NOTHING is colder than absolute zero, but there's a whole world beyond that point. For the first time, atoms have been put in a state where they can take on a range of negative absolute temperatures. This could deepen our understanding of subatomic particles - and perhaps even mysterious dark energy.
Although we're used to talking about negative temperatures, such as -10 °C, all temperatures on an ordinary thermometer are positive when measured in kelvin. On this scientific scale, which starts at absolute zero (-273.15 °C), temperature is determined by the kinetic energy of particles. So a gas of slow particles is colder than a gas of fast-moving ones, and absolute zero corresponds to the point at which particles stop moving completely, which is why nothing can be colder.
That does not tell the whole story, though. Temperature also depends on how the particle energies are distributed within a sample, which sets their entropy, a thermodynamic property.
At absolute zero, all particles have minimum energy, and zero entropy. As they heat up, some particles gain lots of energy but most just a small amount. This increase in the energy spread means higher entropy - but only until the particles are evenly distributed between all possible energies, corresponding to maximum entropy (see "diagram").
This is the maximum possible positive temperature, but you can still increase the particles' energy. However, from now on, there are more particles with higher energies than lower energy, so entropy will go down with further energy hikes, a sign that the particles have flipped into the realm of negative temperature, which, bizarrely, is hotter than its positive counterpart.
"The temperature scale as we know it starts at zero and goes up to infinity, but it doesn't stop there," says Ulrich Schneider of the Ludwig Maximilian University of Munich, Germany. The resulting thermometer is mind-bending. It starts at zero, ramps up to infinity, then jumps to the negative side of infinity before increasing through the negative scale to zero.
Previously researchers entered the negative realm by flipping a magnetic field so that an array of mostly aligned atomic nuclei become mostly misaligned. But this only offered two discrete energy states - one negative, one positive. To create a system with a range of negative temperatures, Schneider and his colleagues used moving atoms, following a recipe first proposed in 2005.
They began by cooling atoms to a fraction above absolute zero and placing them in a vacuum. Lasers then placed the atoms along the curve of an energy "valley", with the majority of the atoms in lower energy states. Next, the researchers turned this positive system negative by adjusting the lasers to change the atoms' energy levels so that the majority of the particles were high energy. The atoms can't lose energy and "roll down" the hill because doing so would mean first increasing their kinetic energy, which is not possible because they are in a vacuum and there is no outside energy source. So this flipped the valley into an energy hill, producing the inverse energy distribution that characterises negative temperatures (Science, doi.org/j55). What's more, unlike in the magnetic system, lasers can set the hill at different heights, giving a range of negative temperatures.
Cold atoms are already used to simulate the interactions of subatomic particles, including quarks. The new set-up could create simulations not possible with positive temperatures.
Negative temperature may also have implications for cosmology. Dark energy, thought to explain the accelerating expansion of the universe, exerts negative pressure, which suggests it might have negative temperature.
"It is amazing experimental work," says Allard Mosk of the University of Twente in the Netherlands, who helped outline the 2005 theory that drove the work. "It will give us a deeper understanding about what temperature really is."
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