Absolute zero no longer absolute; next up, dividing by zero?

Not quite absolute zero, but still rather chilly.

Welcome to 2013: We broke physics.

A team of physicists at the Ludwig Maximilian University in Munich, Germany , fixed a quantum gas of potassium atoms in a standard lattice configuration using lasers and magnetic fields. A quick adjustment of the magnetic fields forced the atoms to shift to their highest possible energy state, while remaining stationary in the grip of the laser trapping field—and that energy transition caused the temperature of the gas to drop “a few billionths of a Kelvin below absolute zero”.

Absolute zero, designated as such in 1848 by Lord Kelvin, was defined as the point at which there is no longer any kinetic energy in a system. However, that doesn’t mean there isn’t heat energy. Forcing a system to use that heat energy to flip into higher energy states would also bring the thermodynamic temperature below 0 K.

While a collection of ultra-cold atoms in a laser field doesn’t sound like it could have too many practical applications (although it’s certainly awesome), Wolfgang Ketterle, a Nobel laureate physicist at MIT, lays it out for us in plainer language: High-energy states, difficult to generate at positive temperatures, become stable at negative temperatures. When those states are stable, it’s easier to mess with them and see what they do. He compares it to “stand[ing] a pyramid on its head” without having to worry about it falling over.

An inverted-pyramid-like system, stabilized by negative temperature, will behave quite strangely, as predicted by theoretical physicist Achim Rosch from the University of Cologne, Germany. Some atoms in a negative-temperature cloud will move against, rather than with, the pull of gravity, as Ulrich Schneider—part of the Ludwig Maximilian team—points out, similarly to the way that dark energy is presumed to behave.

Just published in Science on Friday, the negative-absolute-temperature breakthrough could have wide-ranging effects in physics and cosmology, as well as confusing thermodynamics students all over the world.

[Nature via The Verge and physicsphysics.tumblr.com / Photo: Liz West/Flickr (CC BY 2.0)]

Update: Physicists have known since the mid-20th century that sub-absolute-zero temperatures were physically possible, as confirmed by a 1950 experiment by Edward M. Purcell and Robert V. Pound at Harvard in which atoms of a lithium fluoride crystal were forced into a nuclear spin state that brought their temperature below 0 K. Schneider’s team, however, is the first to demonstrate sub-0K temperatures in a gas!

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