Antimatter is matter composed of antiparticles, which have the same masses and opposite values of certain additive quantum numbers compared with their corresponding matter particles. For example, the antiparticle of the electron is the positron. It has the same rest mass as an electron but opposite electric charge. The antiproton has the same mass as the proton but negative electric charge, while the antineutron has the same mass as the neutron and opposite relevant internal quantum numbers.
Antimatter is not simply matter with a negative mass or an exotic form of ordinary matter. Antiparticles are distinct quantum states associated with relativistic quantum fields. When a particle and its corresponding antiparticle meet under suitable conditions, they can annihilate, converting their rest mass and kinetic energy into other forms of energy and particles while obeying conservation laws.
The existence of antimatter emerged theoretically from relativistic quantum mechanics. Paul Dirac's relativistic equation for spin one half particles possessed solutions that led to the prediction of antiparticles. The positron was discovered experimentally by Carl Anderson in 1932 in cosmic ray observations, providing strong confirmation of the theoretical prediction. Antiparticles are now routinely produced in high energy particle interactions. For example, sufficiently energetic collisions can create particle antiparticle pairs when energy and conserved quantum numbers permit the process.
The energy required is at least related to the rest masses of the particles produced, although the complete collision must also satisfy momentum, charge, angular momentum, and other conservation laws. Antimatter therefore fits naturally into modern relativistic quantum field theory rather than being an accidental exception to ordinary matter physics.
Matter and antimatter can have identical masses and opposite charges or other relevant quantum numbers. The electron and positron provide the simplest familiar example. The electron has negative electric charge, whereas the positron has positive electric charge. A proton and antiproton similarly have opposite electric charges. Some particles, such as the photon, are their own antiparticles because they carry no corresponding conserved charge that distinguishes particle from antiparticle in the same way.
Certain neutral particles require more subtle treatment because a particle and its antiparticle can be distinct despite having zero electric charge, while some neutral fields permit self conjugate particles. The relationship between particle and antiparticle is therefore determined by the quantum field and its internal symmetries rather than simply by whether the object has a positive or negative electric charge.
When matter and antimatter annihilate, their total energy and quantum numbers are redistributed into permitted products. Electron positron annihilation at low relative kinetic energy can produce photons, commonly two photons in a suitable centre of mass configuration so that energy and momentum can be conserved. Higher energy annihilation can produce additional particles if sufficient energy is available. Proton antiproton annihilation is more complicated because protons are composite particles made from quarks and gluons. Their annihilation can produce mesons and other secondary particles. The popular statement that matter and antimatter simply convert entirely into energy is therefore an oversimplification. Annihilation converts the initial particle rest energy and kinetic energy into other physical excitations while obeying conservation laws.
Antimatter is also produced naturally. Cosmic rays interacting with Earth's atmosphere can generate antiparticles, and some radioactive processes produce positrons. Particle accelerators routinely create antiparticles in controlled collisions and secondary particle production systems. Antimatter can be trapped under specialised conditions using electromagnetic fields if the antiparticles are charged, while neutral antimatter atoms such as antihydrogen require more complex magnetic trapping techniques because they cannot be confined by a simple electrostatic field. Storage is difficult because antimatter annihilates upon contact with ordinary matter. Extremely high vacuum, electromagnetic trapping, cryogenic systems, and precise control are required for experiments involving significant numbers of antiparticles or antiatoms.
Antimatter research has tested fundamental symmetries and the apparent imbalance between matter and antimatter in the observable universe. According to simple expectations based on early universe particle production, matter and antimatter could have been produced in nearly related amounts, yet the present universe is overwhelmingly matter dominated on large scales. The origin of this asymmetry is known as the baryon asymmetry problem.
The Standard Model contains sources of charge parity violation, but the amount predicted within the Standard Model appears insufficient to explain the observed cosmic asymmetry under conventional scenarios. Researchers therefore investigate additional sources of symmetry violation and mechanisms such as baryogenesis and leptogenesis. Experiments comparing matter and antimatter properties with extremely high precision test whether known physical laws treat them exactly as expected.
Antimatter has practical and scientific applications, although popular descriptions sometimes exaggerate its technological potential. Positron emission tomography uses positron emitting radionuclides for medical imaging. The emitted positron eventually encounters an electron, and their annihilation produces photons that can be detected to reconstruct information about the distribution of the radioactive tracer. Antimatter is also important in fundamental particle physics, accelerator science, and studies of the properties of antihydrogen and other antimatters.
Proposals for antimatter propulsion or energy storage have been discussed because annihilation has an exceptionally high energy release per unit mass, but producing, collecting, and storing macroscopic quantities of antimatter is extraordinarily difficult and energetically expensive. Present technology therefore does not support antimatter as a practical bulk energy source.
Antimatter should be distinguished from dark matter. Dark matter is a hypothesised form of matter inferred from gravitational and cosmological evidence, whereas antimatter consists of experimentally established antiparticles governed by known quantum field theory. Antimatter is also not the same as negative matter, negative energy, or a hypothetical substance that repels ordinary matter gravitationally. Experiments have increasingly investigated the gravitational behaviour of antimatter, including antihydrogen, and current evidence is consistent with ordinary attractive gravitational behaviour within experimental uncertainties.
Antimatter remains important because it provides a stringent test of quantum field theory, discrete symmetries, conservation laws, and cosmological theories. Its existence demonstrates that the quantum description of matter naturally contains corresponding antiparticle states and that particle creation and annihilation are fundamental processes in relativistic physics.
