This chapter describes the theory of gravitational waves. We first introduce gravitational waves and describe how they are generated and propagate through space. We then show how the luminosity, frequency and amplitude of a gravitational wave source can be defined. A brief mathematical summary of how gravitational waves are a natural consequence of Einstein's general theory of relativity is then provided. To finish, we summarise some important quantities that are used to describe gravitational wave signal strengths and the response of detectors to different types of signal.

Listening to the Universe

Our sense of the Universe is provided predominantly by electromagnetic waves. During the 20th century the opening of the electromagnetic spectrum successively brought dramatic revelations. For instance, optical astronomy gave us the Hubble law expansion of the Universe. Radio astronomy gave us the cosmic background radiation, the giant radio jets powered by black holes in galactic nuclei, and neutron stars in the form of radio pulsars. X-ray astronomy gave us interacting neutron stars and black holes. Infrared astronomy gave us evidence for a massive black hole in the nucleus of our own galaxy.

Gravitational waves offer us a new sense with which to understand our Universe. If electromagnetic astronomy gives us eyes with which we can see the Universe, then gravitational wave astronomy offers us ears with which to hear it. We are presently deaf to the myriad gravitational wave sounds of the Universe. Imagine you are in a forest: you see a steep hillside, massive trees and small shrubs, bright flowers and colourful birds flitting between the trees.