Annihila

Meet Annihila, an electron who meets her antiparticle and turns into pure light.

Every type of particle has an antiparticle with the same mass and opposite charge. The positron, the electron's antiparticle, was predicted by Paul Dirac and discovered by Carl Anderson in 1932. When an electron and a positron meet, they can annihilate: both disappear, and their energy appears as gamma-ray photons.

If the pair is nearly at rest, momentum must still add to zero, so the most common outcome is two photons flying off in opposite directions. Each carries the rest energy of one electron, m_ec² ≈ 511 kiloelectronvolts. No mass is lost from the total energy; rest energy simply turns into photon energy.

PET (positron emission tomography) scans use this. A patient receives a tracer that emits positrons. Each annihilation sends two 511 keV photons back to back, and detectors around the body work out where it happened.

Annihilation into three photons also occurs, more rarely, and at high energies other particles can be produced. Why the universe holds far more matter than antimatter is an open question.