# Quantum Gas Below Absolute Zero

1. Jan 6, 2013

### F1225

Quantum Gas Below Absolute Zero!!!

http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146

I am a fresh undergraduate student and not expertise in fields. The link above shows an interesting research on negative temperature. I've visited wikipedia and found out that negative temperature existed for awhile(not new discovery).

Can anyone explain briefly about what happened in negative temperature? Does the negative temperature means negative kelvin temperature? Wouldn't that contradicts the Laws of Thermodynamics?

2. Jan 6, 2013

### K^2

Re: Quantum Gas Below Absolute Zero!!!

Negative temperatures simply refers to inversion in energy level population. If you look at Maxwell-Boltzman statistics, for example, the levels are populated proportionally to $e^{-E/k_bT}$. If you substitute a negative quantity for T, higher energy levels will have higher populations.

Typical example is the population inversion in a lasing medium. There you have a higher population of excited states due to their metastability compared to an even higher energy state to which the pump excites the system.

These guys figured out how to prepare a system so that there are more particles with higher kinetic energies than low kinetic energies. This gives you the same kind of population inversion and, effectively, a negative temperature.

3. Jan 6, 2013

### DiracPool

Re: Quantum Gas Below Absolute Zero!!!

Ummm...so does this mean that absolute zero isn't absolute zero? There's been two threads on this and I've read two popular articles on the subject and I'm still not sure what it means. The last article I read said that it is negative temperature and also infinitely hot at the same time. What does that mean? If there's anything you thought you could count on in this universe, it was absolute zero.

How specifically, is it manifested? The atoms stop moving at zero and then start moving in a reverse vibe? I'm not getting the "take home" visualization here.

4. Jan 6, 2013

### soothsayer

Re: Quantum Gas Below Absolute Zero!!!

To add to K^2's response, you can kinda think of "negative temperature" as more of a cool trick of quantum statistical mechanics, rather than it being due to making a gas actually that cold via a cooling process. Luckily, it does not contradict the laws of thermodynamics.

Even stranger is that these negative temperature gases are actually "hotter" than positive temperature gasses. If you exposed two such gases to each other, heat energy would flow from the negative temperature to the positive one. A careful study of the entropy in these systems will show that this is inevitable, and that the laws of thermodynamics are still upheld.

5. Jan 6, 2013

### soothsayer

Re: Quantum Gas Below Absolute Zero!!!

Think about energy levels in a gas. Particles fill up the ground state energy levels first, and if a gas reaches absolute zero, all the particles are guaranteed to be in the ground state. As the gas heats up from there, the particles jump up and up towards the higher energy levels, but more particles are still in the ground state than anything else, and this is true all the way up to any temperature.

"Negative" temperature means that there is some energy level above the ground state that actually has more particles in it then the ground state. A careful study of the statistical mechanics (Maxwell-Boltzmann statistics) involved shows that this state must be considered to have a "negative" temperature.

You are trying to think about it the wrong way. You believe this gas has been super cooled even past the absolute zero phase. This is not correct. In classical thermodynamics, this would be the only way to achieve negative temperature, and would be impossible, but in a study of quantum thermodynamics, it is possible, but not via super cooling (this gas is actually really hot) but by careful preparation of energy levels. This is exactly what happens in a gas laser (Helium-Neon, for example), via quantum manipulations, we get more particles in a higher energy level than a lower one, and that causes a cascade which produces a powerful, focused, single-wavelength light beam we call a laser.

Don't think of negative temperature as "super cooled", that is not the right way to think about it. A gas cannot actually go to absolute zero and then beyond, and you cannot get a gas to go directly from a super cooled state to a negative temperature state. In terms of how much energy something has, absolute zero is still the very lowest possible state. I know it confuses you that this phenomenon is called "negative temperature" when it doesn't really seem to have anything to do with temperature, but a careful study of quantum statistical physics will help you understand exactly what is meant by "temperature", "entropy", etc. beyond the simple classical description you are likely more familiar/comfortable with.

Last edited: Jan 6, 2013
6. Jan 6, 2013

### DiracPool

Re: Quantum Gas Below Absolute Zero!!!

Phew, I feel better now. Thanks. And thanks for the lucid explanation. It'll be interesting to see where this goes.

7. Jan 6, 2013

Staff Emeritus
Re: Quantum Gas Below Absolute Zero!!!