The Surprising Physics of the Mpemba Effect
In 1963, a curious teenager from Tanzania discovered that a hot ice cream mixture freezes faster than a lukewarm one. As sometimes happens in science, the effect he discovered changed the way we understand reality.
Almost all equations and phenomena in physics, and in the natural sciences in general, are named after scientists who dedicated their lives to the in-depth study of a particular field. And yet, alongside this distinguished list of figures, including Newton and Pascal, Hawking and Einstein, Galileo and Feynman, there is also a 13-year-old boy from Tanzania. The curious student Erasto Mpemba discovered in 1963, quite by accident, a physical effect that has been named after him ever since, even though it was known to the ancient Greeks.
Mpemba noticed a surprising phenomenon during a cooking class in high school, seemingly the last place where a new science should be born. It all started when Mpemba and his classmates were asked to make ice cream from a mixture that had been boiled and then cooled to room temperature before being placed in the freezer. Mpemba did not have time to cool his mixture well, but he was also afraid that if he waited too long, there would be no room left for it in the freezer. In his distress, he rushed to put the hot mixture directly into the freezer, alongside those of his friends who had managed to cool down. And lo and behold, his hot mixture froze faster.
From School Experiment to Scientific Debate
Mpemba repeated the experiment with the same results. But when he asked the teacher for an explanation, he was met with ridicule. His classmates laughed and referred to the story as "Mpemba's physics." A few years later, the young Mpemba met the British physicist Dennis Osborne, who had spent years teaching in the developing academic system of the "Dark Continent." Osborne listened to the young man with seriousness and decided to test it in the laboratory himself. In 1969, they published their paper, titled "Cool?", which discussed the phenomenon from an observational point of view without providing a complete scientific explanation.
In a series of experiments, they placed two jars containing 70 ml of water at different temperatures into the freezer and saw that when the initial temperature was above 30 degrees Celsius, an increase in the initial temperature actually led to a shortening of the time required for the water to freeze.
Why Does This Happen?
In another paper, the Canadian physicist G.S. Kell tried to provide a basic explanation. He argued that hot water loses mass quickly due to evaporation, so less energy is needed to bring the remaining water to the freezing point. Additionally, the evaporation process removes a lot of heat. The problem is that the Mpemba effect also occurs in closed vessels, where the amount of material remains constant.
Furthermore, the phenomenon contradicts Isaac Newton's law of cooling, which links the rate of heat loss to the temperature difference between the body and the environment. According to classical physics, hot water should first cool down to the temperature of cold water and then freeze. This gap in understanding left the Mpemba effect for a long time as a nice story, behind which a great theoretical controversy lurked.
Statistical Mechanics and New Horizons
In recent years, the first cracks have appeared in the armor of skepticism, thanks to the development of non-equilibrium statistical mechanics. This field tries to understand how multi-particle systems evolve before reaching a stable state. It has become clear that the physics of the Mpemba effect is a "zoo" of effects appearing in polymers, magnetic, and even quantum systems.
In 2017, physicist Oren Raz from the Weizmann Institute of Science and his colleague Zhiwei Lu from the University of North Carolina showed that when a system is pushed far from equilibrium, it may find unique shortcuts where the slowest trajectories are not expressed. Thus, the system reaches equilibrium very quickly.
From Ice Cream to Quantum Computing
In 2023, a research team, including doctoral student Shahaf Aharoni-Shapira, found expressions of the Mpemba effect in quantum systems of ions and lasers. Moreover, they recorded an "inverse Mpemba effect," where a cold system heats up faster than one that was initially hotter.
Recently, a team of physicists led by John Goold from Trinity College Dublin formulated a single theoretical framework for the effect. They showed that systems using a larger amount of a physical resource to reach a destination are capable of consuming it at a very fast rate. Having created a new paradigm, the physics of the Mpemba effect is now moving to the next stage, where researchers will continue to decipher the remaining secrets of this surprising natural phenomenon.
