A test of general relativity from the three-dimensional orbital geometry of a binary pulsar

Binary pulsars provide an excellent system for testing general relativity b ecause of their intrinsic rotational stability and the precision with which radio observations can be used to determine their orbital dynamics. Measur ements of the rate of orbital decay of two pulsars have been shown(1,2) to be consistent with the emission of gravitational waves as predicted by gene ral relativity, but independent verification was not possible. Such verific ation can in principle be obtained by determining the orbital inclination i n a binary pulsar system using only classical geometrical constraints. This would permit a measurement of the expected retardation of the pulse signal arising from the general relativistic curvature of space-time in the vicin ity of the companion object (the 'Shapiro delay'). Here we report high-prec ision radio observations of the binary millisecond pulsar PSR J0437-4715, w hich establish the three-dimensional structure of its orbit. We see the Sha piro delay predicted by general relativity, and we determine the mass of th e neutron star and its white dwarf companion. The determination of such mas ses is necessary in order to understand the origin and evolution of neutron stars(3).