
Quantum gas goes below absolute zero - evgen
http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146
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Xcelerate
I don't really like the misleading headline, although it seems as though the
actual paper does it as well (<http://arxiv.org/abs/1211.0545>). Temperature
is conventionally defined as the average kinetic energy of a collection of
particles.

However, in statistical mechanics, you derive thermodynamic relations from the
principles of quantum mechanics. It turns out there is a term β which for most
purposes seems to match up with 1/kT, where k is the Boltzmann constant and T
is classical temperature. According to this article, there are some occasions
where "conventional" T and quantum mechanical T do not match up.

The most famous relationship in statistical mechanics is:

S = k ln Ω

S is entropy. Ω is the partition function for the microcanonical ensemble, and
it corresponds to the number of quantum states available to a system. (In
English: Ω is the number of different configurations that a system of
particles can possible have.)

Another relation that can be derived is:

1/kT = ∂ln(Ω)/ ∂E

Substituting the first equation into the second, we get

1/T = ∂S/∂E

So what would a negative temperature imply? That increasing energy leads to
decreasing entropy. My guess is that these researchers have created some
system that does exactly that.

~~~
Xcelerate
After thinking about the article's headline some more, I've come to the
conclusion that to get a lot of attention and hype you need to:

    
    
        1. Take a word that has a commonly accepted meaning
        2. Redefine the word to a more general case using advanced physics or chemistry
        3. Find an obscure property that this new definition allows
        4. Shock and amaze!
    

(See "mass" for similar phenomena.)

~~~
aphyr
I know you're being sarcastic, but this is a wonderful thing about science!
Commonly held, intuitive, time-honored ideas about temperature, space, mass,
time, turn out to break down in edge cases, and those breakdowns drive the
development of new mathematics, physics, and chemistry. The universe is so
much deeper, stranger, and more beautiful than we ever imagined.

If you aren't compelled by ideas alone it can feel like much ado about nothing
(who cares about tiny variations in spacetime curvature?), but without general
relativity, we couldn't have built the GPS. Without quantum mechanics we
couldn't have lasers--and without lasers, no modern internet.

Pretty cool time to be a human being, I think. :)

~~~
pm90
I think the parent was criticizing the unfortunate naming of these discoveries
(designed to capture the imagination of laymen) rather than the discoveries
themselves.

~~~
mvaliente2001
But in this case, it seems to be correct. So, we, laymen, should update our
concepts and throw away the idea that "0° is the lowest temperature."

~~~
cygx
But 0K (notice there's no °) _is_ the lowest temperature - it's just that it's
also the hottest temperature, depending on which direction you're coming from.

Temperature has a nice interpretation as average energy for classical systems,
but in a sense thermodynamic β = 1/kT is the more 'natural' quantity that has
a pole at T = 0, corresponding to infinite hotness or coldness respectively.

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jawns
I'm not sure I understand this part:

"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."

If more particles have higher energy, then wouldn't the average energy
increase as well? And if the average energy increases, then shouldn't that
still keep things in positive, rather than negative, territory?

~~~
jules
Temperature is not really the average kinetic energy any more. The temperature
of a system says something about how much the entropy increases as you add
energy. Negative temperature simply means that the entropy decreases as you
add energy.

This is possible if the number of high energy states is small, because then
adding energy will result in the system being in one of a smaller number of
high energy states, thus having lower entropy (the entropy of a system is
basically the logarithm of the number of states we think the system could be
in, so if the number of states is lower, then the entropy is lower). Another
way of saying this is that is as follows. For positive temperatures our
knowledge of the state of the system would _decrease_ if we added energy
(think about a box full of neatly arranged balls and giving it a kick). For
negative temperatures our knowledge of the system would _increase_ if we added
energy. In the extreme case if the number of possible high energy states is 1,
then adding energy can force the system into exactly that state, thus we would
know all you can know about the system.

In particular in the experiment described in the article, if you removed
energy from the system the particles would not be in the lattice arrangement
and instead would go around randomly. Thus the entropy would increase if you
removed energy.

~~~
zeidrich
So I understand it conceptually:

A thought experiment where a cup of some substance manages to be observed at a
stable negative temperature.

This substance is a liquid at its current temperature. But it is colder than
the ambient temperature of the room. As the liquid warms up from the ambient
room temperature, it freezes. But taken outside on a cold day, it will melt
and then evaporate?

I know you're not going to get a cup of this stuff in reality, but am I at
least understanding the idea? The "temperature" of the liquid would remain
negative, it's not like it could ever "warm up" to the point it was positive?
It could reach an equilibrium state with its environment, but it would still
be negative?

~~~
jules
Yes, that's the right idea (assuming it has lower entropy when it's frozen
than when it's liquid). In the context of the liquid you describe, and
assuming it has otherwise normal properties then yes you can't warm it up (=
add energy) and make it positive again. And if you took it outside, the
temperature would first become more and more negative and then flip to
positive. However, as long as we are talking about exotic stuff it is
conceivable that as you add energy, the entropy first decreases (i.e. negative
temperature) but if you add more and more energy, the entropy starts to go up
again. For example lets say that if you add a lot of energy to the frozen
stuff, it becomes a gas. I'm not sure if a system like that where you go from
positive to negative to positive is physically possible, but I don't currently
see any reason why it wouldn't be.

