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The dependence of a coupling g(μ) on the energy-scale is known as "running of the coupling". The theory of the running of couplings is given by the renormalization group , though it should be kept in mind that the renormalization group is a more general concept describing any sort of scale variation in a physical system (see the full article ...
Richard Feynman, one of the originators and early developers of the theory of quantum electrodynamics (QED), referred to the fine-structure constant in these terms: There is a most profound and beautiful question associated with the observed coupling constant, e – the amplitude for a real electron to emit or absorb a real photon. It is a ...
written in terms of the fine structure constant in natural units, α = e 2 /4π. [2] This beta function tells us that the coupling increases with increasing energy scale, and QED becomes strongly coupled at high energy. In fact, the coupling apparently becomes infinite at some finite energy, resulting in a Landau pole. However, one cannot ...
This series diverges asymptotically, but in quantum electrodynamics (QED) at the second order the difference from experimental data is in the order of 10 −10. This close agreement holds because the coupling constant (also known as the fine-structure constant) of QED is much less than 1. [clarification needed]
The running of the coupling constants, i.e. their scale dependence described by the renormalization group (RG), is thus special in its UV limit in the sense that all their dimensionless combinations remain finite. This suffices to avoid unphysical divergences, e.g. in scattering amplitudes.
The scale anomaly, which gives rise to a running coupling constant. In QED this gives rise to the phenomenon of the Landau pole. In quantum chromodynamics (QCD) this leads to asymptotic freedom. The chiral anomaly in either chiral or vector field theories with fermions. This has close connection with topology through the notion of instantons.
Because the theory is "sick" for any negative value of the coupling constant, the series does not converge but is at best an asymptotic series. From a modern perspective, we say that QED is not well defined as a quantum field theory to arbitrarily high energy. [30] The coupling constant runs to infinity at finite energy, signalling a Landau pole.
The model belongs to the Griffiths-Simon class, [1] meaning that it can be represented also as the weak limit of an Ising model on a certain type of graph. The triviality of both the ϕ 4 {\displaystyle \phi ^{4}} model and the Ising model in d ≥ 4 {\displaystyle d\geq 4} can be shown via a graphical representation known as the random current ...