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Was Bohr's version of quantum mechanics deeply flawed

  1. Mar 30, 2009 #1
    Steven Weinberg in "Einstein's Mistakes", Physics Today, November 2005, page 31, said:

    All this familiar story is true, but it leaves out an irony. Bohr's version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wave function (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?
    Considerable progress has been made in recent years toward the resolution of the problem, which I cannot go into here. It is enough to say that neither Bohr nor Einstein had focused on the real problem with quantum mechanics. The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wave function, the Schrödinger equation, to observers and their apparatus.
     
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  3. Mar 31, 2009 #2

    clem

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    Yes, Bohr and Einstein argued so much because each was using incorrect classical concepts and language to personalize quantum mechanics.
     
  4. Mar 31, 2009 #3

    alxm

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    Yes, I agree with Weinberg's statements entirely. (Which is probably a good idea when it comes to physics. He's the one with the Nobel prize, not me..)

    I've said it before, but I'll say it again, the Copenhagen interpretation, the "Many worlds" interpretation and all that, all assume a premise (separation of 'measured' and 'measuring' systems) which is essentially classical, and therefore incorrect.

    It kind of annoys me the copious amount of posts relating to 'interpretations' that you see on this board. You'd think that it's the most important or central issues in quantum physics, when it is not (IMO). I think it's a pedagogical failure when every other pop-sci article on QM says something about the Copenhagen interpretation and all that, whereas a very complete textbook like Landau-Lifgarbagez doesn't mention it once.
     
  5. Mar 31, 2009 #4

    dx

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    The very first section of Landua-Lifgarbagez is a discussion of the Copenhagen interpretation.
     
  6. Apr 1, 2009 #5

    Fra

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    We have a deterministic equation (law) that determines the evolution of the state vector in between measurements. And this descprition presumes an observer and the observed.

    It is claimed that the separation between observer and observer, is a sort of semi-classical idea, and thus not quite consistent.

    So far that's a reasonable comment, but I claim that the resolution here is not unique. Most people seem to want to resolve this by somehow removing the observer and treat the observer + system equally. I think this is a serious mistake, and there is another way.

    First, then inmplicitly you are picturing a second observer, or a birds or gods view, when you talk about observer + system. You are not longer having a measurement theory, instead you have some kind of entanglement theory, wich is attached to somd birds view, governed by a universal evolution law.

    This if anything is a manifestation of old classical realist thinking!

    Lee Smolin raised the same arguments in his evolving law arguments, that a birds view with universal and timeless laws are a sort of realist way of thikning, the last remnant of realist world views.

    The other way out, that is much more consistent with the more scientific ideals of focusing on observations from real inside observer, is to analyse the notion of physical law, and the determinism of the evolution law itself.

    This could instead suggest an evolving model where the observer / observed distinction is maintained (in Bohrs spirit), however the observer itself is evolving and along with it the (time evolution law) also evolves.

    When you allow the observer to by a player in a larger evolutionary scheme, this introduces an indeterminacy in the expected time evolution (ie law). So such an evolutionary scheme, is an alternative solution of the observer problem. so that instead of REMOVING the observer, which really makes no sense IMO from a scientific point of view, one simply considers an evolving observer, and also evolving law.

    So I see two main routes forwards here...

    a) Get rid of the observer and hold a realist view of physical law
    b) Consider en evolving observer, an an evolving view of physical law as seen from an inside observer, and toss out the realist birds view of physical law.

    I know my pick :)

    /Fredrik
     
  7. Apr 1, 2009 #6
    c) Get rid of the observer but explain who an observer gets an illusion of classical reality (QM+Quantum Decoherence)
     
  8. Apr 1, 2009 #7

    Fra

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    Not to start any detailed arguments but this is what I would classify as a version of a.

    Your explanation, would involved a realist view of the physical law. It's a birds view explanation of how inside observer experience things.

    I suspect that doesn't bother you though, but it bothers me. To me it's a reasoning that's inconsistent with the idea that physics deals not with what nature is but what we know of nature.

    In that context, to me, the birds view stands out like a realist fetish :wink:

    This birds view is the constructing principle of all known physics to date. This why Smolins evolving law, as well as similar reasoning from others is such a radical change of reasoning.

    /Fredrik
     
  9. Apr 1, 2009 #8
    Exactly, so it requires serious justification and I dont see any.
     
  10. Apr 1, 2009 #9

    Fra

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    Then at least we agree to disagree. This is healthy diversity IMHO.

