Antimatter is matter comprising particles that are equal in mass to the particles comprising ordinary matter—neutrons, protons, electrons, and so forth—but with opposite electrical properties. An antiproton has the same mass as the proton, but negative charge, an antielectron (positron) has the same mass as an electron, but negative charge, and an antineutron has the same mass as a neutron, but with a magnetic moment opposite in sign to the neutron's. Antiparticles are built up of antiquarks, which come in six varieties that mirror the six varieties of quarks that make up ordinary matter. In theory, antimatter has almost exactly the same properties as ordinary matter.
When particles and antiparticles approach closely, they annihilate each other in a burst of high-energy photons. A complementary or reverse process also occurs: high-energy photons can produce particle-antiparticle pairs. Antiparticles are also given off by the fissioning nuclei of certain isotopes.
The existence of anti-matter was proposed by British physicist Paul Dirac (1902–1984) in 1927. The positron was first observed in 1932 and the antiproton and anti-neutron in the 1950s. Antimatter does yield energy; the annihilation process that occurs when matter and anti-matter meet provides a 100% matter-to-energy conversion, as opposed to the extremely small efficiencies achieved in nuclear fission and fusion reactors. However, antimatter cannot be used as a practical energy source because there are no bulk sources of antimatter. Antimatter can only be collected painstakingly, a few particles at a time.
Antimatter's rarity is one of the great mysteries of cosmology. The production of antimatter from energy must always, so far as present-day physics can determine, produce matter and antimatter in equal quantities. Therefore, the big bang—the explosion in which the Universe originated—should have yielded equal quantities of matter and antimatter. However, the Universe seems to consist entirely of matter. Where did all the antimatter go?
For decades, some physicists theorized that it might still be out there—that some galaxies might consist of antimatter. Because antimatter should absorb and emit photons just like normal matter, it would be impossible to tell a matter galaxy from an antimatter galaxy by observing it through the telescope.
Theoretical physicists now argue that this is unlikely, and that the entire Universe almost certainly does consist entirely of ordinary matter—although they still don't know why. If matter and antimatter had segregated into separate galactic domains after the big bang, these domains must have gone through a process of mutual annihilation along their contacting borders; this would have produced large quantities of gamma rays, and these which would still be observable as a gamma-ray background everywhere in the sky. A cosmic gamma-ray background of uncertain origin does exist, but is only about one fifth as intense as it would be if the formation of matter and antimatter domains were responsible for it. Nevertheless, if antimatter galaxies do exist, some of the very-high-speed atoms from space that are known as cosmic rays should consist of antimatter. A device to detect antimatter cosmic rays was tested briefly on a space shuttle flight in 1998, and found no antimatter; it is scheduled to make a more thorough investigation, operating on the International Space Station, in 2003.
Antimatter, despite its scarcity, has practical uses. Radioactive isotopes that give off positrons are the basis of PET (positron-emission tomography) scanning technique. When administered to a patient, a positron-emitting isotope of (say) oxygen is utilized chemically just like ordinary oxygen; when it breaks down, it emits positrons. Each positron cannot travel far before it encounters an electron. The two particles annihilate, giving off two identical, back-to-back gamma rays. These can be picked up by a detector array, and a map of positron-emission activity can be created by a computer that deduces the locations of large numbers of positron-electron annihilations from these gamma-ray detections. PET produces real-time images of brain activity of importance to neurology, and has also been used to probe the structure of nonliving systems (engines, semiconductor crystals, etc.).
The latest goal of antimatter research is the creation of cool antihydrogen atoms (composed of an antiproton orbited by a positron). These early antihydrogen atoms, however, were formed moving at high speed, and quickly struck normal atoms in the apparatus and were destroyed. In 2002, researchers at CERN (Conseil Europée pour la Recherche Nnucléaire) produced thousands of antihydrogen atoms cooled to about 50 degrees Kelvin. The goal of such research is to isolate the antihydrogen from ordinary matter long enough to observe its properties, such as its absorption and emission spectra; cool atoms, which are moving more slowly, are easier to confine. The properties of antihydrogen are predicted by theory to be identical to those of ordinary hydrogen; therefore, if they are not, revisions to basic physics will be required. CERN researchers hope to obtain their first antihydrogen spectra in late 2003.
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