Why is it that MWI's problem with special relativity is rarely discussed? From: http://plato.stanford.edu/entries/qm-everett/ A final problem is that it is unclear how to formulate a splitting-worlds reading of Everett that is compatible with the constraints of special relativity. Suppose one opts for a strong sort of splitting, contrary to what Everett seems to suggest, where there are more physical systems after a typical measurement than before. If this involves somehow the creation of an entirely new universe (a complete copy of spacetime with an event structure, say) then when is the new universe created? One problem is in giving a frame-independent description of the creation event in the original universe, another is in making sense in relativity of an event that creates a new spacetime when all events, including the creation event, are supposed to be characterized by the local features of a particular fixed spacetime. Those who favor a decoherence account of splitting worlds sometimes seem to imagine some sort of "unzipping" of spacetime that occurs along the forward light cone of the spacetime region that contains the measurement interaction. While decoherence effects can be expected to propagate along the forward light cone of the region that contains the interaction event between the measuring device and the object system, and while there is no problem describing the decoherence effects themselves in a way that is perfectly compatible with relativity, there is a problem in imagining that such a splitting process somehow physically copies the systems involved. A strong picture of spacetime somehow unzipping into connected spacetime regions along the forward light cone of the measurement event, would not be compatible with special relativity insofar as relativity presupposes that all events occur on the stage of Minkowski spacetime. And if we give up this assumption, then it is unclear what the rules are for compatibility with special relativity. Alternatively, one might imagine that there is only one non-branching spacetime but many, equally real, physical copies of the relevant systems being created by the splitting process. But then it is unclear how this might be compatible with the conservation laws associated with the symmetries of the Lorentz group; and insofar as conservation of energy is violated, the theory is strongly incompatible with special relativity. Of course, one might suggest giving up or modifying special relativity; but such a sacrifice would only be attractive if the benefits clearly outweighed the cost, and the cost of sacrificing relativity is high given its long track-record of empirical and explanatory success. If one takes seriously the option of giving up or changing relativity in order to accommodate one's favorite reading of Everett, then it is again unclear what the rules are for theory evaluation. What are the opponents and proponents opinions? I'm tired of hearing people say Bohm is a "ugly interpretation" because of the nonlocality, when infact MWI has a unresolved problem with relativity too.