Binary-pulsar tests of strong-field gravity

This talk is based on my work in collaboration with Thibault Damour since 1991. Unified theories, like superstrings, predict the existence of scalar partners to the graviton. Such theories of gravity can be very close to general relativity in weak-field conditions (solar-system experiments), but can deviate significantly from it in the strong-field regime (near compact bodies, like neutron stars). Binary pulsars are thus the best tools available for testing these theories. This talk presents the four main binary-pulsar experiments, and discusses the constraints they impose on a generic class of tensor-scalar theories. It is shown notably that they rule out some models which are strictly indistinguishable from general relativity in the solar system. This illustrates the qualitative difference between binary-pulsar and solar-system tests of relativistic gravity.