Superradiance
= Superradiance
<Superradiance> is amplification of a scattered wave by extracting rotational or other available conserved energy from a background. For a rotating <Kerr black hole>, a bosonic mode of frequency $\omega$ and azimuthal integer $m$ can have negative horizon Killing-energy flux when $0<\omega<m\Omega_H$. The reflected energy can then exceed the incident energy. A stationary Killing flow need not define a globally positive particle energy.