Scientists push atoms below absolute zero threshold

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.