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In the path integral quantization formulation of quantum field theory the correlation functions (expectation values) are schematically path integrals of the form
Therefore, as for ordinary moments (and explicitly so under Wick rotation, if possible), there is generating functional for the correlators of the schematic form
Here in the exponent one may regard
as a new action functional defined on a larger space of fields that also contains the parameters $J$ as fields. In this context one calls $J$ a source field.
This is in the corresponding equations of motion of $S'$ $J$ will act like a source term. The Euler-Lagrange equations for the modified action are:
Notably if $EL(S)$ is a homogeneous wave equation (as for a free field theory) then $J$ is the inhomogeneous term in such a wave equation which describes indeed a “source” of wave excitations.
Created on July 2, 2013 at 23:59:33. See the history of this page for a list of all contributions to it.