  We consider a string with uniform energy density and tension and with a
number of pointlike masses attached at fixed interdistances. We evaluate the
effective interaction forces between these masses induced by the quantum
fluctuations of the string. For interdistances large compared to the thickness
of the string and small compared to the total length, these forces are
universal and attractive and fall off, for large distances as $1/r^3$ and for
small distances as $1/r$. The attractive nature of these forces creates an
instability under which masses added to the string tend to aggregate.
