1. Klein-Gordon Equation The Klein-Gordon equation is a relativistic wave equation, related to the Schrödinger equation. It is second order in space and time and manifestly Lorentz covariant. It is a quantized version of the relativistic energy–momentum relation. Its solutions include a quantum scalar or pseudoscalar field, a field whose quanta are spinless particles. The […]
Wick’s theorem is a way to reduce a time-ordered operator into the sum of normal-ordered operator and various contractions. Then, where is the time-ordered operator from? Why we need this reduction?