The term "absolute zero" conveys a crystal clear idea: This is the coldest it gets. Ain't gettin' no colder. Except, well, science just figured out how to reach temperatures lower than absolute zero, or 0 Kelvin. Negative temperatures, here we come.
Live Science writes "To comprehend the negative temperatures scientists have now devised, one might think of temperature as existing on a scale that is actually a loop, not linear. Positive temperatures make up one part of the loop, while negative temperatures make up the other part. When temperatures go either below zero or above infinity on the positive region of this scale, they end up in negative territory."
So, in a way, atoms with negative temperature are actually infinitely hot, and thus warmer than 0 Kelvin. If that just elicited a "huh" from you, Live Science explains:
"As one might expect, objects with negative temperatures behave in very odd ways. For instance, energy typically flows from objects with a higher positive temperature to ones with a lower positive temperature — that is, hotter objects heat up cooler objects, and colder objects cool down hotter ones, until they reach a common temperature. However, energy will always flow from objects with negative temperature to ones with positive temperatures. In this sense, objects with negative temperatures are always hotter than ones with positive temperatures.
"Another odd consequence of negative temperatures has to do with entropy, which is a measure of how disorderly a system is. When objects with positive temperature release energy, they increase the entropy of things around them, making them behave more chaotically. However, when objects with negative temperatures release energy, they can actually absorb entropy."
German scientists encountered negative temperatures by placing about 100,000 atoms in a vacuum to control the environment and then lowering their temperature to a few nanokelvin, just a handful of billionths above absolute zero. Using lasers and magnets, they were able to rigidly control the movement and potential energy of each atom, arranging them in a unique optical lattice. The result was a gas in which atoms attracted one another more than repelling one another, which runs counter to how a gas (which naturally expands) typically acts.
Now that science has laughed in the face of absolute zero's limits, what can we do with their achievement? In addition to telling us all sorts of interesting things about matter, negative temperatures could theoretically improve the technology of the combustion engine to more than 100 percent efficiency. If an engine draws energy from heat, negative temperatures could help them draw energy from cold atoms, too. Because cold atoms with negative temperatures are actually infinitely hot. Way to go, science.