ANNIHILATION

in physics, reaction in which a particle and its antiparticle collide and disappear, releasing energy. The most common annihilation on Earth occurs between an electron and its antiparticle, a positron. A positron, which may originate in radioactive decay, usually combines briefly with an electron to form a quasi-atom, called positronium, composed of the two particles spinning around each other before they collide. After annihilation, two or three gamma rays (similar to X rays) radiate from the point of collision. The amount of energy (E) produced by annihilation is equal to the mass (m) that disappears multiplied by the square of the speed of light in a vacuum (c)i.e., E = mc2. Thus annihilation is an example of the equivalence of mass and energy and a confirmation of the special theory of relativity, which predicts this equivalence. Other annihilation reactions also occur. Nucleons (protons and neutrons), for example, annihilate antinucleons (antiprotons and antineutrons), and the energy is carried away as pions and kaons, particles in the class of mesons. Annihilation of matter and antimatter has been invoked by some cosmologists to explain the enormous energy of quasars (quasi-stellar sources) and to explain the apparent expansion of the universe.