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Many-worlds: When does the universe split?

  1. Oct 16, 2013 #1

    I don't quite understand some central points of the many-world interpretation: When does the splitting happen, and is there superposition in a single universe? I see two alternatives:

    1. There is superposition in a single, and this universe splits when we measure. This means we can start with only one universe which branches out more and more.
    2. There is no superposition in a single universe. We just find out about which universe we're in when we measure.

    Both of these alternatives are not satisfactory to me at all. In the first, I can't see the "advantages" over Kopenhagen, because the measurement is still a very special process, namely it causes a whole universe to split!
    The second is even weirder, it would imply that from the beginning there are already enough universes (an enormous number) for all possible superpositions to come. But how do these universes know in which basis we are going to measure? We would have to give up every bit of free will. On the positive, a measurement would really be nothing special at all.

    Which of the two is more accepted (or are there more interpretations)? Both seem very implausible to me.
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  3. Oct 16, 2013 #2


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    In the MWI, decoherence is what splits the worlds and this is neither restricted to measurements nor is it special in the sense that it is introduced by hand like the measurements in Copenhagen. It can be derived from the Schrödinger equation.

    What's special in the MWI is the interpretation of the decohered state and not the kind of processes which occur. MWI is the most literal interpretation of QM. It is what you get if you apply QM to the measurement process itself. However, not everyone agrees that it is a valid interpretation.
  4. Oct 16, 2013 #3


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    Personally, I think "implausible" is an extremely generous description of the various many-worlds / multiverse concepts. I tend to use words that are less acceptable in polite company.
  5. Oct 16, 2013 #4


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    I think the basic idea is pretty compelling but it is not clear to me how it explains the probabilities we observe in experiments. Or if it is even meaningful to request such an explantion within the context of the MWI.
  6. Oct 16, 2013 #5
    In many-worlds, the central object is the wave function of the universe. The wave function evolves with time according to the Schrodinger equation. The wave function tends to contain localized lumps of probability. We can call these localized lumps "worlds." Each lump corresponds to a possible configuration of the universe, and each lump evolves independently.

    It can happen that a localized lump of probability amplitude within the wave function splits in two. Then we can say that one of the worlds has split into two worlds. For example, one lump might describe a configuration of the universe in which a measuring device readout displays "electron with spin up," while the other lump might describe a configuration of the universe in which a measuring device readout displays "electron with spin down."

    The point is that measurement is not special: the wave function of the universe simply evolves according to the Schrodinger equation at all times. Sometimes a lump of probability amplitude will split into two; if we like we can call this a "measurement."
  7. Oct 16, 2013 #6


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    I'm trying to understand what MWI is is really about. The sentence "MWI is the most literal interpretation of QM. " leaves me completely blank. I don't get the need for lumpy splitting to explain decoherence and the apparently classical world that results. What am I missing?
  8. Oct 17, 2013 #7


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    The starting point of Everett was to wonder what happens if an observer A, who performs a measurement on a physical system, is observed himself by an external observer B. After collapse, A has a different wavefunction than B. This implies either that wavefunctions have no physical reality or that QM can not be applied to big systems like observers.

    The MWI is the most literal interpretation of QM in the sense that it doesn't introduce additional elements (like dBB) but still disagrees with both statements. The wavefunction is the real thing (and not just knowledge about more fundamental unknown properties), and QM applies to everything.

    Also note that the MWI doesn't explain decoherence. Decoherence is a measureable feature of QM which is present regardless of the interpretation.
  9. Oct 17, 2013 #8


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    You have it about face. MWI doesn't explain decoherence - its part of the formalism of QM regardless of interpretation - it makes use of it.

  10. Oct 17, 2013 #9
    To me, the MWI is obviously the best interpretation of QM, because it is the simplest. It is nothing more or less than the idea that everything is described by QM and the Schrodinger equation, including measuring devices. Given the (remarkable) absence of any evidence to the contrary, I see no reason to doubt that, and I would say anyone that does has the burden of proof squarely on their shoulders.

    As for when the universe splits - first, this is really a continuous process, so there's no perfectly well-defined "when". But more or less it happens whenever macroscopic objects (such as measuring devices or people) interact with anything. In other words, pretty much all the time.

    It's also worth bearing in mind that given what we know about physics, all interactions are local, meaning that the split affects only the lightcone of an event.
  11. Oct 17, 2013 #10
    The whole point of MWI is to explain the apparent random results you get from decoherence. Instead of looking for a cause for QM decoherence results, MWI allows us to say that there is no answer. From my way of looking at it, it's bad scientific attitude.

    What is normally done in science is to presume that there is a modelable explanation for a measured result, hypothesize, and then test. So if you presume MWI, you have stopped doing science because you are resting on an untestable "theory".

