spontaneous emission

Spontaneous emission is the process by which an atom sitting in an excited state drops down to a lower-energy state all by itself, releasing the energy difference as a photon of light. No outside push is needed and no warning is given; after some unpredictable wait the atom simply emits and falls. It is why a glowing gas eventually fades and why excited atoms do not stay excited forever.

What makes it subtle is the word 'spontaneous'. If you write down an isolated atom with nothing around it, the excited state looks like it should sit there unchanged forever. The decay is triggered by the ever-present quantum vacuum: even empty space teems with fluctuations of the electromagnetic field, and these fluctuations coax the atom into emitting. Spontaneous emission is, in this sense, emission stimulated by the vacuum itself.

The timing of any single emission is genuinely random — you can never say exactly when a particular atom will let go — but the average rate is precise and calculable, governed by Fermi's golden rule and the relevant selection rules. Einstein first described the process phenomenologically in 1917 through his A and B coefficients, well before the full quantum theory of light could explain why it must happen at all.