In physics, a coupling constant, usually denoted g, is a number that determines the strength of the force exerted in an interaction. Usually, the Lagrangian or the Hamiltonian of a system describing an interaction can be separated into a kinetic part and an interaction part. The coupling constant determines the strength of the interaction part with respect to the kinetic part, or between two sectors of the interaction part. For example, the electric charge of a particle is a coupling constant.
A coupling constant plays an important role in dynamics. For example, one often sets up hierarchies of approximation based on the importance of various coupling constants. In the motion of a large lump of magnetized iron, the magnetic forces are more important than the gravitational forces because of the relative magnitudes of the coupling constants. However, in classical mechanics one usually makes these decisions directly by comparing forces.
Read more about Coupling Constant: Fine-structure Constant, Gauge Coupling, Weak and Strong Coupling, Running Coupling, Beta Functions, QED and The Landau Pole, QCD and Asymptotic Freedom, QCD Scale, String Theory
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... Each perturbative description of string theory depends on a string coupling constant ... However, in the case of string theory, these coupling constants are not pre-determined, adjustable, or universal parameters rather they are dynamical scalar ...
... a certain probability (which is roughly the coupling constant) ... Coupling constant in quantum field theory this is, roughly, the probability for one particle to emit or absorb another particle, the latter typically being a ... In string theory, the coupling constant is no longer a constant, but is rather determined by the abundance of strings in a particular mode, the dilaton ...
... Every force has a coupling constant, which is a measure of its strength, and determines the chances of one particle to emit or absorb another particle ... For electromagnetism, the coupling constant is proportional to the square of the electric charge ... is a reasonable approximation only if the coupling constant is small, which is the case for electromagnetism ...
... A renormalization scale must be introduced in the process, and the coupling constant and mass become dependent upon it ... The dependence of a coupling constant g on the scale λ is encoded by a beta function, β(g), defined by the relation This dependence on the energy scale is known as the running of the coupling parameter, and theory ... most commonly perturbation theory, where one assumes that the coupling constant is small ...
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