Temperature has long been understood as a measurement of the average energy of a set of atoms, with the absolute zero temperature representing a system in which the particles no longer have any kinetic energy at all, having ceased all movement. For this reason, while we frequently talk about negative temperatures in the Celsius and Fahrenheit scales, the idea of a negative temperature on the Kelvin temperature scale was believed to be a nonsensical concept. After all, you can't have less motion than none, can you?
Well, new research may have found a way around that.
A Clever Experimental Tour de Force
According to the journal Nature, Ulrich Schneider of Munich's Ludwig Maximilian University (along with colleagues, of course) has succeeded in taking a gas of potassium atoms and reducing them to a state that has a temperature lower than absolute zero.
In order to understand how this is possible, you need to think about temperature in a slightly different way than we're used to. (I'm frankly still trying to get my head around it myself!) Think of a gas that's in a lattice, where the temperature represents the average motion of the particles in that lattice. As it cools, the temperature of the majority of the gas is going to be very low, with a handful of particles in higher energy states. (Temperature is, after all, the average energy of the system.) Doing this with the potassium atoms in the experiment, contained using lasers and magnetic fields, physicists created a situation in which the atoms were repelling each other but the whole set-up was stable and in its lowest-energy state, just above absolute value.
Now here comes the trick. The physicists quickly switched the magnetic field, so now the atoms were attracting each other instead of repulsing them. Instead of being in the lowest-energy state, the system was now in its highest-energy state ... with a handful of particles in lower energy states. Under normally circumstances, the particles would attract each other together and it would all collapse down into its lowest-energy state ... but the experimentalists were able to use trapping lasers to prevent that. It ended up being more favorable for the atoms to remain in this state rather than collapse.
The overall impact of this is that the temperature of the potassium atom gas went from slightly above absolute zero Kelvin to a few billionths of a degree below 0 degrees Kelvin!
Surprisingly, this isn't the first time that physicists have done something like this. In a story that somehow escaped my attention, physicists were able to "demonstrated negative absolute temperatures in a magnetic system" back in 2011. Wolfgang Ketterle, the Nobel laureate who was responsible for this 2011 work, called the new research an "experimental tour de force."
Now that the phenomenon has been shown using two different approaches, things are no doubt going to get very interesting as experimenters begin trying to tackle this amazing atomic behavior in new and inventive ways.
What Does It Mean?
The immediate question that comes to my mind is what this means for the way that we consider the entropy of the situation. The second law of thermodynamics isn't in any danger, though, because this is clearly not a closed system. There are magnetic fields and trapping lasers required in this experimental set-up, after all, so there's energy being expended from outside to maintain the situation, and if the entropy does decrease it'll likely do so in a way that's fully consistent with what our basic understanding of thermodynamics.
This doesn't mean there aren't serious questions, though, because the equations that we use to discuss entropy incorporate temperature, and if our understanding of temperature on the absolute scale is somehow flawed, then our equations could break down in these situations. As a friend of mine enthusiastically opined on Facebook, if this is true, whole textbooks may have to be rewritten! It's an exciting possibility, to be sure ... but I'm not quite ready to throw out my old textbooks yet.
On the other hand, there's also some speculation that these discoveries might lend us insights into one of the greatest scientific mysteries of the last two decades. As far as we know, the expansion of space itself should be contracting (or, at the very lease, slowing down) and it isn't, it's actually speeding up ... due to a largely unknown substance that we've dubbed dark energy. In fact, all evidence suggests that about 75% of our universe should be made up of dark energy, even though we still don't have a clue what it actually is!
This is so speculative, I'll leave it to the researcher himself (and Nature) to present the case:
Schneider notes that the attractive atoms in the gas produced by the team also want to collapse inwards, but do not because the negative absolute temperature stabilises them. "It's interesting that this weird feature pops up in the Universe and also in the lab," he says. "This may be something that cosmologists should look at more closely."
- Nature - Quantum Gas Goes Below Absolute Zero
- Nature - Negative Absolute Temperature for Motional Degrees of Freedom (abstract at Nature, full article available on ArXiv)
- Mashable - Scientists Create Gas Colder than Absolute Zero (video & article)
- New Scientist - Cloud of atoms goes beyond absolute zero
- ArXiv - Spin gradient demagnetization cooling of ultracold atoms (2011)