    I see the justification. Smolin seems to see it. And I'm sure there are few others.
    But there are possibly more that don't.

    Let's see which principle is more successful in the future, as opposed to the past.

    /Fredrik
     
  11. Apr 1, 2009 #10
    Any good links to explanation of Smolins ideas - like 20-30min reading?
     
  12. Apr 1, 2009 #11

    Fra

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    First I might say that there are variations within this style of reasoning. For example his black hole focus in CNS - which is personally not what I think is the most important thing, but anways, for a starter, smolin raises good points (he doesn't answer all of them though).

    There is one talk, available as mp3 with slides. Where Smolin does a lecture on time and evolution of law. An in the end of the lecture, the audience also raises a few questions and objections that smolin elaborates on.

    "On the reality of time and the evolution of laws"
    http://pirsa.org/08100049/
    (Sometime last year it was Marcus who daw my attention to this specific talk)

    If you have an mp3 player this is neat. I don't remember but I think it's some hour or so.

    Also, a related reasoning is the issue of background independence, in a general sense - not just background metric, or even background topologies. The issues of background independnce, is related to the idea of evolving law. I don't remember without rereading if smolin in this paper makes this connection or not but the notion of background in the general sense does apply to a birds view. Anyway, there are different uses of the word here.

    "The case for background independence"
    http://arxiv.org/abs/hep-th/0507235

    The arguement for evolving law is also related to the fine tuning problem and choosing initial conditions.

    /Fredrik
     
  13. Apr 1, 2009 #12

    Fra

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    About Smolin: More than anything, I think he raises good questions, and causes the reader or listener to starting thinking. He does not have all answers, neither does he have a complete realisation. Instead he argues around why a new way of reasoning might be needed. To appreciate it, he first in that talk presents some of the problems of the ideas associated with eternal laws. Such as initial conditions etc.

    In some parts of his paper he also spends some energy to differentiate himself from antrophic reasoning that some people favour in the context of the string landscapes etc.

    /Fredrik
     
  14. Apr 1, 2009 #13

    Fra

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    Also, the link I make between evolution of law and the foundations of QM, is not something that I recall Smolin making, at least not explicitly in the above references. It's part of my persona flavour of this. But I still think that for a starter smolins arguments are worth reading and listening too.

    But that problem can be analyzed by the same reasoning. But there are still more explicit papers on this that remains to be written I'm sure.

    /Fredrik
     
  15. Apr 2, 2009 #14
    I disagree with the "considerable progress" if he means decoherence: Decoherence in itself is a nice technical tool for certain applications, but it's application in fundamental physics is misguided and makes things worse. In http://arxiv.org/abs/0903.4657" [Broken] Copenhagen appears much better than the modern "pure" interpretations.

    The Copenhagen rules work, and it was clearly not Einstein's claim that they don't work. So I don't agree as well with the point that "neither Bohr nor Einstein had focused on the real problem with quantum mechanics".
     
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  16. Apr 2, 2009 #15
    Fra, thank you for the video.

    1. Regarding the claim "the only way to explain why our universe has that specific set of laws is to accept the evolving law" is wrong - the Multiverse Level IV is another possible answer.

    2. he had really good point about an absence of NOW in physics. I see NOW as a symmetry breaking of timescale between NOW and NOT NOW, but I think that our consciousness is responsible for that symmetry breaking, not the Universe

    3. The rest of it is just pure handvawing - no explananion how laws can 'change', everywhere/in some places, gradually (constant tuning) or dramatically (another equations), sorry.
     
  17. Apr 2, 2009 #16

    Fra

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    I agree, the argumentation is not certain, there are other possibilities. I think regardless of how he have formulated himself at times, I'm sure he would agree. If there was only one possibility, and everyone agreed upon that we would not have this discussion.

    I'm not sure what this means, your brain is still part of the physical universe right?

    Yes, he is trying to convey a way of reasoning (handwaving indeed), but then we are dealing with difficult problems to which noone yet has generally accepted answers. Smolins reasoing is IMHO not as clear at it could be, and there are probably better arguments that could be put forward, but it was an easy and accesible tlak which I had fresh in memory.

    You are right that the implementation of his reasoing, into a concrete model is still not completed. But then, there are neither any other equally ambitious approaches which have complete models. The fact that it's mostly handwaving is because it's a young idea. just picture if the same effort had been made on this as on string theory the last 50 years?

    It's difficult for me to convey the arguments I see to you though. Eventually with progress, someone will lay them out I am sure.

    /Fredrik
     
  18. Apr 2, 2009 #17
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