What binary pulsar system provided the first indirect evidence for gravitational waves?

The Hulse–Taylor binary provided the first indirect evidence for gravitational waves. This system, also known as PSR B1913+16, contains a pulsar whose precisely timed radio pulses revealed that its orbit was gradually shrinking.

Russell Hulse and Joseph Taylor discovered the system in 1974 at Arecibo Observatory. The pulsar’s changing pulse arrival times showed that it was orbiting a compact companion, now understood to be another neutron star. General relativity predicts that such an accelerating binary should lose energy and angular momentum by emitting gravitational radiation.

Long-term timing measurements found that the orbital period decreased at the rate predicted by Einstein’s theory. This was indirect evidence because scientists were observing the binary’s orbital decay rather than detecting spacetime ripples directly. Direct detection had to wait until LIGO observed the merger of two black holes in 2015. Hulse and Taylor received the 1993 Nobel Prize in Physics for discovering the binary pulsar and using it to test relativistic gravity. The Double Pulsar later supplied additional high-precision tests, but it was not the first system to provide this evidence.