Gravitational waves have a solid theoretical basis, founded upon the theory of relativity. 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.[3] Indirect observational evidence for their existence first came in 1974 from measurements of the Hulse–Taylor binary pulsar, whose orbit evolves exactly as would be expected for gravitational wave emission.[4] Richard Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for this discovery.[5] Subsequently, many other binary pulsars (including one double pulsar system) have been observed, all fitting gravitational-wave predictions.[6]