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I Wheeler's delayed choice doesn't change the past

  1. Jan 3, 2018 #121
    Niels Bohr (Source: https://en.wikiquote.org/wiki/Niels_Bohr):

    I consider those developments in physics during the last decades which have shown how problematical such concepts as "objective" and "subjective" are, a great liberation of thought. The whole thing started with the theory of relativity. In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived. For all that, we have come a long way from the classical ideal of objective descriptions.

    In quantum mechanics the departure from this ideal has been even more radical. We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice. Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation. To that extent, every physical process may be said to have objective and subjective features. The objective world of nineteenth-century science was, as we know today, an ideal, limiting case, but not the whole reality. Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will.


    Is there any “mystic” in this reasoning? Wheeler has put Bohr's view in a nutshell: "No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon.” Bohr never wanted to find something behind the phenomena.


    Niels Bohr (Source: https://en.wikiquote.org/wiki/Niels_Bohr):

    No, no, you are not thinking, you are just being logical.” (In response to those who made purely formal or mathematical arguments)
     
  2. Jan 3, 2018 #122

    vanhees71

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    Well, what's the statement by Bohr? I always wonder what he really wants to say in writing a lot of complicated text. Are the subjective elements in "modern physics" or not?

    My personal opinion is very clear: There are none! Physics is an empirical science and deals with objective reproducible facts and theoretical reasoning about such facts.
     
  3. Jan 3, 2018 #123

    zonde

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    I don't get it, why do you consider delayed choice experiment as tough for dynamical explanation. If you are satisfied with 50% visibility you even do not need any non-locality and can explain it with shared hidden variables.
    Ok, if you want more that 50% visibility (as can be observed in experiments) you need non-locality or something IMO more drastic.
    Hmm, there is no physics that contradicts my apparently dynamical view of reality.
    .
     
  4. Jan 3, 2018 #124

    Demystifier

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    I wouldn't say that finding something "behind the phenomena" has much to do with emotions or religion. If finding the "behind the phenomena" should not be classified as a part of natural sciences, then it's quite obvious that it should be classified as philosophy.
     
  5. Jan 3, 2018 #125

    vanhees71

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    You don't even need any hidden variables. Standard QED does an excellent job. After thinking for a long time about these issues, I don't understand anymore, why one must invent interpretations of standard QT that create problems instead of simply using the one provided by Born's probability interpretation and taking it seriously. QT, interpreted in this way, is not more mysterious than any classical theory of physics and it's very successful.

    As any hitherto discovered theory of physics it's incomplete in not providing a consistent description of quantum gravity nor does it give a clear hint at what observable consequences of a quantum theory of gravitation to expect. That's the real issue, not some quibble of some philosophers who don't want to accept that the natural sciences force us to learn how nature behaves and that this is not always according to our always preliminary and incomplete worldviews. Due to this anti-science attitude of philosophers/theologians people like Giordano Bruno were burnt!
     
  6. Jan 3, 2018 #126

    Demystifier

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    Well, one must invent non-minimal interpretations precisely because one wants to see (as you nicely expressed it) behind the phenomena. In classical physics the view behind the phenomena is almost automatic, but in quantum physics it isn't.

    Now if you want to ask why does one want to see behind the phenomena, nobody expressed it better than Einstein:
    "I want to know how God created this world. I'm not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts, the rest are details."
    And to avoid miss-conclusions, it has to be said that for Einstein "God" is a philosophical, not a religious concept. What he calls "God" is not very different from the contemporary concept of the "Theory Of Everything".
     
  7. Jan 3, 2018 #127

    zonde

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    I can understand your viewpoint and it seems rater sensible, but I have my reasons to look for interpretation. I am interested what hides behind statistical nature of QT.
    Have you any idea for a topic to discuss along these lines?
     
  8. Jan 3, 2018 #128

    vanhees71

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    Sure, Einstein was a kind of pantheist, mostly inspired by Spinoza, but that's well beyond the realm of objective science.

    Also, why are you saying "In classical physics the view behind the phenomena is almost automatic"? ##\dot{\vec{p}}=\vec{F}## is as abstract an description as Schrödinger's equation, and both are simply justified by describing the phenomena (within their realm of validity only of course). In which sense let's me Newton's equation of motion let view behind the phenomena almost automatically, while the Schrödinger equation doesn't? I don't think that any scientific theory can tell us "how God created this world". It's not even a question you can sensibly pose within the natural sciences! It's another level of human experience, and it's clearly a individual subjective one, which is precisely the realm the natural sciences do not consider.
     
  9. Jan 3, 2018 #129

    vanhees71

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    What makes you think there should hide anything behind the statistical nature of QT. Why shouldn't nature (or rather our observations of phenomena) be inherently probabilistic?

    [/QUOTE]
    Well, I've no clue. I think without any clue of a quantum theory of gravity nor any empirical hint at quantum effects concerning gravity, it's wild speculation anyway!
     
