Binary pulsars and relativistic gravity

Spinning freely on their axes, and emitting radio noise detectable over interastellar distances, pulsars make extraordinarily stable natural clocks. Detailed comparisons of""pulsar time" with time kept by atomic clocks on Earth have opened the way for tests of gravity under conditions much more relativistic than found anywhere within the solar system. Among other results,these experiments have demonstrated the existence,quadrupolar nature, and propagation speed of gravitational waves.