Why is the degree of zero infinite

Colder is hotter

"If we imagine a normal gas - the particles around us, the air moving around us - then it is so that temperature is linked to the movement of the particles. The faster the particles move, the hotter they are And then it is very clear: there is a lower limit. If the particles stand still, if they all stand still, then we have zero Kelvin. That is the absolute zero point and obviously nothing can be colder than that. Because less than even the particles cannot move. "

Ulrich Schneider's team achieved something in the laboratory that initially sounds like a physical impossibility: The physicists at the University of Munich and the Max Planck Institute for Quantum Optics were able to bring gaseous potassium atoms to a state of minus one billionth of a Kelvin. But that does not mean that the particles are actually in a colder state than absolute zero, ...

"... but what it actually means is: we are actually hotter than infinite temperature. If you imagine the normal, real numbers, you know them: they run from minus infinity through zero to plus infinity. And for temperature that's actually wrong. At the temperature you actually have to cut them in the middle at zero and stick them together the other way round. "

At plus infinitely high temperature, the Kelvin scale jumps by definition to minus infinity and thus to negative values. Ulrich Schneider likes to explain this jump using balls or particles in a landscape of energy mountains and energy valleys. When the temperature is positive, the balls spread out in the landscape, mainly in valleys. At an infinitely high temperature, they would be equally likely to be found anywhere in the landscape. If it were possible to continue supplying energy, the balls would mainly collect on the mountains of energy. This is the point at which negative values ​​are reached on the scale.

Since a system cannot absorb an infinite amount of energy, Schneider and his team had to use a trick to make the sentence negative: They cooled the potassium atoms down to a billionth of a Kelvin and captured them in energy valleys using a laser grid.

"And there they really are at the bottom of this valley - and what we are doing now is: We are now suddenly switching this valley into a mountain. That means we are really deforming the landscape, we are making a mountain and the particles out of the valley Those who were just sitting down in the valley are now on the top of the mountain. We turn the energy of the particles over it and that is why the particles are suddenly sitting at the high energy.

And that means at minus one billionth of a Kelvin. In contrast to an infinitely positive temperature, the particles here are not distributed over the entire landscape, but are highly organized on the mountains of energy.

The importance of this result can be shown particularly well in a heat engine - such as an internal combustion engine - with a cold and a warm reservoir. At plus Kelvin degrees, energy and disorder can only be drawn from the warm medium. At a negative temperature, due to the ordered structure, not only could energy be given off, but disorder could also be absorbed at the same time. This means that energy could be obtained from both the warm and the cold reservoir. The efficiency of such a machine would be over 100 percent.

"It's nothing where we'd say it can be built anytime soon. It is even questionable whether this can ever be built or whether there is simply no naturally occurring matter that can assume negative temperatures. It is important, it is a thought experiment and it shows us some new properties of thermodynamics. "

Two years ago theoretical physicists from Cologne predicted that and how one could raise ultracold atoms in an optical lattice to the negative range of the Kelvin scale. Ulrich Schneider and his colleagues have now provided experimental proof of this.

"So the result in the retrospective is not surprising at all, but exactly what we expected."