Observation and Reality

Discussion in 'Physics & Math' started by Bowser, Dec 30, 2016.

  1. James R Just this guy, you know? Staff Member

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    No. The observer could be an electrical detector or some other piece of apparatus, for example.
     
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  3. river Valued Senior Member

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    Could but is the the observer an electrical detector , in actual fact ?
     
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  5. exchemist Valued Senior Member

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    Quite. In fact as I recall, the word "measurement" is often used rather than observation, thereby stressing it is the interaction of the QM system with the detector that determines the state.

    I am not aware of any serious interpretation of QM that suggests the readings on the dial change when the experimenter goes away to get a cup of coffee.

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  7. river Valued Senior Member

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    detector or some other piece of apparatus, for example.


    So where does that put us exchemist ?
     
  8. Magical Realist Valued Senior Member

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    How can observation occur with a non-conscious piece of equipment? Don't you mean measurement? But then measurement seems to be a conscious act too.
     
    Last edited: Jan 8, 2017
  9. danshawen Valued Senior Member

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    That would then require two observations (one BY the instrument and one OF the instrument) to determine the state change of a single event. An instrument is not an event, although it may change state in response to an event. Clumsy way to do science.
     
  10. exchemist Valued Senior Member

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    Don't be ridiculous. It's the only way we have to do it.
     
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  11. Bowser Life is Fatal. Valued Senior Member

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    I'll throw another curiosity in the mix...Quantum Entanglement. I hope it doesn't derail the OP, but I think it's also another interesting idea.
     
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  12. James R Just this guy, you know? Staff Member

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    The philosophical question is really about when, exactly, the "collapse of the wavefunction" occurs.

    Suppose we measure a quantum event of some kind with a piece of experimental apparatus. Some people (myself included) would say that once that apparatus makes a detection sufficient to register the quantum event in a way that can be "amplified" to give a macroscopic-level reading, then this is enough to collapse the wave function to an eigenstate of the system under observation.

    Others go on to insist that the macroscopic apparatus itself remains in a quantum superposition of states until a conscious observer looks at the readings on the dial (or whatever the readout is). This is the sort of argument implied by the Schrodinger's Cat thought experiment.

    To me - and this is a personal preference, somewhat informed by having read up on the matter to some extent - it doesn't make a lot of sense that a conscious observer would be necessary before any quantum events could be resolved. If you subscribe to that theory, then you must believe that the whole universe must have been in some complex superposition of quantum states prior to the first conscious being evolving in the universe. Or, alternatively, you might believe that a supernatural being (God, most likely) may have intervened to collapse wavefunctions up to some point at which conscious beings like human beings could take over the job. This would not, of course, preclude God continuing to act as Chief Collapser of Wave Functions in an ongoing capacity. But this second explanation is obviously more mystical than scientific.
     
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  13. Magical Realist Valued Senior Member

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    "In his 1932 book The Mathematical Foundations of Quantum Mechanics, John von Neumann argued that the mathematics of quantum mechanics allows for the collapse of the wave function to be placed at any position in the causal chain from the measurement device to the "subjective perception" of the human observer. In 1939, Fritz London and Edmond Bauer argued for the latter boundary (consciousness).[1] In the 1960s, Eugene Wigner[2] reformulated the "Schrödinger's cat" thought experiment as "Wigner's friend" and proposed that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function, independent of any realist interpretation. See Consciousness and measurement. The non-physical mind is postulated to be the only true measurement apparatus.[3] Rudolf Peierls was also a proponent of this interpretation.[4]

    This interpretation has been summarized thus:

    The rules of quantum mechanics are correct but there is only one system which may be treated with quantum mechanics, namely the entire material world. There exist external observers which cannot be treated within quantum mechanics, namely human (and perhaps animal) minds, which perform measurements on the brain causing wave function collapse.[3]

    Henry Stapp has argued for the concept as follows:

    From the point of view of the mathematics of quantum theory it makes no sense to treat a measuring device as intrinsically different from the collection of atomic constituents that make it up. A device is just another part of the physical universe... Moreover, the conscious thoughts of a human observer ought to be causally connected most directly and immediately to what is happening in his brain, not to what is happening out at some measuring device... Our bodies and brains thus become...parts of the quantum mechanically described physical universe. Treating the entire physical universe in this unified way provides a conceptually simple and logically coherent theoretical foundation..."-----https://en.wikipedia.org/wiki/Von_Neumann–Wigner_interpretation
     
  14. James R Just this guy, you know? Staff Member

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    I recommend readers who are interested in this also read the various objections to the interpretation that Magical Realist just posted (see the same wikipedia link).

    Magical Realist has chosen to leave out half of the story. (Why would that be, I wonder?)
     
  15. Magical Realist Valued Senior Member

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  16. exchemist Valued Senior Member

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    Agreed. For one thing, as I queried on another thread in which this issue came up, what would qualify as a conscious observer? Would an ape? A cat? An insect? Or, more intriguingly, would even a person incapable of understanding what they were looking at be "conscious" in the required sense?

    My understanding has always been that it is the interaction of the QM system with something (matter and/or a field or radiation) that resolves the probability distribution into an actual value. For example, when a free electron is captured by a detector its state changes and thus so too does its state function.

