Antiparticles

Antiparticles, particles that are counterparts of the elementary particles that make up ordinary matter. At a very tiny scale all matter on Earth—plants and animals, rocks and sand, air and water—is made of atoms. Atoms in turn are made of subatomic particles such as protons, electrons, and neutrons. Antiparticles are almost identical to their corresponding subatomic particles but have at least one property that is completely opposite. Antiparticles are extremely rare in nature but can be created in laboratories by particle accelerators.

Physicists can tell one subatomic particle from another by measuring properties such as mass, electric charge, spin, and magnetic moment (an electromagnetic property that determines the force that acts on a particle as it moves through a magnetic field). The mass of an antiparticle, and the magnitude of its spin, are identical to those of the corresponding particle but electric charge and magnetic moment are exactly opposite. A proton, for example, has an electric charge of +1 and a positive magnetic moment. The proton’s antiparticle, the antiproton, is identical in most other respects, but its electric charge is –1 and its magnetic moment is negative. Both the neutron and its antiparticle, the antineutron, have no electric charge, but their magnetic moments are opposite.

If an antiparticle and the corresponding particle of ordinary matter get too close together, both will disappear in a burst of energy. This property of antiparticles is called annihilation. For example, an electron and a positron (the electron’s antiparticle counterpart) approaching each other will be completely converted to energy, giving off two gamma rays, which are high-energy photons. The masses of the particle and antiparticle determine the energy of the photons given off. According to the famous equation E=mc² by Albert Einstein, the greater the masses of the particles, the greater the energy the particles will give off when they annihilate. (In this case, E stands for the energy the two particles give off, m is their combined masses, and c is the speed of light.)

Physicists have discovered antiparticle counterparts for almost all of the hundreds of subatomic particles they have discovered. Neutrinos have antineutrinos, for example; the negative muon has the positive muon; and the negative tau, the positive tau. Even quarks, the smallest building blocks of matter that make up protons, neutrons, and other particles, have corresponding antiquarks. As physicists using particle accelerators periodically discover ever more exotic particles that are unstable and rarely or never seen in nature, the discovery of the corresponding antiparticle is usually quick to follow. Sometimes the antiparticles are discovered first.