Temperatures below absolute zero

SEXPAND

It used to be that scientists all agreed that it was impossible to achieve temperatures below absolute zero. It was literally the coldest anything could ever get. Late last year, though, a team of scientists from the Max-Planck-Institute in Germany blew that out of the water: finally, they'd cooled a cloud of gas atoms to below −273.15°C. In fact, the result was as much a quirk of the definition of temperature as anything else, and the way it relies on both energy and entropy (the measure of disorder of particles). New Scientist explains:

In principle [it's] possible to keep heating the particles up, while driving their entropy down. Because this breaks the energy-entropy correlation, it marks the start of the negative temperature scale, where the distribution of energies is reversed – instead of most particles having a low energy and a few having a high, most have a high energy and just a few have a low energy.

It's this curious logic that allowed the Max-Planck-Institute researchers to cool a variety of atoms in a vacuum, for the first time ever, to below absolute zero. So far, though, they haven't managed to work out what to do with the chilled particles.