 In 1974, 58 years after Einstein predicted the existence of gravitational waves, two radio astronomers, Joe Taylor and Russell Holtz, were looking for new pulsars using the 305-meter Ericebo radio telescope in Puerto Rico. They found one. It's named PSR B1913-plus-16, and it led to the first indirect verification of Einstein's prediction. You'll recall from the globular clusters in supernova chapter, in the How Far Away Is It video book, that if pulsar is a rapidly rotating neutron star with a powerful magnetic field, the result is a sort of magnetic lighthouse, which, if aligned correctly, flashes in our direction twice each cycle. These signals are highly regular, in fact, pulsars are some of the best clocks in nature. And this allows extremely precise measurements of their motion. This one was pulsing every 59 milliseconds, indicating that the pulsar rotates 17 times per second. But Holtz and Taylor noticed that the pulsars varied regularly every 7 and 3 quarters hours. With pulses arriving 3 seconds earlier at some times relative to others. This meant that the pulsar was in an elliptical orbit with another neutron star. This was the first binary neutron star ever discovered. Using the orbital motion, they calculated the star's masses, their closest approach called a peristron, and their furthest distance apart, called a apistron, as well as the system's inclination. With this information, and the gravitational wave equations, they were able to calculate the amount of gravitational radiation, the expected decay of the orbit due to the lost gravitational energy, and the corresponding reduction in the time it takes per orbit. This graph maps the accumulated reduction in orbital periods against time, assuming that Einstein's equations are correct. Holtz, Taylor, and others have studied this binary system for 40 years now. This graph records their measurements. We see that the measurements fit the theory perfectly. This gave scientists confidence that Einstein's gravitational waves do indeed exist.