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Have physicists ventured below absolute zero?

By Hamish Johnston

“What is temperature?” is the sort of question that a seven year old would ask – and a physicist would struggle to answer in a simple way. That’s why a paper published today in Science about “negative temperature” seems very puzzling at first glance.

One way of looking at temperature is as a way of describing how energy is distributed among a collection of particles. Most particles will have a small amount of energy and the probability that a particle has a higher energy will drop exponentially with energy – the familiar Maxwell–Boltzmann distribution of an ideal gas. Temperature times Boltzmann’s constant is the parameter that fits the distribution to experimental data. Implicit to this distribution is that there is a minimum energy (zero) and no maximum energy.

Now, a team of physicists has used ultracold atoms to create what is essentially a mirror reflection of this familiar scene – a system with a maximum energy and no minimum energy. Furthermore, the probability that a particle in this system has an energy approaching this maximum is very high and drops off exponentially as the energy decreases.

So if you interpret this in terms of the Maxwell–Boltzmann distribution, you get a negative temperature (or perhaps a negative Boltzmann’s constant).

Ulrich Schneider and colleagues at the Max Planck Institute for Quantum Optics in Munich created this system by using an ultracold quantum gas in which the individual atoms repel each other. In this system the atoms want to move apart from each other but are trapped by laser light.

The researchers then adjust the laser light to “freeze” the atoms into a state called a Mott insulator, in which the atoms are stuck in a solid-like lattice. The interaction between atoms is then flipped to be an attractive one and the trap is switched to an “anti-trap” – the laser light tending to push the atoms apart.

The researchers then return the atoms to the gaseous state. The anti-trap provides the maximum energy, to which most of the atoms push against as they try to get closer to each other. And, hey presto, the system behaves as if it has a negative temperature.

So have Schneider and colleagues ventured below absolute zero? No, but they have done a nifty experiment!

You can read Schneider’s paper here.

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