 Solution

GW170817 supplied the first direct gravitational wave observation of a binary neutron star inspiral. The chirping waveform tests compact-binary general relativity, constrains masses and tidal effects, and links a class of known compact binaries to observable sources of gravitational waves. The electromagnetic counterpart identifies the host and distance, enabling complementary source and cosmological inference. This is distinct from the earlier binary-black-hole detections; it established a system involving matter that could also be observed electromagnetically.^[ref]