  To serve as a dispersion relation, a cotangent bundle function must satisfy
three simple algebraic properties. These conditions are derived from the
inescapable physical requirements to have predictive matter field dynamics and
an observer-independent notion of positive energy. Possible modifications of
the standard relativistic dispersion relation are thereby severely restricted.
For instance, the dispersion relations associated with popular deformations of
Maxwell theory by Gambini-Pullin or Myers-Pospelov are not admissible.
