Over the past decades, researchers have made significant progress in cooling objects closer to absolute zero, the temperature at which all molecular motion reaches its minimum. This has allowed them to study unusual states of matter, like Bose-Einstein condensates, which behave quite differently from the materials we're familiar with. But absolute zero is as low as a temperature can get, and we can't actually reach it, so progress will ultimately be limited.

Maybe not.

As thermodynamics defines temperature, it's theoretically possible to have a negative value. Yesterday, a team of German researchers reported that they were actually able to produce a system with exactly that. They found that the negative temperature system was stable for hundreds of milliseconds, raising the prospect that we can study a radically different type of material.

To understand how temperatures can go negative, you have to think in terms of thermodynamics, which is governed by energy content and entropy. In a normal system, there's a lower limit on energy content—absolute zero—but no upper limit. If you start with a system at absolute zero and add energy, the atoms or molecules it contains start occupying higher energy states. With more energy, they start spreading out evenly among these states. This in turn increases the entropy of the system, since fewer and fewer atoms are in the same energy state.

Now imagine a system where there's an upper limit on the energy state an atom can occupy. As you add more energy, more and more atoms start occupying the maximum energy state. As this happens, entropy actually starts to go down, since an increasing fraction of the atoms begin to occupy the identical energy state. In thermodynamic terms, you've reached negative temperatures.

This has some pretty bizarre consequences. If you could maximize the entropy in the system, temperature becomes discontinuous—it jumps from positive to negative infinity. Strange things would happen if you bring it together with a system that has a normal temperature. "In thermal contact," the authors write, "heat would flow from a negative to a positive temperature system. Because negative temperature systems can absorb entropy while releasing energy, they give rise to several counterintuitive effects, such as Carnot engines with an efficiency greater than unity."

To create one of these systems, the authors set up an optical lattice of potassium atoms, chilled to near absolute zero. Under normal circumstances, these atoms repel, and thermodynamics behaves as we've come to expect. But the authors were able to switch things so that the atoms had attractive interactions. This created something that could be viewed as an "anti-pressure," which should cause the collection of atoms to collapse. It's only the negative temperature that keeps the cloud of atoms from collapsing in this set of circumstances.

It's important to emphasize that this negative temperature isn't some state "below" absolute zero. The atoms in this system still have energy, and the negative temperatures are reached through a sudden transition, rather than by gradually shifting to negative values by going past absolute zero.

Still, the system is more than just a quirky consequence of how we define temperature, since it really behaves quite differently from normal systems. The authors suggest it may help us model dark energy, in which the expansion of the Universe is a product of the sort of "negative pressure" that this system displays.

Science, 2012. DOI: 10.1126/science.1227831 (About DOIs).