導語

Dense crowds form some of the most dangerous environments in modern society. Dangers arise from uncontrolled collective motions, leading to compression against walls, suffocation and fatalities. Our current understanding of crowd dynamics primarily relies on heuristic collision models, which effectively capture the behaviour observed in small groups of people. However, the emergent dynamics of dense crowds, composed of thousands of individuals, remains a formidable many-body problem lacking quantitative experimental characterization and explanations rooted in first principles. Here we analyse the dynamics of thousands of densely packed individuals at the San Fermín festival (Spain) and infer a physical theory of dense crowds in confinement. Our measurements reveal that dense crowds can self-organize into macroscopic chiral oscillators, coordinating the orbital motion of hundreds of individuals without external guidance. Guided by these measurements and symmetry principles, we construct a mechanical model of dense-crowd motion. Our model demonstrates that emergent odd frictional forces drive a non-reciprocal phase transition towards collective chiral oscillations, capturing all our experimental observations. To test the robustness of our findings, we show that similar chiral dynamics emerged at the onset of the 2010 Love Parade disaster and propose a protocol that could help anticipate these previously unpredictable dynamics.

Overview

Crowd oscillations may emerge spontaneously due to collective behavior without any external guidance. Thanks to thorough experimental analysis of a unique crowd dataset, the authors construct a mean field model of crowds without making any behavioral assumption. Instead the authors rely on first principles and symmetry properties. This work demonstrates how large scale oscillations and phase transitions can arise in active systems, helping identify key principles for experimental study.

Outline

• Topic 1: High Density Crowd Risks: Real-world Background and Scientific Questions

• Topic 2: Research Limitations: From Model Establishment to Multi-body challenges

• Topic 3: Experimental Fundamentsl: High-density Crowd Observation

• Topic 4: Key Findings: Emergent Chiral Oscillations in Crowds

• Topic 5: Theoretical Framework: Mean Field Model and Non-Reciprocal Phase Transition

Key Concepts

Crowd Dynamics

Collective Motion

Emergent Behavior

Many-body Problem

Quantitative Experimental Characterization

Odd Friction

Non-reciprocal Dynamical Instability

About the Speaker

Speaker:Fran?ois Gu，He holds a Ph.D. in Physics and is currently a postdoctoral researcher at the MIT Senseable City Lab where his work focuses on using physics to understand human systems.

He obtained his Ph.D. from the Ecole Normale Supérieure de Lyon in France under the supervision of Professor Denis Bartolo.

His doctoral research centered on active matter and collective dynamics of high-density crowd, discovering emergent collective behavior and self-organization.

References

1. Helbing, D., Johansson, A. & Al-Abideen, H. Z. Dynamics of crowd disasters: an empirical study. Phys. Rev. E 75, 046109 (2007).

2. Ma, J., Song, W., Lo, S. M. & Fang, Z. New insights into turbulent pedestrian movement pattern in crowd-quakes. J. Stat. Mech. Theory Exp.2013, 02028 (2013).

3. Bain, N. & Bartolo, D. Dynamic response and hydrodynamics of polarized crowds. Science 363, 46–49 (2019).

4. Fruchart, M., Hanai, R., Littlewood, P. B. & Vitelli, V. Non-reciprocal phase transitions. Nature 592, 363–369 (2021).

5. Scheibner, C. et al. Odd elasticity. Nat. Phys. 16, 475–480 (2020).

Live Broadcast Information

Time Information

10:00–12:00, Saturday, May 9, 2026

Online via Tencent Meeting. Simultaneous live broadcast will be available on WeChat Video Channel and Jizhi Club Bilibili official account.

Interested participants may scan the QR code to sign up for the Swarm Intelligence Reading Group and join the participant group for further communication.

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Swarm Intelligence Reading Group

In partnership with Professor Zhangang Han from the School of Systems Science, Beijing Normal University, Associate Professor Tianfang Zhao from the Computational Communication Research Center of Jinan University, and Associate Professor Yusuf Abibula from the School of Physics Science and Technology, Xinjiang University, Swarma Club co-launches the Reading Group on Swarm Intelligence. Adopting a universal conceptual thread, it seeks to reinterpret bird flocks and ant colonies in nature, collective behaviors in human society, as well as multi-agent systems and swarm intelligence optimization in the AI era within a unified framework.

The reading group kicked off on January 24, 2026, and runs every Saturday from 14:00 to 16:00. We welcome anyone interested in the emergence, interpretation and design of swarm intelligence to join us: come with questions, and leave with more inspiring ones.

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