Quantum gas goes below absolute zero | Hacker News Reader

Quantum gas goes below absolute zero

nature.com

117 pointsthisjepisje36 comments

This was published 18 months ago.

A subsequent publication[0][1] argues that their notion of "negative temperatures" is invalid as they used a flawed definition of entropy.

[0] http://scholar.google.com.au/scholar?cites=41283096248189415...

[1] http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature...

So they did not actually attain below-absolute-0 temperatures?

Isn't it impossible as per laws of physics to achieve less than absolute ZERO temperature?

No. https://news.ycombinator.com/item?id=8094173

It depends on what we mean by "temperature".

If we think that temperature is a measure of how much the atoms move around then it is not possible to have less than zero movement.

But if we think temperature as related to the ratio of how much a system's entropy changes when a certain amount of energy is added (or removed) to the system [1],

1/T = dS/dq

then it's possible to construct systems with a negative T.

[1] Eqs. 8–10 in http://en.wikipedia.org/wiki/Temperature#Second_law_of_therm...

If I understand correctly, it's like saying a car has negative speed because it's going backwards. If you accelerate it forwards you actually reduce its speed towards 0, but that does not mean that the car is more immobile at -5km/h than at 0km/h.

It's just a matter of convention, a negative temperature just means that the temperature gets closer to 0 as you add energy. Did I get that correctly?

Negative temperature actually has some nuance to it. By the relation 1/T = dS/dq where T is temperature, S is entropy and q is heat added to the system, a negative temperature means that entropy decreases when you add heat to the system and increases when it emits heat. By the second law, entropy always goes up, so a negative temperature object will always emit heat. In that way, something with a negative temperature is extremely hot.

In fact, it's a negative absolute temperature because it's hotter than infinity.

> If I understand correctly, it's like saying a car has negative speed because it's going backwards.

Well, kind of, but I think that would be an oversimplification.

Negative temperatures have a concrete physical meaning, being "hotter than infinity", in the sense that if we bring in contact 2 systems: one with an arbitrarily high positive temperature, and one with a negative temperature, energy (heat) will flow from the negative system to the positive.

This looks tidier if we use the thermodynamic beta (beta = 1/T) instead of temperature. Then we can say that heat always flows from a system with a smaller beta to systems with a larger beta.

http://en.wikipedia.org/wiki/Thermodynamic_beta

Maybe debt is a somewhat useful metaphor here. If someone has a debt of 5 apples, in some ways if makes sense to say that this person owns -5 apples. But in some other ways it makes no sense: a person with 5 apples can eat them to get less hungry. A person with -5 apples cannot eat them to get more hungry.

That's why I always read comments before following a link, thank you for sharing this :)

There are a lot of cool things about this, but here are two that caught my attention:

>> Exotic high-energy states that are hard to generate in the laboratory at positive temperatures become stable at negative absolute temperatures

and

>> ... 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

Yes, as I read the article at the second part you quoted, a flying saucer immediately popped up into my mind... I am sure I am not the only one.. :)

>> There are a lot of cool things about this...

Pun intended? If so, nicely done.

https://en.wikipedia.org/wiki/Negative_temperature

A little rephrasing: In a system with a negative temperature the particles have more energy than a system with 0K. A system has a negative temperature if making it hotter/adding energy increases the order in it (delta q/delta S is negative). For example if there is a maximum energy state for each particle increasing the average energy at some point makes it more uniform, as more particles reach said state.

That's a very nice and clear explanation.

Am I guessing correctly that this isn't at all related to negative energy? When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do? Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties? I'm thinking a more intuitive approach would be to put an additional system defined factor into equations defining a relation between temperature and entropy - this factor would usually be +1, but can be -1 under certain circumstances.

But hey, IANAP.

> Am I guessing correctly that this isn't at all related to negative energy?

Yes.

> When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do?

No. Suppose you're doing a Fluid Dynamics Simulation. You need to assign a temperature and pressure to each point in space to simulate gas flows. Using the negative temperature will correctly predict the evolution of the gas flow.

This is actually a strength of physics, having equations that can take on values that to your knowledge are unphysical (such as negative refraction indices for example) but still work. This means that when someone manages to figure out how to build such a system; we don't need to rework all of our theorems (conservation of energy; momentum; etc) to see which are still valid. We also have the ability to investigate the applications of such materials; should we ever be able to find a way to create them.

http://en.wikipedia.org/wiki/Metamaterial

> Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties

There are plenty of discontinuous transitions in thermodynamics, phase diagrams are an example. This is hardly an argument to that the concept of negative temperature is a broken one.

http://en.wikipedia.org/wiki/Phase_diagram

Thanks for your detailed response. Your example with fluid dynamics is an interesting one. If it really works as advertised and you can put these negative temperatures into every equation that takes a temperature, then yes, I can see the value. I guess I was just skeptical that it would actually work that way, since that's quite an amazing result ;-).

