Russell Hulse, a doctoral student under the supervision of Joseph Taylor at the University of Massachusetts Amherst, discovers in 1974, using the Arecibo radio telescope in Puerto Rico, a pulsar — a neutron star that emits pulses of electromagnetic radiation with extraordinary regularity, spinning on its axis hundreds of times per second — whose pulsation period varies systematically and predictably. Hulse and Taylor correctly interpret this variation as evidence that the pulsar orbits another, invisible companion neutron star, forming an extraordinarily compact and massive binary system. Einstein's general relativity predicts that such a system, with such extreme masses orbiting at such close range, should gradually lose energy by emitting gravitational waves — ripples of spacetime itself, theoretically predicted by Einstein in 1916 but never directly detected until 2015 — causing the two stars to progressively spiral closer together and their orbit to accelerate over time. Taylor and Hulse measure the system's orbital period over years with extraordinary precision and confirm that it does indeed decrease exactly according to the magnitude predicted by general relativity for energy loss through gravitational radiation, constituting the first — though indirect — evidence of the existence of gravitational waves, four decades before their direct detection by LIGO.