The ice that changed physics | NOVA
Not every scoop of ice cream can be called “fatal”. But a batch of ice cream that Erasto Mpemba made as a teenager in Tanzania in 1963 made physical waves that are still being felt nearly 60 years later. Indeed, it seemed to be evidence of a strange and counter-intuitive idea: that a hot liquid can freeze faster than a cold liquid.
Homemade ice cream was a popular snack when he was a student at Magamba Secondary School, Mpemba wrote in a newspaper article published in 1969. mixing it with sugar and putting it in the freezing chamber in the refrigerator, after it had first cooled almost to room temperature,” he explained. But the competition for the freezer was intense. afternoon, he and another boy took two different shortcuts as they searched for space. Mpemba’s classmate mixed his milk with sugar and poured it straight into an ice cube tray without the boil at all. Not to be outdone, Mpemba boiled her milk, but skipped the step of letting it cool so she could grab the last ice cube tray. An hour and a half later, “my milk tray turned to ice when his was still a thick liquid,” Mpemba wrote.
A few years later, Mpemba asked his high school science teacher why this might be – why hot milk would freeze faster than cold milk, going against Newton’s law of cooling. The teacher’s response was: “All I can say is that this is Mpemba physics and not universal physics. The incident became a running joke in the class. Whenever Mpemba got a math problem wrong, the teacher and classmates called it “Mpemba’s math.”
Determined to find an explanation, Mpemba repeated the experiment with hot and cold water. And when physicist Denis Osborne visited his high school, he also asked him about the incident. Intrigued, Osborne invited Mpemba to visit what is now the University of Dar es Salaam and discuss the matter further, then set up the related research which was eventually published. The article helped a principle that Aristotle, René Descartes and Sir Francis Bacon had all observed over the centuries to become known as the Mpemba effect.
Mpemba and Osborne’s claims created decades of controversy in the world of physics because they challenged fundamental theories about the behavior of matter. Many researchers have attempted to recreate their results, with limited success. In 2016, physicist Henry Burridge of Imperial College London and mathematician Paul Linden of Cambridge University published an extensive review of numerous studies that had attempted to confirm the phenomenon, “unfortunately” reporting that they did not had found no evidence of an Mpemba effect. Worse, they concluded, all of these studies, including Mpemba’s original experiment, could have been easily skewed by minute experimental factors such as the configuration of equipment insulation or the placement of thermometers.
Chill and chaos
As of 2017, a new contingent of studies have finally turned the corner to confirm Mpemba’s sighting, suggesting the explanation lies in the mysterious mechanics of chaos. And, it turns out, the water itself may have been a major hurdle in proving the larger theory. It behaves differently than most other substances, especially since it changes state between solid, liquid and gaseous. The scientists of the Mpemba Effect case therefore sought to remove water from the equation altogether.
In an abstract experiment intended to focus on the forces at play, physicist John Bechhoefer and his colleagues heated microscopic glass beads (intended to replace water molecules) with lasers and examined the rate of cooling. They found that not only did some hot marbles cool faster than their cold counterparts, they sometimes did so exponentially faster. “The simplicity of the study is part of its beauty,” theoretical physicist Marija Vucelja told Science News. “It’s one of those very simple setups, and it’s already rich enough to show that effect.”
Soon after, another group of physicists published a follow-up paper suggesting a more abstract framework for understanding the Mpemba effect, which involved modeling the random dynamics of particles. The results suggest that the key to the Mpemba mystery is a dose of chaos. In particular, a liquid moving rapidly from hot to cold is said to be “out of equilibrium”, meaning that it is a system that does not follow the linear rules that we (or Newton) might expect .
“We all have this naive image that says the temperature should change monotonically,” study author Oren Raz told Quanta magazine (meaning we could assume that a cooling liquid continues to cool). steadily moving in one direction without making significant reversals). “You start at a high temperature, then a medium temperature, and you go to a low temperature.” But in an unbalanced system, “you can have weird shortcuts,” Raz said.
Various publications have offered evocative metaphors to explain these shortcuts: Science News has compared hot liquid cooling under the Mpemba effect to “how a hiker could get to the destination faster from further away, if that starting point allows the hiker to ‘avoid an arduous ascent’. above a mountain. Alternatively, Physics Today suggested it’s a bit like someone using springboards to cross a river, writing, “If you have the right starting energy, you can jump straight from first to third without ever landing on the second. Since a hot liquid is more unbalanced than a cold liquid, it might have just the right energy to jump over rocks.
Another word for it is kurtosis, a statistical term that refers to deviation from a mean, which appears to play an important role in behavior related to the Mpemba effect. The temperature of a fluid generally refers to the medium velocity of its molecules, but each fluid will have aberrant molecules behaving very differently from the others. In cases where the Mpemba effect occurs, these outliers seem to play an outsized role, Antonio Lasanta, a physicist who has published several papers confirming the phenomenon, told Cosmos. By accounting for kurtosis in experiments related to this type of cooling and heating, “we can make analytical calculations of how and when the Mpemba effect will occur,” Lasanta said. It’s certainly a step towards solving the mystery of Mpemba, although there’s still a lot to be said about when the effect kicks in and how strong it is when it does.
Erasto Mpemba grew up to work as a hunting officer in Tanzania’s Ministry of Natural Resources and Tourism and died around 2020 after his ice cream was vindicated. While there’s still a lot we don’t know about the effect that bears his name, it seems to be “Mpemba’s physics” after all.