Fine-structure constant

The fine-structure constant is a dimensionless number (~1/137) measuring the strength of the electromagnetic interaction between charged particles. Being dimensionless, it would have the same value anywhere in the universe.

Used in

• Atomic energy level splitting (fine structure)
• Quantum electrodynamics coupling strength α = e²/(4πε₀ħc)

Source: SI defining constants / CODATA 2018