Spontaneous Emission
The process by which an excited atom or quantum system spontaneously decays to a lower energy state, emitting a photon of random phase and direction.
Governing formula
Rate = A₂₁ N₂
SI unit
s⁻¹ (Transition Rate)
In depth
Driven by zero-point quantum vacuum field fluctuations. Einstein's A coefficient A₂₁ quantifies the probability per unit time of spontaneous photon emission.
Examples in the real world
Fluorescent lamp phosphors and glowing hot metal objects emitting incoherent light.