I wonder what happens if you have trapped hydrogen atoms and you apply a magnetic field. We could observe the Zeeman effect; some electrons would gain energy and some other would lose energy due to the magnetic field. Say an electron gained some energy. Now you remove the magnetic field, the electron could return to its previous state, losing the energy it gained from the magnetic field. My question is... in what form does this electron lose its "tiny" energy? I think the only answer would be a photon (in case of a magnetic field of a few teslas the photon might be in the microwave or radio wave range of the EM spectra, I believe) but I'm not 100% sure. I understand that if I apply an EM field (flux of photons), the electrons will only change their quantum number n, absorbing or emiting a photon in the process. But in the case of a magnetic or electric field, I have no idea. EDIT: Some other thoughts. There can't be emission/absorption of photon when the electron change their quantum number "m". Because the M field, as far as I know, doesn't contains photons. So when you remove the M field, some electron will gain some energy but if there's no photon, I have no idea how the electrons "absorb" the energy.