Quantum gas goes below absolute zero

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…

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...

The experiment is interesting, but the press article has a misleading title. The problem is that negative (Kelvin) temperatures are hotter than positive (Kelvin) temperature. So the title should be: "Quantum gas goes above infinite to a negative absolute temperature (nature.com)" More details: https://en.wikipedia.org/wiki/Negative_temperature