Fine-structure constant
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 that quantifies the strength of the electromagnetic interaction between elementary charged particles. It is a dimensionless quantity (dimensionless physical constant), 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 α = e 2 ( 4 π ε 0 ℏ c ) − 1 {\textstyle \alpha ={e^{2}}\left(4\pi \varepsilon _{0}\hbar c\right)^{-1}} .