Atoms at negative absolute temperature could be used to create an engine with more than 100% efficiency.

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 (-460°F or -273°C) the particles stop moving and all disorder disappears. Thus, nothing can be colder than absolute zero on the Kelvin scale.

Physicists of the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Garching have now created an atomic gas that has negative Kelvin temperature.

These negative absolute temperatures lead to several striking consequences: Although the atoms in the gas attract each other and give rise to a negative pressure, the gas does not collapse – a behavior that is also postulated for dark energy in cosmology.

Supposedly impossible heat engines can be realized with the help of negative absolute temperatures, such as an engine with a thermodynamic efficiency above 100%.

The cold gas is actually hotter than at any positive temperature – the temperature scale simply does not end at infinity, but jumps to negative values instead.

At a negative absolute temperature the energy distribution of particles inverts in comparison to a positive temperature. Many particles then have a high energy and few a low one.

This corresponds to a temperature which is hotter than one that is infinitely high, where the particles are distributed equally over all energies.

Matter at negative absolute temperature leads to the possibility of a heat engine with an efficiency above 100%. The machine would absorb energy from the hotter substance and the colder. The work performed by such an engine could therefore be larger than the energy taken from the hotter substance alone.

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