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Grains, sheep and soldiers: How one scientist is studying the physics of crowds

ELISSA NADWORNY, BYLINE: When a crowd becomes especially big and dense, like at a pilgrimage or a concert, things can get dangerous fast. A Spanish researcher is using physics to understand how they move, and hopefully, how to protect people. Here's science reporter Ari Daniel.

UNIDENTIFIED SOCCER FANS: (Chanting in non-English language).

(SOUNDBITE OF DRUM BEATING)

ARI DANIEL, BYLINE: I'm standing outside Pamplona's massive soccer stadium.

(CHEERING)

DANIEL: The local team is winning.

(SOUNDBITE OF WHISTLE BLOWING)

IKER ZURIGUEL: It's 20-, 25,000 people inside the stadium. We came here because there is a crowd.

DANIEL: Iker Zuriguel is an applied physicist at the nearby University of Navarra. He studies the movement of such crowds to optimize their flow and comfort and improve public safety.

ZURIGUEL: A lot of people trying to move too fast in a huge crowd can be dangerous.

DANIEL: Dangerous and even deadly, since people can die from being trampled in the crush.

UNIDENTIFIED SOCCER FANS: (Chanting in non-English language).

DANIEL: The game ends. Within seconds, a river of fans in red shirts spills out of the stadium onto the street.

ZURIGUEL: Let's go to that corner where I think it's denser.

DANIEL: Zuriguel and I wade through a crowd teeming with folks thinking they're making their own decisions. But when there's a mass of people like this, certain patterns emerge, and Zuriguel is interested in what those patterns are.

ZURIGUEL: As individuals, we can think and we can react. But when we start increasing, increasing, increasing the density, sometimes it's difficult to do what we want to do because the crowd is there.

DANIEL: And that's where the laws of physics, rather than free will, start to take over.

Back in his office, Zuriguel says he used to focus on the movement of particles, doing things like sending grains down little silos. But some years ago, a paper caught his eye. Researchers had put 20-odd people in a room and told them to evacuate as quickly as possible, as if there were an emergency. And they found when they placed an obstacle near the exit door, the flow improved.

ZURIGUEL: The room is evacuated faster.

DANIEL: Zuriguel wondered whether that effect was due to human behavior or pure physics - whether the individuals were acting more like people or particles. So he turned to his grains, placing a small obstacle above the silo exit. Up to that point, the grains had always clogged the outlet every two or three seconds. But with the obstacle, they kept going for three minutes.

ZURIGUEL: And then I said, oh, my God. It was for me unbelievable.

DANIEL: The result could be explained by physics alone. The obstacle reduced pressure from building at the exit, preventing the grains from jamming up. Next, Zuriguel turned to sheep.

ZURIGUEL: Sheep came to my mind because my grandpa was shepherd.

DANIEL: And as his shepherd grandfather knew well, sheep follow one another, even if they have to push through a narrow opening.

(SOUNDBITE OF SHEEP BLEATING)

DANIEL: Zuriguel and his colleagues worked with a shepherd in the mountains.

ZURIGUEL: (Speaking Spanish).

DANIEL: In this video from the experiment...

ZURIGUEL: Ay, ay, ay, ay, ay (ph).

DANIEL: ...A hundred sheep push and cram their way through a doorway to receive food on the other side.

ZURIGUEL: Flow, jam, clog, flow, jam - this kind of behavior.

DANIEL: But when they put a concrete cylinder a few feet from the door, it cut the longest clogs, which can be the most dangerous, by more than 90%. At last, Zuriguel was ready to try this with people, and a captain in the Spanish army was eager to participate, saying to Zuriguel...

ZURIGUEL: A group of 200 soldiers are available to push as hard as you want, like an exercise. So we did it.

DANIEL: But alas - nada.

ZURIGUEL: I'm sorry, that's life. The obstacle has no effect.

DANIEL: To be fair, humans aren't shaped like sheep or grains. And unlike the earlier work with the 20-some people, Zuriguel believes that all those forceful soldiers built up so much pressure on the exit, shoving as hard as they could in the presence of an obstacle, that shear forces may have taken over, spinning them about and making a clean exit harder.

ZURIGUEL: People is facing backwards and they want to rotate a lot.

DANIEL: So his search continues for how to improve a crowd's rapid departure from a small space. In the meantime, Zuriguel has also wanted to analyze crowds in the real world, and he hasn't had to go far to do it.

(SOUNDBITE OF BELLS RINGING)

DANIEL: Zuriguel takes me to the heart of Pamplona, not two miles from his office, to a plaza.

ZURIGUEL: It's tiny, right?

DANIEL: The main drag is a bit smaller than the area of an Olympic swimming pool. Every morning for eight days in early July, the running of the bulls passes through here.

ZURIGUEL: The bulls go uphill there, and they turn around here.

DANIEL: But before all that, things kick off with the festival of San Fermin. All that morning, people pour into this little plaza.

ZURIGUEL: The density grows, grows, grows, grows. What everybody's waiting is just the starting of the festival at noon. Just a single firework - boom - at noon.

DANIEL: At which point, some 6,000 people are crammed so tightly, Zuriguel says it can be hard to breathe. For several years, he and his colleagues have filmed the masses from a balcony above.

ZURIGUEL: I've been within this crowd before, thinking many times, that's chaotic. It's a mess.

DANIEL: But within this apparent chaos, the footage revealed a pattern. Each person in the crowd, getting jostled this way and that, repeatedly traced out a rough circle on the ground about the size of a car.

ZURIGUEL: That was really surprising. I wasn't expecting this.

DANIEL: These orbital motions consistently lasted 18 seconds, a length of time the research team says is likely due to the shape of the plaza. In fact, just after the firework, when a traditional band cuts through the middle of the crowd...

ZURIGUEL: So the square is split in two.

DANIEL: ...Those same orbital motions are also cut in half, to 8 or 9 seconds. Zuriguel's now exploring the pressure waves that can ripple through this crowd - people pushing against each other from the back of the plaza to the front. Such waves elsewhere can be deadly, but Zuriguel says there's never been an injury or fatality here. The analysis isn't finished, but he thinks those orbital motions might help break up the pressure waves.

ZURIGUEL: If we understand why this happens, I think we will be able to apply some strategies in other places to prevent these kind of disasters.

DANIEL: Thereby translating the jitters of a sangria-soaked crowd into recommendations that may save people's lives.

(SOUNDBITE OF BELL RINGING)

DANIEL: For NPR News, I'm Ari Daniel, Pamplona, Spain.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

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Ari Daniel
Ari Daniel is a reporter for NPR's Science desk where he covers global health and development.