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 twice that in total. One photon alone could never carry away both the energy and the momentum, which is why two leave, and hospitals catch those twin rays to spot illness in a PET scan.
The film runs backwards too. A photon with at least twice one particle's mass energy, passing near a nucleus, 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.
Rewritten readings were written for this dataset.
More to read
Matter and antimatter reactions
Particles have antiparticles with equal mass and opposite electric charge when charged. Some neutral particles are their own antiparticles. An electron's partner is a positron; a proton's is an antiproton. Annihilation converts a particle-antiparticle pair into other particles, often photons, conserving total energy.
The reverse process, pair production, can occur when a gamma photon interacts near a nucleus. Creating two particles of mass m requires at least 2mc²; recoil can raise the threshold slightly. These reactions must conserve charge and, in ordinary reactions, baryon and lepton numbers. Such constraints rule out proposed reactions that fail to balance them.
Why PET detects photon pairs
A nearly stationary electron-positron pair commonly annihilates into two gamma photons. In the pair's rest frame, the photons have equal energies and travel in opposite directions. Each carries half the pair's total energy, approximately one electron's rest energy when the particles move slowly. Their momenta cancel, matching the pair's initial zero momentum. One photon alone cannot conserve both energy and momentum for an isolated pair. Three-photon annihilation is also possible.
PET scanners use detectors arranged around the body to register opposing photon pairs. Each detected pair identifies an annihilation along the line between the detectors. Combining millions of detections maps where the tracer accumulated.
Quark partners and medical imaging
A proton contains three valence quarks. There are six quark flavours, each with an antiquark carrying opposite electric charge and baryon number. Mesons contain a quark-antiquark pair; ordinary baryons contain three quarks, and antibaryons three antiquarks.
Quark-pair creation requires enough energy for both masses, at least 2mc², though confinement prevents free quarks from emerging. Annihilation can produce photons, gluons or other particle pairs, subject to conservation laws.
PET uses electron-positron annihilation. An injected tracer contains a positron-emitting isotope. Emitted positrons meet electrons, usually producing two opposing photons. The detector ring records these pairs to locate the tracer.
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Where this leads
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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.