The process of gaining and losing energy is normally random, so under typical conditions, a large group of atoms is unlikely to be in a suitable state for this reaction. Lasers depend on some sort of setup that results in many electrons being in the desired states, a condition known as a population inversion. An easy to understand example is the ruby laser, where there is a metastable state where electrons will remain for a slightly longer period if they are first excited to even higher energy. This is accomplished through optical pumping, using the white light of a flash lamp to increase the electron energy to a blue-green or ultraviolet frequency. The electrons then rapidly lose energy until they reach the metastable energy level in the deep red. This results in a brief period where a large number of electrons lie at this medium energy level, resulting in a population inversion. At that point any one of the atoms can emit a photon at that energy, starting the chain reaction.