Temperature as a game of marbles: The Boltzmann distribution states how many particles have which energy, and can be illustrated with the aid of spheres distributed in a hilly landscape. At positive temperatures (left image), most spheres lie in the valley at minimum potential energy and barely move; they therefore also possess minimum kinetic energy. States with low total energy are therefore more likely than those with high total energy – the usual Boltzmann distribution. At infinite temperature (centre image) the spheres are spread evenly over low and high energies in an identical landscape. Here, all energy states are equally probable. At negative temperatures (right image), however, most spheres move on top of the hill, at the upper limit of the potential energy. Their kinetic energy is also maximum. Energy states with high total energy thus occur more frequently than those with low total energy – the Boltzmann distribution is inverted (Photo : LMU and MPG Munich)
How cold can we go? While most would say absolute zero, some scientists from the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Garching will tell you that it's colder. The scientists in Germany were able to create an atomic gas that exhibits properties only possible below absolute zero.
Absolute zero refers to zero on the Kelvin scale, and is equivalent to -273 degrees Celsius. Temperatures in the Kelvin scale are measured by the amount of movement the particles in the medium exhibit, and at absolute zero, particles stop moving and all disorder is lost. The gas created by scientists, however, has some strange properties only possible through a negative absolute temperature.
Typically, when kinetic energy is added to particles, there will be more of them with low energy than with high energy - this is known as the Boltzmann distribution. This allocation was actually reversed in the scientists' atomic gas, which ended up having more high-energy particles than low-energy particles.
"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."
The implications are enormous. The "infinite" nature of negative absolute temperature systems theoretically means that heat engines with a thermodynamic efficiency of over 100 percent can be achieved. Harnessing this kind of power is far off, but the findings offer insight into another one of the universe's mysteries - dark energy.
Dark energy is believed to be the omnipotent force causing our universe to expand at a faster and faster rate. The behavior of particles in a negative absolute temperature system may help explain how such a phenomena exists when general theory holds that the masses in the universe should be attracting each other rather than repelling each other.
Read the full study published in Science.