One of the easiest experiments is to take two cups of water, one hot, and one cold. If you want to know which one will freeze first, place both in a freezer. According to common sense, the water will be cold. Some people have observed that hot water may cool quicker. Hot water pipes burst in the cold weather. Physicists have been arguing for more than 50 years about whether something like this actually happens.

The Mpemba effect is a term used to describe hot water freezing faster than cold water. They were able to observe the effect, but follow up experiments failed to replicate it. Researchers often have trouble figuring out if they have accounted for all the variables in a precision experiment.

Over the past few years, as the controversy continues about whether the Mpemba effect occurs in water, the phenomenon has been seen in other substances Researchers are able to see the complicated dynamics of systems that are out of equilibrium. A group of physicists have predicted that the Mpemba effect should occur in a wide variety of materials. These ideas seem to have been confirmed in recent Experiments.

Water is proving to be the slipperiest of all the substances.

The most solid observations of the Mpemba effect have been made by John Bechhoefer, a physicist at Simon Fraser University in Canada. It isn't easy once you start thinking about it.

‘That Cannot Happen’

Erasto B. Mpemba is the person who is going to tell you about his discovery. There is a 1969 paper in the journal physics education in which Mpemba describes an incident when he and his classmates were making ice cream at school.

A middle-aged man in a suit holding an ice cube in his palm

Erasto Mpemba started researching whether hot water freezes quicker than cold water.

The stock photo is from PA Images.

In the rush to get the last available ice tray, Mpemba decided to skip waiting for his boiled-milk-and-sugar concoctions to cool to room temperature and went straight to the fridge. His mixture was frozen into ice cream within an hour and a half. He was told by his teacher that he was confused. That can't be done.

The high school physics class was visited by Osborne later. The teenager asked if he could take two beakers with equal volumes of water, one at 35C and the other at 100C, and put them into a fridge, the one that started at 100C would freeze first. What is the reason? Evidence for the effect that bears Mpemba's name was found when they worked with a technician at the University College. The tests were crude and more sophisticated experiments were needed to figure out what was happening.

A photo of a school auditorium with blue walls and rows of empty wooden desks.

The Mpemba effect has been explained by a variety of theories over the years. Solid and liquid phases can coexist at the same temperature when water is less dense than liquid. It has been suggested that heating water could cause a breakdown in the loose network of weak polar hydrogen bonds between the water molecule and the sample, which in turn could cause the amount of energy it takes to cool the sample to be lowered. The time it takes to freeze is explained by the fact that hot water is more likely to evaporate than cold. The freezing point of cold water could be lowered. A layer of frost in a freezer acts as an insulation, keeping heat from leaking out of a cold cup, whereas a hot cup will melt the frost and cool faster.

The explanations all assume that the effect is real. Some people are not sure.

An x-y plot of the time it takes water to freeze as a function of the water’s initial temperature indicates that hot water freezes faster than cold.

Physicist Henry Burridge of Imperial College London and mathematician Paul Linden of the University of Cambridge did an experiment that showed how sensitive the effect is to the particulars of measurement They thought that hot water might form ice crystals first. They noted how long it took water to reach zero degrees Celsius because it's difficult to measure. The readings were dependent on where they placed the thermometer. The Mpemba effect wouldn't appear if they compared the temperatures of cups at the same height. The false evidence of the Mpemba effect could be produced if the measurement was off. The classic study of the Mpemba effect found that it was too pronounced to be attributed to measurement error.

The findings show how sensitive these experiments are even if you don't freeze them.

Strange Shortcuts

A good number of researchers believe that the Mpemba effect can happen. Even before the invention of sensitive thermometers, the benefits of putting water in the sun for cooling were obvious. Mpemba was able to see the difference between his frozen ice cream and his classmates'. The Mpemba effect is hard to pin down because physicists don't understand what happens when the water is out of equilibrium.

An equation with three parameters can be used to describe a fluid in a bottle. If you put that bottle in the freezer, you're toast. The particles at the outer edge will be plunged into an icy environment. Pressure and temperature are not well defined anymore.

