  We calculate the gravitational potential energy between infinitely long
parallel strings with tensions \tau_1 and \tau_2. Classically, it vanishes, but
at one loop, we find that the long range gravitational potential energy per
unit length is U/L = 24G_N^2\tau_1\tau_2/(5 \pi a^2) + ..., where a is the
separation between the strings, G_N is Newton's constant, and we set \hbar = c
=1. The ellipses represent terms suppressed by more powers of G_N \tau_i.
Typically, massless bulk fields give rise at one loop to a long range potential
between p-branes in space-times of dimension p+2+1. The contribution to this
potential from bulk scalars is computed for arbitrary p (strings correspond to
p=1) and in the case of three-branes its possible relevance for cosmological
quintessence is commented on.
