Fine-structure Constant

In physics, the fine-structure constant (usually denoted α, the small Greek letter alpha) is a fundamental physical constant, namely the coupling constant characterizing the strength of the electromagnetic interaction. Being a dimensionless quantity, it has constant numerical value in all systems of units. Arnold Sommerfeld introduced the fine-structure constant in 1916.

The current recommended value of α is 7.2973525698(24)×10−3 = 1/137.035999074(44).