Number 161 (Story #2), January 24, 1994 by Phillip F. Schewe and Ben Stein PICOKELVIN TEMPERATURES have been achieved in rhodium nuclei. The temperature of a physical system, whether it be an ice cube or a collection of nuclei, can be defined as the amount of disorder, or entropy, in the system. If the spins in a group of nuclei are distributed over a wide range of directions, then the system's disorder (and its temperature) is high; in a low temperature system the spins would tend to be organized in a single direction corresponding to a low-energy state. Pertti Hakonen and his colleagues at the Helsinki University of Technology in Finland aligned the spins of rhodium nuclei in an external magnetic field, dropping them to picokelvin temperatures. Then the researchers quickly reversed the direction of the field in a way that kept the amount of entropy constant. The result was that most of the nuclear spins were opposed to the field, and therefore in a high-energy state; since the spin distribution was now exactly the inverse of what it was in the previous case, the sample of nuclei was considered to have a "negative temperature." Hakonen believes that femtokelvin temperatures (positive and negative) may be possible in future experiments. (Scientific American, January 1994.)