  The force between like sign BPS saturated objects generally vanishes. This is
a reflection of the fact that BPS states are really massless uncharged
particles with nonvanishing momenta in compactified directions. Two like sign
BPS objects with zero relative velocity can be viewed as a boosted state of two
neutral massless particles in a state of vanishing relative motion. By contrast
two unlike sign BPS particles may be thought of as colliding objects moving in
opposite directions in compact space. This leads to complicated interactions
which are totally intractable at present. We illustrate this by considering the
potential between opposite sign zero-D- branes .
