The media are abuzz with news of an experiment that manages to create negative absolute temperature. Phys.org:
In order to bring water to the boil, energy needs to be added to the water. During heating up, the water molecules increase their kinetic energy over time and move faster on average. Yet, the individual molecules possess different kinetic energies – from very slow to very fast. In thermal equilibrium, low-energy states are more likely than high-energy states, i.e. only a few particles move really fast. In physics, this distribution is called Boltzmann distribution. Physicists around Ulrich Schneider and Immanuel Bloch have now realized a gas in which this distribution is exactly inverted: Many particles possess large energies and only a few have small energies. This inversion of the energy distribution means that the particles have assumed a negative absolute temperature.
The Boltzmann distribution can be illustrated with balls that are distributed on a hilly landscape, which provides both a lower and upper bound for the potential energy of the balls. At positive temperatures (left figure), as they are common in everyday life, most balls lie in the valley around minimum potential energy. They barely move and therefore also possess minimum kinetic energy. States with small total energy are therefore more likely than those with large total energy – the usual Boltzmann distribution. At infinite temperature (central figure) the balls spread evenly over low and high energies in an identical landscape. Here, all energy states are equally probable. At negative temperatures (right figure), however, most balls wander on top of the hill, at the upper limit of potential energy. Also their kinetic energy is maximal. Energy states with large total energy are occupied more than those with small total energy – the Boltzmann distribution is inverted.