Fine-structure Constant
The coupling constant arises naturally in a quantum field theory. A special role is played in relativistic quantum theories by coupling constants which are dimensionless, i.e., are pure numbers. For example, the fine-structure constant,
(where is the charge of an electron, is the permittivity of free space, is the reduced Planck constant and is the speed of light) is such a dimensionless coupling constant that determines the strength of the electromagnetic force on an electron.
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