



Video: Mosh pit simulation recalls random motion in gases

Rock fans and air molecules: spot the connection? (Image: Kevin Nixon/Classic Rock Magazine via Getty Images)

Metalheads in mosh pits act like atoms in a gas. That’s the conclusion of the first study of the collective motion of people at a rock concert.

The finding could add to the realism of computer-generated crowd scenes in films and games.; More importantly, it could help architects design buildings that ease the flow of chaotic crowds in an emergency.


Research into how humans behave in crowds had mostly been limited to fairly organised situations, like pedestrians forming lanes when walking on the street. But when Jesse Silverberg, a graduate student at Cornell University in Ithaca, New York, took his girlfriend to her first heavy metal concert a few years ago, he witnessed a different and surprising form of crowd behaviour.

“I didn’t want to put her in harm’s way, so we stood off to the side,” he says. “I’m usually in the mosh pit, but for the first time I was off to the side and watching. I was amazed at what I saw.”

Metal fans’ favoured dance style is called moshing and mostly involves bodies slamming into each other. Silverberg wondered if the mathematical laws that describe group behaviour in flocks of birds or schools of fish could apply to moshers as well.

Like a random gas

Together with another grad student and two physics professors at Cornell, he pulled videos of mosh pits off YouTube and used software developed for analysing particles in a fluid to track the moshers’ motions. They found that the dancers’ speeds had the same statistical distribution as the speeds of particles in a gas. Such particles move around freely, interacting only when they bounce off one another.

“This presented a bit of a mystery,” Silverberg says. What makes a crowd of people with independent decision-making powers behave like a random gas?

To investigate, the team simulated a mosh pit with a few basic rules: the virtual moshers bounce off each other when they collide (instead of sticking or sliding through each other); they can move independently; and they can flock, or follow each other, to varying degrees. Finally, the team added a certain amount of statistical noise to the simulated moshers’ movements – “to mimic the effects of the inebriants that the participants typically use”, says co-author Matthew Bierbaum.

They found that by tweaking their model parameters – decreasing noise or increasing the tendency to flock, for instance – they could make the pit shift between the random-gas-like moshing and a circular vortex called a circle pit, which is exactly what they saw in the YouTube videos of real mosh pits. Their simulation is available online.

“These are collective behaviours that you wouldn’t have predicted based on the previous literature on collective motion in humans,” Silverberg says. “That work was geared at pedestrians, but what we’re seeing is fundamentally different.”

“The fact that human beings are very complex creatures, and yet we can develop a lifeless computer simulation that mimics their behaviour, really tells us that we’re understanding something new about the behaviour of crowds that we didn’t understand before,” says co-author James Sethna.

Lane formation

The team also found a third mosh-pit mode that they hadn’t seen on YouTube, which they call lane formation. “If you increase the flocking or decrease the density of the simulated moshers, the active participants can break down the circle and just stream through the crowd,” Bierbaum says. “I’d be excited to see this, but it would have to be at a very large venue, so that the ends didn’t collide with each other to form a circle pit.”

Although the project was mostly for fun, the researchers think it could have real-world implications for crowd animators and architects.

“When you have earthquakes or buildings on fire, people tend to panic when they escape. We don’t have a good way of experimentally seeing what’s going on,” Silverberg says. “By going to these heavy-metal concerts, we’re able to ethically and safely observe how humans behave in these unusual excited states.”

“That’s how we justify it after the fact, by talking about safety,” Sethna adds.

The correlation between moshers and random gases “seems very fitting to me”, says Jon Freund, drummer in Ithaca-based metal band Thirteen South. “It’s kind of the same thing – a pure expression of energy that’s just random.”

But he says knowing the physics behind it won’t change how he moshes – mostly because these days he stays out of the pit. “I’m a hide-on-the-stage-and-play-my-drums mosher,” he admits. “I don’t want to get hit in the head.”

Silverberg notes that the study’s main limitation was the quality of the data. “YouTube videos are typically shaky and taken from a poor viewing angle,” he says. What’s more, staff at venues “tend to baulk when you walk in with a camera”. He hopes to convince at least one venue to let him film with a camera suspended over the crowd. “There really is so much to do and so much we don’t know yet. It’s really just beginning.”

Journal reference: arxiv.org/abs/1302.1886