The fine-structure constant, α is the ratio of the quantum unit of electromagnetic force to the electron-based quantum unit of inertial force.

It plays a key part in Quantum Electrodynamics theory in which the electromagnetic force is transmitted by photons. In that theory the g-factor, or magnetic moment, of an electron is modified by a series of terms involving the fine-structure constant.

The fine-structure constant is so called because it is related to the fine-structure apparent in atomic spectra. These spectral lines are due to the transitions of electrons between available energy levels.

The Rydberg constant Roo which appears in the Rydberg equation relates the spectral lines to energy level transitions. It is given by

where me is the electron mass, c is the speed of light, h is the Planck's constant and α again is the fine-structure constant.

Thus, one way of determining α is by observing the atomic spectrum of hydrogen (for its simplicity) and determining Roo.

In recent years researchers have been checking if the fine-structure constant is a universal constant, immutable in time and space, by observing the atomic spectra of quasars billions of light years distant.

Recent results have indicated that α may not be a constant.