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From Cat Paradox to Multiple Worlds: What Gives?

  1. Aug 13, 2009 #1
    Einstein, champion of the incompleteness of quantum mechanics, wrote a letter to Schrodinger describing a situation where gunpowder is in a half-exploded half-stable state. This was a reductio ad absurdum argument for the incompleteness of QM, since, according to Einstein, "in reality there is just no intermediary between exploded and not-exploded."

    Schrodinger reformulated this reductio ad absurdum argument against the completeness of QM as the famous cat paradox.

    How is it, that 80 years later, some physicists are still using this argument against QM to support such absurd unfalsifiable ideas as the existence of infinite multiple universes?

    Is it understood that holding on to the completeness of QM means that it is in principle impossible to ever come up with a better theory? Do the same people who support the idea of a unified theory or theory of everything also believe (inconsistently) in multiple universes and the completeness of QM?

    Are physicists just so personally tied to the theory of QM that they would believe, with no empirical evidence, in infinite multiple universes, before admitting QM's incompleteness? Isn't it infinitely more simple to just admit, even if a better theory is epistemologically beyond our grasp, that at least in principle it could be out there?

    Clearly I've missed something here. So what is it? Why can't we admit, as we have with every other probabilistic theory in history, that QM is incomplete?

    Please start with your basic assumptions. I think too much confusion in QM has already come from starting at misunderstood intermediate steps. I want to understand how I'm wrong.
     
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  3. Aug 13, 2009 #2

    Hurkyl

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    Quantum mechanics is almost surely incomplete -- but that's a completely irrelevant fact. Do not make the mistake of using it as a "loophole" to rationalize beliefs that contradict quantum mechanics.
     
  4. Aug 13, 2009 #3
    Hurkyl,

    Do you see any statements contradicting QM in what I wrote, or are you making a general statement that one should be careful of that trap?

    Isn't the completeness of QM a necessary premise of the multiple worlds interpretation and every other realist interpretation?
     
  5. Aug 13, 2009 #4

    Hurkyl

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    It was a general statement, applying (as a specific example) to Schrödinger's cat.

    The empirical success of quantum mechanics obliges us to seriously consider its predictions. If it looks like QM predicts something, such as indefinite outcomes, then it's not reasonable to say "I don't like indefinite outcomes, so I'm going to believe QM is incomplete instead".
     
  6. Aug 13, 2009 #5
    I agree that if QM predicts something we should take it seriously unless there is some evidence to the contrary. The problem here is that QM doesn't predict indefinite outcomes unless it is already presumed to be complete (and is interpreted in a realist framework). An incomplete QM simply describes indefinite information about a situation and gives a range of possible outcomes for that situation. Even in a realist framework, an incomplete theory is simply a useful tool with no ontological weight.

    Since the possibility (inevitability?) of multiple worlds, for example, is claimed to be a logical implication of the completeness of QM, I think we should absolutely consider that information in deciding whether or not QM is complete.

    Why shouldn't the logical implications of a statement be used to evaluate its truth?
     
  7. Aug 14, 2009 #6

    Hurkyl

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    That is certainly fair. For the sake of argument, let's assume it is inevitable. So, we are now compelled to ask:
    Is there any empirical evidence contradicting the existence of multiple worlds?​


    Because of the way relative states and decoherence works, it turns out that multiple worlds should be fairly indistinguishable from a universe with definite outcomes. e.g. The answer to
    How would a cat in a mixture of dead and alive states look?​
    would be something like:
    It would look like it was dead, or it would look like it was alive.​


    So, we don't really have any evidence at all that there aren't multiple worlds -- and thus this does not lead to an evidence-based argument that QM is incomplete.
     
    Last edited: Aug 14, 2009
  8. Aug 14, 2009 #7

    ZapperZ

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    Since you are very keen in "empirical evidence", can you please show me the empirical evidence that the majority of physicists actually buy into this "many world" scenario for quantum mechanics? Because without such evidence, aren't your rant here completely moot?

    Zz.
     
  9. Aug 14, 2009 #8
    ZapperZ,

    I never claimed that a majority of physicists, or even many at all, buy into this idea. That's not the point. Neither was empirical evidence a focus of my post. A quick reading of the quantum physics forum, though, will get you discussions of many worlds with representative supporters of the theory. A search of arxiv reveals a couple of many worlds papers posted in the past week, with reference to a new book on the topic. This thread was actually motivated by some posts on the quantum forum, but I felt that this was a more appropriate forum for this topic.

