Not every scoop of ice cream can be described as “deadly.” But a batch of ice cream Erasto Mpemba, made as a teenager in Tanzania in 1963, made waves in physics that can still be felt nearly 60 years later. That’s because it seems to prove a strange and counterintuitive idea: that hot liquids may freeze faster than cold ones.
Homemade ice cream was a popular snack when he was a student at Magamba Middle School, Mpemba wrote in a 1969 journal article. In the refrigerator, after it first cooled to near room temperature,” he explained. But competition for the refrigerator was fierce. One afternoon, he and another boy took two different shortcuts as they scrambled for space. Mpemba classmates mixed the milk and sugar and poured it directly into the ice tray without boiling at all. Not to be outdone, Mpemba boiled his milk – but skipped letting it cool so he could grab the last ice tray An hour and a half later, “my plate of milk was frozen into ice cream, while his was just a sticky liquid,” Mpemba wrote.
A few years later, Mpemba asked his high school science teacher why this happened—why hot milk freezes faster than cold milk, violating Newton’s law of cooling. The teacher’s answer was: “I can only say that it is the physics of Mpemba, not the physics of the universe.” This became a joke in the class. Whenever Mpemba gets a math problem wrong, the teacher and his classmates call it “Mpemba’s Math”.
Determined to find an explanation, Mpemba repeated the experiment with hot and cold water. When physicist Dennis Osborn visited his high school, he also asked him about it. Out of curiosity, Osborne invited Mpemba to visit what is now the University of Dar es Salaam and discuss the issue further before building on the resulting published research. This article contributes to a principle that Aristotle, Rene Descartes and Sir Francis Bacon have observed over the centuries, the Mpemba effect.
Mpemba and Osborne’s claims sparked decades of controversy in the physics community as they challenged fundamental theories about the behavior of matter. Many researchers have tried to reproduce their results, with little success. In 2016, physicists Henry Burridge of Imperial College London and mathematicians Paul Lyndon of the University of Cambridge published a comprehensive review of the many studies trying to confirm the phenomenon, “regrettably” reporting that they were unable to find Any evidence of the Mpemba effect. To make matters worse, they concluded that all of these studies — including Mpemba’s original experiment — were susceptible to tiny experimental factors, such as the setting of equipment insulation or the placement of thermometers.
Cool and mess
Beginning in 2017, a new team of researchers finally turned the tables, confirming Mpemba’s observations, suggesting that the cause lies in mysterious chaos mechanics. And, it turns out, water itself may be a major obstacle to proving a larger theory. It behaves differently than most other substances, especially when it changes state between solids, liquids and gases, so scientists studying the case of the Mpemba effect wanted to remove water from the equation entirely.
In an abstract experiment aimed at zeroing out the force, physicist John Bechhoefer and his colleagues heated tiny glass beads (used to replace water molecules) with a laser and observed how fast they cooled. They found that some hot beads not only cooled faster than cold beads, but sometimes they cooled exponentially faster. “The simplicity of this study is part of its beauty,” theoretical physicist Marija Vucelja told Science News. “It’s one of these very simple setups, and it’s rich enough to show off this effect.”
Soon after, another group of physicists published a follow-up paper that proposed a more abstract framework for understanding the Mpemba effect, which involved modeling the stochastic dynamics of particles. It turns out that the key to the Mpemba mystery is a dose of chaos. In particular, a liquid that moves rapidly from hot to cold is called “unbalanced,” which means it’s a system that doesn’t follow the linear rules we (or Newton’s) might expect.
Study author Oren Raz told the journal Quanta: “We all have this naive picture that temperature should vary monotonically” (meaning we might assume that a cooling liquid would go in one direction) flow steadily without significant reversal). “You start with high temperatures, then moderate temperatures, then low temperatures.” But in an imbalanced system, “you can have weird shortcuts,” Raz said.
Various publications offer evocative metaphors to explain these shortcuts: Science News likens thermal liquid cooling under the Mpemba effect to “how hikers can pass further starting points if the starting point allows them to avoid difficult ascents.” Get to your destination faster over a mountain.” Or, Physics Today suggests it’s a bit like someone crossing a river with stepping stones, writing: “If you have the right starting energy, you can go straight from the first Jump to the third without landing on the second.” Since the hot liquid is more unbalanced than the cold liquid, it may have just the right amount of energy to jump over the stone.
Another word is kurtosis, a statistical term referring to deviation from the mean, which appears to play a significant role in Mpemba effect-related behavior.The temperature of the fluid is usually average The speed of its molecules – but every fluid will have anomalous molecules that behave very differently from others. In the case of the Mpemba effect, these outliers appear to play a huge role, physicist Antonio Lasanta, who has published multiple papers confirming the phenomenon, told Cosmos. By accounting for kurtosis in the experiments associated with this cooling and heating, “we can perform analytical calculations to understand how and when the Mpemba effect will occur,” Lasanta said. This is certainly a step towards unraveling the Mpemba mystery, although much remains to be done to figure out when and how strong the effect is.
Erasto Mpemba grew up working as a gaming officer in Tanzania’s Ministry of Natural Resources and Tourism and died around 2020, his ice cream proved right. While there’s still a lot we don’t know about the effect named after him, it seems to be “Mpemba’s physics” after all.