What do you get when you toss together a mild winter evening, several thousand people roughly between the ages of 20 and 40 (some dressed up, some dressed down, some in rabbit, dragon, and duck costumes), a festive atmosphere, and no clear focal point in Tokyo’s Shibuya crossing on New Year’s Eve?
A human scale demonstration of 2-D random particle physics.
It was crowded, loud, but basically calm until midnight – at which point the mass of humanity began heaving and surging in different directions. Some people trying to leave, some trying to enter, and some content to be in the midst and roll with the tide whichever way it drug them. Groups separated and reunited as the flow merged and dispersed individuals into flow structures with wakes, eddies, and stagnation points serving to slow and speed the mass with a full compliment of laminar and turbulent flows between sources and sinks.
Intervention. A group of baton-wielding police moved in to clear a path. Calm, friendly, perfectly polite and speaking textbook English to us “I’m so sorry, you must retreat” as one hand held a baton and another shook hands, but persistently moving ahead, clearing a way.
Momentarily calmed, the police departed, and the heaving began anew, but less forceful, less energized.