Yet again I feel the need to scold New Scientist for crappy reporting on physics. Ulrich Schneider and colleagues have gotten some atoms to have negative temperatures.  That's nothing new!  What's new is that they're doing it not using the atoms' spins, or other discrete quantities: they're doing it using the motion of the atoms.   But:

1) The article doesn't say that.

2) The blurb here says the existence of negative temperatures "could help us understand dark matter."  Well, anything is possible, but this is not at all what Schneider's new work is about!   The article says dark energy has negative pressure - true, according to our theories - "which suggests it might have negative temperature."  Oh yeah?  That seems very unlikely to me.   Clearly this aspect was emphasized just to catch our attention.

3) They also say this new work "might allow us to build ultra-efficient heat engines."   But when New Scientist says something about physics "could" or "might" be true, they often mean "with probability 0.1%".

4) They begin by saying "Nothing is colder than absolute zero..." This is a botched version of a commonly heard sloppy statement of the Third Law of thermodynamics, which goes "it's impossible to reach absolute zero."    But negative temperatures are possible, for systems that have a maximum allowed energy.  And Schneider's work is one of many examples of this.

5) In modern physics what's more fundamental than temperature is the reciprocal of temperature, which I call coolness.  Coolness can be positive, negative or zero.  When coolness goes from positive to zero to negative, the temperature gets big, then becomes infinite, and then becomes negative.   That sounds weird in terms of temperature!   But it's perfectly reasonable in terms of coolness.  The sloppy statement of the Third Law also becomes more reasonable: it just says infinite coolness is impossible.

The New Scientist article tries to explain this as follows: "The resulting thermometer is mind-bending with a scale that starts at zero, ramps up to plus infinity, then jumps to minus infinity before increasing through the negative numbers until it reaches negative absolute zero."   This emphasizes the weirdness rather than the reasonableness of what's going on.  It also suggests that a temperature of "negative zero" is different than zero, and allowed where zero is not.

For more on negative temperatures, try the FAQ:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/neg_temperature.html

I didn't write it - it's from the early days of the internet, when a few of us physics people got tired of answering the same questions over and over.

#bigness  ﻿
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