Introduction

The detection of gravitational waves with frequencies less than 1 kHz appears to be impossible on earth, due to the magnitude of the earth's seismic noise at these frequencies. These waves, therefore, will only be seen in space-based detectors.

A simple gravitational wave detector in space can be created by setting up two free masses and using an electromagnetic signal passing from one to the other as a probe of the spacetime curvature of the region between them. This is the fundamental idea involved in several gravitational wave detectors in space, including pulsar timing, two-way Doppler tracking of interplanetary spacecraft, and spaceborne interferometers. In this article we will discuss the theory and practice of such detectors.

The outline of this chapter is as follows. In section 17.2, we will briefly discuss the sources for the gravitational waves that are to be the targets of the space-based detectors. Then, in section 17.3, the effect of a plane gravitational wave on the arrival time of electromagnetic signals is derived. Our derivation follows that of Hellings (1983) and gives the same result as that first found by Estabrook and Wahlquist (1975). In sections 17.4 and 17.5, these results are used to discuss existing results from pulsar timing experiments and spacecraft Doppler tracking experiments. Finally, prospects for space interferometers are discussed in section 17.6.