Negative temperature: Atoms beyond absolute zero may be a new form of matter

A cloud of atoms with a temperature beyond absolute zero – which can also be bizarrely hotter than any optimistic temperature possible – may be a mysterious new quantum state of matter.

Luca Donini on the University of Cambridge and his colleagues have put hundreds of potassium atoms into this seemingly paradoxical scenario by exactly manipulating their power ranges and quantum states. “Quantum mechanics allows you to do this, while classical thermodynamics would never allow it,” he says.

Normally, if…