Shoemaker: To precisely measure key physical parameters such as the Hubble Constant and access all black hole binaries of approximately 100 solar masses requires third-generation gravitational-wave observatories that could be operable by the 2030s. The requisite major investments would yield a rich stand-alone science harvest and would also complement the anticipated palette of electromagnetic, cosmic ray, and neutrino instruments likely operating in the 2030s. (For a session on Multi-messenger Astrophysics: Insights from Combining Gravity and Radio Waves at AAAS February 2019; https://aaas.confex.com/aaas/2019/meetingapp.cgi/Session/21772 )