  Elastic active matter or active solid consists of self-propelled units
embedded in an elastic matrix. Active solid resists deformation; the
shape-preserving property and the intrinsic non-equilibrium nature make active
solids a superior component for self-driven devices. Nonetheless, the
mechanical properties and emergent behavior of active solids are poorly
understood. Using a biofilm-based bacterial active solid, here we discovered
self-sustained elastic waves with unique wave properties not seen in passive
solids, such as power-law scaling of wave speed with activity. Under isotropic
confinement, the active solid develops two topologically distinct global motion
modes that can be selectively excited, with a surprising step-like frequency
jump at mode transition. Our findings reveal novel spatiotemporal order in
elastic active matter and may guide the development of solid-state adaptive or
living materials.
