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[Phys-l] Negative kelvin temperatures



On another list, the claim was made that in a system with a finite
number of energy levels, population inversion implies a negative kelvin
temperature. The following Wikipedia article was cited as evidence:

http://en.wikipedia.org/wiki/Negative_temperature

I had never heard this claim of negative kelvin temperature before. My
first reaction is that a system with population inversion is not in
thermal equilibrium, so it doesn't make sense to determine the
temperature of a system based on population inversion. I'm not even
sure that the system has a well defined temperature. Can someone
familiar with this concept please discuss its merits?

Thanks,

Daniel Crowe
Loudoun Academy of Science
dan.crowe@loudoun.k12.va.us