Heating something up will always be quicker than cooling it down on a microscopic scale, according to a proposed new principle of thermodynamics. The two processes, long thought of as two sides of the same coin by physicists, seem actually to be fundamentally different.
While most people have an intuitive understanding of what temperature is, physicists have argued over a precise definition for centuries. A school textbook might say it is a measure of how much atoms jiggle around in a system. But thermodynamics, the study of the relationship between heat and other forms of energy, describes temperature as a measure of how many different arrangements of values, such as speed or energy, all the atoms in a system can have. These arrangements are called microstates.
Based on this understanding, conventional thermodynamics says that heating and cooling are essentially mirror images of each other. However, this theory assumes that temperature changes happen either slowly or over small increments.
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When systems heat up or cool down over very large intervals, the physics is less well understood 鈥� and the outcomes can be counterintuitive. For example, hot water freezes faster than cold water, a phenomenon called the Mpemba effect.
Now, at the Max Planck Institute for Multidisciplinary Sciences in G枚ttingen, Germany, and his colleagues have found that a microscopic sphere of silica that is rapidly heated or cooled by an electric field appears to do so in a lopsided way, heating up faster than it cools.
鈥淭his is very surprising,鈥� says Godec. 鈥淪o far, we know that this is true because we have shown it, but I don鈥檛 think we can claim that we understand why this is the case.鈥�
Godec and his team placed the tiny sphere in water and trapped it in place using a laser. They then applied an electric field to heat or cool it and measured how much the particle jiggled and moved. They repeated this process tens of thousands of times.
Measuring a single particle in this way is equivalent to measuring a single microstate. This is impossible to do for a material consisting of many particles because of the vast number of possible configurations they can take. But by making many measurements for a single microscopic particle, the team was able to map out the possible number of microstates it can take.
The researchers then measured how many different microstates the particle would have to go through when transitioning between two temperatures by heating or cooling. They found it had to travel through fewer possible microstates when heating up as opposed to when cooling down, which translated to a faster heating speed.
While it isn鈥檛 clear why there should be this fundamental difference, it should be present in any system that heats or cools by a sufficiently large amount, says Godec, though it would usually be difficult to see. This is because such large temperature changes normally induce phenomena in the system itself, such as freezing or boiling, that obscure this newly observed effect. Despite this, the asymmetry could be important for improving the efficiency of microscopic systems such as tiny heat engines or motors, he says.
鈥淚t鈥檚 really interesting work,鈥� says at the University of Exeter, UK. 鈥淚t鈥檚 really important to think about what this could explain in nature.鈥�
The effect that Godec and his team have discovered could almost be considered an extra law of thermodynamics, says Anders. It expands upon the second law of thermodynamics, which says that hot things always cool unless you do something to stop them. 鈥淭he second law doesn鈥檛 say anything about speed; it says something about possibility,鈥� she says. 鈥淭his second-and-a-half law, as I鈥檓 calling it, says you can do all these things, but some of them will take a lot longer than the inverse.鈥�
Nature Physics
