It sounds like a contradiction in terms but scientists have reached temperatures that go beyond absolute zero in a lab, and get hotter as they do so.
When they cooled an atomic gas to extreme lows, known as ‘ultracooling’, physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Germany created a gas that went beyond absolute zero.
They found that the atoms in the ultracooled gas attract each other and give rise to a negative pressure. Instead of standing still when they go beyond 0 K, the gas becomes hotter.
"The gas is not colder than zero kelvin, but hotter," says physicist Ulrich Schneider, lead author on the paper that is published in the journal Science.
"It is even hotter than at any positive temperature."
This strange behavior has everything to do with how energy is spread throughout the atoms in a gas known as the ‘Boltzmann distribution’. A gas at any temperature will have different amounts of energy spread amongst its atoms. In a gas that is cooled, the majority of the particles will have low energy states although a few will have higher energy states.
When the Kelvin temperatures become negative in the ultracooled gas, the distributions of energy is the opposite way round so that most of the particles have very high energy states while very few have low ones. In this case, the Boltzmann distribution is said to be ‘inverted’ so that the normal state of affairs is reversed.
“The inverted Boltzmann distribution is the hallmark of negative absolute temperature; and this is what we have achieved,” says Schneider.
Their finding suggests that the previously impossible idea of a combustion engine that is 100 percent efficient may actually be achievable. Their finding offers a tantalizing insight into how 'dark energy', the elusive force that cosmologists believe is responsible for the expansion of the universe, might work.
As the Universe should be contracting under the force of gravity, rather than expanding as measurements suggest, the authors believe that dark energy could cause the expansion of the Universe by behaving in the opposite way to what is expected from the force of gravity. In the same way that the gas particles attract each other at negative temperatures rather than being repelled, dark energy may cause the expansion of the Universe by acting as a sort of negative gravity.