General relativity tested anew

The double pulsar system PSR J0737-3039A/B is unlike anything else observed in our galaxy. The binary’s two members, called A and B for short, are radio pulsars that orbit in their mutual strong gravitational field. As a pair of cosmic clocks, the double pulsar offers unique opportunities to test general relativity and competing gravitational theories in the strong-field regime. In the latest of those tests, graduate student René Breton of McGill University and an international team determined the precession of B’s spin axis around the binary system’s total angular momentum. To do that, they measured the radio flux from A as it was periodically eclipsed by B. The data fit a model that included, among other parameters, the direction of B’s spin axis and the opacity of the plasma surrounding B, where A’s radio emission is absorbed. The deduced precession rate, about 5° per year with an uncertainty of about 13%, is consistent with the prediction of general relativity. Moreover, the precession rate can be related to a strong-field spin–orbit coupling not just for general relativity but for any Lagrangian-based theory of gravity whose action is unchanged by Lorentz boosts, rotations, and translations. Thus the new double-pulsar measurements determine a parameter that presents a test for generic alternatives to general relativity. (R. P. Breton et al., Science 321 , 104, 2008 http://dx.doi.org/10.1126/science.1159295 .)