Correlation function (quantum field theory)

In quantum field theory, the (real space) n-point correlation function is defined as the functional average (functional expectation value) of a product of n {displaystyle n} field operators at different positions In quantum field theory, the (real space) n-point correlation function is defined as the functional average (functional expectation value) of a product of n {displaystyle n} field operators at different positions For time-dependent correlation functions, the time-ordering operator T {displaystyle T} is included. Correlation functions are also called simply correlators. Sometimes, the phrase Green's function is used not only for two-point functions, but for any correlators. The correlation function can be interpreted physically as the amplitude for propagation of a particle or excitation between y and x. In the free theory, it is simply the Feynman propagator (for n=2).