Block and Weinberger show that an arithmetic manifold can be endowed with apositive scalar curvature metric if and only if its $\rationals$-rank exceeds2. We show in this article that these metrics are never in the same coarseclass as the natural metric inherited from the base Lie group. Furthering thecoarse $C^\ast$-algebraic methods of Roe, we find a nonzero Dirac obstructionin the $K$-theory of a particular operator algebra which encodes informationabout the quasi-isometry type of the manifold as well as its local geometry.
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