Jammed packings of soft particles are rigid despite having no thermal energy. Push them below a critical deformation, and they push back elastically. Push harder, and they yield — the packing unjams and flows. The yielding transition in passive systems is well-characterized: a continuous softening of the energy landscape, predictable from the Hessian matrix of the potential.
Caporusso and colleagues (arXiv:2602.20776) study jammed packings of infinitely persistent active particles — particles that push in a fixed direction forever. These self-propelled particles create force networks fundamentally different from passive packings, yet the jammed phase retains a surprising amount of passive structure.
The surprise: redistributing the active forces through a Laplacian framework reveals a scaling law consistent with the passive limit — not just near the unjamming transition, but throughout the entire jammed active phase. The passive scaling persists deep into the active regime. Activity doesn't dismantle the passive structure; it coexists with it.
But activity does change the yielding mechanism. Active plasticity — irreversible rearrangements under increasing active force — emerges abruptly. The Hessian matrix, which captures the curvature of the energy landscape and predicts yielding in passive systems, does not soften continuously toward the transition. The landscape doesn't gradually flatten; it ruptures. The Hessian still predicts relaxation times accurately, but it misses the plasticity onset entirely.
The critical active force for yielding scales with pressure as f_c ~ p^1.17 — close to but distinct from the passive exponent. Activity shifts the boundary but not by much.
The general observation: a system's static structure (force network scaling) can survive the introduction of activity even when its dynamic response (yielding mechanism) changes qualitatively. The statics are passive; the dynamics are not. The same packing obeys passive scaling laws while failing in a distinctly active way. Structure and response live on different axes of the active-passive spectrum.