 They were first predicted by Einstein in 1916; although a specific consequence of general relativity, they are a common feature of all theories of gravity that obey special relativity.However, after 1916 there was a long debate whether the waves were actually physical, or artefacts of coordinate freedom in general relativity; this was not fully resolved until the 1950s.Indirect observational evidence for their existence first came in the late 1980s, from monitoring of the Hulse–Taylor binary pulsar (discovered 1974); the pulsar orbit was found to evolve exactly as would be expected for gravitational wave emission.