Is the Many Worlds Interpretation deterministic? Considering that you can determine all of the possible outcomes of a wavefunction (pretty much impossible but still), the only random thing that remains is in what version of the universe you will proceed.

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    $\begingroup$ In MWI there is no wave-function collapse. What else did you think might be non-deterministic? $\endgroup$
    – WillO
    Commented Dec 2, 2016 at 16:17
  • $\begingroup$ I didn't say there is a collapse. I said that the only non-deterministic thing is the branch of the universe that you will go to. $\endgroup$ Commented Dec 2, 2016 at 16:26
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    $\begingroup$ The wave-function collapse is the only non-deterministic part of quantum mechanics. If there's no wave function collapse, there's no place for non-determinism to creep in. $\endgroup$
    – WillO
    Commented Dec 2, 2016 at 17:49
  • $\begingroup$ There isn't a yes/no answer, because defining determinism is subtle. There isn't even a simple yes/no answer as to whether Newtonian mechanics is deterministic. (Search on keywords like "Norton's dome," "noncollision singularity," and "staccato run.") $\endgroup$
    – user4552
    Commented Dec 2, 2016 at 18:38

1 Answer 1


As WillO has pointed out, the many worlds interpretation is deterministic. This follows from the fact that the MWI requires that the unitary evolution of the wavefunction is never violated. since unitary evolution is deterministic, MWI must also be deterministic.

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    $\begingroup$ Yes, deterministic, you just don't don't which world you're in, so practically random. Sinc ew e seem to approximate classical determinism, MWI must be wrong. $\endgroup$
    – Bob Bee
    Commented Dec 3, 2016 at 6:08
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    $\begingroup$ I don't see how the second assertion follows $\endgroup$ Commented Dec 4, 2016 at 18:25
  • $\begingroup$ I won't argue the second point, mostly my dissatisfaction with MWI. But you agree the randomness of which world you wind up in, or are in, is basically random? By positing that the multiple words exist and that they all occur on any measurement, it's clear that the result is random. Calling it deterministic makes no sense. It is different in normal QM/QFT, where the wavefunction or state evolves Unitarily always, even during measurements which are simply ascribed to multiple interactions and decoherence due to so may interactions and our inability to follow each unitary microscopic process. $\endgroup$
    – Bob Bee
    Commented Dec 5, 2016 at 1:04
  • $\begingroup$ @BobBee I think it would be nice if some currently unknown effect would exist to "re-unify" the zillions of the worlds. It would mean in the experimental sense, that the Gambler's fallacy would be true after a lot of experiments. $\endgroup$
    – peterh
    Commented Dec 5, 2016 at 3:04
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    $\begingroup$ @BobBee Suppose you were born as a twin. Would you say that which twin body you ended up in was basically random? Would you say that the process of birthing twins was non-deterministic? $\endgroup$ Commented Dec 5, 2016 at 17:08

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