Feynman diagram

In a Feynman diagram, straight lines typically represent fermions, like electrons or positrons, while wavy or spiral lines represent bosons, such as photons. The points where these lines intersect, known as vertices, represent the interactions between the particles, where a particle emits or absorbs a boson. Time generally flows from left to right or bottom to top.

Virtual particles are the intermediate particles that exist momentarily within the internal lines of the diagram, between the initial and final states. Unlike real particles, virtual particles do not need to obey the strict energy-momentum relation and are considered "off-shell." They are mathematical constructs used to calculate the probability amplitudes of quantum interactions.

The diagrams were invented by the American theoretical physicist Richard Feynman in 1948. He introduced them as a visual bookkeeping tool to help calculate complex interactions in quantum electrodynamics (QED). Feynman later won the Nobel Prize in Physics in 1965 for this foundational work in quantum mechanics.

The diagrams provide a visual shorthand for the incredibly complex mathematical equations found in quantum field theory. Each diagram represents a specific term in a perturbation series, making it easier for physicists to organize and sum the probabilities of different interaction pathways. The more vertices a diagram has, the lower its contribution to the overall probability amplitude due to smaller coupling constants.