Einstein's General Theory of Relativity predicts that gravitational waves are so weak that they are almost impossible to detect. In fact, these imperceptible ripples in space-time are so feeble by the time they reach Earth that sophisticated experiments have not yet made any direct detections.

But radio astronomers have detected gravitational waves indirectly. In 1974, while searching for pulsars with the giant Arecibo radio telescope in Puerto Rico (shown in the movie Contact starring Jodie Foster), Joseph Taylor and Russell Hulse discovered two neutron stars in orbit around each other. One of these neutron stars aims its beams directly at Earth, making it a pulsar. This system is officially known as PSR 1913+16 for its sky coordinates, but is more commonly known as the Binary Pulsar (even though only one of the two neutron stars is actually a pulsar).

Over the following decade, radio observations of the pulsar's signal showed that the orbital period of the system (currently 7.75 hours) is decaying at a rate in exact accordance with the predictions of General Relativity. Einstein's equations predict that the two neutron stars should emit gravitational waves as they orbit a common center of mass. The waves carry away orbital energy, causing the two bodies to draw slowly closer together. The perfect match between the observed orbital decay and Einstein's prediction is so exquisite that it has convinced almost all physicists and astronomers that gravitational waves must exist. The Nobel Prize committee was also convinced; it awarded the 1993 physics prize to Taylor and Hulse for their great discovery.

More recently, radio astronomers have found several other binary neutron star systems that are also behaving in accordance with Einstein’s predictions. Most notably, the two neutron stars in the binary system PSR J0737 are both pulsars, and their 2.45-hour orbital period is even shorter than the pair in the Taylor/Hulse system. The shorter period means that the effects of General Relativity are stronger. The pulsars are emitting stronger gravitational waves, and the orbital period is shrinking faster (7 millimeters per day). This binary system is the best known laboratory for testing Einstein's theory. Precise timing measurements of both pulsars in PSR J0737 show that the orbital period is also shrinking at the exact rate predicted by General Relativity, bolstering what was already a convincing case that gravitational waves are for real.