Particles, Antiparticles and Annihilation
Every particle has an antiparticle of the same mass and opposite charge. A pair meeting turns entirely into photons, and a photon of enough energy can create a pair, both governed by E = mc squared.
What a learner can do afterwards
- Finds the least photon energy needed for pair production of a stated particle
- Explains why annihilation gives two photons rather than one
- Names positron emission tomography as a use built on annihilation
1 · Read
Every particle owns an antiparticle with the same mass and opposite charge. The positron is the electron's mirror: positive where the electron is negative, yet matching it in mass and spin. A few neutrals like the photon are their own antiparticles.
Meet and vanish is the rule. An electron and positron at rest turn entirely into two gamma photons flying back to back. Each particle holds 511 keV of mass energy, so the pair pays out 1022 keV in total. One photon alone could never carry away both the energy and the momentum, which is why two leave.
The film runs backwards too. A photon with at least twice one particle's mass energy can create a particle plus its antiparticle, and the bookkeeping follows E equals m c squared. Hospitals exploit the forward film: a tracer emits positrons, each meets an electron, and the twin gamma rays mark the spot in a PET scan.
Test any proposed reaction with three tallies: charge, baryon number, and lepton number. All three must match on both sides. A lone electron appearing from one photon fails charge and lepton counts at once, so nature refuses it.
Antiparticles mirror mass with opposite charge, and their meetings trade mass for photons under strict tallies.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.