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.

Such new approaches, scientists say, might reveal previously unknown ways in which matter behaves at the quantum level. “With these atom systems you can mimic various states of matter and do stuff that is otherwise not possible,” says team leader Achim Rosch, a physicist at the University of Cologne in Germany.

To understand negative temperature, think in terms of energy states rather than markings on a thermometer. Atomic particles in what physicists conside