Most of us equate negative temperatures with being cold, but scientists at the Ludwig Maximilian University of Munich in Germany have demonstrated how to create a gas that's mind-blowingly cold: it has a negative absolute temperature.
For those in need of a quick physics refresh, absolute zero was supposed to be as cold as it could get – it is defined as 0K on the Kelvin scale, or -273.15ºC. So pretty chilly then.
When it comes to measuring the temperature of a gas, scientists typically regard the absolute temperature as the average energy of the particles, so typically most have average or close-to-average energy, with a handful whizzing around at higher energy levels.
Ullrich Schneider and his colleagues realised that had important implications for creating a negative absolute temperature.
They used lasers, magnetic fields and an ultracold gas made from potassium atoms to hold individual atoms in a lattice arrangement. In this configuration, the atoms were at their most stable, low-energy state repelling other atoms. But by suddenly reversing the magnetic field, the atoms were made to attract, causing their atoms to change in to a high-energy state – all while being ultra-cool.
In effect, their absolute temperature became negative – albeit a few billionths of Kelvin below absolute zero.
This work isn't just some neat laboratory trick: the researchers believe it could open the doors the creation of exotic new material; it may even help us improve our understanding of inexplicable phenomena such as dark matter. The experiment was reported in this week's issue of Science.