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B Unitary and Branches

  1. Oct 25, 2016 #1
    What theorem would be violated if there is communication between the different branches? I read branches could be just possibilities.. so it's not hard for there to be communications between the possibilities. It is when you try to imagine the branches as worlds that you would find it hard to imagine for there to be communications. but it seems more of a category error... It's just like saying part of the cpu processing in your computer (the pipeline) are located in different worlds and so there is huge barrier between them. But if you think of them as possibilities like many do.. then it's no problem. But what theorem forbids the communications between branches...do you see it as like engenstates already formed and so there should be no communcations between the eigenstates? But what if the eigenstates have not yet formed.. it is because you assume the branches are eigenstates of the global superposition that you think they are separate already but if the eigenstates are like possibilities then they could still intermingle.
  2. jcsd
  3. Oct 25, 2016 #2


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    Staff: Mentor

    Where did you read this? Unless you tell us this, we have no way of telling you whether you've read and incompletely understood a good source, or whether you've been misled by a bad source.
  4. Oct 25, 2016 #3
    Ah, It's from the book The Schrodinger Rabbits the Many Worlds of Quantum. In the chapter Harnessing Many Worlds, the chapter starts with:

    "In the final chapter, we will look at some controversial tests that might prove the correctness of the many-worlds interpretation beyond doubt. But there is one kind of experiment that has already been done successfully and could be said to demonstrate not only that worlds in which history unfolds differently are real, but also that communications between worlds is possible, at least in a carefully defined and limited way.
    The basic procedure is known as the Elitzur-Vaidman experiment...."

    Again I was just interested what theorem would be broken if there is communications between the branches. Or can someone just give me all papers about such communications? Also I read Zurek saying the branches are just possibilities, not real worlds, so what would be wrong for communications between possibilities.. who knows the Elitzur-Vaidman experiment or others like it could be just possibilities influencing one another... this is more believable than if there were really solid worlds and solid barriers.
  5. Oct 25, 2016 #4


    Staff: Mentor

    The theorem that says MW has exactly the same formalism as standard QM.

    They dont exist because MW is formally no different to standard QM. If they did exist and are proven then standard QM would be wrong and MW the only theory.

  6. Oct 26, 2016 #5
    How do you expressed this mathematically in the standard QM.. that the eigenstate or born rule when applied can no longer contact with one another?

    But while the wave function is in unitary or not orthogonal state, it is free to choose any state it wants.. so before worlds are spawned.. it can compute (in superposition).

    Is saying that branches or worlds can communicate the same as the mathematical statements as saying it is still possible after eigenstate or after born rule applied (environment traced out) for any branches to communicate (so I can see how and why it is wrong).

  7. Oct 27, 2016 #6


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    Communication between the branches would violate linearity of QM.
  8. Oct 27, 2016 #7
    What is the synonym of linearity? What does a non-linear wave function looks like? Are you talking about born rule being reversed back to unitary hence impossible? What is the mathematical expression of this linearity or non-linearity? Thanks man.
  9. Oct 27, 2016 #8


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    Linearity is equivalent to the superposition principle, of which you can find more here:
  10. Oct 27, 2016 #9
    If linearity means superposition, why would communications between branches violate the superposition principle? Were you referring to pure state or the entangled state? In pure state, there are not even branches yet but just like quantum computing, there is more information in the state vector (although you can only access them when born rules applied as in Shor Algorithm). So when you say superposition (linearity) is violated if there is communications between branches.. were you referring after born rule applied or prior?
  11. Oct 27, 2016 #10


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    Entangled states are pure. Communication assumes a Born rule.
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