Path integrals

Abstract An elegant alternative formalism of quantum mechanics was given by R. P. Feynman in 1948. The importance of this formalism, known as the path integral, functional integral, etc. approach, first of all lies in its vivid, intuitive description of quantum fluctuations. At the same time, it provides us with a formidable tool of calculus, of both perturbative effects (Feynman diagrams) and non-perturbative effects (e.g., tunnel effects, instantons).