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Quantum Zeno effect rules out many-worlds?

  1. Oct 16, 2013 #1
    Hi,

    I've read somewhere that the Quantum Zeno effect, should it really exist, rules out the many-worlds interpretation. However I did not understand the argument (actually there was no argument, seems the author considered it obvious). Could someone please elaborate on this?

    BTW, what's the current status about the Quantum Zeno effect? I know it has been observed, but I've heard about controversies whether the experimenters really only measured or if they influenced the system in a stronger way.
     
  2. jcsd
  3. Oct 16, 2013 #2

    ZapperZ

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    This is not acceptable by the standards that we require in this forum. Please cite the exact source/paper.

    Zz.
     
  4. Oct 17, 2013 #3

    Demystifier

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    Quantum Zeno effect has been measured, but it does not rule out many worlds.

    I guess someone might (incorrectly) argue as follows. The QZ effect uses the assumption of wave-function collapse. MWI assumes that the wave-function collapse does not exist. Therefore, QZ effect is not compatible with MWI.

    Such a reasoning is wrong because MWI assumes that there is no TRUE collapse, but can explain an APPARENT collapse. For the QZ effect to work the apparent collapse is sufficient, so MWI is compatible with QZ.

    An incorrect interpretation of QZ effect, similar to the one above, can also be found in an otherwise excellent quantum-mechanics textbook by Ballentine, who argues that collapse does not exist and therefore QZ effect also cannot exist. Apparently, Ballentine also seems to fail to distinguish between true and apparent collapse.
     
  5. Oct 17, 2013 #4

    bhobba

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    You noticed that as well?

    I had been meaning to do a post about it for ages.

    Its an excellent book, my favorite by a long way, but perfect it aren't eg I find his argument against Copenhagen a bit 'loose' to say the least.

    Thanks
    Bill
     
  6. Oct 17, 2013 #5

    Are you saying that you think MWI can indeed explain apparent collapse? I'm referring to your blogpost about the basis issue seeming to be unresolved in MWI
     
  7. Oct 18, 2013 #6

    Demystifier

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    To be precise and consistent with everything I said about that, MWI explains the apparent collapse PROVIDED that the system-environment split problem is resolved in some other way. The Born-rule problem of MWI is also a problem, but not relevant to the explanation of the apparent collapse.
     
  8. Oct 18, 2013 #7

    Demystifier

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    Then you may have also noticed another weak point of his book. (Which does not change my opinion that it is still the best general QM textbook.) He never mentions the important concept of decoherence, especially in the first sentence after Eq. (9.13) where it is particularly natural to mention it.

    The lack of understanding of the concept of decoherence seems to be correlated with the lack of understanding of the difference between true and apparent collapse.
     
  9. Oct 18, 2013 #8

    bhobba

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    Yes - noticed that.

    Its even worse than that though. Check out:
    http://download.springer.com/static/pdf/538/art%253A10.1007%252Fs10701-008-9242-0.pdf?auth66=1382257058_3f1aa6821a14f4993b4db48367f384fa&ext=.pdf [Broken]
    'In this paper I have tried to clear up some common misconceptions about the scope and limitations of the decoherence program. Decoherence and its avoidance are very important in interference experiments, and so decoherence theory has proven to be very useful in practical matters. But the claims for decoherence theory in the conceptual problems of QM have been exaggerated. Decoherence theory is of no help at all in resolving Schrödinger’s cat paradox or the problem of measurement. Its role in establishing the classicality of macroscopic systems is much more limited than is often claimed. It succeeds in eliminating certain interference patterns in macroscopic systems, but these are anyhow much too fine to be observed. The claim that the environment will decohere an extended wave function into a mixture of localized wave packets remains unproven. Even if that claim were (improbably) proven correct, it would only confirm the ensemble interpretation of quantum states, and would not restore a version of the individual interpretation. As was said in “It is not correct to assert that environmental decoherence is the root cause of the appearance of the classical world.”'

    He thinks that decoherence basically has little to say about the measurement problem.

    If it solves the measurement problem is hotly debated - and IMHO it doesn't - but it most definitely has something to say about it.

