Exchange Particles and Reading a Feynman Diagram
Every interaction between particles is carried by an exchanged boson, and which boson it is decides the range and the timescale. A Feynman diagram is the bookkeeping for that exchange, with the vertices doing the conserving and the internal line naming the force.
What a learner can do afterwards
- Names the exchange particle for each of the four interactions and links its mass to the range of the force
- Draws the diagram for beta-minus decay, beta-plus decay and electron capture, with the W labelled and the arrows right
- Checks charge, baryon number and lepton number at each vertex of a diagram they did not draw
1 · Read
Every interaction is carried by an exchanged boson, and which boson it is decides the range. The photon carries electromagnetism, the gluon carries the strong force, the W and Z carry the weak force, and the graviton for gravity stays hypothetical. Heavy carriers can borrow their energy for only a tiny moment, so they travel almost no distance, while the massless photon has unlimited range.
Beta-minus decay shows the bookkeeping in action. Time runs upward and a W boson leaves the vertex, turning a down quark into an up quark, while the W becomes an electron plus an antineutrino. Arrows carry meaning too: an arrow pointing backward in time is an antiparticle moving forward, with its charges flipped.
At every vertex you audit three small account books: charge, baryon number, and lepton number. Each total going in must equal the total coming out, and that rule alone kills most impossible diagrams. Electron capture uses the same exchange: an atomic electron joins a proton to make a neutron plus a neutrino, with a W doing the carrying.
To tell electron capture from beta-plus decay, look at the start: capture pulls an atomic electron into a proton, while beta-plus emits from the nucleus. Both balance the same three books through a W.
Bosons carry each force, vertices balance three books, and the W runs beta decay.
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.