Huge Story! Cloud Of Atoms Made To Go Lower Than Zero Temperatures!

[Aethelwulf]

I can't stress how revolutionary this is!

 

http://www.newscientist.com/article/dn23042

[sigurdV]

Nice of you to post it.

[Moontanman]

I've been told it is mostly hype and that this effect has been known since the 1950's...

[Aethelwulf]

I've been told it is mostly hype and that this effect has been known since the 1950's...

 

I am afraid I can't varify that. But I do believe you. It is easy to get lost in the hype of articles.

[JMJones0424]

I'm not even going to pretend to understand the meaning or implications of this experiment. However, one of the science bloggers I read frequently has a layman friendly explanation of what is meant by "negative temperature" that may be helpful to other members.

 

What follows is a cut and paste hackjob from What Does "Negative Temperature" Mean, Anyway?, at the Uncertain Principles blog. Please follow the link for the complete article.

 

...

...Temperature is a measure of how fast atoms are moving, so negative temperature means they’re moving backwards.

Yeah, not so much... In the atomic model, the temperature of a gas is a measure of the average kinetic energy of the atoms making it up. The kinetic energy depends on the speed of the atoms only (the speed squared, provided they’re not moving too fast), not the direction.

 

But if that’s the case, how can it possibly be negative?

It can’t, which is the reason for the “In the atomic model” at the start of the previous paragraph. The relationship between speed and temperature isn’t an absolute fact, just a convenient way to think about temperature in the simple case where you have a gas of atoms that behave like tiny little classical particles. The reality of temperature is a good deal more complex, with a lot of subtleties. It’s not just about the speed at which things are moving.

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Mathematically, physicists talk about the temperature as a property of a distribution of energies, typically a Maxwell-Boltzmann distribution...The important thing to take away is that this is an exponential function. If you double the energy E that you’re looking for, keeping the temperature the same, the probability of finding an atom in the gas with that energy drops off by a factor of [math]e^{-2} \approx 0.14[/math]... It’s also exponential in the temperature, but in the other direction. If you double the temperature, the probability of finding an atom at a given energy goes up by a factor of 7.4, and so on.

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...So, a negative temperature system is one where the probability of finding atoms in a given energy state increases as you increase the energy?

Precisely. It increases exponentially, in fact. The rate of increase still depends on the temperature, but with a twist– the probability increases faster for negative temperatures that are small than for negative temperatures that are large (that is, a temperature of -0.1 K produces many more atoms at a given high energy than a temperature of -10 K).

 

So, this negative temperature stuff is just about probability distributions?

Pretty much, yes. Which may not seem all that much more exciting than just making the atoms move backwards, but it’s actually a pretty clever experiment. The tricky thing with making negative-energy systems is that you need to somehow cap the energy, otherwise the Boltzmann factor demands that you have huge numbers of atoms at arbitrarily high energy. Which is what makes this experiment novel– people have demonstrated “negative temperature” systems before for properties like spin, where there’s an intrinsic limit imposed by the system. In this case, they’re dealing with the velocity– that’s what they mean by “motional degrees of freedom”– and have imposed an upper limit on the velocity by putting the atoms in an optical lattice.

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