Heavy Elements and Fundamental Physics from Neutron Star Mergers*

Just over one year ago, the merger of two neutron stars was detected in both gravitational and electromagnetic radiation. The unprecedented event illuminated the fundamental physics of gravity and dense matter and addressed long standing questions as to the cosmic origin of the heavy elements. I will review our  understanding of mergers -- grounded in theoretical simulations and experimental data -- and will describe how material expelled in these events can assemble into heavy nuclei via rapid neutron capture (the "r-process"). The radioactive glow of these freshly synthesized isotopes allows us to directly study r-process elements at their production site. I'll explain how joint observations of gravitational waves, high energy photons, and optical/infrared emission can be used to constrain the speed of gravity, the equation of state of dense matter, and the formation of heavy nuclei, and will look ahead to what future "multi-messenger" observations, combined with data from nuclear experiments, can tell us about the physics of compact object mergers and the cosmic origin of the heavy elements.