  We analyse perturbatively, whether a flat background with vanishing G-flux in
Horava-Witten supergravity represents a vacuum state, which is stable with
respect to interactions between the ten-dimensional boundaries, mediated
through the D=11 supergravity bulk fields. For this, we consider fluctuations
in the graviton, gravitino and 3-form around the flat background, which couple
to the boundary $E_8$ gauge-supermultiplet. They give rise to exchange
amplitudes or forces between both boundary fixed-planes. In leading order of
the D=11 gravitational coupling constant $\kappa$, we find an expected trivial
vanishing of all three amplitudes and thereby stability of the flat vacuum in
the static limit, in which the centre-of-mass energy $\sqrt{s}$ of the
gauge-multiplet fields is zero. For $\sqrt{s}>0$, however, which could be
regarded a vacuum state with excitations on the boundary, the amplitudes
neither vanish nor cancel each other, thus leading to an attractive force
between the fixed-planes in the flat vacuum. A ground state showing stability
with regard to boundary excitations, is therefore expected to exhibit a
non-trivial metric. Ten-dimensional Lorentz-invariance requires a warped
geometry. Finally, we extrapolate the amplitudes to the case of coinciding
boundaries and compare them to the ones resulting from the weakly coupled $E_8
\times E_8$ heterotic string theory at low energies.
