The fine-structure constant has an approximate value of 1/137.
Usually written as α, it is a dimensionless constant that measures the strength of the electromagnetic interaction between charged particles. Its modern value is approximately 0.00729735, or about 1/137.036, so 1/137 is a useful rounded form rather than an exact identity. Because it has no units, its value is independent of the measurement system used.
The constant first appeared in Arnold Sommerfeld’s 1916 analysis of the fine structure of hydrogen spectral lines. In quantum electrodynamics, it also acts as the electromagnetic coupling parameter: calculations are organized as powers of α, and its small size helps make many perturbative predictions remarkably precise.
The number 137 has acquired an unusual cultural status in physics. Arthur Eddington argued in 1929 that the reciprocal should be exactly 137, but increasingly accurate measurements disproved that claim. The fine-structure constant is not simply “the probability that an electron emits a photon,” another popular oversimplification; it is a fundamental parameter governing electromagnetic interaction strength.
Its value can be determined through experiments involving the electron’s magnetic moment, atom interferometry, and the quantum Hall effect.