excited states quanta fields equation of motion action Lagrangian Feynman diagrams perturbation theory in quantum mechanics quantum mechanics
QFT treats particles as excited states (also called quanta) of their underlying quantum fields, which are more fundamental than the particles. The equation of motion of the particle is determined by minimization of the action computed for the Lagrangian, a functional of fields associated with the particle. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding quantum fields.  Each interaction can be visually represented by Feynman diagrams according to perturbation theory in quantum mechanics.
