MG11 
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The Effacing Principle for Extended Bodies in the Post-Newtonian Mechanics

Newtonian gravity maintains the effacing principle stating that an extended, spherically-symmetric body moves in external gravitational field in the same way as a point-like particle with the same mass. We analyze the effacing principle in the post-Newtonian approximation of general relativity and in a class of scalar-tensor theories of gravity and demonstrate that this principle is not valid due to the non-linear interaction of the internal structure of the body with the external gravitational field. Our conclusion suggests that the point-particle model of a coalescing binary system used currently for calculation of templates of gravitational waves may be not an adequate physical paradigm.

Testing gravimagnetic field using radio links with the Cassini spacecraft

The most precise test of the Shapiro time delay in the solar system has been achieved in measurement of the frequency shift of radio waves to and from the Cassini spacecraft as they passed near the Sun. The relativistic model of a radio wave propagation that has been used to fit the Cassini data was derived in the heliocentric reference frame where the Sun is at rest but, in fact, it moves around the barycenter of the solar system. We analyze the effect of this motion on the propagation of a radio wave passing closely to the solar limb and demonstrate that it can affect the measured value of the PPN parameter gamma characterizing deviation of gravity from a pure geometry. According to general relativity the orbital motion of Sun generates gravimagnetic (non-stationary) components of the solar gravitational field which measurement would be a new test of general relativity in addition to gamma since the numerical value of gamma-1=0.000021 published in Nature by Bertotti, Iess and Tortora is affected by the solar gravimagnetic field at the level of uncertainty amounting to 0.0001. Experimental feasibility to detect the gravimagnetic field of the moving Sun should stimulate re-processing of the Cassini experimental data to make the measurement of gamma more robust.