New Atomic Gas Goes Below Absolute Zero
January 5, 2013

Physicists Create First Negative Absolute Temperature State For Moving Particles

April Flowers for - Your Universe Online

People take it for granted in most northern climes the temperatures will drop below zero at some point during the winter. What is normal in nature is impossible in physics: a minus temperature. Minus temperatures are only surprising on the Celsius scale during the summer. On the Kelvin, or absolute temperature scale used by physicists, it is impossible to go below zero. At least, it is impossible to get colder than zero Kelvin. Physically, the chaotic movement of its particles determines the temperature of a gas. The colder the gas, the slower the particles; at zero Kelvin (minus 273 degrees Celsius), the particles of the gas stop moving and all chaos disappears. Therefore, nothing can be colder than absolute zero on the Kelvin scale.

A new study by physicists at the Ludwig Maximilians University (LMU) and the Max Planck Institute of Quantum Optics describes the creation of a new atomic gas in the laboratory that has negative Kelvin values. Apparently, these negative absolute temperatures have several absurd consequences. For example, even though the atoms in the gas attract each other and give rise to negative pressure, the gas does not collapse. This is a behavior that has been postulated for dark energy in cosmology, as well. Negative absolute temperatures can help to realize supposedly impossible heat engines such as a combustion engine with a thermodynamic efficiency of over 100%.

To get water to boil, energy needs to be added. The water molecules increase their kinetic energy over time as the water heats up, and move faster and faster on average. The individual molecules, however, possess different kinetic energies from very slow to very fast. High-energy states are less likely than low-energy states; in other words, only a few particles move really fast. This distribution is called the Boltzmann distribution. The team, which includes Ulrich Schneider and Immanuel Bloch, has created a gas in which this distribution is precisely inverted. Many particles possess high energies in the new gas, and only a few have low energies, meaning 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. It is even hotter than at any positive temperature — the temperature scale simply does not end at infinity, but jumps to negative values instead.”

To understand the meaning of absolute temperature, picture rolling spheres in a hilly landscape. The valleys symbolize low potential energy and the hills for high. Faster moving spheres have higher kinetic energy as well: if one starts at positive temperatures and increases the total energy of the spheres by heating them up, the spheres will increasingly spread into regions of high energy. If the spheres could be heated to infinite temperatures, there would be an equal probability of finding them at any point in the landscape, regardless of the potential energy. If you added more energy at this point, which would heat up the spheres even further, they would gather at high-energy states and would be even hotter than at infinite temperature. The temperature would be negative because the Boltzmann distribution would be inverted. It seems counterintuitive  a negative absolute temperature is hotter than a positive one, however this is simply a consequence of the historic definition of absolute temperature. If it were defined differently, the contradiction would not exist.

It is not possible in water or any other natural system to invert the population of energy states because the system would need to absorb an infinite amount of energy, which is impossible. If, however, the particles possess an upper limit for their energy, like the top of the hill in the potential energy landscape, it would be a completely different situation. The research team, following theoretical proposals by Allard Mosk and Achim Rosch, has realized a system of an atomic gas with an upper energy limit.

The team first cooled approximately a hundred thousand atoms in a vacuum chamber to a positive temperature of a few billionth of a Kelvin. They captured these atoms in optical traps made of laser beams. The atoms are perfectly thermally insulated from the environment by the ultrahigh vacuum surrounding them. A so-called optical lattice is created by the laser beams, in which the atoms are arranged regularly at lattice sites. The atoms in the lattice can still move from site to site via the tunnel effect, but their kinetic energy has an upper limit and therefore possesses the required upper energy limit.

Temperature relates not only to kinetic energy, however, but also to the total energy of the particles. In this case, that includes interaction and potential energy. The researcher's system also sets a limit to both of these. The scientists in Munich and Garching pushed the atoms to this upper boundary of the total energy, creating a negative temperature, at minus a few billionths of a Kelvin.

It is an obviously stable state if the spheres possess a positive temperature and lie in a valley at minimum potential energy. However, if they are located at the top of a hill at maximum potential energy, they will usually roll down, converting their potential energy to kinetic energy.

“If the spheres are at a negative temperature, however, their kinetic energy will already be so large that it cannot increase further,” explains Simon Braun, a doctoral student in the research group. “The spheres thus cannot roll down, and they stay on top of the hill. The energy limit therefore renders the system stable!”

This makes the negative temperature the team has created as stable as a positive temperature state.

“We have thus created the first negative absolute temperature state for moving particles,” adds Braun.

There is a whole range of astounding consequences for matter at negative absolute temperature: with its help, for example, one could create heat engines such as combustion engines with an efficiency of more than 100%. However, this does not imply that the law of energy conservation has been violated. In fact, the engine could not only absorb energy from the hotter medium, but from the colder medium as well.

The colder medium inevitably heats up in contrast at purely positive temperatures, therefore absorbing a portion of the energy of the hot medium and thereby limiting the efficiency. It is possible to absorb energy from both media simultaneously if the hot medium has a negative temperature. The efficiency of the engine is greater than 100% because the work performed is greater than the energy taken from the hotter medium.

The work of this research team could possibly have implications for cosmology as well, since the thermodynamic behavior of negative temperature exhibits parallels to so-called dark energy. Astronomers postulate dark energy as the force that accelerates the expansion of the Universe, even though the cosmos should contract because of the gravitational attraction between all masses. The atomic cloud in the team's lab behaves in a similar fashion, the experiment relies upon the fact the atoms in the gas do not repel each other as in a usual gas, but instead interact attractively. The atoms exert a negative pressure instead of a positive one, therefore, the atom cloud wants to contract and should collapse — as one would expect the Universe to do because of gravity. The negative temperatures of the atom cloud prevent this from happening and like the Universe; the gas cloud is saved from collapsing.