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B Wigner's Friend

  1. Jun 30, 2016 #1

    gva

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    Wigner's Friend was supposed to be the arguments that wave function collapse can't be really occurring, but there seemed to be counterarguments that refuted the illustration of the Wigner's Friend. What is it? I can't seem to find them at google. Thank you.
     
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  3. Jun 30, 2016 #2

    jambaugh

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    Here is one criticism to both Wigner's Friend and the original Schrodinger's Cat experiment, for a wave-function/ hilbert space vector description to be valid, you must be dealing with a sharp or nearly sharp quantum system. This means you must refrigerate the system. The "alive" observable is rather difficult to define for a cryogenic cat, and one would also question the efficacy of the vial of poison on the Cat-sicle. No less difficult is the definition of a "happy" observable for the cryogenic friend.
    (This point was made to me during graduate seminar.)

    This is not just glibness. Living organisms and classical scale recording devices (hence the measurement process itself) are both fundamentally thermodynamic processes. Measuring a quantum system involves amplifying its state (creating the copies which constitute the measurement record). Attempts to incorporate these processes into your system description means you must expand it to include "mixed state" cases i.e. you must work with density operators instead of wave functions/Hilbert space vectors.

    In my opinion this makes it clear that "wave function collapse" is no different in its ontological character from classical probability distribution collapse upon updating one's assumptions about what is know about the system in question. In short, the wave function collapse occurs in conceptual space, not physical space. I say this because you can just as easily express the outcome of a classical random experiment with density operators and then consider the questions of how this description changes when you assume a specific outcome of a given trial.
     
  4. Jun 30, 2016 #3

    Nugatory

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    The Wigner's Friend thought experiment builds on Schrodinger's infamous cat, so has much the same status. When Schrodinger proposed his thought experiment, he was not suggesting that the cat wpuld be in a superposition of dead and alive, nor that the question of whether the cat was dead or alive was any different than whether a tossed coin is heads or tails before we look. We know perfectly well that the cat is as surely either dead or alive as the coin is either heads or tails, with no quantum weirdness involved at all. Schrodinger's point was that there was nothing in the then-current understanding of quantum mechanics that said it had to turn out that way, and that this was clearly a weakness in the theory. Wigner then extended the thought experiment by adding his friend and introducing (with von Neumann) the notion that consciousness causes collapse.

    However, that was all based on the state of understanding as of 1935. In the decades since then, the discovery of quantum decoherence has cured this weakness. It turns out that the evolution predicted by the theory will naturally lead to either a dead cat or an alive cat, so the fact that we never observe live/dead superpositions is consistent with the prediction and there is no longer any need to ascribe special significance to a conscious observer.
     
  5. Jun 30, 2016 #4

    gva

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    I wrote this thread because I read Bill Hobba archive message saying the interpretation that the wave function is real and consciousness causes collapse is still valid.. but just weird.. so I wonder how he handles the Wigner's Friend scenerio. Ping Hobba!
     
  6. Jul 7, 2016 #5

    gva

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    Attention dear Hobba, I wrote this thread because I read in your archive message where you said the interpretation that the wave function is real and consciousness causes collapse is still valid.. but just weird.. so I wonder how you handle the Wigner's Friend scenario. Well?

    @bhobba
     
    Last edited by a moderator: Jul 7, 2016
  7. Jul 7, 2016 #6

    A. Neumaier

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    Is Wigner's friend still alive? If so, he should be very old now; Wigner himself died in 1995. In any case, collapse is observable independent of Wigner's friend, and can be explained by the influence of the environment. Thus there is nothing to worry abut.
     
  8. Jul 7, 2016 #7

    bhobba

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    The cat is never ever in a superposition of alive and dead so Wigners friend is has no validity just like Schrodingers cat is of no validity.

    They were both proposed before we know as much of QM, especially decoherence, as we now do.

    If you would like to find out why that is, ie why the cat can never be in such a superposition, start another thread or do a search - it has been discussed many times.

    Thanks
    Bill
     
  9. Jul 7, 2016 #8

    bhobba

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    It is.

    But its a very very fringe interpretation these days with many issues.

    Thanks
    Bill
     
  10. Jul 7, 2016 #9

    gva

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    Issues like? But it makes better sense than MWI or Bohmians. In the 1930s. Big Bang cosmology is also very fringe.
     
  11. Jul 7, 2016 #10

    bhobba

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    What is sense for one person is nonsense to another and is very individual.

