Absolute temperature is usually bound to be positive. Under special conditions, however, negative temperatures—in which high-energy states are more occupied than low-energy states—are also possible.

Temperature is typically thought of as the average energy of individual atoms or molecules within a given collection. For atoms of similar mass, this "kinetic temperature" would basically be their ...

Nothing is colder than absolute zero, so it seems nonsensical to talk about negative temperature – but now there is a substance that must have just that. The revelation could shake up our ideas about ...