  10. Jan 3, 2018 #130

    Demystifier

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    In classical physics, we know that it is not really about force F or momentum p, but about particle trajectories x(t). F and p are just auxiliary quantities that help to get the thing we are really interested about, that is x(t). The two crucial properties of x(t) are
    1) It is a quantity that we directly observe, e.g. as a trajectory of a planet.
    2) It is objective in the sense that, according to the theory, it does not depend on whether we observe it or not.

    If classical physics is ultimately about x(t), then what is quantum physics ultimately about? If it is about the actual values of observables as functions of time, then the problem is that we don't have an explicit formula for that. If it is about probabilities of observables at given time, then the problem is that probabilities are not objective, in the sense that the theory does not say what is a probability of an observable when it is not measured.
     
  11. Jan 3, 2018 #131
    ... and so finally ending up in the "ensemble interpretation" due to the implicit assumption that quantum randomness stems not from utter lawlessness but from hidden causes.
     
  12. Jan 3, 2018 #132

    Fra

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    This analogy between relativity and QM is great.

    I think the REASON why Bohr insists for the NEED for a classical measurement device, in order to define the experiements, is simply that of objectivity. Even though it is subjective in the sense of conditional upon the actual choice and settings of the device, it complies to objectivity(*) in the sense that in the classical world different observers can easily communicate without distorting each other.

    So I think that Bohr is right that quantum theory as it stands requires a classical backdrop, for attaching all the things, like probability concepts etc.

    (*) I see complications there, but they elaborations that i think we could not expect anything in 1935 or so to be aware of. But when looking at unficiations and gravity things do get more complex. And the question of "objectivity" actually takes on a whole new level, far beyond Bohr and Einsteins ideas. This is to question the objectivity in inferred laws of physics, and what if there exists no classical measurement device, say at unification energies at big bang? then what happens to these ideas? But that belongs to the BTSM anyway so i will not do more than hint. But I think that in even in that light, the insights of Bohr in the early days was extremely sound and clear. Even though i also agree that some of the actual papers are sometimes hard to parse.

    /Fredrik
     
  13. Jan 3, 2018 #133

    zonde

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    There are clicks in detectors from which experimentalist calculates statistics. There is at least that much behind the statistical nature of QT (this part is actually not very hidden). And actually you don't have to go any further to run into problems with "inherently probabilistic clicks in detectors"
    I think there is enough empirical hints (of course I might be wrong) to think about quantum gravity.
    Say "charge" of gravitation field is mass. But formation of bond state in QM releases some mass and reduces gravity "charge" of component particles.
    .
     
  14. Jan 3, 2018 #134
    What do we expect from a theory of the physical science ? give us rules/axioms to build predictives models or tell us something about the physical world ? In other words, are we frustrated to constat that quantum mechanics has given up the ambition of providing explanations (causal assignment) to stick to the predictive function only ( probabilistic inference ) ?

    Best regards
    Patrick
     
    Last edited: Jan 3, 2018
  15. Jan 3, 2018 #135

    OCR

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    If you think the writings of Bohr are, sometimes, a bit hard to parse... try parsing this:
    Please, can some of you slow down your thinking to match your key strokes, and make some attempt to proofread... just a little ?

    Thank you, and carry on.
     
    Last edited: Jan 3, 2018
  16. Jan 3, 2018 #136

    vanhees71

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    Hm, I just asked a question. What's wrong with that?
     
  17. Jan 3, 2018 #137

    vanhees71

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    No, it's much simpler: There are no hidden causes, but nature behaves just fundamentally in a random way with the laws for the probabilities for measurement results given by quantum theory.
     
  18. Jan 3, 2018 #138

    vanhees71

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    Classical mechanics is ultimately about a trajectory in phase space, given by the dynamical evolution (since the state of the system in classical mechanics is represented by a point in phase space).

    Quantum mechanics is ultimately about the evolution of the probabilities (or probability distributions) given by the statistical operator and the eigenvectors of observables in any picture of time evolution.
     
  19. Jan 3, 2018 #139

    Fra

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    In isolation that looks scrambled grammatically yes, but if you read it "in context" a human parser has little problem to understand it. But i figure posts here are not as polished as as wordings in a formal paper.

    I also think I see Demystifiers point which caused the question to be asked: With some exceptions, in classical mechanics the it is easier to create an intuitive picture of what "really happens" as one can often make causal pictures in 4D of "mechanical mechanisms". This is surely much harder in quantum mechanics, as mechanistic intuition obviously fails.

    /Fredrik
     
  20. Jan 3, 2018 #140

    Fra

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    As I see it, question or "choosing observables" imples a change in the internal structure of the observer (or measurement device, or information processing agent if you prefer).

    Thus technically, asking totally different questions correspond to different observers, so i do not see a problem with this. This is why its "subjective". But its not subjective in a mystical way imo?

    /Fredrik
     
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