Physics
Feynman Diagram Basics
Quick fact
Richard Feynman originally created these diagrams as a shorthand for his own calculations; he later realized they could represent entire mathematical equations for quantum processes.
Why this is interesting
You’ve seen physicists scribble squiggly lines and arrows on blackboards—those simple doodles contain the secrets of how particles interact. But why do these diagrams carry so much power?
Read the full explanation
Understanding Feynman Diagram Basics
Imagine two particles, like an electron and a positron, approaching each other. They interact by exchanging a photon—a particle of light. In a Feynman diagram, this is shown as two straight lines (the incoming and outgoing particles) meeting at points (vertices) connected by a wavy line (the photon). Each element of the diagram corresponds to a precise mathematical term. The diagram visualizes a story: particles travel, they interact at vertices, and new particles emerge. The arrows on lines indicate the flow of charge or time, and external lines represent real particles, while internal lines represent 'virtual' particles that exist only briefly during the interaction. By following the diagram, you can reconstruct the entire mathematical expression for the probability of the process.
A deeper explanation
Feynman diagrams originate from perturbation theory in quantum field theory. In this framework, interactions are treated as small deviations from free particle motion. The probability amplitude for any process can be expanded as a series of terms, each term corresponding to a diagram. The more vertices a diagram has, the smaller its contribution—like adding finer details to a sketch. Each diagram obeys 'Feynman rules': for every particle line, assign a propagator; for each vertex, a coupling constant; and for each interaction, conserve energy and momentum. The sum of all possible diagrams for a process gives the total probability. Feynman diagrams are not just pictures—they are computational tools. They allow physicists to systematically calculate cross sections and decay rates, test the Standard Model, and even discover new particles (like the Higgs boson) through predicted signatures. Their elegance lies in turning abstract algebra into intuitive visual logic.