Why is a population inversion necessary for laser action?

A population inversion is essential for laser action because it reverses the natural statistical distribution of atoms, which typically favors lower-energy states.12

Under normal conditions, a system in temperature equilibrium has more atoms in lower-energy states than in higher ones. Consequently, a spontaneously emitted photon is more likely to be absorbed by an atom in a lower-energy state than to trigger stimulated emission from an excited atom. As long as lower-energy states predominate, any stimulated emission will be absorbed and die out.2

By "pumping" energy into the system to create a population inversion—where more atoms occupy higher-energy states than lower ones—this process is reversed. In this state, spontaneously emitted photons are more likely to encounter excited atoms and trigger a chain reaction of stimulated emissions. This cascade of photons, when amplified within an optical resonator, produces the coherent, monochromatic light characteristic of a laser beam.1234