I'm not exactly sure what I'm trying to ask, and I'm not very keen on laser physics, so bear with me. If I have a system in an excited state that can decay by spontaneous emission with a certain lifetime, what is the effect of placing it in a cavity with a mode resonant with this transition? Does it increase or decrease the likelihood of decay into the lower energy state? If the cavity is isolated and the photon that is emitted into the cavity has to stay, does the energy "flip" back and forth between the system and the cavity producing something like a superposition of the excited and excited state? If it is evident that I have some misconception about what a cavity or spontaneous emission is, please enlighten me. As I said, I'm not sure if I'm asking the right thing.