The fine-structure constant (α), also called the Sommerfeld constant, is a dimensionless fundamental physical constant that quantifies the strength of electromagnetic interaction between charged particles. Named by Arnold Sommerfeld in 1916, its value is approximately 1/137.036 with high precision, though why it has this particular value remains unexplained. It can be measured through quantum electrodynamics methods including the quantum Hall effect, electron anomalous magnetic moment, and atom interferometry.