A team of physicists at the University of Munich, headed by Ulrich Schneider, have generated negative temperatures. We’re not talking Fahrenheit here folks, and I don’t mean Celsius. I am talking about Kelvin. The scale that starts at a true zero temperature, absolute zero that is. The point at which atoms are said to have zero kinetic energy, zero entropy; they are at a standstill. So how do you get below this?
It has to do with the way temperature and our universe work. At any temperature, the kinetic energy is just an average value. The majority of atoms exist at very low energy levels while a couple more energetic atoms are at higher levels. The distribution of these is represented by the Boltzmann distribution:
It shows that there is a range of velocities, with certain velocities having greater probabilities than others. The lower the temperature, the greater probability of finding atoms of a particular velocity (the red graph is not as wide, and has a lower average velocity, with a higher peak at that temperature). This distribution of atoms can be thought of using potential wells and hills.
At a low temperature, the majority of atoms, with low kinetic energies, will settle into the potential well as shown on the left figure. As temperature is increased, the Boltzmann distribution widens greatly, as seen in the first figure. This means that there is a very large range of kinetic energies for the atoms, represented by the middle of the second figure; atoms exist in a chaotic pattern, with many velocities, therefore particles exist at all energy levels, in the well, on the plain, and on the hill.
Looking at these two figures, we can picture what zero kelvin as well as infinite kelvin would look like. At zero kelvin, all of the particles would exist in the potential well. The probability graph would not be a curve. Every particle would have a velocity of exactly zero. At an infinite temperature, the probability graph would be spread out to the point that every velocity would have the same, extremely low, probability. It would look like a flat line close to zero on the y-axis extended forever in the x-direction. The particles therefore exist at all possible locations on the “well and hill” picture, similar to the middle of the second figure. So if that’s what infinite temperature looks like, how is the right most configuration of that picture formed?
It is negative kelvin.
The scientists at the University of Munich discovered a process that makes this possible.
“Schneider and his colleagues began by cooling atoms to a fraction above absolute zero and placing them in a vacuum. They then used lasers to place the atoms along the curve of an energy valley with the majority of the atoms in lower energy states. The atoms were also made to repel each other to ensure they remained fixed in place.
Schneider’s team then turned this positive temperature system negative by doing two things. They made the atoms attract and adjusted the lasers to change the atoms’ energy levels, making the majority of them high-energy, and so flipping the valley into an energy hill. The result was an inverse energy distribution, which is characteristic of negative temperatures.”
– Jacob Aron
As I said earlier, the majority of atoms in any system exist with low kinetic energies, with a few particles at higher energy levels. The scientists at the University of Munich switched this, creating the existence of a system where the majority of the particles have high energies and only a few are at lower energy levels. This creates the configuration at the right of figure two.
“The resulting thermometer is mind-bending, with a scale that starts at zero, ramps up to plus infinity, then jumps to minus infinity before increasing through the negative numbers until it reaches negative absolute zero, which corresponds to all particles sitting at the top of the energy hill.”
– Clay Dillow
This negative zero value is important because it is actually the highest possible temperature. All of the particles exist on the top of the hill, but they don’t like most hill representations do. Rather than being an unstable equilibrium, it is actually stable. Energy must be added in order to get the particles away from the stable equilibrium. Because the system was created in a vacuum, there is no energy for the particles to gain energy, therefore they stay in the equilibrium. Another way to look at it is in terms of kinetic energy. If the particles were to “roll” down the hill, they would have to gain kinetic energy. The problem is still the same though, there is no energy for them to gain in the vacuum.
And if you know anything about science, you probably know the next question that is asked (you are probably thinking it yourself). Why does it matter?
Studying this negative temperature scale is believed to give us insights into dark matter, which scientists believe have negative temperatures as well as negative pressures. Some hypothesize that this fact is why our universe is expanding at an accelerating rate. On the more practical side, scientists believe that the discovery of negative temperatures can lead to super efficient engines that can absorb energy from both hot and cold substances.