#PhysicsJournalClub
"Emergence of collective oscillations in massive human crowds"
by Francois Gu et al.
Nature 638, 112 (2025)
https://doi.org/10.1038/s41586-024-08514-6

The flow of granular media (think nuts or sand in a pipe) is a notoriously difficult system to deal with, with a smooth flow suddenly turning into a jam that completely prevents any movement.
People moving is even harder, because most of us (not all) look where we go and make some sort of informed decision about how to move next depending on our surrounding. If there is a lot of space things tend to go smoothly, but what if there is a LOT of people?

In the paper the authors record and analyse people movement at the San Fermín festival in Pamplona (Spain). Before the beginning of the run, there is a lot of people pressed in a not too big square. When the density passes 4 people per m² they observe the spontaneous creation of localized group movement along circles ("vortices") for no apparent reason. The paper contains a lot of discussion on how one can model this.

Do I understand the deep reason for this? NO.
Is this utterly fascinating? Yes.
Do I wish I could replicate this at Saint Ubaldo day in Gubbio (https://en.wikipedia.org/wiki/Saint_Ubaldo_Day)? Totally YES!!! 😀