Negative temperature, infinitely hot

Physicists have described a new way of making one of the most counterintuitive phenomena known: negative temperature, which despite its name means a system that is almost infinitely hot.

Negative temperatures have been seen before, but only in very limited applications. In a paper to appear in Physical Review Letters, theorists propose broader and more intriguing ways to confirm negative temperatures, by taking pictures of atoms as they change from positive to negative temperature.