Some Considerations Regarding Lorentz-Violating Theories
  We investigate the compatibility of Lorentz-violating quantum field theories with the requirements of causality and stability. A general renormalizable model for free massive fermions indicates that these requirements are satisfied at low energies provided the couplings controlling the breaking are small. However, for high energies either microcausality or energy positivity or both are violated in some observer frame. We find evidence that this difficulty can be avoided if the model is interpreted as a sub-Planckian approximation originating from a nonlocal theory with spontaneous Lorentz violation. The present study thereby supports the validity of the standard-model extension as the low-energy limit of any realistic string theory that exhibits spontaneous Lorentz breaking. 
Ralf Lehnert