Rainer Weiss, at MIT, designs from the 1970s onward the technical concept of a gravitational-wave detector based on laser interferometry: two perpendicular arms, each several kilometers long, through which laser beams are sent and recombined at the end, so that any minute distortion of spacetime caused by a passing gravitational wave would alter the relative length of both arms by a detectable amount via the resulting interference pattern — a required precision on the order of one ten-thousandth the diameter of a proton. Kip Thorne, at Caltech, develops over decades the theoretical framework needed to predict which specific gravitational signals should be searched for, particularly those produced by the merger of pairs of black holes or neutron stars. Barry Barish, also at Caltech, reorganizes and leads the LIGO project (Laser Interferometer Gravitational-Wave Observatory) from the 1990s onward, transforming it from a fragmented experimental effort into a massive international collaboration capable of building and operating the two twin observatories needed in the United States. On September 14, 2015, LIGO detects for the first time in history a direct gravitational wave, produced by the merger of two black holes with masses thirty times that of the Sun that occurred 1.3 billion years earlier, confirming a hundred years later Einstein's 1916 theoretical prediction and opening an entirely new observational window onto the universe, complementary to light, capable of detecting cosmic events invisible to any conventional electromagnetic telescope.