Negative temperature, infinitely hot

Physicists propose creating thermodynamics puzzle routinely in the lab

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 consider positive temperature — which includes most ordinary experiences, from the sun’s surface to Antarctica’s ice — like to be in the lowest energy states possible. But in systems with negative temperature, particles prefer to populate high-energy states instead of low-energy ones.

Scientists have made negative-temperature systems before, using the spins of atomic nuclei. Picture a line of atoms, each with a spin that can point up or down. In the lowest possible energy state, all spins point down. Add energy to the system and the spins will start to flip up — reaching maximum entropy, or disorder, when half the spins are up. Adding more energy after that will shift the system into negative temperature, whose high-energy states are the only way to accommodate the extra energy.

In place of atomic spins, Rosch’s team now proposes using ultracold atoms, like those used in many laboratories to study matter at the quantum level. In such extreme experimental conditions, the atoms lose their collective identities and begin to interact with one another in weird ways. By tweaking energy inputs and other factors, the scientists say, atoms that are millionths of a degree above absolute zero on a thermometer scale could be pushed past maximum entropy into the range of negative temperature.

By making images of the probability of each atom’s location, researchers propose that theoretically they could see the atoms shift from sticking together to flying apart once they crossed the boundary from positive into negative temperature. That change would constitute “a clear signature” of negative temperature, says Immanuel Bloch, an experimental physicist at Ludwig-Maximilians University in Munich and the Max Planck Institute for Quantum Optics in nearby Garching.

Bloch, who works with ultracold atoms, plans to soon try coaxing them into negative temperature and measuring them in the way Rosch’s team suggests. “It’s an exciting proposal which challenges our perception of thermodynamics,” he says.

For instance, if a negative temperature system were plopped down next to a positive one, the heat of the high-energy states would continually flow from the negative to the positive system. In that sense, the negative temperature one is infinitely hot.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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