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Wave function of an observer, 'always collapsed'?

  1. Nov 30, 2009 #1
    Would it be correct to say, that the observer's wave function is 'always collapsed'? I.e. that the observer can always be completely described by a bit string, while everything else only by a 'qbit string'.
  2. jcsd
  3. Nov 30, 2009 #2
    For practical reasons, yes

    If you are asking on the fundamental level, then it is interpretation dependent.

    In collapse theories, like CI, answer is "YES", but nobody could ever explain why some configurations of atoms are "observers" while others are not.

    In non-collapse theories, the answer is N/A :)
  4. Nov 30, 2009 #3


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    First I would like to ask what you mean by "the observers wave function".

    1) if you are considering a second observer then the original observer is just a sub-system just like any other system, and can generally be in superposition with respect to the third observer. The practical exceptions to this are IMO special cases, where the first observer has enough confidence in his state to be questioned by the second observer without beeing destabilized and eventually reach a steady state where the original observers is in equilibrium with it's closest environment (but then you are ignoring the equilibration process, which may be acceptable for practical purposes in cases where it's almsot instant)

    2) OTOH, if you are considering something like the observers description of himeself, I don't think that doesn't make much sense since from the inside view, all there is IMO is a view of the outside. Ie. all that is one the inside, is an image of the outside. So the image of the outside, "IS" more or less the identity of the observer if you see in the sense of Zureks (what the obsever is, is indistinguishable from what the observer knows)

    3) if you are considering some kind of birds view of the observer, then I really don't know what you mean as I see no realizable physical setup that corresponds to that situation except for special cases. In a sense the human laboratory is almost like a birds view of what is going on inside an accelerator. But again this is a special (limiting case).

    Assuming (1) above, I would choose to say no. The yes case, is IMO a special case, not a general rule.

  5. Dec 2, 2009 #4
    Why N/A? It seems to me, that the observer can be completely described either by a bit string or a qbit string. There are simply no other choices. And it should not be interpretation dependent. If the answer is "YES" in the CI it should be "YES" in the MWI as well (of course that would be different string in each 'branch').

    There is a reason why it is interesting. If a bit string or a qbit string is associated with an observer, the statistical mechanics can be applied to it, thus defining entropy, temperature, etc.

    To Fredrik: A second observer view (1) is not really interesting (for the reasons you've mentioned). So I'm considering (2), the observers description of himself, identity.
  6. Dec 2, 2009 #5
    N/A because:
    1. In non-collapse, there is no difference between the observers and any other systems. And clearly, you see that not all systems are always 'collapsed'
    2. If there is no collapse, how you can ask if wavefunction is 'collapsed'?
    3. I am not sure that observer can observe itself *to the full extent* - https://www.physicsforums.com/showthread.php?t=337197
  7. Dec 2, 2009 #6
    I guess it should read 'always collapsed from the point of view of the observer'. But you are right this 'wavefunction collapse' talk IS interpretation depended, so it is best to avoid it.

    On the other hand it seems to me, that the requirement to use bits or qbits in the description of the observer should not depend on the interpretation.
  8. Dec 2, 2009 #7


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    It sound like if you have an idea looking for some way of the observer to do instrinsic "statistics" and thus define a subjective entropy etc, making them subjective quantities, and thus make statistical inferences?

    I support that quest, but IMO there is no answer to it(assuming I read you right) within the standard framework, making the discussion unsuitable for this thread I suppose?

    Also I agree with Dmitry that self-observation might need clarification. I'm not sure what you mean. In one sense, querying yourself as a whole doesn't make sense, unless you consider partial observations in the sense of internal processes. But I think the general case is much more complex than eithre a bitstrong only or a qubit string only.

    But for the simplest possible intrinsic starting point, which I like to think of as distinguishability, a classical bit seems more fundamental since it contains much less information that a qubit they way I see this.

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