  We consider systems of two free particles in de Sitter invariant quantum
theory and calculate the mean value of the mass operator for such systems. It
is shown that, in addition to the well known relativistic contribution (and de
Sitter antigravity which is small when the de Sitter radius is large), there
also exists a contribution caused by the fact that certain decomposition
coefficients have different phases. Such a contribution is negative and
proportional to the particle masses in the nonrelativistic approximation. In
particular, for a class of two-body wave functions the mean value is described
by standard Newtonian gravity and post Newtonian corrections in General
Relativity. This poses the problem whether gravity can be explained without
using the notion of interaction at all. We discuss a hypothesis that gravity is
a manifestation of Galois fields in quantum physics.
