# Fine-Structure Constant

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## fine-structure constant

[′fīn ‚strək·chər ′kän·stənt] (physics)

A fundamental dimensionless constant, equal to

*e*^{2}/(4πε_{0}ℏ*c*) in International System (SI) units and to*e*^{2}/(ℏ*c*) in centimeter-gram-second (cgs) electrostatic units, where*e*is the elementary charge, ℏ is Planck's constant divided by 2π,*c*is the speed of light, and ε_{0}is the electric constant; numerically, it is equal to 0.007 297 352 533 ± 0.000 000 000 027 or to 1/(137.035 999 76 ± 0.000 000 50); symbolized α. Also known as Sommerfeld fine-structure constant.## Fine-Structure Constant

(α), a dimensionless quantity formed from universal physical constants: α = *e*^{2}/*hc* ≠ 1/137, where *e* is the elementary electric charge, *h* is Planck’s constant, and *c* is the speed of light in a vacuum. According to the most accurate measurements, which are based on the Josephson effect, α^{–1} = 137.0359 ± 0.0004. The constant α is called the fine-structure constant because it determines the fine structure of the energy levels of an atom in the sense that the magnitude of the fine-structure splitting is proportional to α^{2}. In quantum electrodynamics, α is a natural parameter that characterizes the strength of the electromagnetic interaction.