QFT - 21: Dirac, Feynman Propagators & Feynman Rule (Prof. Dr. Binil Aryal, TU / 27 June 2021)

Propagator is a function that specifies the probability amplitude for a particle to travel from one place to another in a given period of time, or to travel with a certain energy and momentum. In Feynman diagrams, which serve to calculate the rate of collisions in quantum field theory, virtual particles contribute their propagator to the rate of the scattering event described by the respective diagram. Propagators are Lorentz invariant, they give the amplitude for a particle to travel between two spacetime points. The most common use of the propagator is in calculating probability amplitudes for particle interactions using Feynman diagrams. These calculations are usually carried out in momentum space. In general, the amplitude gets a factor of the propagator for every internal line, that is, every line that does not represent an incoming or outgoing particle in the initial or final state. In the class, I discussed about the importance of propagators in the Feynman diagram calculation.