Quantum gas goes below absolute zero

[Steve001]

I thought you all might find this interesting

Quote:

It may sound less likely than hell freezing over, but physicists have created an atomic gas with a sub-absolute-zero temperature for the first time1. Their technique opens the door to generating negative-Kelvin materials and new quantum devices, and it could even help to solve a cosmological mystery. http://www.nature.com/news/quantum-g...e-zero-1.12146

[fuelair]

YES and thank you very kindly!!! This is way cool!!!

[marplots]

From the article:

Quote:

Technically, you read off the temperature of a system from a graph that plots the probabilities of its particles being found with certain energies. Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature, explains Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany.

I don't understand what they are getting at. Could someone rephrase that?

[Farsight]

I don't either. The temperature of a gas is a measure of average kinetic energy. So negative temperature sounds a bit like negative motion, which doesn't feel at all right. Maybe it's to do with binding energy, which is described as negative energy even though it's actually less positive energy. Anyway, this looks like the paper on arXiv: Negative Absolute Temperature for Motional Degrees of Freedom by Simon Braun, Jens Philipp Ronzheimer, Michael Schreiber, Sean S. Hodgman, Tim Rom, Immanuel Bloch, Ulrich Schneider. It's 5 pages, plus supplements. I'll have a read of it and see if I can make some sense of it.

[Mister Earl]

The article I read about this said that this theoretically makes over 100% efficiency engines a possibility. My first thought was, "Great. Wonder which over-unity kook is going to latch onto this first?"

[Dancing David]

Quote:

This result, described today in Science, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.

I will not try to mangle the actual concepts in explaining it.

[DC]

O.o i thought absolute zero means, no movements anymore and you can't get lower :/

sccience can be very confusing at times especially when you only have a simplified grasp of it like i do

[Farsight]

This bit looks relevant:

In Fig. 1A we schematically show the relation between entropy S and energy E for a thermal system possessing both lower and upper energy bounds. Starting at minimum energy, where only the ground state is populated, an increase in energy leads to an occupation of a larger number of states and therefore an increase in entropy. As the temperature approaches infinity, all states become equally populated and the entropy reaches its maximum possible value Smax. However, the energy can be increased even further if high-energy states are more populated than low-energy ones. In this regime the entropy decreases with energy, which, according to the thermodynamic definition of temperature [8] (1/T = δS/δE), results in negative temperatures. The temperature is discontinuous at maximum entropy, jumping from positive to negative infinity. This is a consequence of the historic definition of temperature."

So a really hot gas where all the molecules are moving really fast at the same speed has a high negative temperature does it? That's a new one on me!

ETA:

So's this:

A continuous and monotonically increasing temperature scale would be given by -β=-1/kBT, also emphasizing that negative temperature states are hotter than positive temperature states, i.e. in thermal contact heat would flow from a negative to a positive temperature system. As negative temperature systems can absorb entropy while releasing energy, they give rise to several counterintuitive effects such as Carnot engines with an efficiency greater than unity [4].

Entropy isn't something you can absorb. It isn't like energy, it's just "sameness". And over-unity breaches conservation of energy. I wasn't fond of this either:

Via a stability analysis for thermodynamic equilibrium we showed that negative temperature states of motional degrees of freedom necessarily possess negative pressure [9] and are thus of fundamental interest to the description of dark energy in cosmology, where negative pressure is required to account for the accelerating expansion of the universe [10].

I don't think that's right. Negative pressure is tension. The universe expands. If it was under tension it would be contracting, not expanding. Sounds to me like there's some deliberate headline-grabbers here.

[BenBurch]

Relativity. Temperature is how the molecules move relative to each other.

A coherent beam of atoms on parallel courses might be hot in our reference frame, as in they could be used as a welding torch or similar, but not in their own frame.

[sol invictus]

Originally Posted by marplots

From the article:


I don't understand what they are getting at. Could someone rephrase that?

The definition of temperature (T) isn't in terms of the motion of anything. For an ordinary gas temperature is related to the motion of the constituent molecules, but that's just for that case. Generally, temperature is defined by two other quantities: total entropy (S) and total energy (E), by the relation T=dE/dS.

