Binary Pulsars as a Test of General Relativity
A pulsar is a rotating neutron star that produces periodic signals in Earth detectors as its beam of radiation sweeps over Earth once per rotation. A binary pulsar may have the additional feature of a measurable decrease in the orbital period as the two pulsars spiral inward toward each other. This change in the orbital period can be attributed to energy loss due to the emission of gravity waves, and the rate of energy loss can be calculated from Einstein's general relativity. Therefore the measurement of the rate of change of the orbital period provides a test of general relativity.
The above illustration is patterned after one presented by the Nobel committee when they announced the 1993 Nobel Prize presented to Hulse and Taylor for the discovery of the first binary pulsar. Their discovery was based upon measurement of the Doppler shift of the radio frequency as the emitting pulsar orbited an unseen companion. The mean period of this pulsar (PSR 1913 + 16) was 0.05903 seconds, implying that this neutron star with a radius on the order of 10 kilometers but a mass comparable to the Sun is spinning on its axis almost 17 times per second! But that period varied several tens of microseconds with an orbital period of less than eight hours, corresponding to an orbital distance only several times the Moon-Earth distance. For these orbital characteristics, the general relativity correction to the perihelion motion would be 4 degrees per year compared to 43 seconds of arc per century for the perihelion of Mercury, the best example in our solar system.
After four years of observation of this pulsar, it was determined that the orbital period was declining by about 75 microseconds per year, agreeing within about 0.5% with the rate predicted by general relativity.
Reference: The Nobel Prize in Physics 1993