An Etymological Dictionary of Astronomy and Astrophysics
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A measure of the strength of → interaction between a → charged particle and the → electromagnetic field. It is a → dimensionless number expressed (in → cgs units) by α = e²/ħc, where e is the → electron charge, ħ is the → reduced Planck's constant, and c is the → speed of light. It is approximately equal to 1/137 or 7.3 × 10^-3. The smallness of this number is of great importance since it determines the size of → atoms and the → stability of → matter. Same as → electromagnetic coupling constant.

→ fine structure; → constant.