# Fine-structur constant

In pheesics, the fine-structure constant, anaw kent as Sommerfeld's constant, commonly scrieved by α (the Greek letter alpha), is a dimensionless pheesical constant chairacterisin the strenth of the electromagnetic interaction atween elementary chairged pairticles. It is relatit tae the elementary chairge e, whilk chairacterises the strenth o couplin o an elementary chairged pairticle wi tha electromagnetic field, bi the formula ε0ħcα = e2. Being a dimensionless quantity, it hae the sel an same nummerical vailyie in aw seestems o units, whilk is approximately 1/137 .

## Defineetion

Some equivalent defineetions o α in terms o ither fundamental pheesical constants are:

$\alpha ={\frac {1}{4\pi \varepsilon _{0}}}{\frac {e^{2}}{\hbar c}}={\frac {\mu _{0}}{4\pi }}{\frac {e^{2}c}{\hbar }}={\frac {k_{\text{e}}e^{2}}{\hbar c}}={\frac {c\mu _{0}}{2R_{\text{K}}}}={\frac {e^{2}}{4\pi }}{\frac {Z_{0}}{\hbar }}$ whaur:

The defineetion reflects the fact thae α is relatit tae the permeability o free space µ0, whilk equals $\mu _{0}={\frac {2\alpha }{e^{2}}}{\frac {h}{c}}.$ in the 2019 redefineetion of SI base units, 4π × 1.000 000 000 82 (20) 10−7 H·m−1 is the value for µ0 based upo' mair accurat meisurements o the fine structur constant.