A1 Stampede / Mass Panic

Topic

Crowd dynamics is the third track of *Psychophysics*, and unlike the previous two, it describes short-term collective physical behavior—measured in hours or days—such as panic, stampedes, riots, and collective euphoria; all are phenomena occurring among people physically present in the same location. In this sense, it is the exact counterpart to social diffusion and contagion, which explicitly described cascade and network-threshold physics rather than that of a physical crowd; here, a physical crowd is indeed gathered in a shared space.

The stampede, or mass panic, is the anomaly describing what happens when such a physical crowd attempts to evacuate a space and the congestion itself becomes the danger. The variable triggering this anomaly is crowd density crossing the critical threshold for safe flow within an escape route: as long as density remains below that threshold, people can move toward the exit at a rate the escape route can accommodate; once density crosses that threshold, the escape route can no longer handle the flow of people attempting to pass through it, and the crowd itself becomes the obstacle preventing the very evacuation that triggered the situation.

This anomaly should not be confused with the "informational panic" described under social diffusion and contagion—even though both share the word "panic." The latter is a collective overreaction to an ambiguous signal propagating through a social network, where individuals need not be physically present in the same location; the former is a purely physical phenomenon triggered by the density of an actual crowd relative to the capacity of a physical escape route—a process that may or may not have been preceded by an ambiguous signal spreading through a network. However, a panic-inducing piece of information originating on social media can end up triggering a physical stampede if it drives a crowd that has already gathered toward the same escape route simultaneously—even though the mechanism transforming that reaction into a stampede is, from that point on, purely physical.

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