**AAS Meeting #193 - Austin, Texas, January 1999**

*Session 42. Gamma-Rays/Gravitation*

Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1
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## [42.14] Astrometric test of general relativity

*Slava G. Turyshev (Caltech/Jet Propulsion Laboratory)*

Recently considerable interest has been shown in the
physical processes occurring in the strong gravitational
field regime. It is known that the classical description,
provided by general relativity, breaks down in a domain
where the curvature is large, and, hence, a proper
understanding of such regions requires new physics. The
tensor-scalar theories of gravity, where, the usual for
general relativity tensor field, coexists together with one
or several long-range scalar fields, are believed to be the
most promising extension of the theoretical foundation of
modern gravitational theory. Damour & Nordtvedt (1993) have
found that a generic scalar-tensor theory of gravity
contains a `built-in' cosmological attractor mechanism
towards general relativity. Their analysis strongly
motivates the search for possible deviation of the parameter
\gamma from unity. For this reason, a number of specific
space experiments dedicated to measure \gamma with a
precision up to a 10^{-5}-10^{-6} have been proposed. We
stress that the future optical interferometers in space,
such as SIM, would provide this precision as a simple
by-product of their astrometric program. The recent analysis
of the VLBI data by Eubanks *et al.* (1997) has provided
the best current estimate for this parameter as
|\gamma-1|=3\times 10^{-4}. Note that SIM will routinely
operate at this level of accuracy and, therefore, this
effect will have to be necesserily included into the
astrometric model and the corresponding data analysis.
Furthermore, a differential astrometric measurements with an
accuracy of ~ 1 \muas over the instrument's FoV of
15^{\}circ may potentailly provide a precision of about few
parts in a million in determining \gamma due to the solar
gravity.

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