    Again this has been discussed many times times:
    https://www.physicsforums.com/threads/does-consciousness-cause-wave-function-collapse.806278/

    But briefly, the double slit is often done with photographic plates. You preach quite an absurd position if you believe collapse occurs when the plate is developed, maybe even decades later and observed. It gets even worse if you imagine it recorded to computer memory, millions of copies taken, and one of those copies read centuries later.

    Thanks
    Bill
     
  12. Jul 7, 2016 #11
    I've heard that decoherence does not completely solve "measurement problem" i.e. does not explain the final selection of one of the possible alternatives. In this sense may the Shrodinger cat problem be still considered unresolved?
     
  13. Jul 7, 2016 #12

    gva

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    Ah.. that's what you meant by fringe. Yeah.. It sounds more absurd than others. But then maybe without consciousness. Objective Collapse interpretation is invoked (Ambient mode). When there is consciousness.. then the ambient collapse mode is overridden. Now would this combo makes sense?
     
  14. Jul 7, 2016 #13

    gva

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    Thinking it deeper.. I think what I wanted to state was Copenhagen that has objective collapse. That is. Not exactly consciousness causing collapse. But the instrument collapsing the wave function in actuality. So instead of this just occurring in the physicists scratch paper.. the instruments actually collapsing via the born rule. Now what is wrong with this?
     
  15. Jul 7, 2016 #14

    Nugatory

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    You're asking whether we should put the Von Neumann cut in one place instead of another, and the 1930-vintage answer is that you can put it anywhere you please. So nothing (except that you're pushing the limits of the PF rule about personal theories) is wrong with what you're saying .... But you're reworking stuff that was covered three-quarters of a century ago.

    Since then, we've learned that the Von Neumann cut naturally belongs where decoherence comes in.
     
  16. Jul 7, 2016 #15

    gva

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    In Bohr original Copenhagen collapse, he didn't specifically say that it only occured on paper. It could mean the collapse is physical. It is only at present or mostly the physicists of Physicsforums who emphasize the collapse is just on paper. So there is really nothing wrong with believing that the collapse is real and controlled by the born rule and the reason Von Neumann cut can be put anywhere is because the collapse is movable too?
     
  17. Jul 8, 2016 #16

    bhobba

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    :smile::smile::smile::smile::smile::smile::smile::smile::smile:

    Thanks
    Bill
     
  18. Jul 8, 2016 #17

    gva

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    What is the connection of von Neumann cut to whether the collapse is real. Does it mean von Neumann cut being movable proves the collapse isn't real?
     
  19. Jul 8, 2016 #18

    bhobba

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    Collapse is not a postulate or a deduction from the postulates of QM. Its simply something some interpretations have, and some do not. For example its not part of the Ensemble interpretation but is part of Copenhagen. In the ensemble interpretation all that happens when you observe is you prepare a system differently, in Copenhagen the state instantaneously changes to another state which by definition is collapse.

    The Von Neuman cut is where you put the quantum classical cut which may or may not involve collapse.

    You need to study a proper book on QM - see Ballentine:
    https://www.amazon.com/dp/9814578584/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687642

    You will never understand QM from popularization's.

    Thanks
    Bill
     
    Last edited by a moderator: May 8, 2017
  20. Jul 8, 2016 #19

    gva

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    The ensemble interpretation is actually a shut up and calculate interpretation in disguise to dull the mind.
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    "Postulating that although the system-apparatus is in an improper mixed
    state, we can interpret it as a proper mixed state superficially solves the
    problem of outcomes, but does not explain why this happens, how or when.
    This kind of interpretation is sometimes called the ensemble-, or ignorance
    interpretation. Although the state jSAi is supposed to describe an individual
    quantum system, one claims that since we can only infer probabilities
    from multiple measurements, the reduced density operator SA is supposed
    to describe an ensemble of quantum systems, of which each member is in a
    definite state."
     
    Last edited by a moderator: May 8, 2017
  21. Jul 8, 2016 #20

    bhobba

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    The above is a personal opinion - not physics.

    No interpretation is inherently better than any other. They all fit the facts, and until there is a way to decide experimentally between them, all choosing one over the other is espousing personal views. Opinions are like bums - everybody has one - it doesn't make it correct.

    On topic here is what different interpretations say, contrasting them to others and pointing out the difficulties and positives. Opinions that its simply to dull the mind is not on topic. A bit of leeway is given, and mentioning things like that in passing is OK - but we do not enter into long drawn out discussion on opinions, because its basically philosophy that is off topic.

    That said it is no more 'mind dulling' than any other interpretation. Its simply minimal. That's neither good nor bad - nature may simply be like that or it may be explained by something deeper - we simply do not know.

    Thanks
    Bill
     
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