Have Physicists Gone Below Absolute Zero?

Have Physicists Gone Below Absolute Zero?

Hello everybody, Jack here, with my first post for the newly improved Young Australian Skeptics.

Today I’m going to be writing about a recent hullabaloo in science news, about a group of scientists at the University of Munich who had apparently created temperatures below absolute zero. That’s right, below 0 Kelvin. This should immediately sound fishy to any physicist or person with rudimentary knowledge of temperature. I was definitely skeptical when I first heard about it, as this whole issue sounded very similar to some of the online science reporting around the “faster-​​than-​​light neutrinos” story a while back.

I decided to look into it, and found a much more mundane explanation after further research, also much like the neutrinos. What was the explanation, you say? You want me to explain why everybody thinks scientists have gone below 0 Kelvin? Okay then.

The most important thing to understand about this whole topic is that, while scientists created negative temperature systems, they never went below 0 Kelvin. How is that possible? In the winter it goes “below zero” all the time, and the weather man calls that “negative” 5 degrees. “Negative” often means “below zero”, but not in this situation.

In the experiment conducted at the University of Munich, scientists isolated a cluster of about 1000 atoms in a vacuum, and used a web of lasers and magnetic fields to arrange the atoms into a lattice, where they could control everything about them, including temperature. What scientists ended up creating was a new state of matter which they stated was at a “negative temperature”. Okay, they have negative temperature, but why isn’t this below zero?

Well, it’s called negative temperature because of the distribution of atoms described as the Boltzmann distribution. In a normal temperature system (normal is a much better description of non-​​negative temperatures than positive because it erases the idea of positive and negative temperatures), the Boltzmann distribution means that the majority of atoms in a system will be in a relatively low energy state, with a smaller amount of atoms in higher energy states, and the fewest atoms in the highest energy state.

In a negative temperature system, this pattern is the opposite, hence the term “negative temperature”. In this state, the majority of atoms are in the highest possible state, with fewer atoms in lower states and the fewest atoms in the lowest energy state.

The easiest way to think of this is to imagine a negative colour picture. Just as white is replaced by black, red by green, blue by orange and so on, high energy atoms are replaced by low energy atoms, and visa versa. It’s not below zero degrees, but it is negative.

They didn’t do it here, but is there any way to get to negative Kelvin? No. It’s impossible to go below 0 Kelvin. This is because of the definition of temperature. 0 Kelvin means all the atoms are motionless. No movement. The higher the Kelvin, the faster and more excited the atoms are. It’s therefore impossible to go below absolute zero, because atoms cannot move less once they are motionless.

Okay, so we can’t go below 0 Kelvin, but how about getting to precisely 0? Is there some sort of quantum effect we can exploit to get there? No. It’s impossible to get to 0, even with quantum mechanics, thanks to Heisenberg’s uncertainty principle. This is the physical principle that does not allow you to know exactly where an atom is and its exact momentum at the same time. There is always some uncertainty with every atom. This isn’t a limitation with our measuring equipment, this is actually how particles work. Applied to temperature, it means that if something is exactly 0 Kelvin, you know its exact position (because it’s not moving) and momentum (zero, because it’s not moving), which is impossible, so it always has to be slightly above 0 Kelvin.

It’s impossible to remove the last tiny amount of heat in a system, because it would require an infinite amount of energy to do so, exactly like how an infinite amount of energy is required to reach the speed of light in general relativity.

I hope this has cleared up some possible confusion caused by bad science reporting on the internet, and I look forward to writing more for the Young Australian Skeptics in the future. If you wish to learn more about the topic, check out Ars Technica’s writeup.

[Creative Commons licensed Flickr photo by vanderkroew]