Antimatter

Antimatter, matter composed of elementary particles that are, in a special sense, mirror images of the particles that make up ordinary matter as it is known on Earth. Antiparticles have the same mass as their corresponding particles but have opposite electric charges or other properties. For example, the antimatter counterpart of the electron, called the positron, is positively charged but is identical in most other respects to the electron. The antimatter equivalent of the chargeless neutron, on the other hand, differs in having a magnetic moment of opposite sign (magnetic moment is another electromagnetic property). In all of the other parameters involved in the dynamical properties of elementary particles, such as mass and decay times, antiparticles are identical with their corresponding particles.

The existence of antiparticles was first recognized as a result of attempts by the British physicist P. A. M. Dirac to apply the techniques of relativistic mechanics to quantum theory. He arrived at equations that seemed to imply the existence of electrons with negative energy. It was realized that these would be equivalent to electron-like particles with positive energy and positive charge. The actual existence of such particles, later called positrons, was established experimentally in 1932. The existence of antiprotons and antineutrons was presumed but not confirmed until 1955, when they were observed in particle accelerators. The full range of antiparticles has now been observed, directly or indirectly (in 2002 a significant quantity of antimatter was produced, and experimented upon, at the European Laboratory for Particle Physics, Switzerland).

A profound problem for particle physics and for cosmology in general is the apparent scarcity of antiparticles in the universe. Their non-existence, except momentarily, on Earth is understandable, because particles and antiparticles are mutually annihilated with a great release of energy when they meet. Distant galaxies could possibly be made of antimatter, but no direct method of confirmation exists. Most evidence about the far universe arrives in the form of photons, which are identical with their antiparticles and thus reveal little about the nature of their sources. The prevailing opinion, however, is that the universe consists overwhelmingly of “ordinary” matter, and explanations for this have been proposed by recent cosmological theory (see Inflationary Theory).