  As of today, we have directly detected exactly one source in both
gravitational waves (GWs) and electromagnetic (EM) radiation, the binary
neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and
the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect
hundreds of events, including new classes of events such as
neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly
something completely unexpected. As we build this sample, we will explore
exotic astrophysical topics ranging from nucleosynthesis, stellar evolution,
general relativity, high-energy astrophysics, nuclear matter, to cosmology. The
discovery potential is extraordinary, and investments in this area will yield
major scientific breakthroughs. Here we outline some of the most exciting
scientific questions that can be answered by combining GW and EM observations.