That equation says that if when you increase the energy the entropy increases, the temperature is positive. But if when you increase the energy the entropy decreases, the temperature is negative. Entropy is simply a measure of the number of states the system can possibly be in. In an ordinary gas, increasing the total energy means more energy to distribute to the molecules in various ways, so higher average energy/molecule and also higher entropy (more ways to divide up the energy).

But in some systems, there are fewer states with high energy. In those systems, more energy means lower entropy, and hence negative temperature.

[bobwtfomg]

Originally Posted by BenBurch

Relativity. Temperature is how the molecules move relative to each other.

A coherent beam of atoms on parallel courses might be hot in our reference frame, as in they could be used as a welding torch or similar, but not in their own frame.

That's what I thought, in which case the concept of negative temperature doesn't make sense, either the molecules move relative to each other (are at some positive temperature) or they don't (are at absolute zero).

I tried to read the paper farsight linked to but it was a little bit over my head as I only studied physics to A level.

Could someone, preferablly with a relevant Phd either a) explain it and why it's valid or b) debunk it.

[Mister Earl]

Hrm. Interesting. What about the claim (which I'm assuming is a mere headline-grab ploy until shown otherwise) that you could theoretically create an engine with over 100% efficiency with this?

[fuelair]

Originally Posted by DC

O.o i thought absolute zero means, no movements anymore and you can't get lower :/

sccience can be very confusing at times especially when you only have a simplified grasp of it like i do

The most useful part for non-specialists (which completely includes me) is the description of how Kelvin calculated/determined the Absolute Zero point. That is where the possibility of a very small fudge factor - given that NO ONE at the time had heard of quanta/quantum effects - got established. That is also why the measure is a few billionths of a degree instead of a few degrees.

And, note that a lot of energy (for the scale) was involved in making it happen.

[fuelair]

Or, better, what sol invictus said!!!!!!!

[sol invictus]

Originally Posted by bobwtfomg

That's what I thought, in which case the concept of negative temperature doesn't make sense, either the molecules move relative to each other (are at some positive temperature) or they don't (are at absolute zero).

It makes perfect sense. Your mistake is thinking that temperature is defined by the motion of molecules. It's not. See my previous post.

Originally Posted by Mister Earl

Hrm. Interesting. What about the claim (which I'm assuming is a mere headline-grab ploy until shown otherwise) that you could theoretically create an engine with over 100% efficiency with this?

Efficiency can be defined in all sorts of different ways. Having not bothered to read the article I can't say whether the claim is correct or not (or whether there's enough information to tell), but I wouldn't be surprised if there's a way to define efficiency where it can exceed 100% under certain circumstances.

But no matter what, energy is conserved, and entropy never decreases. Those two facts preclude any perpetual motion machine, negative temperatures or not.

[BowlOfRed]

Science has a podcast, and this week it includes a discussion with one of the authors.

http://www.sciencemag.org/content/339/6115/99.2

There is also a transcript (in PDF format) available so listening is not required.

[Farsight]

Originally Posted by bobwtfomg

... Could someone, preferablly with a relevant Phd either a) explain it and why it's valid or b) debunk it.

I've only got A-level physics, but I've learned a bit since. This bit of the paper should be enough for you:

negative temperature states are hotter than positive temperature states

It isn't a black-and-white choice between being valid or bunk. What we have here is deliberate headline-grabbing along with an issue of definition and interpretation.

I imagine it relates back to entropy, and a conflict between thermodynamics and statistical mechanics. For example I said entropy is a measure of "sameness", which is the thermodynamic view related to available energy. Put ice cubes in a glass of warm water. The ice melts and the entropy increases. The sameness increases. But sol invictus said entropy is simply a measure of the number of states the system can possibly be in. That's the statistical mechanics view. They don't seem particularly compatible to me.

[DSo]

Originally Posted by Farsight

I don't think that's right. Negative pressure is tension. The universe expands. If it was under tension it would be contracting, not expanding. Sounds to me like there's some deliberate headline-grabbers here.

Mozina is that you? Good grief, are we going to get more Wikipedia diagrams of two parallel plates and arrows and stuff regarding the Casimir Effect?

