Physics
Pair Annihilation
Quick fact
In electron-positron annihilation, the resulting photons each carry about 511 keV of energy — exactly the energy equivalent of an electron's mass.
Why this is interesting
You know that ordinary matter and antimatter cannot coexist — but what actually happens when they meet? The answer is a spectacular burst of pure energy.
Read the full explanation
Understanding Pair Annihilation
Imagine two strangers who, when they touch, vanish completely and turn into a flash of light. That's essentially what happens in pair annihilation. Every particle has an antiparticle twin with the same mass but opposite charge (or other quantum properties). When a particle meets its antiparticle, they do not simply bounce off; instead, they annihilate each other. All of their mass is converted into energy in the form of high-energy photons (gamma rays). The energy produced follows Einstein's famous equation E = mc², where the mass of both particles determines the amount of energy released. For example, an electron and a positron (the electron's antiparticle) annihilate to produce two gamma-ray photons that fly away in opposite directions. This process conserves both energy and momentum.
A deeper explanation
At a deeper level, pair annihilation is governed by quantum field theory. Particles are excitations of quantum fields, and when a particle and its antiparticle interact, the fields annihilate each other's excitations, releasing energy as photons — the quanta of the electromagnetic field. The annihilation cannot produce a single photon because momentum would not be conserved; at least two photons are required to carry away momentum. The energy of each photon equals the rest energy of the annihilated particle (m = E/c²). This process is not just theoretical: it is observed in particle colliders, in astrophysical phenomena like positron annihilation in the galaxy, and it is harnessed in medical PET scans, where positrons emitted from a radioactive tracer annihilate with electrons in the body to produce gamma rays that create 3D images. Understanding pair annihilation is crucial for exploring why the universe is dominated by matter rather than antimatter, as any imbalance would lead to leftover annihilation radiation.