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jpdoctor
The usual explanation of such things: T is not so important as 1/T. So
negative abs temp is achieved by going through +infinity temp and coming back
via -infinity temp.

Note that this is only allowed in systems which have states that are bounded
in energy. The usual example is a laser: When in inversion, its absolute
temperature (of the electron population) is negative.

~~~
TylerE
If you'll allow a car analogy - isn't this like saying backing up is slower
than being parked?

Yes, it's a negative number in the context of a forward velocity vector, but
it's a higher energy state.

~~~
gus_massa
No. The problem is that for historical reasons we choose the wrong scale and
it's a little late to change all the books.

We choose to measure the temperature in this way

    
    
      (infinityK) ... ->- 10K ->- 1K ->- .1K ->- ... | 0K(impossible) | ... ->-.1K ->- -1K  ->- -10K ->- ... (infinityK)
    

Where ->\- means hotter

In this scale 0K is impossible and impassable. But infinityK is possible (only
one unsigned infinity) and some kind of systems when they get hotter they can
pass from very "big" positive temperatures to very "big" negative
temparatures.

And the negative values are hotter than the positive. For example: -.1 is
hotter than -10K that is hotter than 10K that is hotter than .1K

It's (theoretically) better to measure beta=1/temperature (the usual notation
is beta) In this way:

    
    
      (infinity/K) ... -<- 10/K -<- 1/K -<- .1/K -<- ... 0/K(possible) ... -<-.1/K -<- -1K  -<- -10K -<- ... (infinityK)
    

Where -<\- means hotter

In this scale infinity/K is impossible as expected. But 0/K is possible. And
the order is the correct order hotter=less_than. Using beta they are ordered
more intuitively. Hotter means moving to the right in the line.

~~~
mieubrisse
Thank you; this helped clarify things quite a bit.

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ChuckMcM
Ok, mind officially blown.

Still wrapping my head around the notion that you can flip the energy
signature on atoms at near absolute zero and have that result in mirroring to
the other side of the temperature number line.

Also intrigued by the notion that 'dark matter' is just 'regular matter' but
flipped into the negative temperature side of the scale. Which really makes
one wonder about the nature of heat.

~~~
ChuckMcM
From the article:

 _"Schneider and his colleagues reached such sub-absolute-zero temperatures
with an ultracold quantum gas made up of potassium atoms. Using lasers and
magnetic fields, they kept the individual atoms in a lattice arrangement. At
positive temperatures, the atoms repel, making the configuration stable. The
team then quickly adjusted the magnetic fields, causing the atoms to attract
rather than repel each other. “This suddenly shifts the atoms from their most
stable, lowest-energy state to the highest possible energy state, before they
can react,” says Schneider. “It’s like walking through a valley, then
instantly finding yourself on the mountain peak.”

At positive temperatures, such a reversal would be unstable and the atoms
would collapse inwards. But the team also adjusted the trapping laser field to
make it more energetically favourable for the atoms to stick in their
positions. 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."_

And this:

 _"Another peculiarity of the sub-absolute-zero gas is that it mimics 'dark
energy', the mysterious force that pushes the Universe to expand at an ever-
faster rate against the inward pull of gravity. Schneider notes that the
attractive atoms in the gas produced by the team also want to collapse
inwards, but do not because the negative absolute temperature stabilises them.
“It’s interesting that this weird feature pops up in the Universe and also in
the lab,” he says. “This may be something that cosmologists should look at
more closely.”_

Just sayin'

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yk
As others pointed out, there are two definitions of temprature, usually it is
related to average energy per particle and formally it is T=dS/dE (S is the
entropy, E the total emergy of the system). What I do not understand, is why
is this news? A negative emergy is not very uncommen, for example lasers have
negative temperature.

~~~
splat
I think the new part is that this is very close to absolute zero from the
negative side instead of the positive side. It also seems the other
interesting part is that the system transitions from positive to negative
temperatures very suddenly.

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balsam
Assuming the second law is not violated, the system of lower (negative)
temperature becomes a source of energy (as opposed to sink). If this is right,
that is mighty interesting.

~~~
cygx
Not really - this has been done before and the textbook example are spin
systems. The newsworthy part is that the negative temperature is realized via
motional degrees of freedom.

~~~
balsam
Just checked wikipedia: "By contrast, a system with a truly negative
temperature in absolute terms on the kelvin scale is hotter than any system
with a positive temperature. If a negative-temperature system and a positive-
temperature system come in contact, heat will flow from the negative- to the
positive-temperature system."

Let's define wikipedia to be the absolute zero of information.

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brudgers
Once again I am reminded that half the facts I know are wrong.

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bfe
Maybe the Universe somehow has a natural form of this effect in the
intergalactic medium (or in spacetime) that causes negative pressure and is
the mechanism of dark energy?

"Negative temperatures imply negative pressures and open up new parameter
regimes for cold atoms, enabling fundamentally new many-body states."

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31reasons
Nature's article starting with completely anti-science statement like : "It
may sound less likely than hell freezing over" is pretty disappointing.

~~~
polyfractal
To be fair, it's a news brief designed for the average layperson, not a
research article. I think editorializing the content a bit is fine.

Besides, that particular phrase is commonly accepted as an idiom, not a
serious reference to a Biblical Hell.

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Tycho
Below absolute zero is an anti-concept.

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junio
cool.

