I don't know how to understand this:
- Suppose we measure the spin of an electron with apparatus M. M can yield spin-up or spin-down. According to MWI, M briefly becomes in superposition of measuring spin up and spin down. Extremely quick however, M gets split by the splitting into two universes, in one of which M yields spin-up and in the other spin-down.
- Now, if we split a photon with a half-silvered mirror, then the photon will become in superposition of being let through and being reflected: