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There鈥檚 a whole mirror world of negative temperatures reaching from minus infinity to absolute zero 鈥� now we鈥檙e plumbing those depths for real

DEFINING a temperature scale is easy. Fire is hot, ice is cold, draw a line between the two and there you are.

But what makes fire hotter than ice? The 19th-century physicist William Thomson, later Lord Kelvin, gave one answer. He defined temperature in terms of kinetic energy. In hot bodies, particles are moving around a lot; in cold ones, not so much.

(Image: Emilio Segre VisualL Archives/American Institute of Physics/SPL)

Kelvin鈥檚 scale implied a natural lowest temperature at which particles stop moving completely. It is impossible even to make something reach this 鈥渁bsolute zero鈥�, since that would imply the existence of something even colder to cool it with 鈥� let alone leap to negative temperatures beyond it.

Or is it? It turned out later that temperature depends not just on particle energies, but how that energy is distributed: disorder, or entropy. Increasing entropy also implies heat flow 鈥� and temperature change. Because of the way Kelvin defined his scale, increasing a system鈥檚 energy while reducing its entropy registers as a negative temperature. In fact, there is a whole mirror world of negative temperatures stretching towards absolute zero from negative infinity (see diagram).

鈥淎 whole Mirror-world of negative temperatures exists below absolute zero on the kelvin scale鈥�

Motion and disorder usually increase in lockstep, so states of increasing energy and decreasing entropy are not things we stumble across every day. But physicists can make them. Most recently a team led by of the Ludwig Maximilian University of Munich in Germany created a negative-temperature state by using lasers to manipulate atoms cooled to a fraction above absolute zero.

It鈥檚 a bit of a cheat, really. Absolute zero still represents the lowest energy a system can have; it鈥檚 just that Kelvin鈥檚 scale doesn鈥檛 necessarily reflect that. Those in the know prefer to speak of negative thermodynamic beta, which assigns the value of minus infinity to absolute zero and climbs smoothly upwards with increasing energy. Or maybe we should just stick to Celsius.

Read more: 鈥�Us vs the universe: 8 ways we bend the laws of physics鈥�

This article appeared in print under the headline 鈥淟ess than nothing鈥