# Physicists record dense crowds at San Fermín oscillating in orbits and model the effect

- Published: 2026-11-10 21:30 UTC
- Section: Science › Physics
- Tags: dense crowd dynamics, San Fermín Chupinazo, chiral oscillations, odd friction, non-reciprocal phase transition, Love Parade 2010, crowd monitoring, Denis Bartolo, active matter, Pamplona crowd
- URL: https://realtech.media/news/rt-4d65453681e54dca290934bf_physicists_record_dense/

**Analysis of a confined crowd of over 5,000 in Pamplona showed hundreds of people moving in synchronous orbits; a mechanical model explains the effect.**

![Physicists record dense crowds at San Fermín oscillating in orbits and model the effect](https://realtech.media/media/2026/11/881881b771eb356b.jpg)
*Sciences et Avenir*

Physicists analysed the dynamics of thousands of tightly packed people at the opening ceremony of the San Fermín festival, the Chupinazo, in Pamplona, Spain. According to a paper on nature.com, the crowd can self-organize into macroscopic chiral oscillators [1]. The event gathers more than 5,000 participants every year on 6 July on the Plaza Consistorial, about 50 m long and 20 m wide. The authors filmed from the surrounding buildings in 2019, 2022, 2023 and 2024 [1].

## Measurements: orbital motion without external cues

Before the opening, the mean number density of the crowd grows slowly and monotonically. Past a critical density, the authors report, the confined crowd begins to oscillate spontaneously: hundreds of individuals move in synchronous orbits without external guidance or control [1]. Denis Bartolo, a physics professor at ENS de Lyon, said the motion is not chaotic or turbulent but regular and periodic, repeating every 18 seconds. He also gave the threshold as more than four people per square metre.

Because of sub-optimal imaging in 2019, some measurements were made only on the 2022, 2023 and 2024 datasets [1].

## Model: odd friction and a phase transition

Until now, crowd dynamics has relied mainly on heuristic two-body collision models. These were introduced nearly 30 years ago, describe small groups well and are used in evacuation safety models [1]. The authors instead built a mechanical model from symmetry principles, without behavioural hypotheses. In it, emergent odd frictional forces drive a non-reciprocal phase transition towards collective chiral oscillations, and the model captures all the experimental observations [1].

Bartolo explained the mechanism by friction with the ground: a mass of people pulled south is shifted not only south but also east or west. He compared this to the Lorentz force on electrons in a magnetic field.

To test robustness, the authors show that similar chiral dynamics appeared at the onset of the 2010 Love Parade disaster [1]. They also propose a protocol to monitor the motion of dense crowds in real time and anticipate their uncontrolled collective dynamics [1]. So far the observations come from simple geometries such as rectangular plazas. How crowds behave in more intricate urban layouts remains unknown.

## Sources

1. [nature.com](https://www.nature.com/articles/s41586-024-08514-6)
2. [Sciences et Avenir](https://www.sciencesetavenir.fr/fondamental/mecanique-des-foules-l-oscillation-spontanee-a-partir-d-une-certaine-densite_196012)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-4d65453681e54dca290934bf_physicists_record_dense/