Résumé
Abstract The fine-structure constant α is a dimensionless constant introduced by Sommerfeld in 1916.This number was interpreted as a way of quantifying the strength of the electromagnetic interaction. Since then, several physicists expressed their dissatisfaction waiting for a more direct interpretation.Here we propose a one-to-one correspondence between 1/α and the variance of the photon number δN^2_α inside a black-body of given volume and temperature. As a broader perspective we propose that any atomic spectral-line is associated with a structure-line constant that provides the macroscopic statistical-information on the photon gas inside the corresponding black-body cubic cavity.From this point of view, the expression of the Sommerfeld constant is viewed as a particular case that coincides exactly with our results when the black-body temperature corresponds to the equivalent temperature of theemission line of T=45 μeV/k B = 0.516 K and with the cubic size of the black body L=3.3 cm.