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 R_{oo} which appears in the Rydberg equation relates the spectral lines to energy level transitions. It is given by

R_{oo}=α^{2}m_{e}c/2h

where m

_{e} 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 R_{oo}.

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.