Spherically symmetric spacetimes, which we discussed in Chapter 8, do not admit gravitational radiation. Once we relax this symmetry restriction, as we shall do in the following chapters, we will encounter spacetimes that do contain gravitational radiation. In fact, simulating promising sources of gravitational radiation and predicting their gravitational wave signals are among the most important goals of numerical relativity. These goals are especially urgent in light of the new generation of gravitational wave laser interferometers which are now operational. A book on numerical relativity therefore would not be complete without a discussion of gravitational waves.

In this chapter we review several topics related to gravitational waves. We start in Section 9.1 with a discussion of linearized waves propagating in nearly Minkowski spacetimes and the role that these waves play even in the case of nonlinear sources of gravitational radiation. In Section 9.2 we survey plausible sources of gravitational waves, highlighting those that seem most promising from the perspective of gravitational wave detection. We briefly describe some of the existing and planned gravitational wave detectors in Section 9.3. Finally, in Section 9.4 we make contact with numerical relativity, and review different strategies that have been employed to extract gravitational radiation data from numerical relativity simulations.

Linearized waves

Most of this book deals with strong-field solutions of Einstein's equations, including black holes, neutron stars, and binaries containing these objects. As long as these solutions are dynamical and nonspherical, they will emit gravitational radiation.