Persistent Singularities and Crack Motion Robert Deegan Physics, U Texas, Austin Singularities are a common feature of the solutions to partial differential equations and constitute a breakdown of the long-wavelength description of the system. Often the macroscale behavior of the system is unaltered by the microscale physics that regularizes the singularity. However, for some systems, such as contact line motion, the physics within the singularity determines the systems long-wavelength evolution. I will argue based on my results from two recent experiments that fracture is also such a system; its long-wavelength behavior is dominated by the stress singularity at the crack tip. In the first of these experiments, silicon is fractured at 80o K. In the absence of strong thermal fluctuations, I find that cracks cannot travel slower than a certain threshold, due to the atomic discreteness of a solid. In the second experiment, silicon is fractured by a slow moving thermal gradient. I find that the crack path is qualitatively different than in an isotropic solid, indicating a strong dependence on the underlying crystal lattice.