Heating and cooling are not opposites, but they are fundamentally different things

According to thermodynamics, heating and cooling are opposites, but the small silica ball experiment now suggests something different.

If you look on a microscopic scale, heating something will always be faster than cooling it down. So says a new rule in thermodynamics. The two processes, which physicists have long believed to be two sides of the same coin, appear to be fundamentally different.

Difficult definition

Although most people have an intuitive idea of ​​the concept of temperature, physicists have been puzzling over a specific definition of temperature for centuries. Textbooks often state that temperature is a measure of the amount of movement of atoms in a system.

But thermodynamics, or heat theory, describes temperature differently. This looks at the different values ​​of a quantity, such as speed or energy, that all atoms in a system can have – and all possible variants of them. The potential arrangements are also called “mini-states” of the system.

Accordingly, thermodynamics states that heating and cooling are mirror images of each other. But there is a caveat to this: The theory assumes that temperature changes occur slowly, or in small steps.

When something heats or cools in large steps, its physics are not well understood. So what you see happening may be counterintuitive. For example, warm water freezes faster than cold water. This phenomenon is called the Mbemba effect.

It heats up faster than cold

Now we have physicists Jazz Judic from the Max Planck Institute for Interdisciplinary Science in Göttingen, Germany, and his colleagues have discovered another strange effect. It caused a microscopic ball of silicon dioxide to rapidly heat or cool. They discovered that these processes do not occur symmetrically: the ball heats up faster than it cools. The team published the discovery in the scientific journal Nature physics.

“This is very surprising,” Godek says. “We know this is true because we showed it. But we cannot claim to understand why it happened.”

Godek and his team placed the small ball in the water. A laser beam on the ball holds it in place. The researchers then turned on an electric field that heated or cooled the ball. They measured how much the particle oscillated and moved. They repeated this process tens of thousands of times.

Small countries

Measuring a single particle in this way is equivalent to measuring a single microstate. This is impossible to achieve with materials consisting of many molecules, because the number of possible arrangements in which the atoms can be is very large. But by making many measurements on a single particle, the team was able to precisely map the possible number of microstates.

The physicists then measured how many different microstates a particle must go through when transitioning between two temperatures. They determined that when the ball heats up and when it cools down. They discovered that the particle goes through fewer precise states when it is heated than it does when it cools. This translates to a faster heating rate.

new law

It is not clear why this difference exists. In principle, it should be present in any system that gets hot or cold enough, Godek says, although in practice it would usually be difficult to see. This is because large changes in temperature usually cause changes in the system itself, such as freezing or boiling. This will hide this hidden effect.

However, Godek says this asymmetry may be important for improving the efficiency of microscopic systems, such as small heat engines.

“It's really interesting work,” says the physicist. Janet Anders From the University of Exeter, United Kingdom. “It's really important to think about what could explain this in nature.”

Anders says the effect discovered by Godek's team can be considered an additional law of thermodynamics. It's an extension of the second law of thermodynamics, which states that hot things always cool unless you do something to stop them. “The second law says nothing about speed, but something about probability,” she says. “This two-and-a-half rule, as I like to call it, says that all of this is possible, with some exceptions [processen] It takes much longer than reversing it.