How Can Systems Achieve Negative Temperature Without Crossing Absolute Zero?

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tannerbk
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I've read recently about systems with the ability to reach negative temperature: where entropy decreases as energy increases. I have a couple question regarding this. First, how is it possible to reach negative temperature without crossing absolute zero? Is there some discontinuity like a phase transition that allows this to happen? How can negative temperature be related to the kinetic energy of system?

Additionally, wouldn't this allow heat to flow from negative to positive temperature, leading to more than 100% efficient engines?

Finally, could this have anything to do with negative pressure associated with positive vacuum energy density?
 

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Do a forum search. This has been discussed here not too long back.
 
I tried before I posted this, can you provide a link?
 
phinds said:
DO A SEARCH --- use "negative temperature" in quotes

Use the "Search PF via Google" option at the SEARCH pulldown menu at the top of the page. Then click the thread hit from 3-Jan-2013.

:smile:
 
berkeman said:
Use the "Search PF via Google" option at the SEARCH pulldown menu at the top of the page. Then click the thread hit from 3-Jan-2013.

:smile:

Thanks!