On Brane-Antibrane Forces

In this note, we will discuss two aspects of brane-antibrane forces. In one aspect, we generalize the force calculation of D0-${\bar {\rm D}}$0 of Banks and Susskind to Dp-${\bar {\rm D}}p$ for $1\le p \leq 8$. In particular, we find that the force is also divergent for p = 1 while for the other cases ($p \ge 2$) the forces are in general finite when $Z \to 0^+$, where $Z = \frac{Y^2}{2\pi^2\alpha'} - 1$ with Y, the brane-antibrane separation. However, the forces are divergent for all cases when Z < 0, signalling the occurrence of open string tachyon condensation in this regime. The other deals with the puzzling static nature of the supergravity brane-antibrane configurations. We will show that the force on a brane probe due to a brane-antibrane background vanishes when the probe is placed at the location of the coincident brane-antibranes, thereby providing a direct evidence for the existence of general static brane-antibrane configuration in the supergravity approximation.