    A better attitude is to presume that decoherence isn't in principle random - presumably that there are discoverable non-local causes that can be modeled. This is not to say that such a model could be used to actually predict the outcome of decoherence event, but that model could be used to make other testable predictions.
    Last edited: Oct 17, 2013
  12. Oct 17, 2013 #11
    I don't recognize what you're saying as the MWI, at least not its modern incarnation. The modern view is that MW is simply a consequence of Schrodinger evolution. The Schrodinger evolution of systems with many particles is decoherent, full stop. The "cause" is, well, Schrodinger's equation.

    Not so, as above.

    That's precisely what the MWI assumes. Differential equations (like the Schrodinger equation) are deterministic. So decoherence in MW is certainly not random.
  13. Oct 17, 2013 #12


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    Today, we have all kinds of explanations for the randomness of QM: hidden variables, many worlds, retrocausality, etc. However, all of them have in common that the entities which are supposed to explain the radnomness can't be measured. My guess is that this will be true also for future theories and interpretations.
  14. Oct 17, 2013 #13
    I hope that I am allowed to agree and disagree with you on the same point at the same time.
    To a certain extent, the light cone statement is true. But for this discussion, I would flip it around: Everything contributing to the outcome of a decoherence event will be contained within the historic light cone of that event.

    However... you still need a form of non-locality to make it happen because the light cone isn't really a cone. It doesn't come to a point. There is no instant in time or point in space where the decoherence takes place. Instead, the transition occurs over a region of time and space.
  15. Oct 17, 2013 #14
    If you can't, in principle, test a theory, then it isn't a theory.
  16. Oct 17, 2013 #15
    There is a big difference between the implications of differential equations and MWI. With the differential equations, you're allowing all of the "many worlds" to interact. With MWI, at decoherence they stop interacting with each other completely.

    Put it another way, if Scroedinger's Cat was really subject to macroscopic QM effects, it wouldn't just come out dead or alive, it would escape with the experience of having interacted with it dead self. That's what the differential equations suggest.
  17. Oct 17, 2013 #16


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    Yes, that's why they are called interpretations. But my guess is that also future theories won't be able to introduce observable elements which get rid of quantum randomness.
  18. Oct 17, 2013 #17
    I am also confident that they "won't be able to introduce observable elements which get rid of quantum randomness" - at least not in the sense that they will make it any more predictable. To do so would be a direct violation of Heisenberg Uncertainty. But they'll be able to demonstrate other side effects that are consistent with there being no net synthesis of information - that is, that the results, though random, can be modeled as fully determined by their historic light cone and are independent of anything like a many-world selection parameter.

    Ahh, that's true. In the many-world theory, there would be a continuous influx of information into the universe - and as a result, there would be a continuous minimum physical size to the universe.

    That would be, at least in principle, testable.

    It would also mean that if you run time backwards, there would be a destruction of information. I'm not sure that's good for the MW theory.
  19. Oct 17, 2013 #18


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    I am very sceptical about this as well because I don't think that future theories will get rid of quantum non-seperability.

    Why do you think so? The entropy of the universe doesn't change under unitary time evolution.
  20. Oct 17, 2013 #19
    What? You seem to think that in MWI something special happens when "worlds" split. Not so. The splitting of worlds, and their evolution before and after the splitting, is described by the Schrodinger equation.

    No! Stop! You don't understand MWI as well as you think you do. Schrodinger's equation is linear, which means that the two elements of a superposition evolve entirely independently of one another, without affecting each other. If you were placed into a superposition of two states, neither version of you would be able to detect or interact with the other version, precisely because of this linearity.

    Pretty sure you've just postulated local hidden variables, which are ruled out by Bell's theorem. If you want the results of measurements to be fully determined by their past light cone, this is the same as having some hidden variable at each point in space with the hidden variables evolving in time according to local interactions. Any such model is ruled out by the observed violations of Bell's inequality.

    What? The fact that worlds are constantly splitting does not imply a minimum size for the universe.

    The fact that worlds merge instead of splitting when you run time backwards is not a bad thing. It's a reflection of the second law of thermodynamics.
    Last edited: Oct 17, 2013
  21. Oct 17, 2013 #20


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    The many worlds interpretation makes lots of sense if we do away with any notion that observers are somehow special, or that we are anything more than the total of all the atoms and their interactions making us up

    That we observers happen to see a random chain of events is simply due to the fact that our memories are material accounts written in patterns of connected neurons, but written with materials made out of atoms nonetheless.

    As far as probabilities go, keep in mind we never determine a probability experimentally with a single measurement. We have to measure many many times and look at the ensemble as a whole. Also for a large number of measurements, these random outcomes overwhelmingly conform to specific probability distributions.

    As for what determines those distributions in the first place, I haven't figured that out yet.
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