Ever heard a crowd referred to as a ‘sea of people’? A new study has found that this phrase might hold more water than previously thought.
Scientists have found that extremely dense crowds behave like liquids, moving in predictable and fluid ‘ripples’ when enough people share the same limited space.
The new findings come from a Spanish team led by Prof Denis Bartolo, who observed crowds at the opening ceremony of the San Fermín festival in Pamplona, Spain, for four years. They monitored the estimated crowds of 5,000 festivalgoers by placing two cameras overlooking the 50m by 20m (164ft by 66ft) plaza chosen for the event.
By applying modern computer-vision techniques to the video footage taken in 2019 and 2022–2024, the team was able to automatically spot festivalgoers and measure factors like how tightly packed the crowd was and how fast they were moving. Looking back at their footage, Bartolo and his team realised the crowd was so dense that they could treat it like a fluid in their mathematical model.
They found that the density of the crowd significantly increased during the festival's opening – from two people per square meter to six people per square meter. When an upper limit of nine people per square meter was reached, pockets of several hundred festival attendees began to move at regular intervals – around every 18 seconds.
This movement creates the effect of 'ripples' moving through the crowd, with people several meters apart shifting in almost perfect harmony. What's interesting is that the researchers found these regular waves of motion happened naturally, without any outside force, like pushing, causing them.
Bartolo’s team hopes their research can help address safety concerns at large events. Dense crowds at festivals or concerts can be dangerous, with so many people packed into one area, potentially leading to crushing, suffocation, or even death in extreme cases.
To establish the nature of their findings, the team contrasted their footage with some captured at the 2010 Duisburg Love Parade in Germany, an incident where several hundred people in a frenzied crowd were injured and 21 people died. They observed similar 18-second oscillations in that crowd, which was almost equally dense, with 8 people per square meter.
The study has provided insights into how larger crowds might be studied going forward. As Dr Antoine Tordeux, professor in the Department of Reliability in Safety Engineering at the University of Wuppertal, Germany said to Nature: “Understanding and controlling crowd dynamics is not straightforward. From a modelling perspective, crowds of people are many-body systems comprising self-driven agents that interact in complex ways and exhibit intriguing collective behaviours.”
Read more: