Definition:Fine-Structure Constant

Definition
The fine-structure constant is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between elementary charged particles.

The fine-structure constant is commonly denoted $\alpha$ (Greek alpha).

It is related to the strength of the coupling of an elementary charge $\E$ with the electromagnetic field, by the formula:
 * $\alpha = \dfrac {\E^2} {2 \pi \epsilon_0 h c}$

where:
 * $\E$ is the elementary charge: $1.60217 \, 6634 \times 10^{-19} \, \mathrm C$
 * $h$ is Planck's constant: $6.62607 \, 015 \times 10^{-34} \, \mathrm J \, \mathrm s$
 * $c$ is the speed of light: $299 \, 792 \, 458 \, \mathrm m \, \mathrm s^{-1}$
 * $\epsilon_0$ is the vacuum permittivity: $8.85418 \, 78128(13) \times 10^{-12} \, \mathrm F \, \mathrm m^{-1}$

Its numerical value is approximately:
 * $\alpha \approx 0.00729 \, 735 \approx \dfrac 1 {137.0360}$

with a relative uncertainty of $1.5 \times 10^{-10}$.

The fine-structure constant is dimensionless.

Also known as
The fine-structure constant is also (rarely) known as the Sommerfeld constant, for.