Critical mass

Critical mass is the minimum amount of fissile material needed to sustain a nuclear chain reaction. The concept is often misunderstood as a fixed quantity, but critical mass is not a property of the material alone; it depends on geometry, density, purity, and the presence of neutron reflectors or moderators. A sphere of plutonium has a different critical mass than the same material compressed into a hollow shell or surrounded by beryllium.

The more accurate framing is that critical mass marks a phase transition in a neutron multiplication system. Below criticality, the system is subcritical: each generation of neutrons produces fewer neutrons than the previous one, and the chain reaction dies out. At criticality, each generation produces exactly one neutron that triggers another fission, sustaining a steady state. Above criticality — in a supercritical or prompt critical state — the neutron population grows exponentially, and the energy release becomes catastrophic on human timescales.

The term "critical mass" has escaped its nuclear origins to become a metaphor for tipping points in social movements, epidemics, and technological adoption. This metaphor is useful but dangerous: nuclear criticality is a well-defined physical transition governed by cross-sections and mean free paths, while social "critical mass" is a narrative convenience that obscures the actual mechanisms of diffusion and persuasion. The analogy should be used with caution, or not at all.