杏吧原创

Metalheads in mosh pits act like atoms in a gas. That鈥檚 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 , 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.

鈥淚 didn鈥檛 want to put her in harm鈥檚 way, so we stood off to the side,鈥� he says. 鈥淚鈥檓 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.鈥�

Metal fans鈥� favoured 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鈥� motions. They found that the dancers鈥� speeds had the . Such particles move around freely, interacting only when they bounce off one another.

鈥淭his presented a bit of a mystery,鈥� 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鈥� movements 鈥� 鈥渢o mimic the effects of the inebriants that the participants typically use鈥�, says co-author Matthew Bierbaum.

They found that by tweaking their model parameters 鈥� decreasing noise or increasing the tendency to flock, for instance 鈥� 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. .

鈥淭hese are collective behaviours that you wouldn鈥檛 have predicted based on the previous literature on collective motion in humans,鈥� Silverberg says. 鈥淭hat work was geared at pedestrians, but what we鈥檙e seeing is fundamentally different.鈥�

鈥淭he 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鈥檙e understanding something new about the behaviour of crowds that we didn鈥檛 understand before,鈥� says co-author .

Lane formation

The team also found a third mosh-pit mode that they hadn鈥檛 seen on YouTube, which they call lane formation. 鈥淚f 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,鈥� Bierbaum says. 鈥淚鈥檇 be excited to see this, but it would have to be at a very large venue, so that the ends didn鈥檛 collide with each other to form a circle pit.鈥�

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

鈥淲hen you have earthquakes or buildings on fire, people tend to panic when they escape. We don鈥檛 have a good way of experimentally seeing what鈥檚 going on,鈥� Silverberg says. 鈥淏y going to these heavy-metal concerts, we鈥檙e able to ethically and safely observe how humans behave in these unusual excited states.鈥�

鈥淭hat鈥檚 how we justify it after the fact, by talking about safety,鈥� Sethna adds.

The correlation between moshers and random gases 鈥渟eems very fitting to me鈥�, says Jon Freund, drummer in Ithaca-based metal band Thirteen South. 鈥淚t鈥檚 kind of the same thing 鈥� a pure expression of energy that鈥檚 just random.鈥�

But he says knowing the physics behind it won鈥檛 change how he moshes 鈥� mostly because these days he stays out of the pit. 鈥淚鈥檓 a hide-on-the-stage-and-play-my-drums mosher,鈥� he admits. 鈥淚 don鈥檛 want to get hit in the head.鈥�

Silverberg notes that the study鈥檚 main limitation was the quality of the data. 鈥淵ouTube videos are typically shaky and taken from a poor viewing angle,鈥� he says. What鈥檚 more, staff at venues 鈥渢end to baulk when you walk in with a camera鈥�. He hopes to convince at least one venue to let him film with a camera suspended over the crowd. 鈥淭here really is so much to do and so much we don鈥檛 know yet. It鈥檚 really just beginning.鈥�

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