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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.

Qmechanic
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Silent_Jager
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    In MWI there is no wave-function collapse. What else did you think might be non-deterministic? – WillO Dec 02 '16 at 16:17
  • 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. – Silent_Jager Dec 02 '16 at 16:26
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    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. – WillO Dec 02 '16 at 17:49
  • 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.") –  Dec 02 '16 at 18:38

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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.

Joel Klassen
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    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. – Bob Bee Dec 03 '16 at 06:08
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    I don't see how the second assertion follows – Joel Klassen Dec 04 '16 at 18:25
  • 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. – Bob Bee Dec 05 '16 at 01:04
  • @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. – peterh Dec 05 '16 at 03:04
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    @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? – Joel Klassen Dec 05 '16 at 17:08
  • @JoelKlassen haha, I was trying to put into words myself how the question there is misformed. It's not random which universe you end in, you end in ALL universes! – TKoL Feb 18 '21 at 16:37
  • Just because the internal mathematics of the model are deterministic doesn't mean the model itself is deterministic. There still remains the key element that connects the internal math to the laboratory predictions, and that part is non-deterministic: in MWI, that part is that the observed results are in one world, and you can´t know which one, so it's not deterministic. – Juan Perez May 17 '21 at 19:57
  • @JuanPerez It depends in your point of view. From the viewpoint of you, a conscious individual whose brain is a quantum system, MWI is non-deterministic because you are following branches at random. But from the viewpoint of a hypothetical all-seeing and all-knowing physicist, which is the normal viewpoint for discussing physics, it’s deterministic because they can see that all the possible versions of you exist and there is no room left for randomness. – Mike Scott Jul 19 '21 at 11:49