The complementary particle to every subatomic particle, identical in mass and, if charged, identical in charge magnitude but with opposite sign. If a particle has a magnetic (dipole) moment, its antiparticle has an equal and opposite magnetic moment. For example, the electron and positron comprise a particle–antiparticle pair. Because the electron was discovered first (and electrons are more abundant), the electron is the particle and the positron the antiparticle (antielectron), but this designation is arbitrary. The electron could just as well be called the antipositron. Collectively, antiparticles are called antimatter. When a particle and its antiparticle interact, they are annihilated, their energy living on in the form of gamma rays. The inverse process, pair production, is possible: A gamma ray photon interacting with an atomic nucleus may be annihilated, its energy taken up by an electron–positron pair. Some neutral particles are their own antiparticles (e.g., the photon and the neutral pion).