# Third Law Of Thermodynamics Proven By Finding The Maximum Speed Of Cooling

The third law of thermodynamics is responsible for the definition of absolute zero, the lowest possible temperature that can be obtained. While the concept is well-known, an intense debate regarding it has been taking place in academic papers where famous scientists debated ideas, proofs, and the consequences of such a law.

A new paper, published in Nature Communication, aims to clarify this debate by proving one of the famous interpretations of the third law, by using the principle of quantum mechanics. The researchers, from University College London, studied the impossibility to cool a system to absolute zero with a finite number of steps, what’s known as the unattainability principle, and discovered that it is possible to define a speed limit by how quickly a system is cooled down.

“You can’t actually cool a system to absolute zero with a finite amount of resources and we went a step further,” lead author Dr Lluis Masanes told IFLScience. “We say that it is impossible to cool a system to absolute zero in a finite time and we established a relation between time and the lowest possible temperature. It’s the speed of cooling.”

The speed of cooling is not universal (like the speed of light) but depends on the speed of sound in the environment and how quickly energy can be injected in it.

The solution comes from the world of quantum information. The main insight from this research is that a cooling process can be seen as a computation. A cooler system has lower energy and it can arrange itself into fewer states. So in a system with a lot of energy, particles can be organized in many different configurations. In a way, there’s a lot of ignorance since you can't be certain what the state is of these particles. At absolute zero, one knows exactly what the system looks like.

“Thinking in this term, the task of cooling is an information problem and our main insight was to understand the complexity of the task,” Masanes added.

This information theory is closely linked to the second law of thermodynamics, where quantum information has already been successfully used to prove a variety of versions. But the third law wasn’t as straightforward.

This derivation of the third law might have some technological applications, but the researchers stress that its theoretical value is currently a lot more important.

“We derived a speed limit for cooling and it is very, very fast, while we are currently at the state of horses and carts,” co-author Professor Jonathan Oppenheim told IFLScience. “Technology is currently not even close to getting near the speed limit. Although, it does give a framework for particular cooling machines.”

The research is in a way similar to the discovery of the speed of light. Knowing that there’s a limit is important even if we are nowhere near reaching it.