  The mobility of two interacting particles in a random potential is studied,
using the sensitivity of their levels to a change of boundary conditions. The
delocalization in Hilbert space induced by the interaction of the two particle
Fock states is shown to decrease the mobility in metals and to increase it in
insulators. In contrast to the single particle case, the spectral rigidity is
not directly related to the level curvature. Therefore, another curvature of
topological origin is introduced, which defines the energy scale below which
the spectrum has the universal Wigner-Dyson rigidity.
