In other words, the idea is that there’s a fourth level of parallel universes that’s vastly larger than the three we’ve encountered so far, corresponding to different mathematical structures. The first three levels correspond to noncommunicating parallel universes within the same mathematical structure: Level I simply means distant regions from which light hasn’t yet had time to reach us, Level II covers regions that are forever unreachable because of the cosmological inflation of intervening space, and Level III, Everett’s “Many Worlds,” involves noncommunicating parts of the Hilbert space of
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