Gravitational Wave Astronomy

The theory of General Relativity predicts that objects moving in a gravitational field should emit gravitational waves. This is analogous to the emission of electromagnetic waves by moving charged particles, but the effect is much weaker. Einstein himself at first doubted the reality of the effect, and later, although recognizing the effect, thought that it wouild be too weak ever to detect. In this chapter I give a summary description of the steps which led up to the detection of gravitational waves by the LIGO experiment in 2015, and the increasing role gravitational wave detection is playing in astrophysics. The timing of pulsars in binary star systems from 1974 onwards, had already shown indirectly that gravitational energy was being radiated. In this chapter you will see how the major technical challenges of detecting gravitational waves were overcome, and see the solutions embodied in the LIGO and VIRGO experiments. The initial source was identified as the merger of two black holes each with a mass some 30 times that of the Sun. Now these experiments are producing a regular harvest of detections, mostly of stellar mass black hole mergers, but including a merger of two neutron stars which could be observed over a wide range of wavelengths by conventional telescopes. The construction of new ground based experiments, to be followed by satellite borne detectors marks a new era in astronomy, an era epitomised by the term “multimessenger astronomy”, in which black hole mergers from the entire universe should come within range.