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 4πε0ħcα = e2. Being a dimensionless quantity, it hae the sel an same nummerical vailyie in aw seestems o units, whilk is approximately 1/ .
- e is the elementary chairge (:= 1.602176634×10−19 C);
- π is the mathematical constant pi;
- ħ = h/ is tha reduiced Planck constant (:= 6.62607015×10−34/ J s);
- c is the speed of light in vacuum (:= 299792458 m/s);
- ε0 is the electric constant or permittivity o free space;
- µ0 is the magnetic constant or permeability o free space;
- ke is the Coulomb constant;
- RK is the von Klitzing constant;
- Z0 is the vacuum impedance or impedance o free space.
The defineetion reflects the fact thae α is relatit tae the permeability o free space µ0, whilk equals 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.