Fine-structure constant

Value of α
0.0072973525643(11)
Value of α−1
137.035999177(21)

In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by α (the Greek letter alpha), is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between elementary charged particles.

It is a dimensionless quantity, independent of the system of units used, which is related to the strength of the coupling of an elementary charge e with the electromagnetic field, by the formula 4πε0ħcα = e2. Its numerical value is approximately 0.00729735256431/137.035999177, with a relative uncertainty of 1.6×10−10.[1]

The constant was named by Arnold Sommerfeld, who introduced it in 1916[2] when extending the Bohr model of the atom. α quantified the gap in the fine structure of the spectral lines of the hydrogen atom, which had been measured precisely by Michelson and Morley in 1887.[a]

Why the constant should have this value is not understood,[3] but there are a number of ways to measure its value.

  1. ^ "2022 CODATA Value: fine-structure constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 18 May 2024.
  2. ^ Sommerfeld, Arnold (1916). "Zur Quantentheorie der Spektrallinien". Annalen der Physik. 4 (51): 51–52. Retrieved 6 December 2020. Equation 12a, "rund 7·10−3" (about ...)
  3. ^ Cite error: The named reference Feynman1985 was invoked but never defined (see the help page).


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