    And indeed he doesn't seem to understand it does explain apparent collapse - not actual collapse - and much of the debate is if apparent collapse is good enough. I believe it is - but that is very hotly debated.

    But I also have to say some people that post on this forum that I would judge as knowing a lot more QM than I do SEEM to not get it either.

    And peace be to those folks - I don't want to get into another heated discussion about it - its been done to death - its quite possible it's my error.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  10. Oct 18, 2013 #9

    bhobba

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    I wish others that go on about the factoring problem got it as well.

    Its part of ongoing research if that can or cannot be done.

    Thanks
    Bill
     
  11. Oct 18, 2013 #10

    Demystifier

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    Thanks bhobba for sharing your viewpoints (which appear to be very similar to mine), and to drawing my attention to the Ballentine's paper (which I was not aware of).

    Let me also add that I partially agree with Ballentine, i.e., I agree with the following part of his statement: "The claim that the environment will decohere an extended wave function into a mixture of localized wave packets ... would only confirm the ensemble interpretation of quantum states, and would not restore a version of the individual interpretation."
     
  12. Oct 18, 2013 #11

    bhobba

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    Since that is the interpretation I adhere to of course I agree.

    Its advantage is it gives the APPEARANCE that the state was there before observation and you can think of it that way. Its a fiction of course - but since it appears that way its quite understandable that we would interpret it that way.

    Thanks
    Bill
     
  13. Oct 18, 2013 #12
    But "another way" would introduce additional postulates outside of "pure qm math", so is it then correct to still call it the "MWI" ?
     
  14. Oct 18, 2013 #13

    kith

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    Sure because it would still be an interpretation with many worlds. The discovery of additional assumptions is always interesting but it depends on the nature of these assumptions to determine if they make a theory / interpretation less attractive than other ones.

    That said, I'm not sure how serious the factorization issue really is. My self-awareness for example introduces a natural split between me and the rest of the world.
     
  15. Oct 18, 2013 #14

    bhobba

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    At this stage its simply the observation that the derivations that show decoherence resolves the preferred basis problem make an assumption dividing the system into observed, environment and observational apparatus. To really solve the problem you need to show it doesn't depend on that. It hasn't disproven anything - simply saying it needs to be investigated further and we need to await the results of ongoing research.

    Its also interesting to note standard textbooks like my go-to book Decoherence and the Quantum to Classical Transition by Schopenhauer don't even mention it, saying the preferred basis problem has been solved. That could be due to a number of reasons, but one is they consider it a bit of a non issue - when we solve problems in physics its usual and natural to decompose systems in a natural way to make problems simpler - the assumption being the physics shouldn't really depend on it.

    There are a number of open questions in the decoherence program, this is just one of them, and research is ongoing. I have no doubt in 10 to 20 years time the situation will be greatly clarified.

    The biggest issue IMHO is not this one - its why we get outcomes at all. Schopenhauer, and others, correctly IMHO, point out that issue is left untouched.

    Thanks
    Bill
     
  16. Oct 19, 2013 #15

    Yeah, I worded my sentence a bit bad. I added "the MWI" as the Everettian/Oxfordian MWI. But yes obviously any interpretation that adds something will still have a many worldian character, but I don't think it's good to call it MWI as that would lead to massive confusion.

    There are quite a lot of serious people who has and continue to propose that there could be something like Many Bohmian Worlds etc. but then I think it's important to distinguish them by name.


    I'm not sure what you mean with your self awareness being "split" from the rest of the world. How does that play into the factorization issue?
     
  17. Oct 19, 2013 #16
    Well given all that we already know we can almost safely say that it's not possible without ADDING to the hypothesis, which is considered almost sinful because the only reason some people like MWI in the first place is the simplicity and lack of additional assumptions.


    I haven't read the book, but I've read plenty of others and papers where authors make such mistakes all the time. People usually write books (even textbooks) from their point of view.
    I don't understand how anyone could possibly say that it is a non-issue though.

    Could you elaborate?
    I'm not sure what you are referring to here.
     
