  The past four years have seen a scientific revolution through the birth of a
new field: gravitational-wave astronomy. The first detection of gravitational
waves---recognised by the 2017 Nobel Prize in Physics---provided unprecedented
tests of general relativity while unveiling a previously unknown class of
massive black holes, thirty times more massive than the Sun. The subsequent
detection of gravitational waves from a merging binary neutron star confirmed
the hypothesised connection between binary neutron stars and short gamma-ray
bursts while providing an independent measurement of the expansion of the
Universe. The discovery enabled precision measurement of the speed of gravity
while shedding light on the origin of heavy elements. At the time of writing,
the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European
partner, Virgo, have published the detection of eleven gravitational-wave
events. New, not-yet-published detections are announced on a nearly weekly
basis. This fast-growing catalogue of gravitational-wave transients is expected
to yield insights into a number of topics, from the equation of state of matter
at supra-nuclear densities to the fate of massive stars. The science potential
of 3G observatories is enormous, enabling measurements of gravitational waves
from the edge of the Universe and precise determination of the neutron star
equation of state. Australia is well-positioned to help develop the required
technology. The Mid-term Review for the Decadal plan for Australian astronomy
2016-2025 should consider investment in a scoping study for an Australian
Gravitational-Wave Pathfinder that develops and validates core technologies
required for the global 3G detector network.
