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In the Lagrangian of a prequantum field theory the those terms that involve various fields describe the coupling or interaction of these fields with each other. Hence the prefactors of these terms encode the strength of this coupling, and hence are called coupling constants.
In perturbative quantum field theory the coupling constant (as well as Planck's constant which indicates the strength of quantum effects) is regarded as tiny, in fact as infinitesimal, in that all observables are expressed as (generally non-converging, asymptotic) formal power series in the coupling constant.
See also
Last revised on May 18, 2019 at 14:13:25. See the history of this page for a list of all contributions to it.