The emission of a photon by an atom as it makes a transition from an excited state to the ground state. Spontaneous emission occurs independently of any external electromagnetic radiation; the transition is caused by interactions between atoms and vacuum fluctuations (see vacuum state) of the quantized electromagnetic field. The process of spontaneous emission, which cannot be described by nonrelativistic quantum mechanics, as given by formulations such as the Schrödinger equation, is responsible for the limited lifetime of an excited state of an atom before it emits a photon. See also quantum theory of radiation; Einstein coefficients; laser.