PSR B1913+16 (or J1915+1606) is a binary pulsar, or a pulsar in a binary star system. A binary pulsar consists of two stars orbiting around each other (around a common center of mass based on the two stars).
This particular binary pulsar, PSR B1913+16 was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of Princeton University. They were later awarded the 1993 Nobel Prize in Physics based on a claim in the abstract of their paper that this binary pulsar confirms the Theory of General Relativity due to its loss in energy over time, which Hulse and Taylor attributed to the radiation of gravitational waves under the Theory of General Relativity. The body of the paper itself showed that assumptions could be made about the physical attributes of the system (masses, periastron, angular momentum, etc.) in order to fit the data to the theory. The "fit" to General Relativity was about 0.2%.
- Fitting observed data to match a theory based on those data is a common practice. Kepler's laws of planetary motion don't say what the orbit of Mars is. They make general statements about what the orbits have to look like. Kepler took observational data and fit them into his laws by setting the appropriate values for major and minor axes, perihelion point, etc. If the laws had been incorrect (perhaps because the gravitational field were inverse cube instead of inverse square) he would not have been able to fit the observed orbital behavior. Hulse and Taylor similarly took the rather scant data about pulsar period fluctuation and fit it to physical data about the pulsars and their orbits. They could only "see" one of the two pulsars; the pulsation beam of the other one did not strike Earth.
The observations were made in the mid-1970's. As time went by, the precession of the orbiting pulsars moved the beam out of the Earth's line of sight. The signals became fainter, and were nearly gone by 2003. (It had been a serendipitous occurrence that the signals were detectable in the first place.)
In 2004, Professor Taylor co-authored a paper reviewing new data from this binary pulsar, and pointing out the loss of signal. They also speculated on factors that could explain the 0.2% discrepancy. The calculations required more detailed knowledge of "galactic constants" (size and shape of the galaxy, the Earth's actual location within it, the pulsars' distance, the pulsars' proper motion, etc.) than had been available. They noted that, since the pulsars had swung out of view by that time, "it seems unlikely that this test of relativistic gravity will be improved significantly."
Further observations of this sort will have to wait until another binary pulsar pair swings into view.
- J. M. Weisberg and J. H. Taylor, Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis (July 7, 2004).