Antimatter

Antimatter, form of matter composed of elementary particles with properties that make them mirror images of the particles that make up ordinary matter. Ordinary matter is matter as it is known on Earth and is it is thought to exist in most of the universe. According to the standard model in physics, however, matter can exist in two forms—every type of particle of ordinary matter has a corresponding type of antiparticle of antimatter. Antiparticles have the same mass as their corresponding ordinary particles but have opposite electric charges or other properties related to electromagnetism or to color charge, a property associated with the strong nuclear force inside the nucleus of an atom. In all of the other parameters involved in the dynamical properties of elementary particles, such as mass, spin, and decay lifetime, antiparticles are thought to be identical with their corresponding ordinary particles.

For example, the antimatter electron, or positron, has opposite electrical charge and magnetic moment (a property that determines how it behaves in a magnetic field), but it is identical in all 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.

Antiquarks are the antiparticles of quarks, the particles that combine to form protons and neutrons. Antiprotons and antineutrons are made up of antiquarks. In addition to opposite electrical charges, quarks and antiquarks also have opposite color charges.

Physicists are still studying the distinction between neutrinos and antineutrinos, which are thought to have opposite directions of spin. If neutrinos have mass, as some experimental evidence indicates, the distinction between neutrinos and antineutrinos may be more complicated. Some particles, such as the photon and the Z particle, which mediates the weak nuclear force, are considered to be their own antiparticles.

The existence of antiparticles was first proposed by the British physicist Paul Adrien Maurice Dirac, arising from his attempt to apply the techniques of relativistic mechanics (see Relativity) to quantum theory. In 1928 he developed the concept of a positively charged electron but its actual existence was established experimentally in 1932. The existence of other antiparticles was presumed but not confirmed until 1955, when antiprotons and antineutrons were observed in particle accelerators. Since then, the full range of antiparticles has been observed or indicated.

When ordinary matter and antimatter interact, they can mutually annihilate each other, releasing enormous amounts of energy. As indicated by Einstein’s mass-energy equivalence equation E = mc², however, particles and antiparticles can also materialize out of energy. Using particle accelerators, scientists have been able to cause high-energy collisions between electrons and positrons and between protons and antiprotons. The resulting collisions between particles and antiparticles caused other subatomic particles to materialize out of the bursts of energy.

Antimatter atoms were created for the first time in September 1995 at the European Organization for Nuclear Research (CERN). Positrons were combined with antimatter protons to produce antimatter hydrogen atoms. However, these atoms of antimatter existed only for 40 billionths of a second. In 2002 researchers at CERN reported that they were able to study antihydrogen atoms trapped in a magnetic field. Future experiments may reveal if there are differences between normal hydrogen and its antimatter counterpart.

A profound problem for particle physics and for cosmology in general is the apparent scarcity of antiparticles in the universe. According to the big bang theory, equal amounts of ordinary matter and antimatter should have been created during the earliest microseconds after the beginning of the universe. Since particles and antiparticles mutually annihilate each other with a tremendous release of energy, all matter in the early universe might have been quickly destroyed. So far all evidence from astronomy and physics indicates that the modern universe contains much more ordinary matter than antimatter. Researchers want to know why any form of matter survived and why ordinary matter now predominates.

One possibility is that some particles and antiparticles decay in different ways and so violate the conservation of parity that predicts that particles and antiparticles should have identical properties except for charge. Some experiments with particles known as mesons show that the ordinary particles have a higher decay rate than their corresponding antiparticles do. Different decay rates between certain particles and antiparticles might have had a major effect in the early universe.

In 1997 scientists studying data gathered by the Compton Gamma Ray Observatory (GRO) operated by the National Aeronautics and Space Administration (NASA) found that Earth’s home galaxy—the Milky Way—contains large clouds of antimatter particles. Later observations by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) operated by the European Space Agency (ESA) revealed that the large cloud of antimatter at the center of our galaxy is lopsided. Astronomers have found a population of binary stars in the same region as the gas. One possible source of the antimatter cloud may be pairs of stars in which a normal star is being torn apart by the enormous gravity of a neutron star or a black hole. The gas that spirals off the normal star onto the neutron star or black hole is heated to such high temperatures that pairs of positrons and electrons materialize out of the intense radiation generated. When antimatter particles meet particles of ordinary matter, the two annihilate each other and produce a burst of gamma rays. It was these gamma rays that GRO and other instruments have detected.