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Stanford Report, August 9, 2000

Antimatter: Not just for sci-fi anymore

BY DAVID SALISBURY AND MICHAEL RIORDAN

Most people have learned about antimatter from science-fiction television programs such as "Star Trek," where it is used to power warp drives. Many believe antimatter is as fictional as the warp speeds these drives are supposed to generate, but it is in fact quite real. Antimatter is commonly produced in cosmic rays and by particle accelerators. Its existence was predicted in 1928 by the British physicist Paul Dirac and confirmed by the subsequent discovery of many different antiparticles.

Antiparticles are virtually identical to the ordinary, garden-variety particles of everyday life, but their electrical charge and a few other, more obscure properties are reversed. The electron has a negative electrical charge, for example, while its antiparticle, the positron, has exactly the same mass but is positively charged. Similarly, protons and antiprotons weigh the same but have opposite charges. Most neutral particles, like the neutron, also have antiparticles that are distinctively different even though their electrical charge is the same -- zero.

The relationship between a particle and its antiparticle is much like that between a hole and the pile of dirt you get from digging it. In a certain sense, they are opposites, but you make them both at the same time. You cannot have one without the other. Whenever matter and antimatter are created out of pure energy, according to Einstein's famous E = mc2, they appear as particle-antiparticle pairs. Today, antimatter is easily made in the laboratory, at particle accelerators such as the Stanford Linear Accelerator. Although antiparticles are being created continually by cosmic rays, they don't remain around for long because when an antiparticle encounters its opposite number, the two annihilate each other, and their mass reverts back into energy -- at least temporarily. This annihilation process provides the greatest amount of energy per unit of mass theoretically possible. If antimatter could be stored in sufficient quantities and its annihilation with matter controlled (two extremely difficult tasks), it might eventually be used as rocket fuel. This is what makes antimatter so interesting to science-fiction writers. SR