Stellar-sized black hole binaries are an important source of gravitational radiation for ground-based interferometric detectors. The radiation arises from three phases: the inspiral of the two black hole companions, the merger of these two companions to form a single black hole, and the ringdown of this initially distorted black hole to become a stationary Kerr black hole. The gravitational radiation of the black hole inspiral has been discussed in section ; calculations of the late stages of inspiral, the merger, and the early stages of the ringdown have not yet been completed; the radiation produced in the late stages of black hole ringdown is the topic of this section.
At late times, the distorted black hole will be sufficiently ``similar to'' a stationary Kerr black hole that the distortion can be expanded in terms of ``resonant modes'' of the Kerr black hole. By ``resonant modes'' we refer to the eigenfunctions of the Teukolsky equation--which describes linear perturbations of the Kerr spacetime--with boundary conditions corresponding to purely ingoing radiation at the event horizon and purely outgoing radiation at large distances. These resonant modes are also called the quasinormal modes; they are described in the next subsection.