antimatter

Antimatter refers to particles that have the same mass as their corresponding matter particles but opposite electric charge and other quantum numbers. Each particle has an antiparticle: the electron's antiparticle is the positron; the proton's is the antiproton; the neutron's is the antineutron. Antimatter and matter annihilate when they meet, releasing energy commonly in the form of gamma photons and, for annihilating hadrons, pions. The concept is a natural consequence of relativistic quantum theory; antiparticles were predicted by the Dirac equation and are an integral part of the standard model.

Antimatter is produced in high-energy particle accelerators, during cosmic-ray interactions, and in certain radioactive decays. Because antimatter destroys ordinary matter on contact, it is stored only in special devices that use magnetic and electric fields to suspend charged antiparticles in vacuum, such as Penning traps. Containing macroscopic amounts of antimatter remains technologically impractical due to extreme energy costs and leakage.

Antiparticles have practical applications, the best-known being positron emission tomography (PET) in medical imaging, which uses positron-emitting isotopes. In fundamental physics, antimatter studies test symmetries and interactions in the standard model. The positron was discovered by Carl Anderson in 1932; antiprotons were observed in 1955, and antihydrogen atoms were first produced in 1995, with later experiments trapping them in 2010.

Cosmology raises questions about the lack of large-scale antimatter in the visible universe: if matter and antimatter were created in equal amounts in the early universe, their annihilation would have left a signature; the observed predominance of matter implies a matter-antimatter asymmetry, a major area of research in particle physics, for example involving CP violation.