Quantum gas goes below absolute zero

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

by thisjepisje • 12 years ago

117 36 12 years ago

heydenberk• 12 years ago

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

MrBra2• 12 years ago

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.. :)

AnimalMuppet2• 12 years ago

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

Pun intended? If so, nicely done.

omegant2• 12 years ago

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.

avoid3d2• 12 years ago

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.

omegant2• 12 years ago

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?.

forgotketchup• 12 years ago

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.

eloff• 12 years ago

Needs 2013 tag, this is old news.

lelf• 12 years ago

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

1ris• 12 years ago

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.

thisjepisje• 12 years ago

That's a very nice and clear explanation.

m_mueller2• 12 years ago

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.

KhalilK• 12 years ago

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

michaelochurch• 12 years ago

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)?

abecedarius• 12 years ago

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.

sampo3• 12 years ago

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.

idlewords• 12 years ago

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.

waqf• 12 years ago

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

resdirector• 12 years ago

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...

MrBra2• 12 years ago

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

pagejim• 12 years ago

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

androidb• 12 years ago

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

KONAir• 12 years ago

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

Zikes• 12 years ago

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

ThePhysicist• 12 years ago

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

ericfrederich• 12 years ago

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"

Quantum gas goes below absolute zero

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