Negative Absolute Temperature: Defying Gravity?

[Tollendal]

Dr. Achim Rosch, a theoretical physicist at the University of Cologne in Germany, who proposed the technique used by Dr. Ulrich Schneider and his team to create in laboratory negative absolute temperature, have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity.

The matter is still under debate, but in my view it's no more certain to affirmate that gravity is only atractive: one ought to suspend judgement until peer review goes ahead.

Interesting enough, but in my view the most relevant thing about the new research is that it seems to establish a new fundamental limit in Nature, somewhat analogous to c. Absolute zero temperature can't be reached, but can be surpassed!

See also:

Nature
doi:10.1038/nature.2013.12146

Braun, S. et al. Science 339, 52–55 (2013).

Medley, P., Weld, D. M., Miyake, H., Pritchard, D. E. & Ketterle, W. Phys. Rev. Lett. 106, 195301 (2011).

Rapp, A., Mandt, S. & Rosch, A. Phys. Rev. Lett. 105, 220405 (2010).

Mandt, S., Rapp, A. & Rosch, A. Phys. Rev. Lett. 106, 250602 (2011).

 

[nasu]

Did you actually read the references?
If you do, you will see that the effect described is predicted by using the standard model for gravity. So it does not allow to draw any conclusion about some exotic, unknown yet, behavior of gravity.
See the PhysRevLett paper.

 

[Tollendal]

Dear nasu,

I thank you very much your kind observation. In my view the most relevant thing about the new research is that it seems to establish a new fundamental limit in Nature, somewhat analogous to c. Absolute zero temperature can't be reached, but can be surpassed! I'm transposing this discussion to a new tread: "A new limit in Nature".

 

[nasu]

Some of the titles in the science media may give you the wrong idea about the "new" part.
The concept of negative temperature is quite old. I can find papers from 1950s mentioning the concept. Probably is older than that.
So if it is a "fundamental limit", it is not so new.
If you want to call it this and make an analogy with speed of light, is your choice. I don't see the relevance though.

 

[Andy Resnick]

In my view the most relevant thing about the new research is that it seems to establish a new fundamental limit in Nature, somewhat analogous to c. Absolute zero temperature can't be reached, but can be surpassed! I'm transposing this discussion to a new tread: "A new limit in Nature".

If you are not able to read the Phys Rev Lett 2011 article, I recommend reading the ArXiv article (http://arxiv.org/pdf/1005.3545.pdf). Aside from T < 0 not being a 'new fundamental limit', the Hubbard model contains more than simply gravity- it's not correct to claim that particles 'fall up', especially since the T < 0 particles are at the bottom of the cloud. It's not clear from the articles, but it appears that this system also realizes a Brownian ratchet.

To summarize: fermions in an optical lattice are a very interesting model system, displaying many unique and novel features that provide broad relevance to condensed matter.

 

[Dale]

somewhat analogous to c. Absolute zero temperature can't be reached, but can be surpassed!

In addition to the other comments, this seems to be a misunderstanding of c. Photons and other massless particles do reach c, but there are no known real particles that surpass it.