This chapter first introduces gravitational wave detection from a very general point of view, before looking at the particular methods of detection across the spectrum from nanohertz to kilohertz. It finishes by focusing specifically on terrestrial laser interferometers.

Introduction

The discovery of radio waves by Heinrich Hertz in 1886 unleashed the communications revolution which has transformed our lives. Optimisation of radio receivers required understanding and integration of two concepts. The first was the concept of the antenna, which taps energy from a wave freely propagating in space and converts it into a signal which can be amplified and detected. The second was the receiver, which processes this energy by detection (converting it to a slowly time-varying voltage), amplification (increasing its amplitude without changing its frequency) or modulation (changing its frequency).

Designing gravitational wave receivers is analogous to designing radio receivers, except that electric charges moving freely in conductors are replaced by test masses floating freely in space. This concept was illustrated in Figure 1.2 in Chapter 1, showing how a ring of test particles is deformed by a passing gravitational wave. The first gravitational wave receivers were constructed by Joseph Weber in the 1960s. They took the form of large test masses in which gravitational waves could induce quadrupole vibrations. Weber went on to develop the Weber bar, in which one searched for excitations in the fundamental longitudinal vibrational mode of a cylinder. In this case, the receiver can be idealised as a pair of point masses joined by a mechanical spring.