Antiparticle
One of the seminal scientific discoveries of the twentieth century was Louis Victor de Broglie's theory of the wave nature of the electron. By integrating theories by Albert Einstein dealing with mass and energy and Max Planck's Quantum relationship between energy and wavelength, de Broglie posited that any material particle should also exhibit wave like properties, just as Einstein earlier had shown that light, which had been described as a wave phenomenon, also exhibited particle like behavior.
Building on de Broglie's theory, Erwin Schrödinger developed his famous wave equation for both free electrons and those bound to atoms.. A limitation of Schrodinger's equation was that it did not include Relativity. In the late 1920s, the English physicist Paul Adrien Maurice Dirac extended Schrodinger's theory by incorporating Relativity into the wave equation for the electron. He showed that particles, such as. the electron should always exist in two energy states, one positive and one negative. When applied to the electron, the theory suggested that, in addition to the negatively-charged electron already known, there should also exist a positively-charged "twin." The twin would be identical to the electron in every respect except for its electrical charge.
Dirac's prediction, announced in 1930, was confirmed experimentally within two years. In his 1932 studies of cosmic ray interactions, Carl David Anderson found the positive electron that Dirac had anticipated. Anderson suggested the name positron for the new particle. Anderson discovered that the collision of an electron and a positron resulted in the annihilation of both particles, with their mass being converted into energy. Similarly, an electron-positron pair can be created when a high-energy photon interacts with matter. The mathematical equivalence of mass-energy calculated for these conversions provided dramatic confirmation of Einstein's mass-energy equation (e = mc2 ) of 1905.
Dirac's theory applied not only to electrons, but to all other particles. Just as he predicted an antielectron, he also suggested the existence of an antiproton, a particle identical to a proton in every respect except for its electrical charge.
The negatively-charged proton proved to be much more elusive than had been the positron. With a mass more than 1,800 times greater than that of the positron, the antiproton requires far greater energy for its production. Such energies are available in cosmic rays, but the rate of antiproton production from this source is too low to have resulted in their discovery.
It was not until the invention of particle accelerators that a systematic search for the antiproton could be launched. Then, in 1955, the particle was discovered by a research team led by Emilio Segrè and Owen Chamberlain at the University of California. The antiproton was produced when protons from a cyclotron were used to bombard a copper target. As predicted by Dirac, the antiproton was similar to the proton in every respect except for its electrical charge.
Is there also an antineutron? Lacking an electrical charge, an exact analogue to the electron and proton for the neutron is obviously impossible. However, Dirac's original research had anticipated and solved this problem. Along with the existence of antiparticles, Dirac had predicted that all particles possess an intrinsic property known as spin. The spin of a particle is designated as either "up " or "down." An antineutron would, Dirac suggested, be identical to a neutron except for its spin. In the presence of an external magnetic field, an antineutron and a neutron would have equal but opposite intrinsic spins. The first antineutron was discovered at the University of California at Berkeley only a year after the discovery of the first antiproton.
One can imagine atoms that are made of antiparticles: antiprotons, antineutrons, and positrons. Substances composed of such atoms are known as antimatter. Thus far, the antideuteron, made of one antiproton and one antineutron, and an isotope of antihelium-3 have been made in the laboratory.
Theory suggests that, at the moment the universe was created, equal amounts of matter and antimatter were formed. Today, however, we are able to detect only matter in the universe. One of the great questions of modern physics, therefore, is what happened to the original antimatter that was created at the time of the Big Bang.
Modern theory suggests that all particles have antiparticles. For example, the neutrino, hypothesized to explain energy changes that occur during some nuclear reactions, was found in 1963 to have its own antiparticle, the antineutrino. In some cases, the theory produces somewhat bizarre predictions. For example, since the photon has no mass and no electrical charge, it is, in fact, its own antiparticle.
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