Quantum gas goes below absolute zero

[Eugene]

Source: Nature

Quantum gas goes below absolute zero

Zeeya Merali

03 January 2013

It may sound less likely than hell freezing over, but physicists have created an atomic gas with a sub-absolute-zero temperature for the first time1. Their technique opens the door to generating negative-Kelvin materials and new quantum devices, and it could even help to solve a cosmological mystery.

Lord Kelvin defined the absolute temperature scale in the mid-1800s in such a way that nothing could be colder than absolute zero. Physicists later realized that the absolute temperature of a gas is related to the average energy of its particles. Absolute zero corresponds to the theoretical state in which particles have no energy at all, and higher temperatures correspond to higher average energies.

However, by the 1950s, physicists working with more exotic systems began to realise that this isn't always true: Technically, you read off the temperature of a system from a graph that plots the probabilities of its particles being found with certain energies. Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature, explains Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany.

Peaks and valleys

Schneider and his colleagues reached such sub-absolute-zero temperatures with an ultracold quantum gas made up of potassium atoms. Using lasers and magnetic fields, they kept the individual atoms in a lattice arrangement. At positive temperatures, the atoms repel, making the configuration stable. The team then quickly adjusted the magnetic fields, causing the atoms to attract rather than repel each other. “This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”

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Read more: http://www.nature.com/news/quantum-gas-goes-below-absolute-zero-1.12146

[samsingh]

1. we are continually pushing scientific boundaries

[hollysmom]

2. Absolute not so absolute.

[tama]

3. Nothing new

From wiki:
By contrast, a system with a truly negative temperature in absolute terms on the kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system.
That a system at negative temperature is hotter than any system at positive temperature is paradoxical if absolute temperature is interpreted as an average internal energy of the system. The paradox is resolved by understanding temperature through its more rigorous definition as the tradeoff between energy and entropy, with the reciprocal of the temperature, thermodynamic beta, as the more fundamental quantity. Systems with positive temperature increase in entropy as one adds energy to the system. Systems with negative temperature decrease in entropy as one adds energy to the system.
Most familiar systems cannot achieve negative temperatures, because adding energy always increases their entropy. The possibility of decreasing in entropy with increasing energy requires the system to "saturate" in entropy, with the number of high energy states being small. These kinds of systems, bounded by a maximum amount of energy, are generally forbidden classically. Thus, negative temperature is a strictly quantum phenomenon. Some systems, however (see the examples below), have a maximum amount of energy that they can hold, and as they approach that maximum energy their entropy actually begins to decrease.
http://en.wikipedia.org/wiki/Negative_temperature

[unblock]

5. thanks, that's more helpful than the article!

[unblock]

4. i'm having trouble wrapping my head around this one.

ok, if i have 9 particles at 1 kelvin and 1 zipping around at a higher energy state of 2 kelvin, then the average temperature is 1.1 kelvin.
if the situation is reversed and i have 1 particle a 1 kelvin and 9 zipping around at a higher energy state of 2 kelvin, then the distribution is upside down, sure, but the average temperature is 1.9 kelvin.

evidently i'm wrong thinking that temperature of a system is just the average of the temperatures of its particles, none of which individually can be lower than zero kelvin. mathematically, my thinking prevents me from ever getting to a negative temperature.

what am i missing?

[sir pball]

6. See post #3 for an explanation - temperature isn't what you think it is

Fun factoid - lasers, all of them, have a negative temperature inside the lasing cavity; in nerdspeak it's known as a "metastable state". The big news here that I can see is that boffins have done it in an atomic gas, not a more specialized and constrained energy-pumped specifically designed system.

[reteachinwi]

7. Entropy, I think

Entropy is a measure of randomness. Gasses, like water vapor, have converted the energy added into entropy as the gas molecules are arranged more randomly in relation to each other than they were in a liquid or solid state. I don't know what quantum materials are, but apparently when more energy is added the randomness decreases, in other words, the lattice structure is more stable, more orderly if you will, than previously. So it is "cooler" than before. Maybe a DUer with more physics ability than me can confirm or correct this.

[tama]

9. Thermodynamic beta

"Thermodynamic beta is essentially the connection between the information theoretic/statistical interpretation of a physical system through its entropy and the thermodynamics associated with its energy. It can be interpreted as the entropic response to an increase in energy. If a system is challenged with a small amount of energy, then ? describes the amount by which the system will "perk up," i.e. randomize. Though completely equivalent in conceptual content to temperature, ? is generally considered a more fundamental quantity than temperature owing to the phenomenon of negative temperature, in which ? is continuous as it crosses zero where T has a singularity.[1]"

And in fact negative beta is "hotter" than any positive beta, as "If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system."

More: http://en.wikipedia.org/wiki/Negative_temperature

[Festivito]

8. Suppose they are not zipping about. What is their spacing.

And, how much energy is needed to pull one out of its spacing? That would be a negative measure of the ten atomic particles' temperature. The particles might even crystallize further increasing the energy needed to release one.

And, this does not even begin to touch subatomic states.