  Testing deviation of GR is one of the main goals of the proposed {\emph{Laser
Interferometer Space Antenna}}, a space-based gravitational-wave observatory.
For the first time, we consistently compute the generation of gravitational
waves from extreme-mass ratio inspirals (stellar compact objects into
supermassive black holes) in a well-motivated alternative theory of gravity,
that to date remains weakly constrained by double binary pulsar observations.
The theory we concentrate on is Chern-Simons (CS) modified gravity, a 4-D,
effective theory that is motivated both from string theory and loop-quantum
gravity, and which enhances the Einstein-Hilbert action through the addition of
a dynamical scalar field and the parity-violating Pontryagin density. We show
that although point particles continue to follow geodesics in the modified
theory, the background about which they inspiral is a modification to the Kerr
metric, which imprints a CS correction on the gravitational waves emitted. CS
modified gravitational waves are sufficiently different from the General
Relativistic expectation that they lead to significant dephasing after 3 weeks
of evolution, but such dephasing will probably not prevent detection of these
signals, but instead lead to a systematic error in the determination of
parameters. We end with a study of radiation-reaction in the modified theory
and show that, to leading-order, energy-momentum emission is not CS modified,
except possibly for the subdominant effect of scalar-field emission. The
inclusion of radiation-reaction will allow for tests of CS modified gravity
with space-borne detectors that might be two orders of magnitude larger than
current binary pulsar bounds.
