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. Is it possible for temperatures to reach absolute zero in a finite number of steps if entropy can never reach zero?

A new paper, published in Nature Communications, aims to clarify this debate by using the principle of quantum mechanics. The researchers, from University College London, studied the unattainability principle – the impossibility to cool a system with a finite number of steps – and discovered that it’s possible to define a speed limit on cooling that prevents absolute zero from being achieved.

“We show that you can’t actually cool a system to absolute zero with a finite amount of resources, and we went a step further,” Dr Lluis Masanes, one of the two authors, told IFLScience. “We then conclude 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 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 these terms, 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 in the era 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.