I understand that a photon can be 'absorbed' by an electron resulting in the electron jumping to a higher energy level within an atom. I also understand that a photon is emitted when an electron jumps to a lower energy level within an atom. But why does the electron jump to a lower energy level? It seems that in the case of jumping to a higher energy level, there is a cause - a photon interacts with the electron. But I am not able to wrap my head around what causes the opposite - jumping to a lower energy level and emitting a photon. If a lone atom was sitting in an isolated system in an excited state, would it eventually decay to the ground state?