I just read an article on Nature.com about a study by Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany, demonstrating stable sub zero “Quantum Gas”. And when I say “sub-zero”, I mean bellow absolute zero!
In most of my science classes growing up I was taught that it is physically impossible to go below zero Kelvin, so obviously I had to do some digging.
The blog by Nature did a good job of describing the science:
Lord Kelvin defined the absolute temperature scale in the mid-1800s in such a way that nothing could be colder than absolute zero. Physicists later realized that the absolute temperature of a gas is related to the average energy of its particles. Absolute zero corresponds to the theoretical state in which particles have no energy at all, and higher temperatures correspond to higher average energies.
However, by the 1950s, physicists working with more exotic systems began to realise that this isn’t always true: Technically, you read off the temperature of a system from a graph that plots the probabilities of its particles being found with certain energies. Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature.
As I have just started educating myself on the topic I don’t want to speculate too much, but it is all very exciting. But for your reading pleasure I have found online PDFs of the original journal articles referenced in the Nature blog. I know that I always prefer to read the scientific facts rather than journalists interpretations even if they are much more difficult to understand at times 🙂
1. Negative Absolute Temperature for Motional Degrees of Freedom The original article references the article as if it was published concurrently with the blog, but it was published in November. Having been through the publishing process I’m familiar with how it feels like an article gets “published” several times as it goes through several stages of publication (peer review, digital publication, journal publication, and sometimes a separate print publication)
2. Spin gradient demagnetization cooling of ultracold atoms This is the 2011 work of Nobel laureate, Wolfgang Ketterle (though he’s the last name on the article…?) Which is referenced as early demonstration of super cooling below zero Kelvin.
3. Equilibration rates and negative absolute temperatures for ultracold atoms in optical lattices. Referenced as the article where Achim Rosch proposed the method used by Schneider, Published in 2010
4. Interacting fermionic atoms in optical lattices diuse symmetrically upwards and downwards in a gravitational potential Article Published in 2011 suggestion that materials below absolute zero could exhibit anti-gravity and other interesting properties.
I hope to read all of these articles and report back on my thoughts before too long, I’d love to hear your thoughts in the mean time.