Regarding equations, for a physically unrelated but similar cognitive process, see[0] how we (A) set up a physical wave system, use complex numbers to work with wave equations, then liberally discard imaginary parts as if they never existed to get a physical result (B). (A) and (B) are anchored in physical reality, but the mathematical process in between is — as far as we know — not.

As for discontinuities, another non-thermodynamic physical discontinuity is the speed of light. Special relativity predicts it's entirely possible for particles to move FTL, when it's actually crossing the boundary that's a problem.

[0]: http://en.wikipedia.org/wiki/Plane_wave#Complex_exponential_...

S being the entropy and q being the heat, thermodynamics for the win.

Negative temperatures are not at all unheard of in physics, as lasers are based on this very principle: At positive temperatures, states with higher energy will always be (statistically) less occupied than states with lower energy. In a laser one inverts this for some states of the system by pumping energy to meta-stable high-energy states, thus creating a "population inversion".

http://en.wikipedia.org/wiki/Negative_temperature

A curiosity of physics is that you can set up systems with a negative temperature, but you can't ever get to zero temperature, from either direction.

But you can get to infinite temperature from either direction. So it's more a curiosity of notation: instead of temperature T, the more natural thermodynamic concept is β = 1/T.

http://en.wikipedia.org/wiki/Thermodynamic_beta

By the description it seems like they have reduced the energy so that not only intermolecular energy is reduced (or somehow stop), but also it reduces interatomic energy some how. Is this possible? It´s there another atomic absolute zero still to be discovered beyond 0K?

I´m have no idea about physics, so surely what I say doesn´t make sense.

The reason that negative temperatures are hard to explain is that the definition of temperature in the sense used by physicists is not very accessible.

It has to do with how much a certain property of a system (object) changes when you add a certain quantity of heat energy.

In certain edge cases adding energy makes this quantity change in the opposite to usual way and therefore the temperature is negative.

I´ve been reading the negative temperature link, I understand that you reduce entropy reducing energy, till a point, where you start "holding" the molecules down adding energy (I don´t know if that makes sense).

But still is there a way that you could start affecting the forces inside the atom this way?. Or then we are talking about something different?.

This is really on the edge if where I don't know what I'm talking about anymore but I would think not.

The forces holding atoms together and the energies associated with changing those forces are far larger than the amounts that are presented in this study.

To expand on what avoid3d said:

You can make a plot of entropy vs. energy of a system. Further, you can calculate the derivative (slope) of that plot and give it a name. You could also calculate the quantity 1/slope and give that a name, too--and the name we give it is "temperature." So, the temperature of the system is negative any time the slope of entropy vs. energy is negative.

How do we get a negative slope? We need to find a place where entropy decreases when energy is added to the system. Entropy is proportional to the natural log of something called the "partition function (see below)." That means we need to find a place where partition function has a negative slope.

The partition function tells us essentially "how many distinct ways can we arrange (store) a given amount of energy in this system?" For almost all macroscopic systems, there are a greater number of ways to rearrange the system each time a unit of energy is added. However, it is possible to construct a system where adding a unit of energy actually restricts the number of ways you can arrange the system. And that is the basis for negative temperature.

If we could achieve a negative energy/temperature state couldn't we use that to harness zero point energy? [1]

[1] http://en.wikipedia.org/wiki/Zero-point_energy

Hoverboards... if Back to the Future was right, we'll have them in October of next year.

"For instance, 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"

This result, described today in Science1, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.

Correct me if I'm wrong, but doesn't this mean that the gas molecules (at near-0K from negative) were extraordinarily hot? My understanding is that -1/T is the "true measure", ergo super-low negatives are, in fact, "absolute hot".

Does anyone know what are the thermodynamic properties of negative temperatures? For example, a pin-sized blackbody at 10^9 K would probably kill everyone on earth if it remained at that temperature. Would that apply to macroscopic objects at negative temperatures (if that were even possible, and if blackbody mechanics make sense in such states)?

I believe so: to keep a thing at negative temperature while it's in thermal contact with us, you'd need to keep supplying power, just as you need to power a lightbulb. I didn't read the OP, but a familiar example of negative temperature is the population inversion in a laser.

Systems at negative temperature are, in a way, hotter than infinite temperature. Yet, the contain much less energy than the same system at a very large (or approaching infinite) temperature would contain.

They are, after all, created in a lab, so their energy content is limited by how much electricity the lab equipments has consumed.

Bots in the comments section of the article are pretty interesting.

Needs 2013 tag, this is old news.

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