Hi everyone, I would like to understand the answer to the question in the title. I got that when we use optical pumping of a laser, the probability, given an incoming photon of adequate energy, for an atom to absorb it and for an other to produce some stimulated emission out of it are equal. Thus the rate equations for the level population cannot be consistent with a stationary population inversion. However, I couldn't find any justification of why this also applies to non-optical pumping. I thank you in advance for your answers.