Fine-structure constant

The fine-structure constant $\alpha$ , also known as the electromagnetic coupling constant, is the dimensionless coupling constant that determines the strength of the electromagnetic interaction. It is given by

\alpha=\frac{e^{2}}{4\pi\varepsilon_{0}\hbar c}=\frac{1}{137.03\ldots}

where

$e$ is the elementary charge.
$\hbar$ is the reduced Planck constant.
$c$ is the speed of light.
$\varepsilon_{0}$ is the vacuum permittivity.

The exact value of the constant is not an integer, but unless precision is needed, $\alpha\equiv1/137$ is commonly used since it's easy to remember and is only $\sim0.03$ away from the actual value.