Last August, the discovery of the merger of two neutron stars heralded the first time we have detected both gravitational waves (GW) and light from the same event. Gamma-rays measured only seconds after the GW signal put strong constraints on the speed of gravity. Optical and infrared observations over the following days and weeks localized the GW source and revealed the radioactive glow of heavy elements freshly synthesized in the merger and its aftermath. I will review the physics of compact object mergers and their electromagnetic counterparts, and discuss how modeling their radioactive light has allowed us to probe dense matter physics and to identify (finally) an astrophysical site where the heavy elements were made via rapid neutron capture. I'll look ahead to what future "multi-messengers" observations may tell us about the diversity of gravitational wave sources and the cosmic origin of the heavy elements.