Say we send a photon through a beam splitter and let each path decohere, resulting in two separate worlds. Does the entire universe split into two everywhere at the same instant (and what does that mean, given that Einstein’s relativity abolished the notion of a universal “now”) or does it start splitting at the point where the decoherence happens near the beam splitter, and move outward at the speed of light? Opinions differ, and there’s no consensus, even among those who are not troubled by the idea of many worlds.
