Scientists have made a startling achievement long thought to be impossible: a negative absolute temperature.
Using a reversed magnetic field on a laser-stabilized lattice of super-cooled potassium atoms, physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Germany registered a temperature a “few billionths of a Kelvin” below absolute-zero.
Temperature is determined by the movement of atoms within a given material. The colder an object is, the slower its atoms are moving. At absolute zero degrees Kelvin (-459.67 degrees Fahrenheit or -273.15 degrees Celsius), atoms would stop moving altogether, making such a state physically impossible to reach.
The team in Munich cleverly leap-frogged this barrier by cooling about 100,000 atoms of quantum potassium gas inside a vacuum to a few nanokelvin above absolute zero, then reversing the magnetic field surrounding it.
Tweaking the field “suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” said physicist Ulrich Schneider, one of the project leads. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”
At normal (positive) temperatures atoms tend to occupy low energy states, while at an infinite temperature they would be equally likely to occupy all energy states. At the newly achieved negative temperatures atoms are more likely to occupy high-energy states– potentially opening the doors for new types of matter.
Somewhat counterintuitively, another way to look at these negative temperatures is to consider them hotter than infinity. “The gas is not colder than zero kelvin, but hotter. It is even hotter than at any positive temperature — the temperature scale simply does not end at infinity, but jumps to negative values instead,” explained Schneider.
In an exclusive interview with the Science Recorder, Mr. Schneider outlined some of the potential theoretical implications of his group’s discovery.
“This is so far completely unknown and remains to be seen,” Schneider noted. “We could show that our system is stable even though the atoms strongly attract each other– that means they want to collapse but can’t due to the negative temperature.”
“The universe as a whole is also not collapsing under the attractive effects of gravity but its expansion is accelerating,” Schneider continued. “And both our atoms as well as the dark energy are described by a negative pressure. But whether this is just a coincidence or not remains to be seen.”
Concerning the practical applications of his group’s discovery to a hypothetical combustion engine with over 100% efficiency, Schneider soberingly explained, “we do not expect any ‘real’ materials or machines based on this principle.”
“Negative temperatures require the existence of an upper limit for the energy per particle. This limit is not an external limit in the sense that there is no more energy available, but it is an internal limit – the particles cannot absorb more energy even if there is plenty available.”
“This requirement severely limits the applicability of the idea,” Schneider added. “In fact, the idea of negative absolute temperature is old, but was seldom discussed for mobile particles because everybody assumed that it was impossible to realize.”
Mr. Schneider then described the group’s next steps for expanding on their groundbreaking discovery. “We will continue to explore the regime of negative temperatures and use it to enhance and refine our quantum simulations. And as more theorists start to think about the new possibilities, certainly interesting applications will pop up.”
From the development of new types of matter to explanations for the Universe’s expansion, the potential applications of this discovery will certainly be worth keeping an eye on.