Antiparticle

For each kind of particle, there is an associated antiparticle with the same mass but opposite electromagnetic, weak, and strong charges, as well as spin. Particle-antiparticle pairs arise from pure energy and will annihilate one another to give pure energy, usually in the form of photons. Antiparticles are produced by nuclear reactions and cosmic rays. Antimatter is a collection of antiparticles.

The existence of antiparticles was predicted by Dirac a few years before the first one, the antielectron or positron, was found. The idea stemmed from the existence of negative energy states, which in a relativistic universe can not be discarded a priori. Since electrons normally seek the lowest possible energy state, Dirac posited that these extra states must all be filled with what are called [virtual particle]?s. In that case, a virtual particle could be promoted to a positive energy state, creating a real particle and leaving a hole that would behave exactly the same, but with opposite charge.