Plans for a third generation interferometric gravitational wave (GW) detector are epitomised by the Einstein Telescope proposal. We start by describing the motivation for building third generation instruments, followed by a description of the different science objectives that can be achieved by such an observatory. In the next section we discuss the technological challenges that must be met to achieve third generation sensitivities. The final section outlines a possible timeline for the development of this detector and various detector configurations that are being considered.

Introduction to the third generation of GW observatories

As described in the previous chapters and based on the current models of GW sources, the next generation of advanced interferometric GW detectors (the ‘second’ generation of GW interferometers, such as ‘Advanced LIGO'and ‘Advanced Virgo’) promise the detection of GW in the first year of operation close to the target sensitivity. For example, at the nominal sensitivity of these apparatuses, it is expected that a few tens of coalescing neutron stars will be detected each year. But, apart from extremely rare events, the expected signal-to-noise ratio (SNR) of these events, in the advanced detectors, is too low for precise astronomical studies of the GW sources and for complementing optical and X-ray observations in the study of fundamental systems and processes in the Universe.

These evaluations and the need for observational precision in GW astronomy have led the GW community to start a long investigative process into the future evolution of advanced detectors to a new (‘third’) generation of apparatuses (Punturo et al., 2009), with a considerably improved sensitivity.