How self-propulsion changes the glass transition of dense active matter
In plain words
Dense crowds of self-propelled particles, such as packed cell layers or active colloids, become glassy and nearly stop moving, much like a cooling liquid. Whether this active glass follows the same physics as an ordinary glass with an effective temperature, or a different one, is not known.
Precise statement
For dense active Brownian or active Ornstein-Uhlenbeck particles with active force $f_0$ and persistence time $\tau_p$, determine the dynamical arrest line $\phi_g(f_0, \tau_p)$ and whether fragility, dynamic heterogeneity ($\chi_4$) and the relaxation spectrum map onto those of the passive glass at an effective temperature, given reports that $\tau_{\alpha}$ depends non-monotonically on $\tau_p$ at fixed effective temperature. An answer is the arrest diagram with a quantitative test of the effective-temperature mapping and a theory that predicts where it fails.
What would settle it
Simulations over several decades of $\tau_{\alpha}$ and $\tau_{p}$, compared with an active mode-coupling or other theory, and with measurements on dense active colloids or cell monolayers.
Status in the literature
Unverified note
Reviewed in 2019 (Janssen, Active glasses, J. Phys.: Condens. Matter, https://doi.org/10.1088/1361-648X/ab3e90); no consensus theory as of 2026.