In the weak-field limit of General Relativity, gravitational waves obey
linear equations and propagate at the speed of light. These properties of
General Relativity are supported by the observation of ultra high energy cosmic
rays as well as by LIGO's recent detection of gravitation waves. We argue that
two existing relativistic generalizations of Modified Newtonian Dynamics,
namely Generalized Einstein-Aether theory and BIMOND, display fatal
inconsistencies with these observations.Comment: Minor clarifications, title changed, accepted for publication in
Physical Review Letter