Negative temperature: Understanding what happens below absolute zero

By John Hewitt on January 8, 2013 at 3:55 pm

Temperature is typically thought of as the average energy of individual atoms or molecules within a given collection. For atoms of similar mass, this “kinetic temperature” would basically be their speed at equilibrium. For a group of molecules, we have just a little extra accounting to do. Their total energy is also partitioned into the relative motions of their constituent atoms oscillating about their bonds, typically either bending or translating motions.

These familiar ideas of temperature work pretty well for most solids, liquids, and gases, and conform to the general expectation that it should always be greater than absolute zero. What are we to make of a recent claim by a group of German researchers that they have created an experimental system where negative (as in below absolute zero) temperatures can actually be observed and measured?

Despite the near universal desire to find the other-worldly in the everyday, there is unfortunately no real new bizarro with the idea of negative temperature. Negative temperatures were first created back in 1951 by Ed Purcell, who won the Nobel Prize the next year. Among other related pursuits, he had previously been the first person to observe nuclear magnetic resonance (NMR) — the heart of the modern MRI scanner — which uses a large magnetic field to polarize nuclear spins. In fact the negative temperature systems Purcell created were nuclear spins in a crystal of lithium fluoride that was itself at room temperature. The novelty of the negative temperature system created by the German group is that instead of nuclear spins, they used ultracold atoms. They describe their system as having “motional degrees of freedom,” in contrast to nuclear spins which do not move in any conventional sense.

So what is negative temperature, then?

Read more: Negative temperature: Understanding what happens below absolute zero | ExtremeTech.

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