Many physical systems exhibit a transition from a disordered state to an ordered one following the change of an externally controlled parameter. Among these cooperative phenomena are the phase transitions of matter, the development of a convection pattern in a fluid layer heated from below—and the onset of lasing in a laser. Ordering phenomena of systems in thermal equilibrium have been known for a long time; their properties near the transition point were studied intensely in the last decade. A wealth of experimental data has shown marked similarities among what appear to be very different phase transitions: A fluid near its critical point behaves very much like a ferromagnet near its Curie point, or to a binary liquid mixture near the consolution point. To see these similarities we must make the appropriate choice for the corresponding variables. A considerable amount of work has been devoted to explaining the simple, universal behavior of thermodynamic systems in the region of the critical point.
Exploring the analogy of the lasing transition to such critical phenomena as ferromagnetism and the onset of convection in fluids leads to a clearer picture of the behavior of cooperative systems.