  The Galactic disk retains a vast amount of information about how it came to
be, and how it evolved over cosmic time. However, we know very little about the
secular processes associated with disk evolution. One major uncertainty is the
extent to which stars migrate radially through the disk, thereby washing out
signatures of their past (e.g. birth sites). Recent theoretical work finds that
such "blurring" of the disk can be important if spiral arms are transient
phenomena. Here we describe an experiment to determine the importance of
diffusion from the Solar circle with cosmic time. Consider a star cluster that
has been placed into a differentially rotating, stellar fluid. We show that all
clusters up to ~10^4 solar masses, and a significant fraction of those up to
~10^5 solar masses, are expected to be chemically homogeneous, and that
clusters of this size can be assigned a unique "chemical tag" by measuring the
abundances of <~10 independent element groups, with better age and orbit
determinations allowing fewer abundance measurements. The star cluster
therefore acts like a "tracer dye", and the present-day distribution of its
stars provides a strong constraint on the rate of radial diffusion or migration
in the Galactic disk. Sellwood & Binney have argued for strong radial transport
driven by transient spiral perturbations: in principle, we could measure the
strength of this migration directly.
