In
science, specifically
statistical mechanics, a
population inversion occurs while a
system (such as a group of
atoms or
molecules) exists in a state with more members in an
excited state than in lower energy states. It is called an "inversion" because in many familiar and commonly encountered physical systems, this is not possible. The concept is of fundamental importance in
laser science because the production of a population inversion is a necessary step in the workings of a standard
laser.