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  1. Oct 23, 2011 #1
    This quote comes from Paul Davies Information and the Nature of Reality but is written by another author, not Davies:

    I think they're really exaggerating. Yea, it might be true theoretically at some quantum level but I'm skeptical that this fact really makes a difference about reality. Let me know what you think.
     
  2. jcsd
  3. Oct 23, 2011 #2

    xts

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    I don't call 'collapse' a 'phenomenon', as it has no observable implications.
     
  4. Oct 23, 2011 #3

    Bill_K

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    The collapse of the wavefunction is the signal that you've become part of the system. The guy in the next room doesn't know what state you've observed, he only knows that you have become correlated with it. Until you tell him, then he becomes correlated.
     
  5. Oct 23, 2011 #4

    xts

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    @Bill-K:
    A signal?
    Signal is a bit of information sent by someone to be received by someone else.
    How do you receive a "collapse signal"?
     
  6. Oct 23, 2011 #5

    Ken G

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    I think by "signal" he just means "signature". So if you see something you interpret as a collapse to a specified definite outcome, it means your participation in that system has become in some way decisive about how you will describe that system. To the many-worlds proponent, it means you have gotten so entangled into that system that you can no longer see it clearly (you don't see its unitarity or its "many worlds" character). To the CI proponent, it means you are the one who is using language about that system, it is something that has a reality for you because you are experiencing it (so being "part of the system" has a kind of reverse implication from MWI-- instead of losing contact with the full system, in CI the collapse is how you gain contact with that system). To the deBB proponent, it means you have discovered a truth about that system that had previously been kept from your awareness (so becoming "part of the system" here means joining those who are "in the know" about that system, but that information was already true about that system even if it had never been culled out from the other coherent possibilities via contact with the measuring apparatus).
     
  7. Oct 23, 2011 #6

    xts

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    @Ken-G:
    please!
    tell the same in words understandable to humble layperson, rather than making relations: "signal"->"signature", and "collapse" -> ~"entanglement", etc.

    I feel like listening to a lecture on Hegel's philosophy...

    Bertrand Russell's principle: "if the philosophical idea cannot be expressed in common words it must be crap"
    [sorry - I can't find original B.R. quote - that's my back-translation to English...]
     
  8. Oct 23, 2011 #7

    Ken G

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    I'm not sure about what distinctions between the word "signal" and "signature" you are asking me about. Is that distinction not perfectly clear? You interpreted "signal" as meaning information passing between two observers, it seems to me that Bill_K is just talking about how an observer can tell when they should consider themselves to be part of (or "correlated with") the system they are studying. That's the "signature" I'm talking about. The rest of the post was just to point out there is little agreement or demonstrability about just exactly what form that correlation takes.

    Response to Russell-- that quote is basically crap, albeit from a great man. To see that, simply replace the word "philosophical" with the word "physical" or the word "mathematical." Now that makes it perfectly clear does it not? Why would he think philosophy should be able to live up to a standard that physics and mathematics never do? He should have known better, since he knew a lot about all three.
     
    Last edited: Oct 23, 2011
  9. Oct 23, 2011 #8

    xts

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    Both words "signal" and "signature" mean that some information is to be transferred: intentionally - for 'signal' or discovered from 'signature'. The information may be obtained from the object being a 'signal' or 'signature'.

    The idea of 'collapse' is not associated with any information flow between its 'cause' and the 'collapsed object'. It may be only used as a measure of our (yet external observer's) knowledge about the system.
     
  10. Oct 23, 2011 #9

    Ken G

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    Well, I have no real idea what distinction you are objecting to, by my point is simply that you can tell when you are part of a system when you perceive that the system has undergone a collapse, collapse is the signature of becoming part of what you are studying. As you are the only intelligent agent in that story, there is no "signal" passing between intelligent agents there.
    I believe the point Bill_K is making is that when the collapse is perceived, we are not "yet external" any more, that is just exactly when we are "now internal" to what we are studying. If that is what he meant, I agree with him.
     
  11. Oct 23, 2011 #10

    xts

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    I am not making any distinction (relevant for this discussion) between "signal" and "signature" - I am just objecting against using any of those words in context of "observation" or "collapse of wavefunction".

    Let me express it that way:
    - "collapse" is nothing happening in real world - it means it cannot be detected or measured if the "collapse" happened or not;
    - "collapse" is an operation we do mentally to simplify our further description of the object: as soon as we got know the value of some parameter, we substitute it into the formulae describing the world, making it simpler and easier for further calculations.
     
