  We prove a rigidity theorem for the geometry of the unit ball in random
subspaces of the scl norm in B_1^H of a free group. In a free group F of rank
k, a random word w of length n (conditioned to lie in [F,F]) has
scl(w)=log(2k-1)n/6log(n) + o(n/log(n)) with high probability, and the unit
ball in a subspace spanned by d random words of length O(n) is C^0 close to a
(suitably affinely scaled) octahedron.
  A conjectural generalization to hyperbolic groups and manifolds (discussed in
the appendix) would show that the length of a random geodesic in a hyperbolic
manifold can be recovered from the bounded cohomology of the fundamental group.