    I am certainly not claiming that physicists don't know what they are talking about, and I'm sorry you view my post as a rant. I don't view the question as a matter of physics itself. I realize it's not polite to point out the sociological aspects of scientific development, but especially when we are considering untestable metaphysical hypotheses, sociology may play a large role. I could have been more politically correct in my post, but I wanted to get to the point and hopefully spark some discussion.

    I'm more interested in the completeness of QM than in the many worlds interpretation itself. I chose many worlds as a particularly absurd (at least naively) example of what can come from assuming the completeness of QM.

    Also, if no one on this particular forum supports MWI, this thread will probably fade away pretty quickly, so it's not an issue :smile:.

    Do you have any thoughts on the matter?
     
  10. Aug 14, 2009 #9
    Hurkyl,

    Agreed. MWI seems to not be empirically testable. When the choice is between multiple worlds and quantum incompleteness, it would seem to me that Occam's razor suggests quantum incompleteness. This is the argument made by Einstein.

    Proof by Occam's razor is, of course, impossible. Unfortunately, there can be no proof for or against any self-consistent and experimentally consistent metaphysical theories.

    In this case it seems to me that the burden lies with MWI to show its simplicity over an assumption of incompleteness. Anyone want to take a stab at that?

    Alternatively, anyone want to take a stab at quantum completeness from Bohr's point of view of the primacy of Kantian categories of experience?
     
  11. Aug 14, 2009 #10

    ZapperZ

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    Yes. My thought is that you are confusing the interpretation with the formalism.

    You poked holes at QM via the many-world interpretation. That's like attacking Jimmy Webb because you didn't like the disco version of MacArthur's Park by Donna Summer.

    Zz.
     
  12. Aug 14, 2009 #11

    Hurkyl

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    It is actually standard to invoke Occam's razor as favoring MWI over interpretations such as Copenhagen or Bohm.


    Remember, Occam's statement was something like "entities should not be multiplied unnecessarily".

    The "entities" posited by MWI are simply wavefunctions evolving via unitary evolution -- nothing more, and nothing less. Everything else in MWI is an attempt to describe how that behaves.

    Compare with other common interpretations: Bohm posits, in addition to wavefunctions evolving unitarily, we also have these other point particles whose motion is governed by the wavefunction.

    One flavor that goes by the name of Copenhagen posits that at the fundamental level some unknown and unspecified physical behavior is going on -- and the only thing we know about1 it is that it can be effectively described by quantum states evolving unitarily.

    Another flavor of Copenhagen posits that there is some unknown and unspecified physical behavior that looks quantum on "small" scales and Newton on "large" scales that describes how things really behave.

    Yet another flavor of Copenhagen posits that reality is well-described by wavefunctions, but posits there is a nonlinear correction to (equivalent of the) Schrödinger equation. (But we don't yet know what sort of correction would work)


    The above are, I believe, relatively common interpretations -- the distinguishing feature of MWI is that it's the only one that does not make assumptions above and beyond wavefunctions and unitary evolution.


    There are, of course, other interpretations. e.g. you can (IMHO) improve upon MWI by considering that, after conditioning upon "past observation" (whatever that might be), many wavefunctions are observationally indistinguishable, and should be treated as simply being different representations of the same reality -- in this picture, applying a wavefunction collapse is exactly the same in spirit as switching coordinate systems to make a calculation easier.


    1: In particular, if we are trying to dismiss MWI on aesthetic grounds, then this is not a viable argument: there is no reason to think this unspecified behavior is going to be more pleasing than the behavior of wavefunctions evolving unitarily.
     
  13. Aug 14, 2009 #12
    Nope, not poking holes in QM at all. QM has not been falsified to my knowledge, and I believe it's the best theory we have. I personally actually prefer Bohr's interpretation of QM which does state QM is complete. However, the reason for him to claim its completeness is based on totally different reasoning than all other interpretations claiming QM is complete.

    MWI is the most prominent example of a theory supposing the completeness of QM that is not the CI. Even Bohm clearly states that QM is not complete with regard to "deep reality."
     
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