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Boffins at the Ludwig Maximilian University in Munich, Germany have literally turned the Kelvin scale on its head, having produced a quantum gas with a temperature below absolute zero.

Ulrich Schneider and his colleagues created the subzero gas by arranging potassium atoms into a lattice using lasers and magnetic fields, then suddenly adjusting the magnetic fields so that they caused the atoms to attract, rather than repelling each other as they normally would.

That's the opposite of what happens under natural conditions, when most of the particles in such a system would be at average to near-average energies, with only a few particles at a higher energy state.

Because Schneider and his team were able to hold their atoms in an unnatural, high-energy state, however, the temperature of the gas effectively dropped from just above absolute zero to a few billionths of a Kelvin below absolute zero.

That might seem counterintuitive at first; normally you'd expect that adding energy to a system would increase its temperature. But Schneider's exotic, artificial system was built so that it could only hold a certain amount of energy. Adding more energy actually caused the entropy of the system to decrease, resulting in the negative temperature.

Note that this doesn't mean the resulting system is actually cold, however. Nothing can be colder than absolute zero, which is a theoretical state at which particles have no energy at all. On the contrary, a system with a sub-zero absolute temperature is actually hotter than the same system at any positive temperature – a perverse result of how absolute temperature has been defined.

"The temperature scale simply does not end at infinity," Schneider explains, "but jumps to negative values instead."

Pushing a system to such a tiny fraction below absolute zero might not seem significant, but it is. Schneider's techniques open the door to further research into such exotic, high-energy systems, which tend to collapse at positive temperatures but are stable below absolute zero.

As Wolfgang Ketterle of the Massachusetts Institute of Technology told Nature, "This may be a way to create new forms of matter in the laboratory."

Boffins have theorized that such sub-zero matter would have weird properties, such as seeming to defy gravity, and that studying it could yield insight into the cosmic force known as "dark energy".

Schneider and his group published their full findings in the journal Science on Friday. ®

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