Effective field theory - Definition and Overview

In physics, an effective field theory is an approximate theory (usually a quantum field theory) that contains the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale, but ignores the substructure and the degrees of freedom at shorter distances (or, equivalently, higher energies).

Nevertheless, this approximation is often good enough to understand all processes whose typical distance scale is longer than the scale of the effective field theory or whose exchanged energy is smaller than the chosen bound.

For example, an effective field theory describes the nuclear interactions in terms of protons, neutrons, and mesons, even though all these objects are bound states of quarks. More generally, the transition between different effective field theories and the dependence of its parameters on the length scale in particular is described by the renormalization group.