Physicists have been able to create an atomic gas that can attain a temperature below absolute zero, -273.15˚C. They were able to create this gas using negative-Kelvin materials and new quantum devices.
The scientists published their findings in the journal Science¹. The absolute temperature scale dates back to Lord Kelvin, who created it in the mid-1800s. It was designed in a way that nothing could be colder than absolute zero. Later, physicists discovered that the absolute temperature of a gas is related to the average energy of its particles. Absolute zero corresponds to a theoretical state in which particles have no energy, and higher temperatures correspond to higher than average energies.
In the 1950s, physicists working with exotic systems started to realize that this wasn’t always true. Temperatures of a system are read off from a graph that plots the probabilities of its particles being found with certain energies. Most particles have average or near-average energies, with only a few having higher energies. In theory, if more particles have higher energies, the plot would flip around and the sign of the temperature would change from positive to negative.
Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany, and his colleagues reached a sub-absolute zero temperature with an ultracold quantum gas that was made up of potassium atoms. They used lasers and magnetic fields to keep the atoms in a lattice arrangement. At positive temperatures, the atoms repel, making the whole configuration stable. When the magnetic fields were adjusted, causing the atoms to attract rather than repel each other, the atoms shift from their most stable state to the highest possible energy state.
At positive temperatures, such a reversal is unstable as the atoms would collapse inwards. The trapping laser field was also adjusted to make it more energetically favorable, allowing the atoms to stick in their current positions. The gas then shifted from a temperature above absolute zero to a few billionths of a Kelvin below absolute zero.
Exotic high-energy states are hard to generate in the laboratory. These techniques allow these states to be studied in detail, possibly allowing the creation of new forms of matter in the laboratory. Such systems would behave in strange ways. Clouds of atoms that are usually pulled downwards by gravity would be pulled upwards, apparently defying gravity.
A sub-absolute-zero gas mimics dark energy, the force that pushes the Universe to expand at an ever-faster rate, against the pull of gravity.
- Braun, S. et al. Science 339, 52–55 (2013).