Mirembe had married Jabari two years ago. She had renounced her inheritance to marry the man she loved. She never had been poorer, or happier. Their little house at the end of the street stood proudly as a refuge for peace and love. For some reason she never fully understood, the houses in her street were less and less opulent as she walked along it. The first one was occupied by her friend Abimbola and her husband, and it had everything one could wish for; the last one was occupied by Mirembe and Jabari, and it only had everything they could wish for.

With one young couple in every house, it was the liveliest street in the small village. People would dance around and wander among the crowd with no direction. Money made the world go round and the people go around, and it was no wonder that the richest neighbours went out to the city more often. Mirembe, on the other hand, had to work all day long, and stayed home after sunset. But a night in was never a bad night for her.

One morning Mirembe went out and felt unnaturally cold for that time of the year. The warmth of people had disappeared from the streets. She knocked on her neighbours’ door, but only a nervous noise came out from the inside. Suddenly a rifle clicked behind her. A bunch of helmets and uniforms walked towards her, shouting threats in a language she did not understand. She put her hands up. Among all the incomprehensible shouting she could only make out the friendly “Go home” coming out one of the houses. The guns pushed her back home, and finally she collapsed on the floor by the door, embracing it in search of support.

Jabari woke her up and Mirembe looked at him, happy to let go of her bad dream. Suddenly someone knocked on the door, and the shouting from her dream echoed back into reality. Jabari helped her up, and opened the door with caution. There they were again: the guns and the angry shouting. They took them to the square at the end of the street, where more guns were waiting. They took their money and their colourful clothes, and covered them with grey one-piece uniforms. A soldier wielding a razor shaved Mirembe’s head, then Jabari’s, then everyone else’s. By the end of the day, those long hairs and dresses were gone.

Mirembe hadn’t seen Abimbola on the square. Soldiers had settled into her house, and she was nowhere to be seen. “She left the moment she smelled danger”, said someone in the crowd. She was not the only one gone. The neighbours from the richest houses had all evaporated. The military had occupied their houses, and they threatened to expand further. Mirembe and Jabari made it back home, and held each other for an eternity that didn’t last nowhere enough. Another couple came knocking on their door, homeless and hopeless. As hours went by, more couples joined them in their poor house. What would have normally been a reason for celebration felt now like a cold isolation.

Jabari, a quiet and peaceful man, was screaming inside his squeezed mind. He needed to go out, and soon. Others in the house wanted to fight back, to steal the soldiers’ guns. Mirembe didn’t want to hear any of this, and just hoped for things to pass. She had seen what soldiers were capable of, and she did not even dare to go out. This would end some day. It had to.

The discussion about what to do and what not to do went on and on for days. There was no plan or agreement, only shouting as incomprehensible as the one outside. Desperate, unable to hold the pain inside him any longer, Jabari stormed out of the room. He just wanted to go out and shout. And he did go out, but he did not get to shout. A bullet pierced his head and ended his pain in a split second. Mirembe wanted to cry, to shout, to hurt herself, but above all she wanted to hug her husband one last time. Prudence held her back through her neighbours’ arms.

She cried until her eyes were empty, and all meaning had abandoned life. She could not move, and she would not have if other neighbours had stopped coming into her house, but they did not. Out of options, out of space, there was nowhere to go, but outside. She would not go very far on her own, but talking to others would end up in endless wandering. So she did not talk. She stood in the middle of the room, grabbed her friend’s hand at her side, and walked the first step. Other hands joined the chain, and other feet joined the march. Only crossing the door was left.

It was a cold dark night outside. What came out of Mirembe’s house was a bunch of people so close together you could not tell them apart any more. The first soldier who saw it ran scared at the image of the crowd, moving together with one goal in mind. He ran to a second soldier, and the second one to a lieutenant, and after ten minutes they had nowhere to run, no one to run to.

That was not the end of it. It took many neighbourhoods in the city, many sacrifices to get the soldiers out for good. But when those people with their shaven heads and their grey uniforms moved as one, their clothes, their hair, their city was theirs to claim.

The Physics behind the Story

At the beginning, people in the neighbourhood are all different, and we can tell them apart by the house they occupy and their sex. I’m going to assume, strictly for the sake of the analogy, that all couples are made up of a woman and a man. There is a type of subatomic particles in physics that behave in a similar manner; they are called fermions. There may be many identical fermions in a system (e.g. many electrons in an atom), that are distinguishable by the level of energy (that would be the house in this story) and their spin (their sex). Mirembe’s and Jabari’s house is at the bottom, at the lowest energy level, while Abimbola’s one is at the top. It is important to remember that energy is quantised at the subatomic scale, that is, one may live in a house or another, but not between houses.

The moment the soldiers occupy the neighbourhood, people drastically change their behaviour and now behave as a group of bosons. The difference with fermions is that an unlimited number of bosons can occupy the same level of energy and have the same spin. This is portrayed in the story by making people of both sexes look alike by shaving their heads (that would be the same spin) and later occupying the same house (same level of energy). It is important to remark that in real life fermions do not turn into bosons, and this change of particle type is done in the story to illustrate the difference between these two types of particles and their associated behaviours.

The whole process of people moving into Mirembe’s house is analogous to the forming of a Bose-Einstein condensation (see also this link). First of all, the richest people like Abimbola or the bosons in the highest levels of energy disappear from the system, or in real life, they evaporate (metaphor here). Then, the temperature (money in the story) is reduced so the mobility of all people/bosons is limited. They all end up concentrating in the poorest house or in the state of smallest possible speed. Actually the temperature is so low that Bose-Einstein condensates are sometimes referred to as the “coldest place in the Universe”.

At the very end of the process, the Bose-Einstein condensate is properly formed. The number of people allowed to live in other houses is limited by the soldiers, and they are all pushed down to Mirembe’s and Jabari’s house, the lowest level of energy. When the number of people is high enough, as it happens at the end of the story, they behave as a group rather than a collection of separated bosons. This is the moment when they all march as one and get rid of their enemies. It is important to remark that all bosons in a Bose-Einstein condensate are in the same level of energy (Mirembe’s house) but in real life that does not necessarily mean that they are close together in space, even if people are close in the analogy.

Bose-Einstein condensate behaves in a way that is fundamentally different from solids, liquids or gases, and therefore is considered to be a different state of matter. It is not only of theoretical interest, but it can be applied to create new technology, including quantum computers.

Other metaphors

Other analogies have been offered before for the Bose-Einstein condensation. For example, the Science magazine designed a cover with bosons portrayed as soldiers, being the role of soldiers completely different to the one in the story above. Other metaphors describe the process in terms of dancers and cars.

Image from absfreepic.com, used with license CC0 Public Domain.


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