    Whether a bloke in a white coat is watching or not seems to me neither here nor there. But then I side with Johnson rather than Berkeley (even though the latter is buried in my old college).
     
    Last edited: Jan 9, 2017
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  17. James R Just this guy, you know? Staff Member

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  18. James R Just this guy, you know? Staff Member

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    The problem there, though, is: what is part of the "QM system" and what is part of the "something"? In your detected electron example, what is the QM system? The electron by itself? The electron and the detector? Or the electron, the detector and the dude wearing the lab coat? Or something else?

    That is: how do we draw a line between the quantum system and the thing that causes the system to collapse? How do we decide in any given case where to draw that line?

    For example, if consciousness demarks dividing line, then we need to ask exactly the kinds of questions you've asked. Is the consciousness of a fly sufficient, or do we need human-level consciousness? What about a bacterium? Could an artificial intelligence do the trick, perhaps?

    Personally, I lean towards the view that some kind of "decoherence" (vaguely defined) is probably responsible for the apparent collapse of wavefunctions. To tell the truth, I'm not even sure what the wavefunction is. Is it a physical thing that describes a system independently of observers, or is it telling us something about what the universe will allow observers to know about the system at any given time?
     
  19. danshawen Valued Senior Member

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    Ditch the observer. Not important. Use a retina or other sense organ that is sensitive to the transfer of energy.

    You can still predict that no matter where you do the double slit experiment, it will work the same way it always does, whether you understand it or not, and whether the photons propagate through the apparatus in large numbers or one at a time.

    A pattern on a screen is a memory device. It records photon propagation over a time interval, not an instant of time, and time itself is not defined by the propagation of light. Photons can also have spin, and/or be quantum entangled with each other. Entanglement is something we know persists over time and also great distances, which is light travel time. The double slit is composed of bound energy which persists also in part due to entanglement. None of this observed behavior appears strange to me, even slightly.

    If you are confused because what is a bright band with a single slit has a dark band in the middle with a double slit, then you probably aren't paying sufficient attention to the relative exposure times and number of photons needed to make that pattern emerge. Atoms persist over time. The double slit can persist over time because of this, and also because of entanglement. Why shouldn't photons also share a sort of temporal persistence? Photons have inertia in one direction as they propagate. If you change that direction of propagation by means of diffraction with a pair of slits made of bound energy with inertia in every direction at once (and also faster than light propagates), what did you expect would happen?

    Photons aren't little squares, cubes or rectangles. Try the experiment again with a two slit circular diffraction grating.
     
    Last edited: Jan 9, 2017
  20. James R Just this guy, you know? Staff Member

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    danshawen:

    A lot of that post didn't make much sense.

    Tell me how you explain why, in a single-slit experiment with one photon at a time, we see an interference pattern built up over time that shows a series of adjacent bright and dark bands, rather than a single bright band that fades out at the edges.
     
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  21. exchemist Valued Senior Member

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    Dan you do not need to drag bloody entanglement into something as simple as the double slit experiment. Nor your personal obsessions with "bound energy". Let's keep it as simple as we can, shall we, rather than mixing everything together in a huge incomprehensible mess?

    You have an interference pattern that can be shown to build up, quantum by quantum, at low beam intensities. This is what basic QM predicts, by virtue of de Broglie's matter waves and their simple relation to probability density. C'est tout.

    The reason people talk so much about the double slit experiment is that it shows the wavefunction of a single photon or electron must explore both slits - which is counterintuitive if one thinks of the entity as being a "particle" in the classical sense. But this is not an entanglement phenomenon. Entanglement is all about wavefunctions describing groups of QM entities, whereas this experiment is specifically about single entities.

    https://en.wikipedia.org/wiki/Quantum_entanglement
     
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  22. Yazata Valued Senior Member

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    I'm inclined to interpret "the QM system" as the realm in which quantum physics applies and the "something" to be the realm where classical physics applies. The 'micro-scale' and the 'macro-scale'.

    That raises the question of the quantum/classical interface. And regarding that, I'm inclined to think that as our scale expands and more and more matter is included in our "something", the interactions within the matter provide constraints and the quantum weirdness gradually becomes less obvious, so that the the quantum account converges on the classical account. (Of course I'm just a layman and can't give a mathematical account of that.)

    Excellent point. It seems to me to be a mathematical model that spits out probabilities. In that sense it's kind of a black-box. We assume that something, some physical reality (albeit non-classical) is inside the box, but there are any number of hypotheses about what physical reality on the micro-scale might 'look like' and about what might really be going on down there. (The 'interpretations' in your link.)

    This stuff is all way above my pay-grade.

    I do know that I give very little credence to the kind of quantum idealism that MR seems to be pushing.
     
  23. Magical Realist Valued Senior Member

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    "Decoherence has been used to understand the collapse of the wavefunction in quantum mechanics. Decoherence does not generate actual wave function collapse. It only provides an explanation for the observation of wave function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. Specifically, decoherence does not attempt to explain the measurement problem. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble"."-https://en.wikipedia.org/wiki/Quantum_decoherence
     
    Last edited: Jan 9, 2017

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