Fine-Structure Constant


fine-structure constant

[′fīn ‚strək·chər ′kän·stənt] (physics) A fundamental dimensionless constant, equal to e 2/(4πε0ℏ 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 ε0 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 = e2/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 α2. In quantum electrodynamics, α is a natural parameter that characterizes the strength of the electromagnetic interaction.