Fine-structure constant

In physics, the fine-structure constant, also known as Sommerfeld's constant, commonly denoted by α (the Greek letter alpha), is a dimensionless 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 4πε0ħcα = e2. As a dimensionless quantity, it has the same numerical value in all systems of units, which is approximately 1/137 . The inverse of α is 137.035999084(21). While there are multiple physical interpretations for α, it received its name from Arnold Sommerfeld introducing it (1916) in extending the Bohr model of the atom: α quantifies the gap in the fine structure of the spectral lines of the hydrogen atom, which had been precisely measured by Michelson and Morley. Some equivalent definitions of α in terms of other fundamental physical constants are: