The solutions of Dirac's equation are also solutions of this equation 
he deliberately set up his equation so that this would be the case. Whenever one has a quadratic equation,
there are in general two solutions. (For example, x^{2} = 1 has the
solutions x = 1 and x = 1.) This means that the above equation for E has two solutions
for a given p and m  one is the positive square root of the righthand side, and one is the
negative square root.
Rather than just dismissing the negative energy solution as being unphysical, Dirac postulated that it
corresponds to a particle with the same mass as an electron but with the opposite charge. This prediction was
verified a few years later when Carl Anderson, working at the California Institute of Technology, discovered
the socalled positron. The positron is called the antiparticle of the electron. Its mass is equal
to that of the electron, but it has the charge +e instead of e. Whenever an electron and a positron
meet they annihilate, producing electromagnetic radiation.

This is a picture of one of the first positron tracks observed by Anderson. Click on the picture for
more information.

Dirac's prediction of antimatter was a great achievement. There is a problem associated with it, though.
As far as physicists could tell, antimatter and matter should act identically, apart from having opposite
charges. This means that matter and antimatter should have been created in equal amounts in the Big Bang,
and so there should be just as much antimatter as matter in the Universe today. However, the Universe is
overwhelmingly made up of matter, and this is one of the biggest puzzles facing physicists today.
