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Can MWI work without shared histories?

  1. Oct 28, 2014 #1
    If the same particle is measured with spin up in one universe and spin down in another (at the same time), is it absolutely required in MWI that they both have the same history? That is. Why couldn't the paired universes have different histories, but the different histories lead up to the same measurement?
  2. jcsd
  3. Oct 29, 2014 #2


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    Roughly speaking, this is like asking whether it is possible that two identical twins (i.e. two human beings with identical genetic material) do not have common parents? In principle it is possible, but in practice the probability for such a coincidence is negligibly small. The same is with MWI.

    More generally, identical twins are a good analogy to develop intuition about many aspects of MWI.
  4. Oct 29, 2014 #3


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    Is this a bit like asking whether there is a version of BM where every initial condition is realized as a separate universe? An example of such a Bohmian Many-Worlds interpretation seems to be described by Wallace (in the context of describing ideas of Deutsch, Zeh, Brown and Wallace) in http://arxiv.org/abs/0712.0149:

    "Advocates of the Everett interpretation claim that, (given functionalism) the decoherence-defined quasiclassical histories in the unitarily evolving physically real wavefunction describe — are — a multiplicity of almost-identical quasiclassical worlds; if that same unitarily-evolving physically real wavefunction is present in DBB (or any other hidden-variable theory) then so is that multiplicity of physically real worlds, and all the hidden variables do is point superfluously at one of them."
    Last edited: Oct 29, 2014
  5. Oct 31, 2014 #4
    You say it yourself, "the same particle in two universes". What else could "the same particle" mean, as opposed to two separate particles?
  6. Oct 31, 2014 #5

    But from the particle's point of view, that finds itself with spin up in one universe, does it really matter what events transpired in the
    macroscopic world? Isn't that suggesting that the macroscopic world determines the particle's spin?
  7. Oct 31, 2014 #6
    It means its the same particle with different histories, much in the same way that the particle clearly has divergent futures in both universes. Think of two entangled electrons. Is it required for them to share the same history in order to be entangled? No.

    My point is couldn't the particle have divergent past and futures that intersect. And this intersection that is MWI.
    Last edited: Oct 31, 2014
  8. Nov 1, 2014 #7
    Isn't that the Path Integral formulation of QM? The histories/paths diverge then re-converge.
  9. Nov 1, 2014 #8
    Nope, I do not follow. Then it is just two separate particles, coincidentally gotten into the detectors at the same time in each universe. The distinction "same particles" has no meaning then.
  10. Nov 1, 2014 #9
    I guess that would depend on whether the worlds split as the particle is emitted or whether the world splits on measurement.
  11. Nov 1, 2014 #10
    I don't know. I had to Google that one. Thanks though, I'm going research Path Integral some more.
  12. Nov 1, 2014 #11
    Except with non-shared histories and divergent paths there is no need to split the universe. The two universes always existed.
  13. Nov 2, 2014 #12
    So the universes all exist, but some are identical until there is an event which distinguishes them?
  14. Nov 2, 2014 #13
    No. My question is based upon two universes that do not have shared histories. If a particle can exist in two different places at the same time, then why can't it exist in two different universes at the same time- within the framework of MWI?

    ***WARNING: the following paragraphs are stream of thought ***

    Even if the two universes had different histories (and futures) why shouldn't the particle still have spin up in one universe and spin down in the other at the same point in time? Again, there's no need to split the universe because both of them already existed.

    This brings up another question. How would wavefunction collapse work under this scheme? I don't see why wavefunction collapse wouldn't result in the particle having, for instance, spin up in both universes. That is, the particle appears to behave classically.
  15. Nov 3, 2014 #14


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    Without macroscopic world there is no decoherence, and hence no wave-function splitting, and hence no many worlds. So yes, the macroscopic world does determine the particle's spin.
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