The critical temperature is the temperature below which a dilute gas can form a Bose–Einstein condensate.
At sufficiently low temperature, the particles’ matter waves become long enough to overlap substantially. For bosons, which do not obey the Pauli exclusion principle, many particles can then occupy the same lowest-energy quantum state. The transition occurs below a critical temperature that depends on particle density, mass, and interactions.
Eric Cornell and Carl Wieman created the first dilute-gas Bose–Einstein condensate in 1995 using rubidium-87 atoms; Wolfgang Ketterle independently produced one using sodium-23. Their work earned the 2001 Nobel Prize in Physics. The condensate is not simply a very cold ordinary gas: it behaves as a coherent quantum matter wave on a macroscopic scale.
The critical temperature is not universally one fixed number. It varies from system to system. This distinguishes it from absolute zero, which is the lower limit of thermodynamic temperature, and from the Curie temperature, associated with magnetic phase transitions.