Absolute zero, or 0 K, is an interesting concept in science as it represents the lowest possible energy of a system. For a system to reach this state, all movement within the system must stop, which is extremely difficult, and may be impossible to achieve. Researchers at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics however have successfully created a gas with a negative absolute temperature; a temperature below absolute zero.
While the name 'absolute zero' seems to preclude the possibility of negative absolute temperatures, the actual definition of temperature does allow it. Temperature and the Kelvin scale measure the disorder of a system, and typically adding energy to a system increases its disorder. What the researchers have done is created a gas of atoms in which adding energy actually decreases disorder, and such behavior means the system must exist on the opposite side of 0 K. To create this gas the researchers forced it to have an upper total energy limit with lasers, and then maxed-out its kinetic energy. This means that any additional energy will result in an increase of potential energy, which on a graph would appear as the particles coming together at the top of a hill.
Negative absolute energy has some interesting consequences to it, including an attractive interaction between the gas particles. Typical gas particles are repulsed by each other. Another consequence could enable combustion engines with efficiencies above 100%. This is because it the engine would absorb energy from the hotter and colder mediums involved.