  The glassy dynamics of dense active matter have recently become a topic of
interest due to their importance in biological processes such as wound healing
and tissue development. However, while the liquid-state properties of dense
active matter have been studied in relation to the glass transition of active
matter, the solid-state properties of active glasses have yet to be understood.
In this work, we study the structural fluctuations in the active glasses
composed of self-propelled particles. We develop a formalism to describe the
solid-state properties of active glasses in the harmonic approximation limit
and use it to analyze the displacement fields in the active glasses. Our
findings reveal that the dynamics of high-frequency normal modes become
quasi-static with respect to the active forces, and consequently, excitations
of these modes are significantly suppressed. This leads to a violation of the
equipartition law, suppression of particle displacements, and the apparent
collective motion of active glasses. Overall, our results provide a fundamental
understanding of the solid-state properties of active glasses.
