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  The theories of Quantum mechanics and relativity were developed separately in the early part of the 20th century. However, it soon became obvious that a relativistic treatment of quantum mechanics was necessary for a full description of the interactions of elementary particles, as they often move at speeds approaching that of light.

The inclusion of relativity required that the non-relativistic Schrödinger equation, which describes all non-relativistic interactions, be replaced by a relativistic equation. This equation was discovered by Paul Dirac (1902 - 1984), and named after him.

The solutions to the Dirac equation are found to contain a whole class of new particles in addition to the expected solutions. They correspond to negative energy solutions, and are called antiparticles. At first they were not thought to have a physical existence, but soon experiments showed that antiparticles do indeed exist.

In quantum mechanics, all particles are described by a set of quantum numbers, like spin or charge. Each particle has a corresponding antiparticle which has opposite quantum numbers. Thus the electron, which has one unit of negative charge, has as its antiparticle that positron, with one unit of positive charge. Neutrinos and antineutrinos have opposite spin. In theory, it should be possible to construct anti-atoms, completely composed of antimatter. There are some experiments in progress at this time which are attempting to construct anti-hydrogen.

When a particle meets its antiparticle, they annihilate each other, producing a burst of energy. This means that it is very difficult to store antimatter; as soon as it comes into contact with normal matter, it is violently destroyed.

One of the puzzles of the universe is that it contains far more matter than antimatter. This is not what might be expected, for in all interactions where particles are formed, particles and antiparticles are produced in equal amounts. It is surmised that, near the beginning of the universe, different conditions prevailed which ensured that particles were produced in preference to antiparticles. JJ



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