I don't understand where radioactive decay comes from. Everything I've read discusses how the environment can influence the decay (for example, neutrons in different nuclei decay with different speeds), but I couldn't find an explanation of what is the underlying cause of the decay. If the quantum system is in its eigenstate, the Schroedinger equation tells us that it will remain in this state for ever. Then why does an elementary particle - a quantum object - decay? Is it because of some disorder affecting it "from outside"? Would a perfectly isolated elementary particle never undergo decay? What if we isolated a whole nucleus? Can the fact that we're observing the nucleus (or a sample of them) itself be triggering the decay?