  We investigate how forces spread through frictionless granular packs at the
jamming transition. Previous work has indicated that such packs are isostatic,
and thus obey a null stress law which, independent of the packing history,
causes rays of stress to propagate away from a point force at oblique angles.
Prior verifications of the null stress law have used a sequential packing
method which yields packs with anisotropic packing histories. We create packs
without this anisotropy, and then later break the symmetry by adding a
boundary. Our isotropic packs are very sensitive, and their responses to point
forces diverge wildly, indicating that they cannot be described by any
continuum stress model. We stabilize the packs by supplying an additional
boundary, which makes the response much more regular. The response of the
stabilized packs resembles what one would expect in a hyperstatic pack, despite
the isostatic bulk. The expected stress rays characteristic of null stress
behavior are not present. This suggests that isostatic packs do not need to
obey a null stress condition. We argue that the rays may arise instead from
more simple geometric considerations, such as preferred contact angles between
beads.
