linguistic:voice_change
Natural units (particle and atomic physics)

This natural unit system, used only in the fields of particle and atomic physics, uses the following defining constants:

where c is the speed of light, m is the electron mass, ħ is the reduced Planck constant, and ε is the vacuum permittivity.

The vacuum permittivity ε is implicitly used as a nondimensionalization constant, as is evident from the physicists' expression for the fine-structure constant, written α = e /(4 π), which may be compared to the correspoding expression in SI: α = e /(4 πε ħc).