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 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 ΩS 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)/ ∂ESubstituting the first equation into the second, we get1/T = ∂S/∂ESo 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.