  Block and Weinberger show that an arithmetic manifold can be endowed with a
positive scalar curvature metric if and only if its $\rationals$-rank exceeds
2. We show in this article that these metrics are never in the same coarse
class as the natural metric inherited from the base Lie group. Furthering the
coarse $C^\ast$-algebraic methods of Roe, we find a nonzero Dirac obstruction
in the $K$-theory of a particular operator algebra which encodes information
about the quasi-isometry type of the manifold as well as its local geometry.