  12. Oct 23, 2011 #11

    Ken G

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    I'm fine with that, and Bill_K might be too. I don't see any conflict between what you are saying here, and his point (if I interpreted it correctly) that any time we find that collapse is the operation we are doing, we can take that as a "signal" or "signature" that we are becoming part of (or correlated with) what we are trying to study.
     
  13. Oct 23, 2011 #12

    xts

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    OK then ;)

    I just object about words like "signal" as they lead to misunderstandings.
    You (Bill_K?) understand "signal" as something affecting your own mind.
    In my meaning (well, I may be biased, I work for telecom industry...) the "signal" means something real, carrying information, sent by one party to be received by another party.
     
  14. Oct 23, 2011 #13

    Ken G

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    That's exactly why I suggested "signature" instead, because it didn't seem to carry that connotation for me. But we all agree (if Bill_K does) that we are just talking about the thought processes of a single intelligent agent who is doing physics here, so the words simply need clarification.
     
  15. Oct 23, 2011 #14

    xts

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    I don't like 'signature' either.

    "Signature" is something existing independently from the signer, attached to signed object, which may be objectively tested by anybody. It carry information, which may be used by others than signer.

    In your meaning (the meaning we agreed...) there is no information flow between you and anybody else.
     
  16. Oct 24, 2011 #15
    I agree, to me it doesn't make any sense. Anyway, that said 'phenomenon' is the basis of the Schrodinger's cat idea, that says that the cat is in a position of being alive and dead at the same time, until an observer sees if the cat is alive or dead, collapsing the function. The thing i don't understand is that an observer to the incident somehow has some role in dictating that the cat is alive or dead. an observer should not be necessary in the result of the cat surviving or dying. Anyway, that is my take on the idea, in a different perspective of Schrodinger's cat rather than your example.
     
  17. Oct 24, 2011 #16

    xts

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    If you take 'collapse' as nothing real, but describing your knowledge, then there is nothing weird in fact that you don't know if cat is alive or dead, but opening the box you gather this knowledge. Nothing real happens to the cat at the moment of opening the box (except that, if he is alive, he finally regain hope for some whiskas).
    It is your knowledge about cat what changes, not the cat himself.
     
  18. Oct 24, 2011 #17

    Yes. what I am saying is i don't see why human (or other means of) observation collapses the function of why the cat is both alive and dead. The means of observation collapses the function of whether or not the cat is alive or dead. the cat should never have been alive and dead. Human observation should not be neccesary for the cat being alive or dead. It is only necessary for the cat to be known if it is alive or dead. Long story short, the cat is either alive or dead. human observation is only needed to know which one it is; not to MAKE it alive or dead.
     
  19. Oct 25, 2011 #18

    Ken G

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    But "signature" has a more general meaning. You are thinking like a web signature, or a signature on a document. Often, "signature" is used as a synonym to "fingerprint." That's the way I mean it-- the signature, or fingerprint, that an observer has become part of the system is the appearance of a collapse. It can be the observer themself that notices this fingerprint, just as you can see your own fingerprints on a glass you are drinking from-- there is no implied connotation that information is being sent to someone else.
     
  20. Oct 25, 2011 #19

    xts

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    In all those meanings 'signature' is some real property of the object, which may be objectively tested, rather than state of your mind. So it carries the information (regardless if the information had been intentionally sent or if its purpose is to be analysed).

    It can be observer themself, that notice a fingerprint, but it may be also Sherlock Holmes who find it. That is a fundamental difference between 'signals', 'signatures', 'fingerprints', etc, and 'collapse'. The 'collapse' - in the meaning we agreed before - may not be detected by anybody else than observer. So it is not a property of the object - it is rather property of observer.

    So if you like metaphore with fingers and traces - it is rather not a fingerprint you leave on a glass, but dirt on fingers you get touching something something.
     
  21. Oct 25, 2011 #20

    Ken G

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    The state of my mind may also be objectively tested. It can also be perceived, by me. That is all that is required to identify a "signature" that I have become part of what I am studying.
    Yes, of course-- collapse carries the information that I have become part of the system. It carries that information to me, and to anyone else who enters the same system.
    Actually, I think the Sherlock Holmes analogy works fine here-- someone else can determine that I have witnessed a collapse. I can tell them I did, or they can even infer that I did based on their own experiences in similar situations. The signature is there for any to see.
    But a key aspect of quantum mechanics is that if I perceive a collapse, and someone else enters the system too, they must see the same collapse I did. So the collapse can be detected by someone else too, and it is the same collapse.
    It cannot be purely a property of the observer, or we would not have objectivity in quantum mechanics.
    Dirt that someone else can see too.
     
