This concept, within quantum field theory and quantum optics, addresses why spontaneous emission — an excited quantum system's photon-emitting transition to a lower energy state without external stimulus — cannot be explained by the (non-relativistic) Schrodinger equation, since that equation neither quantizes electron energy levels nor the electromagnetic field and is non-relativistic. Because spontaneous emission requires a quantized electromagnetic field and is fundamentally irreversible, explaining it requires extending quantum mechanics into a relativistic, field-quantized framework (via the Dirac equation) — the motivating step from quantum mechanics to quantum field theory. It relates to, but is formally distinct from, spontaneous radioactive decay, which is a nuclear rather than atomic/electromagnetic process.
This concept, within quantum field theory and quantum optics, addresses why spontaneous emission — an excited quantum system's photon-emitting transition to a lower energy state without external stim…