What is Antimatter?

R. Michael Barnett of the Lawrence Berkeley National Laboratory and Helen Quinn of the Stanford Linear Accelerator Center offer this answer, portions of which are paraphrased from their book The Charm of Strange Quarks:

In 1930 Paul Dirac formulated a quantum theory for the motion of electrons in electric and magnetic fields, the first theory that correctly included Einstein's theory of special relativity in this context. This theory led to a surprising prediction--the equations that described the electron also described, and in fact required, the existence of another type of particle with exactly the same mass as the electron, but with positive instead of negative electric charge. This particle, which is called the positron, is the antiparticle of the electron, and it was the first example of antimatter.

Its discovery in experiments soon confirmed the remarkable prediction of antimatter in Dirac's theory. A cloud chamber picture taken by Carl D. Anderson in 1931 showed a particle entering from below and passing through a lead plate. The direction of the curvature of the path, caused by a magnetic field, indicated that the particle was a positively charged one, but with the same mass and other characteristics as an electron. Experiments today routinely produce large numbers of positrons.

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