Résumé
We investigate quantum and quantum-relativistic effects associated with the noise power spectrum and the fluctuation--dissipation relation between current--noise spectra and linear--response conductance at low frequencies of the electromagnetic field. At high frequencies, vacuum catastrophe is shown to be avoided by the presence of Casimir force. At low frequencies, the quantum effect associated with one--dimensional structures under the conditions of ballistic transport typical at the nanometric scale length are briefly reviewed in terms of a universal quasi-particle approach. The case of a photon gas inside an appropriate black-body cavity is found to provide a physical interpretation of the lines spectra of atomic elements within an exact statistical approach based on a physical interpretation of the fine structure constant,$α=1/137.0560$ .