Absolute zero
The lowest possible temperature, 0 K (−273.15 °C), where thermal motion reaches the minimum permitted by quantum mechanics.
Definition
Absolute zero is the zero of the Kelvin scale, −273.15 °C, the temperature at which a system's thermal energy is as small as the laws of physics allow. It is not a state of literally no motion — quantum mechanics forbids that through zero-point energy — but the state of minimum possible energy, in which a system settles into its quantum ground state.
Absolute zero is unattainable. The third law of thermodynamics implies that reaching it would require infinitely many steps or infinite work, because the entropy change available to cool a system shrinks to zero as the temperature does. Laboratories have come extraordinarily close — to within billionths of a kelvin — using laser and evaporative cooling, but never to zero itself.
The concept gives the Kelvin scale its physical anchor and underlies the third law and phenomena such as Bose–Einstein condensation that emerge only in the deep cold. This entry is a stub; the approach to absolute zero and the third law are treated in depth later in the branch.