Rotational Properties of Superradiant Condensates

Superradiance enables scalar fields to extract energy and angular momentum from a rotating black hole (BH), leading to the formation of a BH-condensate system. The condensate appears to exhibit global rotation as a rigid body, with phase propagation occurring only in the azimuthal direction, such a picture leads to unphysical superluminal velocities at large distances. In this work, we demonstrate that the apparent rotation is a collective effect arising from local radial, polar, and azimuthal motions. In the region sufficiently far from the BH, the condensate exhibits a radial velocity magnitude that approaches $ (r_gμ/\bar{n}) \sin (2ωt-2 φ)$, while the polar and azimuthal velocity magnitudes asymptotically decline as $\propto 1/r$.