Every year, on July 6, thousands of people gather in the Plaza Consistorial in Pamplona, Spain, for the opening ceremony of the San Fermín festival. Dressed in white, they dance, sing, and celebrate, packing themselves into a space no larger than a soccer field. To the untrained eye, the crowd seems chaotic, a sea of bodies moving in unpredictable ways. But according to a new study, there’s a hidden predictable order to this movement — one that could help prevent deadly crowd disasters.

Researchers have discovered that dense crowds can spontaneously synchronize into collective oscillations, with hundreds of people moving in coordinated, swirling patterns — like a human vortex. These oscillations, which emerge without any external guidance, are driven by a phenomenon the scientists call “odd friction.”

The findings could lead to a new way to predict and prevent dangerous crowd behavior.

The Science of Crowds

Crowds are complex systems, where the interactions between people give rise to emergent behaviors that can’t be predicted by looking at any single person. For decades, scientists have tried to understand how crowds move, using models that treat people like particles in a fluid. These models work well for small groups, but they struggle to explain the behavior of massive crowds, where thousands of people are packed tightly together.

“Our current understanding of crowd dynamics primarily relies on heuristic collision models, which effectively capture the behavior observed in small groups of people,” the researchers write in their study published in the journal Nature. “However, the emergent dynamics of dense crowds, composed of thousands of individuals, remains a formidable many-body problem.”

To tackle this problem, the team turned to the San Fermín festival, where they could safely observe thousands of people in a confined space. Many festival-goers take part in the ‘running of the bulls’, the controversial centerpiece of the week-long event. So, using high-resolution cameras, they filmed the crowd over several editions of the festival, tracking the movements of individual heads to measure density and velocity. And what they found was striking: as the crowd density increased, the motion of the crowd shifted from random to rhythmic.

“I was like, what is this? Why 18 seconds?” study co-author François Gu said in an interview Nature, referring to how often the circular motion repeated itself.

The Emergence of Collective Oscillations

When the crowd reached a critical density — about four people per square meter — something remarkable happened. The crowd began to oscillate, with groups of hundreds of people moving in synchronized, swirling patterns. Even more interestingly, these oscillations weren’t driven by any external signal, like music or a leader’s direction. Instead, they emerged spontaneously from the interactions between individuals.

“Dense crowds can self-organize into macroscopic chiral oscillators, coordinating the orbital motion of hundreds of individuals without external guidance,” the researchers explain.

There’s a clear physical explanation for these patterns.  The researchers developed a mathematical model that showed how the interplay between confinement and odd friction — a type of friction that can generate rotational motion — drives the crowd into these collective oscillations.

Odd friction is a concept borrowed from the study of active matter, materials that can move on their own, like flocks of birds or schools of fish. In dense crowds, odd friction arises from the way people push against each other and the ground. This friction doesn’t just slow people down; it can also create forces that twist and turn the crowd, leading to the swirling patterns observed in Pamplona.

A New Tool for Crowd Safety?

Crowd disasters, like the 2010 Love Parade tragedy in Germany, are often caused by uncontrolled collective motions. At that event, sadly, 21 people died in a stampede. Understanding how these motions emerge could help prevent future tragedies.

The researchers tested their model on footage from the Love Parade. Remarkably, they found the same kinds of oscillations that they observed in Pamplona. “Crowd quakes are not turbulent but periodic,” they write. This suggests that monitoring for the onset of collective oscillations could provide an early warning system for dangerous crowd behavior.

“When the crowd density reaches its maximal value, groups of several hundreds of confined people can undergo self-sustained oscillations over meters,” the researchers note. “The mass of these macroscopic groups can exceed 10 tons.”

By detecting these oscillations in real time, authorities could take steps to reduce crowd density before it reaches dangerous levels. This could involve redirecting people to less crowded areas or temporarily halting entry to a venue.

The Bigger Picture

This fascinating study is part of a growing body of research that seeks to understand the physics of living systems. From flocks of birds to schools of fish, scientists are discovering that many collective behaviors can be explained by simple physical principles.

“Our study establishes a robust mechanical framework for understanding the dynamics of crowds and animal groups,” the researchers write. “More broadly, it offers a perspective on emergent chirality in non-equilibrium matter.”

Chirality, or handedness, is a property that describes whether a system rotates clockwise or counterclockwise. In the case of the San Fermín crowd, the direction of rotation wasn’t predetermined. It emerged spontaneously from the interactions between individuals. This spontaneous symmetry breaking is a hallmark of complex systems, from chemical reactions to the behavior of galaxies.