Exploiting Binary Pulsars as Laboratories of Gravity Theories

Four decades have passed since the discovery of pulsars [1]. These unique objects have been proven to be invaluable in the study of a wide variety of physical and astrophysical problems. Most notable are studies of gravitational physics, the interior of neutron stars, the structure of the Milky Way and stellar and binary evolution. A number of these studies utilize the pulsar emission properties and/or the interaction of the radiation with the ambient medium. Most applications, however, are enabled by a technique known as pulsar timing. Here, pulsar astronomers make use of pulsars as accurate cosmic clocks where a number of fast-rotating pulsars, so called millisecond pulsars, show long-term stabilities that rival the best atomic clocks on Earth. Being compact massive objects with the most extreme states of matter in the present-day Universe, a number of pulsars are also moving in the gravitational field of a companion star, hence providing ideal conditions for tests of general relativity and alternative theories of gravity. In this review, I first discuss why a continuing challenge of Einstein's theory of gravitation, the theory of general relativity, with new observational data is still necessary. Then I describe pulsars and their use as clocks, in particular for their use as cosmic gravitational laboratories. Finally, I review some classical tests and report on the recent progress such as the discovery of the first double pulsar and look ahead to the future. This text should be read in close comparison to the contribution of Thibault Damour who provides much of the theoretical framework and motivation for the observations described here.