Negative Absolute Temperature: Defying Gravity?

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Discussion Overview

The discussion revolves around the concept of negative absolute temperature and its implications for gravity, particularly whether atoms at negative absolute temperatures can move upwards, seemingly defying gravitational pull. Participants explore the theoretical underpinnings, historical context, and potential new limits in nature associated with this phenomenon.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that negative absolute temperature allows atoms to move upwards against gravity, challenging traditional views of gravity as solely attractive.
  • Others argue that the effects observed can be explained within the standard model of gravity, suggesting no exotic behavior is implied.
  • There is a contention regarding the novelty of the concept of negative temperature, with some noting that it has been discussed since the 1950s, questioning the framing of it as a new fundamental limit.
  • One participant emphasizes that the Hubbard model involves more than just gravity and cautions against oversimplifying the behavior of particles at negative temperatures.
  • Concerns are raised about the analogy between negative absolute temperature and the speed of light, with some asserting that no particles surpass the speed of light, unlike the claims made regarding temperature.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of negative absolute temperature on gravity and the novelty of the concept itself. The discussion remains unresolved, with no consensus reached on these points.

Contextual Notes

Some limitations include the dependence on specific definitions of temperature and gravity, as well as the historical context of the concept of negative temperature, which may not be fully acknowledged in current discussions.

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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).
 
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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.
 
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".
 
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.
 
Tollendal said:
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.
 
Tollendal said:
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.
 
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