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MWI understanding

  1. Feb 23, 2016 #1
    Hey guys,

    I want to understand the basic assumptions of MWI in quantum physics. So one of it is that all possible states exist in different universes, and that's relatively understandable although weird. Now my problem is the splitting of the universes and how and when does it happen.

    Example 1) Schrodinger's cat. In normal circumstances the decoherence occurs at the detector and we observe only one outcome. MWI says that both possibilities exist, just in different universes, continuously, so when and how does the splitting occur, are possibilities already split per se so that a cat is let's say alive in our universe and dead in another?

    Example 2) Macroscopic localization
    The good old is the moon there when nobody looks question. In normal circumstances and if we regard that this universe is the only one that is real, due to decoherence the moon is there or thereabout at any instant, it is highly localized. So this is a bit different than Schrodinger's cat, MWI would assume that the Moon can be at any location in the universe (of course one location per universe), so how does this follow from decoherence which gives moon a constant location throught time (e.g. there was no situation where the moon was spread through space and then "collapsed" to a definite position in our universe leaving other options to realize in another universe)

    Your suggestions are welcome. Thanks.
  2. jcsd
  3. Feb 23, 2016 #2


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    Staff Emeritus
    Science Advisor

    There is no precise moment that a split happens. In MWI, there is a wave function for the whole universe, and it just evolves smoothly. But a "split" is considered to happen when a microscopic change is amplified to make a macroscopic difference.

    For example, let's consider a tiny little universe consisting of nothing but an electron and a detector. The electron has a property, spin (in the z-direction, for concreteness), which can have two possible values: spin-up or spin-down. The detector measures the spin, and either prints out a message saying "spin up" or prints out a message saying "spin down". Suppose that initially, the electron is in a superposition of spin-up and spin-down.

    Before the measurement, the detector is in a state, "waiting for results" and the electron is in a state "superposition of spin-up and spin-down".
    After the measurement, the entire universe is in a superposition of
    1. A state in which the detector printed out "spin up" and the electron is in the spin-up state, and
    2. A state in which the detector printed out "spin down" and the electron is in the spin-down state.
    Speaking informally, the single detector has "split" into two possible states, or alternative worlds. But all that has really happened is that there already was a superposition--the electron was in a superposition. That superposition then "infected" the detector, to put it into a superposition, as well.

    So the "split" just means that the superpositions have started making a difference to macroscopic objects.
  4. Feb 23, 2016 #3


    Staff: Mentor

    Its easy.

    After decoherence you have the mixed state where each outcome occurs with a certain probability. Instead of only one outcome happening each is interpreted as a separate world and everything continues to evolve - no collapse - no nothing.

    Now don't get aught up in what's going on during decoherene - it happens so fast its irrelevant.

    If you want to go down that path you will need to study the modern version based on histories:

  5. Feb 24, 2016 #4
    Well, as you know, this issue about decoherence time has been my biggest obstacle in understanding it all together. When someone says 'it happens so fast that we cannot observe superpositions' that seems pretty vague to me. I understand how theoretically it turns a superposition into a mixed state, but practically I still don't. In a sense that the objects around us seem already decohered, but decoherence is actually still happening, so I don't understand what superpositions are destroyed through the process. Or to state it better, does continuous decoherence manage to keep objects at an almost precise location all the time and the only superpositions that get 'destroyed' are the range of nearby microscopical locations of the object near the object. Theoretically as I say this all seems fine but decoherence as a process in objects that already seem decohered seems fuzzy.
  6. Feb 24, 2016 #5


    Staff: Mentor

    Then I, and I suspect anyone else, cant help you. It utterly simple, but if you don't get it - shrug.

  7. Feb 24, 2016 #6
    You've tried to help and I learned a lot from you, but there still exists a conceptual barierre. If somebody could give me a real life example or analyze mine that would be great.

    For instance I am observing my cell phone and decoherence is occuring constantly, so what superpositions I don't see or what superpositions are destroyed? As I see it, my phone is constantly spreading in space by a tiny amount but decoherence continuously localizes it. I know that may be too simple but I would like to elaborate it with you.
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