Fine-structure constant

Quantum field theory
Feynman diagram
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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 0.0072973525643 ≈ ⁠1/137.035999177⁠, 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.

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