Year in Review: Below absolute zero, but hot

Lab trickery achieves negative temperature


Ulrich Schneider must be a hit at cocktail parties. The physicist at Ludwig Maximilians University of Munich can tell awed guests that he is responsible for creating both the world’s hottest substance and lowest temperature — at the same time.

Schneider’s substance — a gas consisting of about 100,000 potassium atoms — reached a temperature below absolute zero, about –0.000000001 kelvins.

Unlike Fahrenheit and Celsius temperatures, where the zero point is arbitrary, absolute temperature (measured in kelvins) supposedly can go no lower than zero. And in fact, nothing can get colder than absolute zero. But a negative absolute temperature, though technically below zero, is actually infinitely hot.

That’s because a positive or negative sign on the kelvin scale describes the energy distribution of a substance’s particles. Usually, most particles within a system have relatively low energies; only a few occupy the highest rungs of the energy range. In such situations temperature is always positive.

Schneider and colleagues reversed that distribution in their gas of potassium atoms. They used lasers and magnets to confine the atoms to a narrow band of energies. At first, most of the atoms possessed energies at the lower end of that band. But by altering the lasers and magnetic field, the researchers flipped the atoms’ energy distribution. Suddenly most of the atoms were at the upper limit of the allowed energy. In that situation the gas had a negative temperature (SN: 2/9/13, p. 10).

At the same time, the gas was hotter than any substance with a positive temperature. Because of the glut of high-energy atoms, heat would flow from the gas to any substance with a positive temperature. And heat always flows from hotter to colder, by decree of the laws of thermodynamics.

Schneider’s experiment offers scientists the rare opportunity to study a system that gets more orderly with increasing energy — adding energy causes more atoms to cluster at the high-energy limit. The researchers also noted that the potassium atoms, which should have collapsed toward each other, remained stable at negative temperatures. This repulsion might provide insight into dark energy, the mysterious component of the universe that counteracts gravitational attraction and causes the cosmos to expand at an accelerating rate.

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