Fine-structure constant
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Numbers
$\alpha$:
0.0072973525675 +/- 1.09e-11
$\alpha^{-1}$:
137.035999113 +/- 2.03e-7
Definition
The fine-structure constant $\alpha$ is the dimensionless physical constant defined as $\alpha = e^2/(2 \varepsilon_0 c h)$, where $e$ is the elementary charge, $\varepsilon_0$ the electric constant, $c$ the speed of light in vacuum, and $h$ the Planck constant.
$\alpha$ may not be constant [3].