  Remnant low-energy effects of Planck-scale Lorentz breaking in candidate
fundamental theories typically include modified one-particle dispersion
relations. Theoretical constraints on such modifications are discussed leading
to the exclusion of a variety of previously considered Lorentz-violating
parameters. In particular, the fundamental principle of coordinate
independence, the role of an effective dynamical framework, and the conditions
of positivity and causality are investigated.
