Wake up to another seemingly impossible day. A collection of potassium atoms in an optical lattice, which exchanges entropy for energy.
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 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.
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."