## Quantum Gas Goes Below Absolute Zero 264

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."*
## Re:better explanation (Score:5, Interesting)

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 (Score:5, Interesting)

Is this proof of a simulated universe?

## Dark Energy (Score:5, Interesting)

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.”

## Sub Means below? (Score:3, Interesting)

So is this story misleading to say that absolute zero was achieved. Wikipedia The Celsius and Fahrenheit scales are defined so that absolute zero is 273.15 C or 459.67 F. https://simple.wikipedia.org/wiki/Absolute_zero [wikipedia.org]

But in the news story it says SUB and SUB means below, yet there is no mention of the temperature whatsoever in the article and going beyond absolute zero is not possible even out in space! You can get close, but not to absolute zero otherwise you would have created the ultimate weapon!

Enough said.

## Re:better explanation (Score:4, Interesting)

## Re:better explanation (Score:5, Interesting)

Actually, since this is predominantly a computer science crowd, let's try to explain it in purely binary terms (or a simplified pure quantum mechanical model if you wish). Every particle in your system is a binary bit; 0 is ground state (low energy) and 1 is excited state (high energy). Now, our kelvin scale is defined so that 0K is all 0s (there's only one state that satisfies this criterion) and +infinity is a 50/50 mix of 0s and 1s (which has the largest number of possible combinations/states := highest entropy). That worked pretty well for a long time since one never can go higher than 50/50 through ordinary heating. The problems started when people figured out clever tricks to have more 1s than 0s. This is called a population inversion, and LASER and NMR/MRI rely on it. The temperature of such an inverted population would be "beyond infinity", in other words, not representable in the kelvin scale. The solution was to use negative temperatures for these inverted populations: all 1s would be -0K (the fact that -0 is not the same as +0 is not a problem because neither of these states can ever be reached), and the temperature would go down (-1, -2,...) as 0s are introduced, to ultimately reach -infinity at the same 50/50 mix as +infinity (so basically + and - infinity are the same state). This weird system turns out to have mathematically convenient properties. Just to get an idea, if one inverts this temperature scale (ie. define a new new (K^-1) scale that goes with 1/T), the 50/50 state would be 0(K^-1), all 1s is -infinity (K^-1) and all 0s is +infinity (K^-1), so the problems at 0 and infinity are solved.

Remarks: - given the above, I feel it's more correct to state that inverting a population is going through infinity [wikipedia.org] (as opposed to going through zero).

- inverted populations are not stable; when perturbed, they always equilibrate to positive temperature states (and they cannot be maintained through ordinary heating as another reply incorrectly stated, though they can through pumping, as in continuous-wave lasers [wikipedia.org]). This equilibration can, however, take several seconds (in NMR applications) - long enough for practically useful applications.

TL:DR; version: negative temperature matter doesn't contain less energy than 0K; a good deal more in fact.