  18. Oct 19, 2013 #17

    bhobba

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    Its an assumption that's made all the time is solving physical problems. Nothing percular to QM about it. I suspect most think as I do, its so pervasive its basically assumed without further adoo like for axample the assumption in mechanics velocity is differentiable to give acceleration. Its an assumption but most wouldn't elevate it to the status of stating it explicitly.

    It's the problem of definite outcomes - why does QM allow us to measure quantities at all ie how is a potentiality expressed as a probability actualized as a specific outcome. When we throw a dice we know how the specific face that comes up was made apparent - whats the corresponding process in QM? We don't know. We don't even know if its even a meaningful question or nature is simply like that.

    Dechoherence most definitely does not answer that.

    Thanks
    Bill
     
  19. Oct 20, 2013 #18

    kith

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    The factorization issue is that the full state and Hamiltonian alone don't give you a preferred decomposition of the Hilbert space. My self-awareness gives a preferred decomposition H = Hme⊗HRest. All my observations are based on interactions between me and the rest. It doesn't matter wether this is mathematically equivalent to a situation where I don't exist and nothing happens.

    For me, the main point of the MWI is that if I want to be a realist about the systems I observe, I don't have to make additional assumptions like collapse or hidden variables. I can include myself in the quantum mechanical description and interpret the terms in the resulting entangled state as different worlds. Note the repeated occurance of "I" in the previous two sentences.

    It is not clear to me why we should require the MWI to yield the factorization in the first place. Most papers I have read on the MWI postulate a decomposition anyway. (However, I haven't read that much about it and the aspect how unique the worlds are and how MWI people talk about this is something I want to know more about.)

    Also do you know of any peer-reviewed papers about the factorization issue? It is astonishing that I have read about it exclusively in arxiv-only papers and in this forum so far.
     
    Last edited: Oct 20, 2013
  20. Oct 20, 2013 #19

    bhobba

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    Yes - noticed that as well - which is interesting in itself.

    There was a paper I came across a while back that showed for a simple model it didn't matter how you decomposed it the same preferred basis was singled out - but it needs to be greatly extended.

    Thanks
    Bill
     
  21. Oct 20, 2013 #20

    bhobba

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    We do NOT know that at all.

    All we can say is further work needs to be done.

    Precisely why do you believe that the physics depends on how you arbitrarily decompose a system to analyse it? If so that would be big news - the assumption it doesn't is pretty much tacitly assumed in solving many physical problems.

    Added Later:
    Here is a link I came across that gives the state of play with the preferred basis problem - although I haven't gone through it in detail it looks very similar to Schlosshauer's text that is my go-to book on such things:
    http://cdn.intechopen.com/pdfs/2908...chanics_decoherence_and_the_pointer_basis.pdf
    'The studies mentioned above do shed light on the nature of the preferred basis but are inadequate and there is a need to analyze more systems. In particular, it is important to look at systems like the harmonic oscillator apparatus model which is fairly generic and exact solutions make it an interesting candidate to explore experimentally in the context of decoherence and quantum measurements. Also, this example indicates that it seems pertinent to look at a system-apparatus-environment like scenario for measurement to analyse the issue of the pointer basis and the states singled out by the environment.'

    A number of reasonable models show, and others suggest, that a preferred basis is singled out by the interaction Hamiltonian and the consensus seems to be, while everything hasn't been worked out in full generality, decoherence by itself is likely enough to single out a preferred basis. That's the view of Schlosshauer in the previously cited reference.

    The factoring issue is that analysis depends on decomposing the system into observed system and environment. There is nothing unusual about doing that in analysing physical systems - it's done all the time - the tacit assumption being it shouldn't matter ie it's only done for convenience. Most would not even think of stating it explicitly as an assumption.

    But for the claim to be true it still is necessary to show it doesn't depend on that decomposition. That's the issue in a nutshell. Most, including me, don't think its a biggie - its an issue - but in this stuff there are all sorts of areas the full detail hasn't been worked out yet - research is ongoing and it will be better understood, what was it Sir Humphrey said - In the fullness of time Minister.

    Thanks
    Bill
     
    Last edited: Oct 20, 2013
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