ARI SHAPIRO, HOST:
When you're in a stadium or arena rooting for your team, you might find yourself caught up in a wave.
UNIDENTIFIED CROWD: (Cheering).
SHAPIRO: People stand up, then they sit down. If everyone plays along, you can make a wave go all the way around full circle. We've been reporting on all kinds of waves this summer as part of a special series. As NPR's Nell Greenfieldboyce reports, the kind of wave done by sports fans has caught the attention of physicists.
NELL GREENFIELDBOYCE, BYLINE: The wave has been looping around stadiums since at least the early 1980s. Illes Farkas first started pondering it back in 2001.
ILLES FARKAS: It was summer. It was really hot. And there was something in town.
GREENFIELDBOYCE: He can't really remember what - maybe a swimming competition? What he does remember is seeing the audience do the wave and wondering, how did tens of thousands of people burst into this unplanned but highly coordinated movement?
FARKAS: So it was basically out of curiosity, an odd summer project. And then it turned into something very serious.
GREENFIELDBOYCE: Back then, he was a graduate student in physics. Now he's at the Hungarian Academy of Sciences in Budapest. He says it's only natural that he and a couple of scientific pals would be drawn to a crowd doing the wave.
FARKAS: The reason why we got interested in stadium waves was that people, apparently, very often behave like particles.
GREENFIELDBOYCE: Particles obeying a few simple rules can create seemingly complex phenomena, like ice melting.
FARKAS: And in a very similar way, surprisingly, humans do similar things.
GREENFIELDBOYCE: To find out what simple rules produce the wave, he and his colleagues got videos of stadium crowds from TV stations. They analyzed more than a dozen waves. They also built computer models and zeroed in on three key parameters - the distance between audience members, how many neighbors an audience member could see...
FARKAS: And the third one was the readiness or probability of an individual to start standing up assuming that others nearby are already standing.
GREENFIELDBOYCE: What they learned appeared in the prestigious scientific journal Nature. A wave typically rolls clockwise, moving at a speed of about 20 seats per second. To keep going, it needs to be wide, stretching from the top rows to the bottom seats. Interestingly, though, starting a wave doesn't take that many people.
FARKAS: It was surprising that the number of individuals necessary for triggering a wave is actually quite low. So it was on the order of 20, 30, 35 people.
GREENFIELDBOYCE: The key is to strike when the mood of the crowd is just right. A critical moment in a close game is probably not a good time to try.
FARKAS: Waves actually happen quite often when there's nothing interesting happening or when people are very enthusiastic.
GREENFIELDBOYCE: Like when their team is clearly going to win. So if you're out at the stadium and your side is way ahead, get some buddies and try a little wave experiment of your own. Nell Greenfieldboyce, NPR News.
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