Ultracold gas sets record on the kelvin scale

By Andrew Grant

Web edition: January 4, 2013
Print edition: February 9, 2013; Vol.183 #3 (p. 10)

Coaxing a gas to a negative temperature on the kelvin scale has produced, paradoxically, the hottest temperature ever measured. The study, published in the Jan. 4 Science, will help physicists learn about quantum phenomena and perhaps even the strange form of energy that dominates the universe.

A negative kelvin temperature indicates that particles at high energies outnumber those at low energies.

“We are used to positive temperatures,” says Achim Rosch, a physicist at the University of Cologne in Germany who was not involved in the research. “But there’s nothing forbidden about negative temperatures. It’s always fascinating to do something unusual.”

Temperature is commonly interpreted as a measure of the average energy of the particles in a sample. Each of the molecules buzzing around in a pot of boiling water, for example, has more energy on average than a sluggish water molecule within an ice cube.

But for scientists who study matter at quantum scales, temperature is better defined as the energy distribution of the particles in a sample. Just above absolute zero (0 kelvin, or -273° Celsius), almost all of the particles within a sample have energies very close to zero, with little variation. But as temperatures rise, the variation in energies widens — some particles still have very small energies, but others have more.

Physicist Ulrich Schneider at the Ludwig Maximilians University of Munich set out to do something unusual: He wanted to cajole the particles within a substance to be confined to a very high amount of energy. In other words, instead of having the particles start at a minimum energy (corresponding to absolute zero) and spreading out toward higher energies, he wanted to start at a maximum energy and spread toward lower energies. By definition, such a substance would have a negative kelvin temperature.

His team achieved that with potassium atoms chilled to a few billionths kelvin above absolute zero. Through the use of lasers and magnets, the team managed to get the atoms to jump to a high-energy state. By creating a cluster of particles exclusively at high energies, Schneider and his colleagues had a gas at a few billionths negative kelvin.

This temperature is technically not below absolute zero, because negative on the kelvin scale (unlike that on the Fahrenheit or Celsius scale) is a construct that simply indicates something about the energy state of the particles involved. In fact, the new creation is extremely hot because of the high energies of the particles. Heat travels from hot to cold, Schneider says, and heat will always flow away from this gas. “It’s actually hotter than everything we know,” he says.

Despite the semantics involved, this experiment isn’t merely a fun physics trick. Scientists are fascinated by negative-temperature substances because they have other strange properties. The molecules in a typical gas spread out and exert a force on the walls of their container. But a negative-temperature gas also has negative pressure, meaning the particles tend to cave in rather than expand. “It wants to collapse into a single point,” Schneider says.

Negative pressure may be important in another part of the physics universe: Cosmologists believe that dark energy, the mysterious entity that is causing the universe to expand at an accelerating rate, also has negative pressure. Schneider suggests that experimenting with the quantum phenomenon of negative temperature could reveal the nature of dark energy throughout the cosmos.