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. If it were possible to heat the spheres to infinite temperature, there would be an equal probability of finding them at any point in the landscape, irrespective of the potential energy. The faster the spheres move, the higher their 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. The meaning of a negative absolute temperature can best be illustrated with rolling spheres in a hilly landscape, where the valleys stand for a low potential energy and the hills for a high one. “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.” A negative temperature can only be achieved with an upper limit for the energy “The inverted Boltzmann distribution is the hallmark of negative absolute temperature and this is what we have achieved,” says Ulrich Schneider. This inversion of the energy distribution means that the particles have assumed a negative absolute temperature. 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. In physics, this distribution is called the Boltzmann distribution. Low-energy states are more likely than high-energy states, i.e. Yet, the individual molecules possess different kinetic energies – from very slow to very fast. As the water heats up, the water molecules increase their kinetic energy over time and move faster and faster on average. In order to bring water to the boil, energy needs to be added. 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. 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.

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. Thus, nothing can be colder than absolute zero on the Kelvin scale. At zero kelvin (minus 273 degrees Celsius) the particles stop moving and all disorder disappears. 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. 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. On the Celsius scale minus temperatures are only surprising in summer. What is normal to most people in winter has so far been impossible in physics: a minus temperature.

Entropy at absolute 0