Question

We have encountered stimulated emission, (stimulated) absorption, and spontaneous emission. How come there is no such thing as spontaneous absorption?

Short answer

Spontaneous absorption would involve taking energy (a photon) from the ground state of the electromagnetic field

Step-by-step solution

Absorption

The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc. In spectrophotometry, absorption of light at characteristic wavelengths or bands of wavelength is used to identify the chemical nature of molecules, atoms, or ions and t measure the concentration of these species.

A phenomenon in which radiation transfers to matter which it traverses some of or all its energy.

Step 2: Spontaneous absorption involves taking energy

Spontaneous absorption would involve taking energy (a photon) from the ground state of the electromagnetic field. But you can’t do that, because the ground state already has the lowest allowed energy.