

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

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resdirector
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...](http://scholar.google.com.au/scholar?cites=4128309624818941529&as_sdt=2005&sciodt=0,5&hl=en)

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

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

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

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sampo
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...](http://en.wikipedia.org/wiki/Temperature#Second_law_of_thermodynamics)

~~~
simias
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?

~~~
mjfl
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.

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

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heydenberk
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

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MrBra
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.. :)

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lelf
[https://en.wikipedia.org/wiki/Negative_temperature](https://en.wikipedia.org/wiki/Negative_temperature)

~~~
1ris
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.

~~~
m_mueller
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.

~~~
archgoon
> 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](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](http://en.wikipedia.org/wiki/Phase_diagram)

~~~
m_mueller
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 ;-).

~~~
lloeki
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_...](http://en.wikipedia.org/wiki/Plane_wave#Complex_exponential_form)

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ThePhysicist
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](http://en.wikipedia.org/wiki/Negative_temperature)

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idlewords
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.

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waqf
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](http://en.wikipedia.org/wiki/Thermodynamic_beta)

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omegant
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.

~~~
avoid3d
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.

~~~
omegant
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?.

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avoid3d
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.

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Zikes
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](http://en.wikipedia.org/wiki/Zero-point_energy)

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ericfrederich
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"

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michaelochurch
_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)?

~~~
sampo
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.

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

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eloff
Needs 2013 tag, this is old news.