[The Central Scrutinizer]

I often have quantum gas after 50 cent taco night at the pub.

[edd]

Originally Posted by Farsight

I imagine it relates back to entropy, and a conflict between thermodynamics and statistical mechanics. For example I said entropy is a measure of "sameness", which is the thermodynamic view related to available energy. Put ice cubes in a glass of warm water. The ice melts and the entropy increases. The sameness increases. But sol invictus said entropy is simply a measure of the number of states the system can possibly be in. That's the statistical mechanics view. They don't seem particularly compatible to me.

They would maybe seem more compatible if you used a better (correct?) definition of classical entropy.

[edd]

Originally Posted by DSo

Mozina is that you? Good grief, are we going to get more Wikipedia diagrams of two parallel plates and arrows and stuff regarding the Casimir Effect?

No - I think we're not in such a dire situation here. Mozina never accepted even the physical possibility of negative pressure. Farsight (impressively for someone who likes going on about GR so much) simply doesn't understand how negative pressure acts gravitationally, I think.

[bobwtfomg]

OK from the science podcast, ty bowlofred

Quote:

You all know the ordinary temperature, the Celsius scale – you can have positive
numbers, negative numbers – but you also know that there’s the absolute temperature
scale, the Kelvin scale, where you normally only have positive numbers. It’s thought that
zero can go up all the way to infinity, but that’s it. Well, it’s not. In some sense what
we’re doing is we’re extending this to negative absolute temperatures. And the important
thing to remember right away is that this is not colder than zero Kelvin – nothing can be
colder than that – but it’s actually the opposite. It’s even hotter than infinite
temperatures. This is kind of a pretty old and classical problem, so it’s nothing to do with
quantum mechanics. But it’s just when you look at the formulas describing temperature,
you really see that the scale starts at zero, it increases up to infinity, but it doesn’t
necessarily stop there. In fact, what we see is that it jumps from plus infinity to minus
infinity, and then continues growing. So the energy of the system grows forever and
forever and forever until it reaches zero again from below.

So nothing can be colder than absolute zero but it can be hotter than infinitely hot, in which case it has a negative temperature.

[Steve001]

Science Friday has a short talk on this.
http://www.sciencefriday.com/segment...n-the-sun.html

[fuelair]

Originally Posted by Steve001

Science Friday has a short talk on this.
http://www.sciencefriday.com/segment...n-the-sun.html

Beat me to it by about 4 minutes! Well played. I sat in my car in a restaurant parking lot to listen to the whole segment!!!

[Farsight]

Originally Posted by DSo

Mozina is that you? Good grief, are we going to get more Wikipedia diagrams of two parallel plates and arrows and stuff regarding the Casimir Effect?

No, I'm John Duffield.

Originally Posted by edd

Farsight (impressively for someone who likes going on about GR so much) simply doesn't understand how negative pressure acts gravitationally, I think.

Don't start, edd. When we're discussing relativity, it's me pointing to what Einstein said, whilst you dismiss it as "cherry picking".

All: Hamish Jonston the Physicsworld editor gave this paper a mention yesterday, see Have physicists ventured below absolute zero? Here's a couple of fair-use excerpts:

"One way of looking at temperature is as a way of describing how energy is distributed among a collection of particles. Most particles will have a small amount of energy and the probability that a particle has a higher energy will drop exponentially with energy – the familiar Maxwell–Boltzmann distribution of an ideal gas. Temperature times Boltzmann’s constant is the parameter that fits the distribution to experimental data. Implicit to this distribution is that there is a minimum energy (zero) and no maximum energy. Now, a team of physicists has used ultracold atoms to create what is essentially a mirror reflection of this familiar scene – a system with a maximum energy and no minimum energy. Furthermore, the probability that a particle in this system has an energy approaching this maximum is very high and drops off exponentially as the energy decreases. So if you interpret this in terms of the Maxwell–Boltzmann distribution, you get a negative temperature (or perhaps a negative Boltzmann’s constant)...

...So have Schneider and colleagues ventured below absolute zero? No, but they have done a nifty experiment!"

