"In physics, the **fine-structure constant**, also known as Sommerfeld's constant, commonly denoted ? (the Greek letter alpha), is a fundamental physical constant characterizing the strength of the electromagnetic interaction between elementary charged particles. It is related to the elementary charge e, which characterizes the strength of the coupling of an elementary charged particle with the electromagnetic field, by the formula 4??0?c? = e2. Being a dimensionless quantity, it has the same numerical value of about ?1?137 in all systems of units. Arnold Sommerfeld introduced the **fine-structure constant** in 1916.

Arnold Sommerfeld introduced the **fine-structure constant** in 1916, as part of his theory of the relativistic deviations of atomic spectral lines from the predictions of the Bohr model. The first physical interpretation of the **fine-structure constant** was as the ratio of the velocity of the electron in the first circular orbit of the relativistic Bohr atom to the speed of light in the vacuum. Equivalently, it was the quotient between the maximum angular momentum allowed by relativity for a closed orbit, and the minimum angular momentum allowed for it by quantum mechanics. It appears naturally in Sommerfeld's analysis, and determines the size of the splitting or fine-structure of the hydrogenic spectral lines." Wikipedia, fine structure constant

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