The Standard Model and the Four Interactions
Matter is built from quarks and leptons in three generations, and forces are carried by exchange particles with characteristic ranges and strengths. The model states which interactions each particle feels.
What a learner can do afterwards
- Places a given particle in the standard model and states which interactions it feels
- Relates the range of a force to the mass of its exchange particle
- Uses conservation rules to decide whether a proposed reaction can happen
1 · Read
Matter comes in quarks and leptons, arranged in three generations. Quarks feel all three quantum forces; leptons skip the strong one. Each force has its carriers: gluons carry the strong force, photons carry electromagnetism, and the W and Z bosons carry the weak force.
Range follows carrier mass through the uncertainty principle: a heavy carrier lives briefly and travels far less. Photons are massless, so electromagnetism reaches forever. The W and Z weigh near 80 to 90 GeV, so the weak force dies out around 10 to the minus 18 meters. Gluons are massless too, yet the strong force fades near 10 to the minus 15 meters.
Before trusting a proposed reaction, balance the books: charge, baryon number, and each lepton number must match on both sides. A neutron can never decay into just an electron plus a positron: charge works, but baryon number falls from 1 to 0. Strangeness must also match in strong reactions, though weak decays may break it.
Quarks team up color-neutral: three make a baryon like the proton or neutron, while a quark plus an antiquark makes a meson like the pion. Mass itself comes from grip on the Higgs field: stronger coupling means a heavier particle, and the Higgs boson is that field's ripple.
Carriers set each force's reach, and charge plus baryon and lepton tallies decide which reactions may happen.
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Where it sits
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.