= Cavity equilibrium destabilized by a second void
A uniform <Newtonian gravitational field> inside one spherical void can be cancelled at a point by the repulsive field representing another removed sphere. The latter field has a <Jacobian matrix> proportional to $I-3\hat r\hat r^T$: its axial eigenvalue is negative and both transverse eigenvalues positive. The equilibrium is restoring along the axis but unstable transversely. A zero force is thus not enough for stable rest.
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