Quantum Gas Goes Below Absolute Zero

First time accepted submitter mromanuk writes in with a story about scientists at Ludwig Maximilian University of Munich who have created an atomic gas that goes below absolute zero. "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 time. Their technique opens the door to generating negative-Kelvin materials and new quantum devices, and it could even help to solve a cosmological mystery."

Comments

Not as new as it seems

[Grantbridge]

Lasers have had negative temperature for decades!

Re:Not as new as it seems

[140Mandak262Jamuna]

Epic fail. Every Jedi knight builds his own light sabre. What the galaxy is coming to now a days, aspiring jedi knights nonchalantly ordering their light sabre by mail order... What next? Subject verb object order Yoda learns?

Re:Not as new as it seems

[da007]

The Doctor seems to be doing quite well for his age.

Doctor Who?

better explanation

[ssam]

wikipedia has quite a good explanation of negative temperature.
http://en.wikipedia.org/wiki/Negative_temperature

Re:better explanation

[hydrofix]

An interesting quotation from that article:

A substance with a negative temperature is not colder than absolute zero, but rather it is hotter than infinite temperature.

It seems this is a very specific quantum mechanical perversion, and no classical systems can reach the state quantum physicists call "negative temperature".

Re:better explanation

[Rhaban]

So, temperature uses unsigned floats?

Re:better explanation

[Anonymous Coward]

Is this proof of a simulated universe?

Re:better explanation

[Anonymous Coward]

Temperature isn't defined in physics as anything to do with heat, but the derivative of energy with respect to entropy. Absolute zero is the temperature at which there is no energy left in the system. At normal temperatures, it is positive. At absolute zero, it's zero. If you can create a system with dU/dS as negative, it's technically negative temperature, even though the system still has energy.
It's hard to explain due to how things like temperature and energy are defined, not because physicists are being smug. There isn't a proper name for it because it doesn't happen very much. That's why it's news.

Re:better explanation

[Rogerborg]

"Technically correct" is the best kind of correct.

Re:better explanation

[locofungus]

It's a quirk of the way the temperature scale was defined.

One possible definition of temperature:

Put lots of little magnets in a magnetic field. They will line up with the field. At absolute zero there will be no (technically minimal[1]) deviation from them all being perfectly aligned. As you warm them up they will start to be less and less well aligned until at what we call infinite temperature, there is no alignment with the field at all and the alignment is completely random.

But, if instead of warming them up, you flip the magnetic field they will then "cool" through "infinite" temperature.

If we use this definition of temperature then it would make more sense to have absolute zero as negative infinite temperature, infinite as zero and still hotter temperatures as greater than zero.

This makes the unreachability of absolute zero make more sense. "Infinite" temperatures (and greater than infinite) are only unreachable via trying to add more heat.

Lasers utilize population inversion - which is a state that is impossible via naive thermodynamics and also does not have a sensible temperature as a result.

[1] Zero point energy.

Tim.

Re:better explanation

[mrbester]

It's the previously unquantifiable temperature of a McDonald's Apple Pie.

Re:better explanation

[cgaertner]

It seems this is a very specific quantum mechanical perversion, and no classical systems can reach the state quantum physicists call "negative temperature".

This is by no means a quantum perversion, just a natural consequence of the definition of temperature as 1/T = dS/dE. There's nothing mysterious about negative temperatures from a thermodynamical point of view, it just happens that calssical systems don't exhibit this property because they do not come with an upper limit on energy, whereas there are quantum ones that do.

The common interpretation of temperature as average energy per degree of freedom comes in via the equipartition theorem, but breaks down in various edge cases, eg when the energy levels cannot be approximated by continuity (eg heat capacity of diatomic gases) or for non-ergodic systems (some plasmas, I believe).

As to the problem of infinite temperature: In a sense, thermodynamic \beta = 1/kT is the more natural measure of hotness and coldness and has a pole at T = 0. Coming from T > 0, this corresponds to infinite coldness, whereas coming from T < 0, this corresponds to infinite hotness.

Re:better explanation

[serviscope_minor]

This is one of the things I love about slashdot. People with no knowledge of a subject call professionals "retarded" and get modded insightful.

It seems to me to be a retarded description, like calling infinity + 1 a negative number.

Think about reciprocals.

They need to use a proper name for it,

How about "nagative temperature". That's a proper name for it.

Temperature is defined by energy and entropy. Add energy and the entropy increases. That means the temperature is positive. How positive is how big that change is. Nice, straightforward, works well. Does whay you expect.

Then some quantum physicists discovered that adding energy makes the entropy go down. Well, plug that into the definition of temperature and the number comes out negative.

So basically, what you are claiming is that physicists are retarded because you don't like how the maths work out and you would rather they change the perfectly good classical definition of temperature to fit your sensibilities.

Deal with it. Quantum physics is very strange and many ideas you bring with you from the classical world simply don't work.

That doesn't make the professional physicists retarded, by the way.

Dark Energy

[metamarmoset]

Observtions during the experiment could point to new research on dark energy.

From TFA:

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.”

Older hardware

[alendit]

Sadly, our universe runs on a quite old hardware, which allowed the scientists to overflow the temperature variable. Why the Great Programmer didn't use unsigned longs ist beyond me, rookie mistake, really!