Albert Einstein's general theory of relativity, first published a century ago, was described by physicist Max Born as "the greatest feat of human thinking about nature." We report on two major scientific breakthroughs involving key predictions of Einstein's theory: the first direct detection of gravitational waves and the first observation of the collision and merger of a pair of black holes. This cataclysmic event, producing the gravitational-wave signal GW150914, took place in a distant galaxy more than one billion light years from the Earth. It was observed on September 14, 2015, by the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO). arguably the most sensitive scientific instruments ever constructed...