At negative temperatures an engine can do more work

A temperature below absolute zero: Atoms at negative absolute temperature are the hottest systems in the world

Summary: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.

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.

?The inverted Boltzmann distribution is the hallmark of negative absolute temperature; and this is what we have achieved,? 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.?

A negative temperature can only be achieved with an upper limit for the energy

The meaning of a negative absolute temperature can best be illustrated with rolling spheres in a hilly landscape. 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 it were possible to heat the spheres to infinite temperature, , irrespective of the potential energy. If one could now add even more energy and thereby heat the spheres even further, they would preferably gather at high-energy states and would be even hotter than at infinite temperature. The Boltzmann distribution would be inverted, and the temperature therefore negative.

At negative temperatures an engine can do more work

?If the spheres are at a negative temperature, however, their kinetic energy will already be so large that it cannot increase further,?. ?The spheres thus cannot roll down, and they stay on top of the hill. The energy limit therefore renders the system stable!? The negative temperature state in their experiment is indeed just as stable as a positive temperature state. ?We have thus created the first negative absolute temperature state for moving particles,? adds Braun.

Matter at negative absolute temperature has a whole range of astounding consequences: with its help, one could create heat engines with an efficiency of more than 100%.

The achievement could additionally be interesting for cosmology, since the thermodynamic behaviour of negative temperature exhibits parallels to so-called dark energy. Cosmologists postulate dark energy as the elusive force that accelerates the expansion of the universe, although the cosmos should in fact contract because of the gravitational attraction between all masses.

Date: 2016-06-12; view: 95