When he read about the Mpemba effect in middle school, he sneaked into an oil refinery in China where his mother worked and used precision lab equipment to measure temperature as a function of time in a sample of water As a graduate student, he attempted to change his approach to the Mpemba effect. He asked if there was a rule that would prevent something starting further away from the final equilibrium from approaching equilibrium faster than something starting from close.

A pair of portraits of men looking at the camera.

Two scientists from the University of North Carolina and the Weizmann Institute of Science in Israel have shown that hot liquids can find strange shortcut ways to freeze.

Robert Filcsik is at the Weizmann Institute of Science.

Lu met Oren Raz, who studies nonequilibrium statistical mechanics at the Weizmann Institute of Science in Israel, and they began to develop a framework to investigate the Mpemba effect, not just in water. The random dynamics of particles were modeled in a paper by the National Academy of Sciences. The findings suggest that the components of a hotter system are able to explore more possible configurations and thus discover states that act as a sort of bypasses, allowing the hot system to overtake the cool one as both dropped toward a colder final state.

There is a naive picture of temperature changing monotonically. You start at a high temperature and go to a low one. It is not true to say that the system has a temperature, since that is the case.

The Spanish group that began simulation of what are known as granular fluids, collections of rigid particles that can flow like liquids, such as sand or seeds, showed that these can have Mpemba-like effects. Marija wondered how common the phenomenon would be. She wondered if it was useful for optimal heating or cooling protocols. The Mpemba effect could be seen in a significant fraction of the materials studied. There is an enormous variety of possible materials covered in the findings.

To find out if the theoretical hunches had a real-world basis, the two men approached Bechhoefer. They grabbed me after a talk and said, "Hey, we want you to hear about it."

Exploring the Landscape

A stripped down look at a collection of particles under the influence of different forces was created by Bechhoefer and Kumar. A laser creates a W-shaped energy landscape by placing a glass bead in it. A resting place is found in the deeper of the valleys. A particle can fall into the valley and eventually get knocked into the deeper well, but it's a stable state. The trials are equivalent to a system with 1000 particles because the scientists submerged the landscape in water and used optical tweezers to position the glass bead.

An infographic explaining Kumar and Bechoefer’s experiment with glass beads and lasers.

Since hotter systems have more energy and can explore more of the landscape, the glass bead can be placed anywhere. The starting position was limited to a small area near the valleys. During the cooling process, the glass bead first settled into one of the two wells, then spent a longer period jumping between them. The glass bead was cooling when it spent 20% of its time in the metastable one and 80% in the stable one. The ratios were determined by the water's initial temperature and the valleys' size.

The warm system took longer to settle into a final configuration than the hot system. Particles from the hot system settled into the wells quicker. When the experimental parameters were just right, the hot system's particles almost immediately found their final configuration, cooling infinitely faster than the warm system. Experiments showing the inverse Mpemba effect were published in PNAS earlier this year.

Ral Rica Alarcn said that the results were clear. A system that is farther away from the target can reach it quicker than another system that is closer to the target.

A pair of portraits of men standing next to lab equipment.

Experiments with lasers and glass beads show that hot liquids can relax to equilibrium quicker than cold liquids.

Dianne Mar-Nicolle is a faculty member at Simon Fraser University.

Some people are not convinced that the Mpemba effect has been demonstrated. Burridge was not impressed by the write-up. I feel that there is an interesting question as to whether Mpemba-like effects exist in a meaningful way.

It is not known if the Mpemba effect is the only mechanism or how any particular substance undergoes such out-of-equilibrium heating or cooling.

It remains to be seen if the phenomenon happens in water. The Mpemba effect could be achieved through a related mechanism that Raz has previously described with Lu in systems that undergo a second-order phase transition, meaning that their solid and liquid forms can't coexist at the same temperature Bechhoefer described the work as gradually sneaking up on an answer for water, even though water is not a system like that.

Physicists have begun to hold onto nonequilibrium systems that they otherwise don't because of the theoretical work on the Mpemba effect. We don't have a good theory forRelaxation towards equilibrium.

After sparking a decades-long controversy with his teenage interrogations, Mpemba went on to study wildlife management and become a principal game officer in the ministry of natural resources and tourism. Christine said that Mpemba died around 2020. He insists on the effect that bears his name. A lesson from the initial skepticism and dismissal that the schoolboy faced was that it pointed to the danger of an authoritarian physics.