Running Coupling
One can probe a quantum field theory at short times or distances by changing the wavelength or momentum, k of the probe one uses. With a high frequency, i.e., short time probe, one sees virtual particles taking part in every process. The reason this can happen, seemingly violating the conservation of energy is the uncertainty relation
which allows such violations at short times. The previous remark only applies to some formulations of quantum field theory, in particular, canonical quantization in the interaction picture. In other formulations, the same event is described by "virtual" particles going off the mass shell. Such processes renormalize the coupling and make it dependent on the energy scale, at which one observes the coupling. The dependence of a coupling on the energy-scale is known as running of the coupling. The theory of the running of couplings is known as the renormalization group.
Read more about this topic: Coupling Constant
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