  The second-generation interferometric gravitational wave detectors currently
under construction are expected to make their first detections within this
decade. This will firmly establish gravitational wave physics as an empirical
science and will open up a new era in astrophysics, cosmology, and fundamental
physics. Already with the first detections, we will be able to, among other
things, establish the nature of short-hard gamma ray bursts, definitively
confirm the existence of black holes, measure the Hubble constant in a
completely independent way, and for the first time gain access to the genuinely
strong-field dynamics of gravity. Hence it is timely to consider the
longer-term future of this new field. The Einstein Telescope (ET) is a concrete
conceptual proposal for a third-generation gravitational wave observatory,
which will be ~10 times more sensitive in strain than the second-generation
detectors. This will give access to sources at cosmological distances, with a
correspondingly higher detection rate. I give an overview of the science case
for ET, with a focus on what can be learned from signals emitted by coalescing
compact binaries. Third-generation observatories will allow us to map the
coalescence rate out to redshifts z ~ 3, determine the mass functions of
neutron stars and black holes, and perform precision measurements of the
neutron star equation of state. ET will enable us to study the large-scale
structure and evolution of the Universe without recourse to a cosmic distance
ladder. Finally, I discuss how it will allow for high-precision measurements of
strong-field, dynamical gravity.
