Temperature as we know it can be loosely defined as the speed of atoms or molecules, which we experience as hot (fast) and cold (slow). We thought we had this one pretty well figured out: hot things evaporate, cold things condense or freeze, and despite some strange superconducting shenanigans at super-cold temperatures, there’s a limit. Nothing can be colder than absolute zero.
Except it can.
Absolute zero, zero Kelvin (-273.15 degrees C) is the temperature at which everything stops. Atoms and molecules are in their lowest-energy state, nothing moves, and disorder is at a minimum. It’s nature’s equivalent of that person whose desk has nothing on it but a perfectly aligned row of sharpened pencils: everything is orderly and no messy movements or vibrations get in the way. Through years of science classes that got progressively more complicated, absolute zero was one thing that never changed. So when I picked up Science (the Jan. 4 issue, I’m a little behind) this morning and read about negative absolute temperatures, it kind of blew my mind. What’s colder than absolute zero?
Well, nothing you or I will probably ever see. It turns out that the key to getting negative absolute temperatures is creating a system with an energy limit. In the world we see and interact with, particles can move and interact with each other and heat up to their heart’s content, which means that there isn’t any upper limit on the energy they can contain. This keeps things safely in the positive temperatures for us. German scientists in Munich and Garching, though, figured out a way to create a system with an energy maximum. They realized that if you could control atoms’ movement, environment, and interactions, you could limit their total energy and theoretically cause negative absolute temperatures.
But this wasn’t just a really interesting thought experiment and some cool equations. They actually did it. The details of how they did it involve way more quantum mechanics than I can pretend to understand, but here’s the simplified version. They used a magnetic field to trap a bunch of potassium ions in one place inside a strong vacuum. Then come the lasers, because most of the best parts of physics involve playing with lasers. In this case they built a lattice made of light, which holds the atoms in place while they fiddle with things. This was the important step where they changed the conditions so that the system has an energy maximum. When they let the lattice go, the atoms rearranged themselves in the best possible way in the new environment.
Since that’s all pretty abstract, let’s think of the potassium atoms as magnets. You spread them all out in a container, then pour water on them and freeze it. The magnets in the ice, like the potassium atoms in the optical lattice, can’t go anywhere. While they’re frozen, you introduce a new magnetic element: you hold a powerful magnet over the container. When the ice melts, the original magnets will rearrange themselves in reaction to the new magnet that you’re holding, and you can observe their new positions. Similarly, the scientists can change the environment of the atoms while they are locked into place and watch what happens when they let go. And as it turns out, what happens is way cool. The atoms rearrange themselves, showing up in spots where there were no atoms at all before. Since atoms move around until they find the most stable position (like a ball that eventually comes to rest between two hills), this means that the most stable position has changed. The scientists manipulated the system to create something new: negative temperatures.
And here’s the clincher: you might think that negative absolute temperatures mean going even colder, like it does on your thermostat. But if you compare temperatures by looking at heat flow–which object warms up the other object–it turns out that these negative absolute temperatures are hotter than any positive temperature could ever be.
Besides the thrill for people like me who think this is way cool, what’s the big deal? There are a couple of things. For people who work with atoms at low temperatures, this kind of manipulation allows brand-new configurations of atoms and opens up new areas of research. Finding negative absolute temperature also means that negative pressure is possible, and this is a cornerstone of the calculations about the universe expanding and the existence of dark energy. For the rest of us, breaking an established rule of science is always a big deal. If something you always thought was true turns out to be false, in this case that nothing is colder than absolute zero, it can lead to a whole new way of thinking. And if nothing else, maybe the paradox of negative absolute temperatures being hotter than positive will merit an ironic chuckle the next time the temperature drops below zero.
For more details, check out the original Science article (Braun, et. al) and the Perspectives article in the same issue (Carr).
