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/ ].

As mentioned in my previous post (Lincoln On Equality, 6th February, 2013), the Zeroth Law of Thermodynamics enables the concept of temperature and temperature scales to be established.  The Swedish astronomer, Anders Celsius (1701-44) devised a temperature scale on which water froze at 100 degrees and boiled at zero, i.e. the opposite way around to the scale that bears his name today.  Daniel Fahrenheit (1686-1736), a German instrument maker, was probably the first to use a mercury thermometer and he assigned zero to the lowest temperature he could achieve, which was for a mixture of salt and water.  He chose his body temperature as 100 degrees because it was an easily portable standard, but not ideal because it is not totally reproducible.  Fahrenheit’s scale had a temporary advantage because negative numbers were rarely needed given the technology of the day and that water freezes at 32 degrees Fahrenheit.

Somewhat later it was decided that it might be more appropriate to set zero as the lowest attainable temperature, known as absolute zero, which is defined by the Third Law of Thermodynamics as the temperature at which the entropy of all perfectly crystalline pure substances is zero.  This lead to definition of two temperature scales: the Kelvin scale with degrees the same size as on the Celsius scale so that water freezes and boils at 273K and 373K respectively; and the Rankine scale with degrees the same size as the Fahrenheit scale.

Actually, absolute zero is not attainable.  The world record stands at 810 trillionths of a degree Rankine (see http://www.smithsonianmag.com/science-nature/phenom-200801.html).

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