The Fine Structure Constant

The fine structure constant, also known as the coupling constant for electromagnetism, is used to define the strength of the electromagnetic force between two charged particles. It is symbolized by α and is approximately 7.2974e-3 (or approximated as 1/137) and is unitless.


In terms of other physical constants, in this case the elementary charge e, the Planck constant h, the speed of light c, the permittivity of free space (electric constant) ε_o, and the permeability of free space (magnetic constant) μ_o, we can relate α to these quantities.

The amazing thing about the fine structure constant is that any change in one of the fundamental constants will change α. A change of just 4% of the constant will not allow carbon to form, and hence no carbon-based life. An even larger change to α > 0.1 would mean that stellar fusion can't take place and therefore no warm planets, no liquid water, and again no life.


Fun note: the product of the permittivity of free space ε_o and the permeability of free space μ_o is the reciprocal of the speed of light squared.