In quantum physics, Fermi's golden rule, also known as the golden rule of time-dependent perturbation theory, is a formula that can be used to calculate the rate at which an initial quantum state transitions into a final state, which is composed of a continuum of states (a so-called "bath"). This valuable equation has been applied to numerous physics problems, particularly those for which it is important to consider how systems respond to imposed perturbations and settle into stationary states over time.
source https://www.lifetechnology.com/blogs/life-technology-science-news/exploring-the-decay-processes-of-a-quantum-state-weakly-coupled-to-a-finite-size-reservoir
