ANTIMATTER

substance composed of atoms made up of elementary particles that have the mass and charge of electrons, protons, or neutrons, their counterparts in ordinary matter, but for which the charge is opposite in sign. Such particles are called positrons (e+), antiprotons (p), and antineutrons (n), or, collectively, antiparticles. Matter and antimatter cannot coexist at close range for more than a small fraction of a second because they annihilate each other with release of large quantities of energy. It has been suggested that some distant galaxies may be composed entirely of antimatter. The concept of antimatter first arose in analysis of the duality between positive and negative charge. The work of P.A.M. Dirac on the energy states of the electron led to the prediction and, finally, to laboratory production of a particle identical in every respect but one to the electron, that is, with positive instead of negative charge. Such a particle, called the positron (e+), is not found in ordinary matter. The life expectancy or duration of the positron in ordinary matter is very short. Unless the positron is moving extremely fast, it will be drawn close to an ordinary electron by the attraction between opposite charges. A collision between the positron and electron results in their simultaneous disappearance, their masses being converted into energy in accordance with the Einstein relation E = mc2, where c is the velocity of light. This process is called annihilation, and the resultant energy is emitted in the form of high-energy quanta of electromagnetic radiation or gamma rays. The inverse reaction g e+ + e- can also proceed under appropriate conditions, and the process is called electron-positron creation. This last process is the one commonly used to produce positrons in the laboratory. The electrical properties of antimatter are opposite to those of ordinary matter; thus, for example, the antiproton (p) has a negative charge, and the antineutron (n), although electrically neutral, has a magnetic moment opposite in sign to that of the neutron. The Dirac theory of electrons and positrons predicts that an electron and a positron, because of Coulomb attraction, will bind together into an atom just as an electron and a proton form a hydrogen atom. The e+e- bound system is called positronium; its annihilation into gamma rays has been observed. Its lifetime is of the order of 10-7 second or 10-10 second, depending on the orientation of the two particles. These lifetimes agree well with those computed from Dirac's theory. Both protons and neutrons are described by the Dirac equation. Antiprotons can be produced by bombarding protons with protons. If enough energy is available, that is, if the incident proton has a kinetic energy of at least 5.6 GeV (5.6 109 electron volts), extra particles of proton mass appear according to the formula E = mc2. Such energies became available in the 1950s at the Berkeley (Calif.) Bevatron. In 1955 a team of physicists led by Owen Chamberlain and Emilio Segr observed that antiprotons are produced by high-energy collisions. Antineutrons also were discovered at the Berkeley Bevatron by observing their annihilation in matter with a consequent release of high energies. By the time the antiproton was discovered, a host of new subatomic particles had also been discovered; all these particles are now known to have corresponding antiparticles. Thus, there are positive and negative muons, positive and negative pions (also called pi-mesons), the K-meson and the anti-K-meson, plus a long list of baryons and antibaryons. Most of these newly discovered particles have too short a lifetime for them to be able to combine with electrons. The exception is the positive muon that together with an electron has been observed to form a muonium atom. In 1995 physicists at the European Laboratory for Particle Physics (CERN) created the first antiatom, the antimatter counterpart of an ordinary atomin this case, antihydrogen, the simplest antiatom, consisting of a positron in orbit around an antiproton nucleus. They did so by firing antiprotons through a xenon gas jet. Some of the antiprotons collided with protons in the xenon nuclei, creating pairs of electrons and positrons; a few of the positrons thus produced then combined with the antiprotons to form antihydrogen. Each antiatom produced survived for only about forty-billionths of a second before it came into contact with ordinary matter and was annihilated. Many attempts have been made to investigate the importance of antimatter in cosmological problems; theoretical and experimental knowledge of matter and antimatter is relevant to the understanding of the creation and constitution of the universe. Obviously no star can contain a close mixture of matter and antimatter; otherwise it would instantaneously explode with more violence than a supernova. Interstellar gas, and even intergalactic gas, cannot be a mixture, either. This is because among the annihilation products of proton plus antiproton into pions there is a certain amount of neutral pions ( p0), which in turn decay into two energetic gamma rays. Satellite experiments have not detected enough of such gamma rays to suggest a significant amount of antimatter annihilation. One could resort to the hypothesis that matter and antimatter are separated on the scale of clusters of galaxies. The creation of baryon-antibaryon pairs, however, is very localized, the particle and antiparticle being created at distances of approximately 10-13 centimetre. No present understanding of the evolution of the universe can explain the unmixing of matter and antimatter if they had been originally created together. But the presence of large amounts of antimatter in the universe cannot be ruled out completely, nor can the possibility that some cosmic sources of intense radiation might be due to the interpenetration of matter and antimatter. But it can be shown that the total relative amount of antimatter in the Milky Way Galaxy must be less than one part in 107. Soon after the discovery of the antiproton the question was raised as to whether antimatter would be subject to gravitational attraction or repulsion from ordinary matter. This question is of extreme importance because gravitational repulsion between matter and antimatter is inconsistent with the theory of general relativity. The answers to such questions can be obtained experimentally because of the properties of K0 and K0 mesons. Observation of the interference phenomena between K01 and K02 led to the conclusion, by M.L. Good, that the gravitational interaction between matter and antimatter is identical to that between matter and matter.