Studying crowd dynamics could inform strategies that help to prevent dense gatherings from becoming dangerous

February 04, 2025 | 08:00pm

Physics shows how crowds suck people into a vortex

Studying crowd dynamics could inform strategies that help to prevent dense gatherings from becoming dangerous

Huge crowds gather in the Spanish city of Pamplona for the Feast of San Fermín.Credit: Danny Lehman/Getty

Researchers studying the movement of crowds at a traditional Spanish festival have shown that densely packed groups of people form swirling ‘vortex’ patterns never before documented in human gatherings. The discovery, published on 5 February in Nature¹, contrasts with previous studies that have found crowds to move in more-chaotic ways.

“I was like, what is this? Why 18 seconds?” says study co-author François Gu, referring to how often the circular motion repeated itself. The finding, which was the outcome of a computer analysis of video footage, was so puzzling that he spent more than a month double-checking the methods, says Gu, a physicist at the École Normale Supérieure in Lyon, France. He then realized that the swirling was clearly visible in videos of the event, once the footage was sped up.

Tracking of individual heads in this footage of the crowd reveals gradual circular movements.Credit: Bartolo lab, ENS de Lyon

No bull here

The Feast of San Fermín in Pamplona, Spain, is famous for its running of the bulls, in which participants put themselves in harm’s way as the angry animals are released into the city’s streets. But Gu and his collaborators focused instead on the opening of the festival, in which thousands of people gather in the Plaza Consistorial.

Gu and his colleagues set up cameras on balconies overlooking the plaza, and used computer models to analyse the videos. To understand the crowd’s behaviour, they modelled it as a dense continuum, akin to a fluid made up of particles (most previous studies have instead modelled crowds as made up of discrete, individual agents). They found that, as the area filled up in the morning, the crowd reached a critical density of around four people per square metre, beyond which it started — initially very slowly and almost imperceptibly — to form rotating vortices that pushed against one another, each involving hundreds of people.