A Temperature Below Absolute Zero: Atoms at Negative Absolute Temperature Are the Hottest Systems

[n2doc]

Jan. 4, 2013 — What is normal to most people in winter has so far been impossible in physics: a minus temperature. On the Celsius scale minus temperatures are only surprising in summer. On the absolute temperature scale, which is used by physicists and is also called the Kelvin scale, it is not possible to go below zero – at least not in the sense of getting colder than zero kelvin.

According to the physical meaning of temperature, the temperature of a gas is determined by the chaotic movement of its particles – the colder the gas, the slower the particles. At zero kelvin (minus 273 degrees Celsius) the particles stop moving and all disorder disappears. Thus, nothing can be colder than absolute zero on the Kelvin scale. Physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Garching have now created an atomic gas in the laboratory that nonetheless has negative Kelvin values. These negative absolute temperatures have several apparently absurd consequences: although the atoms in the gas attract each other and give rise to a negative pressure, the gas does not collapse – a behaviour that is also postulated for dark energy in cosmology. Supposedly impossible heat engines such as a combustion engine with a thermodynamic efficiency of over 100% can also be realised with the help of negative absolute temperatures.

In order to bring water to the boil, energy needs to be added. As the water heats up, the water molecules increase their kinetic energy over time and move faster and faster on average. Yet, the individual molecules possess different kinetic energies – from very slow to very fast. Low-energy states are more likely than high-energy states, i.e. only a few particles move really fast. In physics, this distribution is called the Boltzmann distribution. Physicists working with Ulrich Schneider and Immanuel Bloch have now realised a gas in which this distribution is precisely inverted: many particles possess high energies and only a few have low energies. This inversion of the energy distribution means that the particles have assumed a negative absolute temperature.

“The inverted Boltzmann distribution is the hallmark of negative absolute temperature; and this is what we have achieved,” says Ulrich Schneider. “Yet the gas is not colder than zero kelvin, but hotter,” as the physicist explains: “It is even hotter than at any positive temperature – the temperature scale simply does not end at infinity, but jumps to negative values instead.”

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http://www.sciencedaily.com/releases/2013/01/130104143516.htm

[tama]

1. I remember

that couple decades ago the cold lab of Helsinki University was making lot of records, and they said that they get also on the other super hot side "below" absolute zero.

[eppur_se_muova]

2. This is just playing games with Boltzmann's formula ...

strictly speaking, temperature is not defined for systems not at equilibrium. So what's the temperature of this system ? It's undefined.

For systems at equilibrium, the temperature is just the average kinetic energy (number) density of the molecules. So it can't be less than zero, regardless of the population inversion.

SO-CALLED negative temperatures have been frequently cited in systems undergoing laser emission, because this results in a partial inversion of energy populations. If you plug a negative value for energy into Boltzmann's formula, this also predicts a population inversion. But since the definition of temperature is number energy density, not population distribution, a population inversion does not imply a negative energy density.

p --> q does not necessarily mean that q --> p.

Science journos need to stop hyperventilating at the mention of negative temps, and go get a second opinion.

[dimbear]

4. So true. This idea has been kicking around a long time, and has been misleading

exactly that long. Scientists need to think more carefully before they name things with misleading names.

[tama]

5. Thermodynamic beta

Wiki explains well:

In physics, certain systems can achieve negative temperature; that is, their thermodynamic temperature can be expressed as a negative quantity on the Kelvin scale. In colloquial usage, "negative temperature" may refer to temperatures that are expressed as negative numbers on the more familiar degrees Celsius or Fahrenheit scales, with values that are colder than the zero points of those scales but still warmer than absolute zero. 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 a positive temperature will increase in entropy as one adds energy to the system. Systems with a negative temperature will 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

Thermodynamic beta gives exactly same results as classical definition, but is more general notion, just like quantum physics is more general than classical mechanics.

[sir pball]

6. It's more like games with the concept of "temperature"..

The world at large uses the classical concept of temperature, the average kinetic energy you mentioned. Boffins use the more rigorous and formal thermodynamic definition - the relationship between added energy and entropy of the system. Negative temperatures are paradoxical to the point of nonsense in a classical sense; in a thermodynamic sense, they simply mean that any energy added to the system will *decrease* the entropy.

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

We covered negative-absolutes in undergraduate thermodynamics, it's not a hard concept at all if it's explained clearly. The problem is the so-called "science journos" who don't seem to have the relatively basic education to understand what they're reporting on.

[Fumesucker]

3. Someone put a thermometer in Dick Cheney's heart?