#### PeterDonis

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I see this claim made fairly often, but it does not seem correct to me. According to the MWI, the dynamics of the wave function is always unitary (there is no collapse), and a unitary process cannot "create" or "destroy" anything. All it can do is entangle things. So, for example, if we consider a measurement with two possible results, under the MWI, unitary evolution induces the state transition (highly schematic since I am ignoring normalization)MWI requires the instantaneous creation of a pair (or infinite number) of parallel universes.

$$

\left( |1> + |2> \right) |R> \rightarrow |1>|U> + |2>|D>

$$

where ##|1>## and ##|2>## are the eigenstates of the measurement for the measured system, and ##|R>##, ##|U>##, and ##|D>## are the "ready to measure", "measured state 1", and "measured state 2" states of the measuring apparatus. The MWI describes this final state as having two "worlds", in one of which the measurement yielded the "1" result and in the other of which the measurement yielded the "2" result; but since the state transition induced by the measurement is unitary, nothing has been "created"; all that has happened is entanglement of the measured system and the measuring device.

Does anyone know of discussions of this in the literature?