Just as everything we know about speed tells us that light is as fast as anything can get, everything we know about temperature tells us that absolute zero is as cold as anything can get. But just as experiments have allowed us to bring matter down to temperatures very close to absolute zero right here on Earth, scientists have given us reason to believe that absolute zero might not be all that absolute after all.
New research summarized by Monash University physicist Tapio Simula and his colleagues has brought new attention to the idea of negative absolute temperature, matter that can actually get colder than perfectly cold energy by being, in effect, anti-hot (and therefore super-hot). As Nature’s Zeeya Merali explains:
Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature, explains Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany …
“This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”
But is negative absolute temperature colder than absolute zero, or just hotter in a different way? It all depends on how you define heat. Using a mathematical definition, negative absolute temperatures are colder than absolute zero because they have negative values, which are by definition lower than zero. But if you define coldness as the absence of motion, matter at negative absolute temperatures has no less heat than matter at or near absolute zero. Indeed, the motion present at negative absolute temperatures resembles the kind of motion we’d associate with heat—and, to the extent that it exceeds the motion traditional heat can generate, is actually hotter than temperatures above absolute zero.
For most purposes, it’s useful and accurate to think of absolute zero as the coldest possible temperature—because to the extent that it represents minimal physical motion on a molecular level, it is still the coldest possible temperature. But the reality of negative absolute temperature serves as an important reminder that our understanding of heat is more than likely fundamentally wrong in some very important ways, and that we’re only beginning to understand how wrong.