Emergence of Hydrodynamics: from extreme matter to the early universe and society
​​​Welcome to the EAAS workpackage 2! Here you will find short accessible descriptions of the scientific papers that have been published in the context of this EAAS workpackage. If something catches your interest, feel free to click the links to explore further!
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What is this workpackage about?
Emergence is how simple parts can come together to create complex, large-scale behaviour. A powerful tool for understanding this is hydrodynamics—a theory originally developed to describe how fluids like water or air flow. Surprisingly, it has been discovered that the same ideas can be used to describe many different kinds of systems, far beyond liquids. Hydrodynamics can help us understand how groups behave as a whole, no matter what they are made of. These groups could consist of particles, cells, bacteria, or even people. In other words, it provides a kind of “universal language” for studying collective behaviour across very different situations and scales.
In this workpackage, we explore both how this approach works and where its limits might be. We also apply it to a wide range of topics: from extreme forms of matter in the universe, such as those found just after the Big Bang or inside neutron stars, to patterns that emerge in human societies, like social segregation. By connecting these very different systems, we aim to better understand the common principles that shape complex behaviour in our world.
Symmetric preferences, asymmetric outcomes: Tipping dynamics in an open-city segregation model
​​Cities can become segregated even when people do not have a strong preference to live among their own kind. This puzzling effect was famously shown by the economist Thomas Schelling, who demonstrated that mild individual preferences can still produce sharply divided neighborhoods.
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In this work, we revisit that question from a new angle and uncover a surprising result: even when two groups have exactly the same preferences, one group can suddenly come to dominate a city.
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Instead of assuming that people consciously optimize their happiness or follow strict rules about when to move, we model residential change as a stochastic process, similar to chemical reactions. People move in or out of neighborhoods at certain rates, influenced by who already lives nearby, but without making deliberate “best” decisions. Vacant homes play an essential role by enabling movement, much like empty seats in a game of musical chairs.
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Using this framework, we find three key behaviors:
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Mixed neighborhoods where different groups are well integrated.
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Segregated neighborhoods where people cluster with similar neighbors, but both groups remain equally represented overall.
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A tipping point where, beyond a critical strength of in-group attraction, the city suddenly shifts to a state where one group dominates, even though both groups follow identical rules.
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​​​This tipping point behaves much like phase transitions in physics, similar to how a magnet suddenly becomes magnetized when cooled below a critical temperature. However, careful analysis shows that this transition does not fit neatly into any well-known physical universality class, suggesting genuinely new collective behavior.
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The broader message is important for urban policy and social science: both large-scale segregation and neighborhood tipping into a dominance of one type can emerge from simple and symmetric rules between individuals.
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Because our model is based on measurable movement rates rather than hidden personal preferences, it also opens the door to closer connections with real housing and mobility data. This makes it a promising framework for understanding, and possibly anticipating, sudden shifts in urban demographics.