Negative Kelvins

Hi Sam_Wormley, You wrote:

PhysLink.COM:

When a cosmologist talks about the 'temperature' of a photon they are
basically describing the equivalent energy of a photon.
PhysLink.COM/education/askexperts/ae210.cfm

Planck's Law
http://scienceworld.wolfram.com/physics/PlanckLaw.html

Good links. There are lots of definitions of temperature.

Although ideal laser ( one that's perfectly coherent ) is at absolute zero,
strong ones can burn through steal ( thanks to their relativistic mass ).
How cool is that ?

Atom lasers employ Bose-Einstein condensates that are near absolute zero.
See:

WikiPedia.ORG/wiki/Bose-Einstein_condensate
WikiPedia.ORG/wiki/Atom_laser

A Bose-Einstein ( laser ) system's population inversion
has negative Kelvins. See:

Since we started with over half the atoms in the spin-down state,
initially this drives the system towards a 50/50 mixture,
so the entropy is _Increasing_ [ dissipated energy ],
corresponding to a positive temperature.
However, at some point more than half of the spins are in
the spin-up position. In this case, adding additional energy
_Reduces_ the entropy [ accumulated energy ]
since it moves the system further from a 50/50 mixture.
This reduction in entropy with the addition of energy
corresponds to a negative temperature.
This phenomenon can also be observed in many lasing systems,
wherein a large fraction of the system's atoms
( for chemical and gas lasers ) or electrons ( in semi-conductor lasers )
are in excited states. This is referred to as a population inversion.
__ WikiPedia.ORG/wiki/Negative_temperature


.

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