Coupling Constant
A dimensionless number quantifying the relative interaction strength between particles or fields in a fundamental force interaction.
Governing formula
α_EM = e² / (4π ε₀ ℏ c) ≈ 1 / 137, α_s(Q²)
SI unit
Dimensionless Parameter
In depth
Coupling constants 'run' (change magnitude) with energy scale Q². Electromagnetic coupling α_EM increases slightly at higher energies, while the strong coupling α_s decreases at high energies (asymptotic freedom).
Examples in the real world
The electromagnetic fine-structure constant α ≈ 1/137 determining atomic spectral line strength.