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Below absolute zero?

http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146

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Yes, it is legitimate, and is published in the most prestigious peer reviews science journal there is, the flag ship of science journals (which attests to the review scrutiny), by reputable physicists. Your link explains it pretty well: at normal temperature most particles are around some average temperature, and there is a distribution (like a Bell curve) around the average for particles with higher energies. There are more particles with the average energy than particles with the higher energy. (In a quantum system think of a two energy level system, in which most electrons are at the ground state and some at the excited state). Under some conditions you can have a population reversal, that more electrons are at the excited state and few are in the ground state. This is not unusual, but it's not a stable state: the electrons would decay to the ground state. If you have a stable population inversion state, the math equations are the same as if the temperature were negative. This experiment created such a state. They interpret it as negative temperature, and I think most physicists would agree with this interpretation, because we physicists like to trust the math. But it is negative temperature only in this very specific technical sense, bear that in mind, and there would be no smooth transition through zero: the temperature jumps from just above to just below zero Kelvin. it's never exactly at zero Kelvin.

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Absolute Zero is the complete absence of any measurable heat... -459.9 degrees farenheit.

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True, but speaking in 2-level Quantum Energy shouldn't it be plausible that the rapid switch from its lowest to highest energy levels cause the freeze temperature to be one billionth of a degree (Kelvin) below zero?
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Perhaps in theory, however, it would be difficult to prove ...
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Ah! but my friend it may have already been proven...
http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146
Which leads to my question, is this artificial creation of absolute zero a credible thing to believe. I am asking for a better understanding of the science behind this experiment.
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