Dec 2, 2010

How to create temperatures below absolute zero

“ABSOLUTE zero sounds like an unbreachable limit beyond which it is impossible to explore. In fact there is a weird realm of negative temperatures that not only exists in theory, but has also proved accessible in practice. An improved way of getting there, outlined last week, could reveal new states of matter.

Temperature is defined by how the addition or removal of energy affects the amount of disorder, or entropy, in a system. For systems at familiar, positive temperatures, adding energy increases disorder: heating up an ice crystal makes it melt into a more disordered liquid, for example. Keep removing energy, and you will get closer and closer to zero on the absolute or kelvin scale (-273.15 °C), where the system’s energy and entropy are at a minimum.

Negative-temperature systems have the opposite behaviour. Adding energy reduces their disorder, and hence their temperature. But they are not cold in the conventional sense that heat will flow into them from systems at positive temperatures. In fact, systems with negative absolute temperatures contain more atoms in high-energy states than is possible even at the hottest positive temperatures, so heat should always flow from them to systems above zero kelvin.

Creating negative-temperature systems to see what other “bizarro world” properties they might have is tricky. It is certainly not done by cooling an object down to absolute zero. It is, however, possible to leap straight from positive to negative absolute temperatures.”

Read more at New Scientist

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