Fear of crowds – it might not be so irrational.

The Atlantic reveals the fluid dynamics of deadly mob disasters that shows how crowds can be so blindly powerful:

“It happens like magic,” says Dirk Helbing, a professor in Zurich, Switzerland, who studies sociology and crowd modeling. “People don’t have to think about it, you don’t need to have legal regulations or policemen to organize the crowd. It just happens, like this invisible hand like what Adam Smith described.”

“At very low density, when everybody can move freely, [crowd dynamics are] like a gas,” he says. “When the density goes up, then eventually peoples’ movements are constrained, and it becomes more like a fluid. And then at very high densities, when people are squeezed in between other bodies, it’s more like a granular material.”

Like sand, or rice, or small pebbles.

Helbing and co-author Pratik Mukerji brought this perspective to an in-depth study of the 2010 Love Parade techno music festival in Duisburg, Germany. The festival was held in a fenced-in freight yard designed to hold about 250,000 people, with only one dedicated entrance and exit. By some accounts, more than a million music fans turned up. And, in the end, 21 of them died and more than 500 were injured in what most news reports called a “stampede.”

The Love Parade followed a series of high-profile deadly crowd disasters over the past several decades, from soccer stadiums in England to rock concerts in the U.S.

As the density increases even further, the forces would be transmitted from one body to the next, and this is the moment where forces start to add up,” Helbing says. “That causes these turbulent waves in the crowd.”

The small movements of so many people aggregate into a powerful force – one that security officials are often helpless to halt – that has the capacity to knock over bodies, shove them together and, ultimately, asphyxiate them.

This sounds impossible, but 21 people died at Love Parade inside a crowd that had essentially been standing still. There was no real crowd rush or dramatic “stampede.” And this is the heart of the mystery to non-scientists as to how such a thing could happen.

“Why do people think it’s panic that causes crowd disasters?” Helbing asks. “They just cannot understand how it can happen that people can die although nobody is behaving in a ruthless way.”

If you want to see what happened at the Love Parade – literally – the disaster has its own YouTube channel.

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