## negative temperature and entropy

we know the entropy in each process increases is it true for absolute negative temperature?

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 Recognitions: Gold Member There is noabsolute temperature below absolute zero. http://en.wikipedia.org/wiki/Absolute_zero

 Quote by Naty1 There is noabsolute temperature below absolute zero. http://en.wikipedia.org/wiki/Absolute_zero
a week ago scientist discover there is negative temperature,if you suppose there is, what is your answer?

## negative temperature and entropy

 Quote by persia7 a week ago scientist discover there is negative temperature,if you suppose there is, what is your answer?
The negative temperature arrives from the definition of temperature.

So if the entropy of the system decreases with increasing energy, then the temperature is said to be negative. This is common in lasers, where population inversion can be considered as a negative temperature.

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 Quote by persia7 a week ago scientist discover there is negative temperature,if you suppose there is, what is your answer?
Negative temperature has been known for a far longer time now. I think it was the first demonstration of negative temperature for an atomic gas which has been published recently. Here adding energy can decrease entropy. However, it is important to note two things:

1) Although the temperature is negative, one cannot translate that into the system being colder than one at absolute zero. It is instead VERY hot.

2) Negative temperatures occur in metastable or quasi-equilibrium systems, NOT in systems which are in equilibrium, so you keep a state stable which is not the ground state. You may imagine such state as similar to the steady state of a laser which involves population inversion. Note the difference between steady state and equilibrium state! As these states are not the equilibrium ones, you do not run into trouble with thermodynamics.

 the principle increasing of entropy is universal , please explain it in negative temperature.

 Quote by Cthugha 2) Negative temperatures occur in metastable or quasi-equilibrium systems, NOT in systems which are in equilibrium, so you keep a state stable which is not the ground state. You may imagine such state as similar to the steady state of a laser which involves population inversion. Note the difference between steady state and equilibrium state! As these states are not the equilibrium ones, you do not run into trouble with thermodynamics.
Well, they are metastable only in the sense that their environment is colder, thus they tend to cool (go toward positive temperatures). This is true for any system (positive or negative temperature), which is hotter than its surroundings.

Some systems (e.g. nuclear spins in magnetic field) can be decoupled from their environment so effectively that a negative temperature remains even days without any additional work.

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