Some scientist aspire to belong to the ‘Impossible Things Before Breakfast’ (ITBB) Club. In my view, modern physics presents a pretty coherent and startlingly understandable picture of the world. For ITBB club members this is anathema! For these iconoclasts, physics must always be paradoxical, requiring radical re-evaluation of everything we thought we ever knew. The latest applicants for ITBB club membership claim they have produced “negative temperatures”. This is nonsense.
- Science Magazine Article Here (requires membership)
- Live Science Report Here
- Nature News Article Here
The temperature of a substance is a remarkable property. It is a single number that describes the distribution of energies of particles within the substance on average. In a gas, the energy of the particles is held as kinetic energy of the molecules and so the temperature describes the distribution of molecular speeds. It describes this distribution in equilibrium. Those last two words mean something.
Even if you set up all the molecules initially with exactly the same speed, or any other distribution of speeds you care to create, once they have interacted with each other a few times – (crudely the interactions are described as “collisions”), their distribution of speeds will approximate a universal distribution, first predicted by James Clerk-Maxwell. The shape of the distribution is described by one single parameter – the temperature. And here is the amazing part – no matter how long the molecules continue to interact, this distribution of molecular speeds does not change. Wow! So temperature describes the distribution of molecular speeds in equilibrium.
If one could somehow take each molecule and instantaneously slow down the fast molecules and speed up the slow ones, then one could transiently describe the distribution of molecular speeds by a curve with a negative temperature. This is what the researchers have done – and its a great trick for which I congratulate them. However, they have not produced a gas with a negative temperature. As they themselves say:
Ulrich Schneider, a physicist at the University of Munich in Germany said “Yet the gas is not colder than zero kelvin, but hotter. It is even hotter than at any positive temperature — the temperature scale simply does not end at infinity, but jumps to negative values instead.”
I agree with Ulrich that the gas is not colder than absolute zero, but he is wrong when he says that the gas is hotter than any positive temperature. What he and his colleagues have produced is a gas which is not well described by any single number called temperature, because it is not in equilibrium
As soon as the molecules begin to interact, the distribution will tend towards the equilibrium distribution which would be described by a positive temperature – slightly higher than they started with. What happened was that the molecules were not in equilibrium with each other and that’s all. Nothing paradoxical, and nothing requiring radical re-assessment of thermodynamics. All that exists is a wanna-be member of the ITBB club seeking publicity and a flattering appearance on Horizon.
This concept is not even new. Non-equilbrium distributions of particle energies are routinely described with a negative temperature. And such distributions are created within the body of any material which supports laser action – so they exist inside every laser pointer. The key thing about these distributions are that they are not in equilibrium. They are not in any meaningful sense colder than absolute zero.
The work is very clever but it will not:
“…lead to new engines that could technically be more than 100 percent efficient, and shed light on mysteries such as dark energy, the mysterious substance that is apparently pulling our universe apart.”
Friends, we live in a beautiful and amazing world. And even more amazingly many aspects of it make some sort of sense – who would have expected that? While I sympathise with these researchers, who after months in their darkrooms emerge seeking the bright lights of fame and funding, it really doesn’t help anybody to make these nonsensical claims.
Move along now… nothing to see here…