  This talk is based on my work in collaboration with Thibault Damour. We
compare the probing power of different classes of gravity experiments:
solar-system tests (weak-field regime), binary-pulsar tests (strong-field
regime), and future gravitational-wave observations of inspiralling binaries
(strong-field effects detected in our weak-gravitational-field conditions).
This is done within the most natural class of alternative theories to general
relativity, namely tensor-scalar theories, in which the gravitational
interaction is mediated by one tensor field (g_munu) together with one or
several scalar fields (phi). Our main conclusion is that strong-field tests are
qualitatively different from weak-field experiments: They constrain theories
which are strictly indistinguishable from general relativity in the solar
system. We also show that binary-pulsar data are so precise that they already
rule out the theories for which scalar effects could have been detected with
LIGO or VIRGO. This proves that it is therefore sufficient to compute the
`chirp' templates within general relativity.
