Binary pulsars act as precise clocks which can move very rapidly in a strong gravitational field of varying amplitude. By monitoring the arrival times of the emitted pulses over long intervals of time, it is possible to measure the orbital parameters of the binary systems with exquisite precision and carry out tests of predictions made by Einstein's theory of general relativity or other theories of gravity.
Progress in this field is made by long-term, high precision timing observations, where synergy effects are particularly beneficial. Currently we use the Lovell, Effelsberg and Westerbork telescopes within the PulSE network, and linking up with Parkes, Arecibo and Green Bank instruments to monitor a wide variety of binary systems.
Our most stringent tests of general relativity to date have been made from monitoring the double pulsar system. From our measurements so far, we have been able to detect a total of five so-called post-Keplerian parameters necessary to fully describe the orbit of this system. One of these parameters is the orbital decay due to the energy loss from the orbit by gravitational wave emission. A testament to the precision of pulsar timing is that the measured rate of decay amounts to only 7 mm per day!
The post-Keplerian parameters are summarized on the diagram shown below in terms of the masses of each pulsar. From the common point of intersection of the various constraints, we conclude that general relativity agrees to within 0.1% of the observed parameters. This remarkable result matches earlier Nobel-prize-winning observations of the original binary pulsar made over a 30-year interval. In our case, the extreme nature of the double pulsar system has made this new test within only 3 years! In the future, we expect to make dramatic improvements in the level of measurement precision and observe hitherto unseen relativistic parameters.