Recently a study has been published in Science [http://www.sciencemag.org/content/339/6115/52] about creating temperatures below absolute zero (see previous post on Arbitrary Zero, 13 February 2013), i.e. negative temperatures on the Kelvin and Rankine scales. Temperature is a measure or indicator of the internal energy of matter which in turn is related to the spin state of electrons. Electrons have two available spin states that are known as ‘up’ and ‘down’. At room temperature more electrons have ‘down’ spin than ‘up’, and as absolute zero is approached all electrons align to have ‘down’ spin which is a configuration that corresponds to zero entropy. When the temperature rises from room temperature, electrons tend to switch from ‘down’ to ‘up’ spin so that at an infinite temperature there are equal numbers of electrons in the two spin states.
The current theory is that at -300 Kelvin more electrons have ‘up’ than ‘down’ spin, i.e. a mirror image of the situation at +300 Kelvin. If the temperature is lowered still further then the ‘up’ spin electrons tend to switch to ‘down’ spin so that at a negative infinite temperature there are equal numbers of electrons with ‘up’ and ‘down’ spin. This state is equivalent to an infinite positive temperature, i.e. the absolute temperature scale can be considered to be circular or to have the negative and positive components joined at zero and infinity.
If you have made it this far then well done! But if you didn’t quite follow everything then try the explanation at Newsy [http://www.newsy.com/videos/scientists-create-negative-kelvin-temperature-gas/ ].
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