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Reference.org
Fine-structure constant
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<table><tbody><tr><th>Value of <i>α</i></th></tr><tr><td>0.0072973525643(11)</td></tr><tr><th>Value of <i>α</i>−1</th></tr><tr><td>137.035999177(21)</td></tr></tbody></table> <p>In <a href="/facts/Physics/a7aH2y08">physics</a>, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by α (the <a href="/facts/Alpha/I51VqVVm">Greek letter <i>alpha</i></a>), is a <a href="/facts/Dimensionless_physical_constant/79vy9fTj">fundamental physical constant</a> that quantifies the strength of the <a href="/facts/Electromagnetic_interaction/oAkmALHT">electromagnetic interaction</a> between elementary charged particles. </p><p>It is a <a href="/facts/Dimensionless_quantity/TINrpQIq">dimensionless quantity</a> (<a href="/facts/Dimensionless_physical_constant/79vy9fTj">dimensionless physical constant</a>), independent of the <a href="/facts/System_of_units/xIsWmFWW">system of units</a> used, which is related to the strength of the coupling of an <a href="/facts/Elementary_charge/lPcj6CoQ">elementary charge</a> <i>e</i> with the <a href="/facts/Electromagnetic_field/MacraS8y">electromagnetic field</a>, by the formula 4<i>πε</i>0<i>ħcα</i> = <i>e</i>2. Its <a href="/facts/Numerical_value/auQSAE2m">numerical value</a> is approximately 0.0072973525643 ≈ 1/137.035999177, with a relative uncertainty of 1.6×10−10. </p><p>The constant was named by <a href="/facts/Arnold_Sommerfeld/cJtpVHvn">Arnold Sommerfeld</a>, who introduced it in 1916 when extending the <a href="/facts/Bohr_model/FmTHjaxy">Bohr model</a> of the atom. <i>α</i> quantified the gap in the <a href="/facts/Fine_structure/6lejlmN2">fine structure</a> of the <a href="/facts/Spectral_lines/HuI4Bj5f">spectral lines</a> of the hydrogen atom, which had been measured precisely by <a href="/facts/Albert_A._Michelson/LHIAKkRN">Michelson</a> and <a href="/facts/Edward_W._Morley/yNp2YFUe">Morley</a> in 1887. </p><p>Why the constant should have this value is not understood, but there are a number of ways to <a href="/facts/Precision_tests_of_QED/ddojHb67">measure its value</a>. </p>