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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.