  22. Oct 25, 2011 #21

    xts

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    The difference is:
    Sherlock Holmes may find (examining the glass only) if it had been touched or not looking at the glass - you (who left the fingerprint) are no longer needed to distinguish between touched and untouched glasses.
    In case of 'collapse' the trace is associated with you, not with the object. Sherlock may interrogate you to find if you know something about the glass, but it cannot detect anything examining only the glass itself.

    Dirt that someone else can see too.
    yes, and it is my point: the dirt is detectable, but not by examining the object, but rather you. 'Collapse' is a change made by examined object to observer, while 'fingerprint' is something opposite: change made by observer to the object.
     
  23. Oct 25, 2011 #22

    Ken G

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    I don't see that, if I do an experiment and don't look at the result, I'll have to treat the outcome as a mixed state, even though I can imagine that something definite happened and I just don't know yet, if I'm using CI. Thus I can treat myself as though I was part of that system, even if I am not actually part of that system, because that is the characteristic of macroscopic systems-- there is just one coherent macroscopic world that we interact with, remember, and communicate within. Similarly, if my collaborator comes in and records the outcome, not knowing if I have already done so or not, it doesn't change their impression of the situation whether I looked or didn't look. The fingerprint is on the apparatus simply by virtue of being a measuring device-- by making itself "part of the system", it collapsed that system for all of us (not in the sense of some physical effect, but in the sense of our decisions about how we are treating the action of that measuring device), we needn't collapse it again for ourselves (we'd have a big objectivity problem if that were not true).
    I feel that relationship is a two-way street, the direction of the relationship is highly ambiguous and possibly unknowable.
     
  24. Oct 25, 2011 #23
    What does it mean to observe or measure something?

    A measurement is the act of determining the value of some observable. But this means the system is left in a state where the uncertainty of the observable vanishes, yes?

    Generally whatever the state of the system before measurement, at the time of measurement the system must be in a state of zero-uncertainty:

    [tex]\triangle O = 0[/tex]

    Every observable is associated with a Hermitian operator so a state of zero uncertainty must have the property:

    [tex](\triangle O)^2 = < \phi | (O - <O>)^2 | \phi > = 0 [/tex]

    Define operator D as

    [tex] D \equiv O - \lambda [/tex]

    Where

    [tex]
    \lambda = < O >
    [/tex]

    By using the hermiticity of D:

    [tex] ( \triangle O)^2 = < \phi | (D)^2 | \phi >
    = < \phi | D | D\phi >
    = < \phi | D | \phi ' >
    = < D\phi | \phi ' >
    = < \phi ' | \phi ' >
    = \int dx \phi '*(x)\phi '(x)
    [/tex]

    Now if:

    [tex] \triangle O = 0 [/tex]

    Then:

    [tex] ( \triangle O)^2 = \int dx \phi^{'*}(x)\phi '(x)
    [/tex]

    And since

    [tex]
    \phi^{'*}(x)\phi '(x) \geq 0
    [/tex]

    for all x

    [tex]

    0 = \phi '(x)
    = D\phi (x)
    = [O - \lambda]\phi (x)

    [/tex]

    So for a zero-uncertainty state we have

    [tex] O\phi (x) = \lambda\phi (x) [/tex]

    This is an Eigenvalue equation. The state does not collapse because we measure it, we simply define an ideal measurement as zero-uncertainty, a collapsed state. Looking at a finger print on a window is not even an ideal measurement.
     
    Last edited: Oct 25, 2011
  25. Oct 26, 2011 #24

    Fra

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    I would agree the collapse is related the observer. A possible resolution to Ke's issue
    is to consider the proper interaction, and evolution.

    Apparent objectivity as in "agreement between observers" is accomplished since the observers are TUNED.

    In my view, the tuning takes place when observers are interacting and the objectivity is emergent on part with a negotiation process.

    The important distinction which is again at the heart of this matter is wether we understand "objectivity" as a kind of mathematical CONSTRAINT or wether it is a RESULT or like a kind ot equilibrium. Thus the "observer objectivity" is possibly more like a property of the steady state population of the observers (read structure of matter).

    /Fredrik
     
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