# Fine-structure constant

## Dimensionless number that quantifies the strength of the electromagnetic interaction / From Wikipedia, the free encyclopedia

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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.

**More information**Value of α, Value of α−1 ...

Value of α |
---|

0.0072973525643(11) |

Value of α^{−1} |

137.035999177(21) |

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 4*πε*_{0}*ħcα* = *e*^{2}. 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.^{[lower-alpha 1]}

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