[RobDegraves]

So...it's still a question of how you define temperature, which is the same thing that the article and talk stated. Nothing like trying to oversimplify a complex subject though. I found the experiment and the actual subject quite interesting.

[edd]

I'll requote you Farsight. Einstein would understand how a negative pressure tries to accelerate an expansion. Also Einstein wouldn't confuse the first and second time derivatives of the scale factor as you have. You don't tend to be as good as you think at pointing to what Einstein said.

Originally Posted by Farsight

I don't think that's right. Negative pressure is tension. The universe expands. If it was under tension it would be contracting, not expanding. Sounds to me like there's some deliberate headline-grabbers here.

[Ziggurat]

Originally Posted by bobwtfomg

So nothing can be colder than absolute zero but it can be hotter than infinitely hot, in which case it has a negative temperature.

In statistical mechanics inverse temperature is actually more fundamental than temperature. Inverse temperature always decreases with increasing energy. For most systems it asymptotically approaches zero (infinite temperature) as energy goes to infinity, but for some systems it can go negative. Inverse temp is always continuous, but when it passes through zero, temperature is discontinuous, going from positive to negative infinity. The energy is never infinite, and negative temps are hotter than positive temps because lower inverse temperature is hotter than higher inverse temperature.

[marplots]

Originally Posted by Ziggurat

In statistical mechanics inverse temperature is actually more fundamental than temperature. Inverse temperature always decreases with increasing energy. For most systems it asymptotically approaches zero (infinite temperature) as energy goes to infinity, but for some systems it can go negative. Inverse temp is always continuous, but when it passes through zero, temperature is discontinuous, going from positive to negative infinity. The energy is never infinite, and negative temps are hotter than positive temps because lower inverse temperature is hotter than higher inverse temperature.

This cries out for a graph.

[dlorde]

Quote:

"Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity."

What's the betting anti-gravity enthusiasts will get excited?

[fuelair]

This is beginning to be real fun!!!

[StankApe]

This will finally lead to the discovery of the mythical Higgs-Beano!!!!

[mijopaalmc]

Originally Posted by marplots

This cries out for a graph.

[fuelair]

Originally Posted by marplots

This cries out for a graph.

Or three!!! See above.......

[marplots]

Originally Posted by fuelair

Or three!!! See above.......

Those graphs actually did help me understand it.
"Draw me a picture, we live in a visual society." -- my older brother in a moment of pith.

[mike3]

Originally Posted by RobDegraves

So...it's still a question of how you define temperature, which is the same thing that the article and talk stated. Nothing like trying to oversimplify a complex subject though. I found the experiment and the actual subject quite interesting.

1. How did you manage to understand the actual subject?

2. "Nothing like trying to oversimplify a complex subject though." Oversimplify... but I don't suppose one can do any better if all they have is a short article or forum posts to express it in.

[Reality Check]

Originally Posted by Farsight

I don't either. The temperature of a gas is a measure of average kinetic energy.

It is a bit complex but the the basic thing you have missed out is the word quantum. This is a quantum gas which meets the ctriteria for the negative temperatures that have already been predicted in other quantum systems. The temperature of a quantum gas is a measure of how thermal energy changes with changes in entropy.

[DallasDad]

For those of us who are really stupid, could someone explain very very basically what this experiment demonstrates? It sounds as you guys are saying the sign flips when temperature exceeds infinitely hot, although the experiment was done with something at nanokelvins above absolute zero. I don't get how the two are related.

If I put a tea kettle on top of their experiment, would the water boil or freeze? If we varied the temperature to +1 nanokelven instead of -1 nanokelvin, would the tea kettle behave differently?

[mijopaalmc]

Originally Posted by Reality Check

It is a bit complex but the the basic thing you have missed out is the word quantum. This is a quantum gas which meets the ctriteria for the negative temperatures that have already been predicted in other quantum systems. The temperature of a quantum gas is a measure of how thermal energy changes with changes in entropy.

I'm pretty that the classical definition of (inverse) temperature is:

[h.g.Whiz]

Originally Posted by mijopaalmc

I'm pretty that the classical definition of (inverse) temperature is:

http://latex.codecogs.com/gif.latex?...tial%7BS%7D%7D

Your looks are irrelavent.