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In [[physics]], the '''fine-structure constant''', also known as the '''Sommerfeld constant''', commonly denoted by {{mvar|α}} (the [[Alpha|Greek letter ''alpha'']]), is a [[Dimensionless physical constant|fundamental physical constant]] which quantifies the strength of the [[electromagnetic interaction]] between elementary charged particles.
It is a [[dimensionless quantity]] ([[dimensionless physical constant]]), independent of the [[system of units]] used, which is related to the strength of the coupling of an [[elementary charge]] ''e'' with the [[electromagnetic field]], by the formula {{math|1=4''πε''{{sub|0}}''ħcα'' = ''e''{{sup|2}}}}. Its [[numerical value]] is approximately {{nowrap|{{physconst|alpha|round=13|ref=no}} ≈ {{sfrac|{{physconst|alphainv|round=9|ref=no}}}}}}, with a relative uncertainty of {{physconst|alpha|after=.|runc=yes}}
The constant was named by [[Arnold Sommerfeld]], who introduced it in 1916<ref name=Sommerfeld-1916>{{cite journal |author=Sommerfeld, Arnold |author-link=Arnold Sommerfeld |year=1916 |title=Zur Quantentheorie der Spektrallinien |journal=[[Annalen der Physik]] |volume=4 |issue=51 |pages=51–52 |url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433090771183&view=1up&seq=65 |access-date=2020-12-06 }}
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