ANTIMATTER (antiparticles)—matter composed of antiparticles, anti-atoms, and antimolecules.
Antiparticles differ from ordinary particles because the their quantum numbers (electrical charge Q, eccentricity or spin S, parity P) have the opposite sign to those of the correponding particles. The antiparticle of the electron (the negatron) for which Q=-1 (in individuals of an elementary charge) is an additional electron (positron) where Q=+1. The antiparticle of the proton where Q=+1 is an antiproton where Q=-1, etc. Antiparticles that have zero values for the numbers Q, S, and P, … are identical to their particles (e.g., π mesons, Σ baryons).
The identifical of the various antiparticles allowed physicists to advance the hypothesis that antiparticles exist in nature. According to this hypothesis antiparticles and antimolecules are physically possible: the anti-atom of hydrogen (created for a moment under laboratory conditions—1995) is composed of an antiproton (the nucleus) and a positron (the electron shell). The anti-atom of helium would be composed of two antiprotons and two antineutrons (the nucleus) and two positrons (the electron shell). A molecule of anti-hydrogen would be composed of two antiprotons and two positrons etc..
Antimatter that would possess the same physical properties as ordinary matter could be built from anti-atoms and antimolecules. Locally antimatter and matter cannot exist beside each other, since they would immediately be annihilated. Scientific investigations have not yet identified antimatter on an astronomical scale. Apart from certain antiparticles, in that part of the universe that can be investigated by science we find the presence of ordinary matter. The absence of antimatter in those parts of the universe that can be investigated from the Earth may be explained as a consequence of the physical processes that occurred in earlier period of the evolution of the cosmos.
H. Alfvén, Världen- Spegelvärlden Kosmologi och Antimatter, Stockholm 1966 (Kosmologia i antymateria [Cosmology and antimatter], Wwa 1973); M. Kubiak, in: Encyklopedia fizyki współczesnej [Encyclopedia of contemporary physics], Wwa 1983, 908–912; P. J. E. Peebles, Principles of Physical Cosmology, Pri, NY 1993, 207, 455–456; G. Tarlé, S. P. Swordy, Cosmic Antimatter, Scientific American 278 (1968), n. 4, 30–35.