The ice cream that changed physics

Not every scoop of ice cream can be called “fatal”. But a batch of ice cream made as a teenager by Erastu Mbemba in Tanzania in 1963 made waves in physics that can still be felt nearly 60 years later. That’s because it seemed like evidence of a strange and counterintuitive idea: that a hot liquid might freeze faster than a cold one.

Homemade ice cream was a popular snack when he was a student at Magamba High School, Mpemba wrote in a newspaper article published in 1969. “The boys at school do this by boiling the milk, mixing it with sugar and putting it in the freezer room of the fridge, after it cools down for the first time. to approximately room temperature. But the competition for the freezer was fierce. One afternoon, he and another boy took two short cuts while racing for space. Mbemba’s colleague mixed milk with sugar and poured it directly into an ice tray without boiling it at all. Not to be outdone, Mpemba boiled his milk – but skipped the step of letting it cool down so he could cut down the last ice pack. An hour and a half later, Mpemba wrote, “The tray of milk froze in the ice cream while it was only a thick liquid.”

A few years later, Mpemba asked a high school science teacher why this was – why hot milk freezes faster than cold milk, which goes against Newton’s law of cooling. The teacher’s response was, “All I can say is that this is Mpemba physics and not general physics.” The incident turned into an ongoing joke in the classroom. When Mpemba misses a math problem, the teacher and his classmates call it “Mbemba math.”

Determined to find an explanation, Mpemba repeated the experiment with hot and cold water. And when physicist Dennis Osborne visited his high school, he asked him about the accident, too. Osborne was impressed by Mbemba’s invitation to visit what is now the University of Dar es Salaam and discuss the issue further, and then prepared the relevant research which was eventually published. The essay aided a principle that was noted by Aristotle, René Descartes, and Sir Francis Bacon over the centuries and became known as the Mpemba effect.

Mpemba and Osborne’s claims created decades of controversy within the physics world, as they challenged basic theories about how matter behaves. Several researchers have attempted to recreate their findings with limited success. In 2016, physicist Henry Burridge of Imperial College London and mathematician Paul Linden of the University of Cambridge published a comprehensive review of several studies that attempted to confirm this phenomenon, reporting “unfortunately” that they were unable to find any evidence for it. Mpemba effect. Worse, they concluded that all of these studies—including the original Mpemba experiment—could easily have been skewed by small experimental factors such as the setting up of equipment insulation or the placement of thermometers.

Cooling and Mess

Starting in 2017, a new set of studies finally turned the corner to confirm Mpemba’s observation, suggesting that the explanation lies in the mysterious mechanisms of the chaos. It turns out that water itself may have been a major hurdle in proving the larger theory. It behaves differently than most other substances, especially since it changes states between solids, liquids and gases, so scientists have looked at the case of the Mpemba effect to remove water from the equation completely.

In an abstract experiment meant to focus forces at work, physicist John Beechofer and his colleagues heated microscopic glass beads (supposed to act as water molecules) with a laser and examined the speed of cooling. They found that not only did some of the hot beads cool more quickly than their cold counterparts, they sometimes did so even 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 settings, and it’s really rich enough to show that effect.”

Shortly thereafter, another group of physicists published a follow-up article proposing a more abstract framework for understanding the Mpemba effect, which involved modeling the stochastic dynamics of particles. The findings suggest that the key to the Mpemba puzzle is a dose of chaos. In particular, a fluid 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 we (or Newton’s) might expect.

“We all have this naive picture that the temperature has to change monotonously” (which means we might assume that a cooled fluid continues to move steadily in one direction without causing major reversals), study author Oren Raz told Quanta. “You start at high temperature, then medium temperature, and then drop to low temperature.” But in an out-of-equilibrium system, “you can have weird shortcuts,” Raz said.

Various publications have provided reminder metaphors to explain these abbreviations: Science News has compared hot liquid cooling under the influence of Mpemba with “how a hiker could reach a destination more quickly by starting farther, if such a starting point would allow a hiker to avoid an uphill climb over a mountain.” Alternatively, Physics Today has suggested that it’s a bit like someone using stepping stones to cross a river, writing, “If you have the right starting energy, you can jump straight from first to third without ever landing in the second.” Since a hot liquid is more out of equilibrium than a cold liquid, it may have the right energy to jump over stones.

Another word for this is kurtosis, a statistical term referring to the deviation from the mean, which appears to play an important role in the behavior associated with the Mpemba effect. Liquid temperature generally refers to modified The speed of its molecules – but each liquid will have outer particles that are very different from the others. In cases where the Mpemba effect occurs, these outliers appear to play a large role, Antonio Lasanta, a physicist who has published several research papers confirming this phenomenon, told Cosmos. By taking into account kurtosis in experiments related to this type of cooling and heating, “we can make analytical calculations to see how and when the Mpemba effect will occur,” LaSanta said. It’s definitely a step toward unraveling the Mpemba mystery, although there’s still a lot to learn when the effect appears and how powerful it is when it does.

Erasto Mpemba grew up to work as a games officer at Tanzania’s Ministry of Natural Resources and Tourism and died around 2020, after his ice cream parlor was acquitted. Although there’s still a lot we don’t know about the effect that bears its name, it appears to be “Memba physics” after all.

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