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Featured Insights The Fundamental Difference in Interpretations of Quantum Mechanics - Comments

  1. Jan 9, 2018 #1


    Staff: Mentor

  2. jcsd
  3. Jan 9, 2018 #2

    Buzz Bloom

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    Hi Peter:

    I very much like the clarity of the dichotomy you present.

    Sometimes I find myself comfortably accepting either of the two points of view at different times. The particular choice I make depends on my recognizing that the context makes one choice more convenient than the other. When I think about my practice of doing this, I interpret this as actually accepting both points of view at the same time, and when I do that I just ignore the apparent contradictions. I summarize this practice with the maxim:
    Reality is fundamentally paradoxical.​

  4. Jan 9, 2018 #3
    Peter, when you write "physically real", do you refer to which definition of it?

  5. Jan 9, 2018 #4


    Staff: Mentor

    There is no precise definition of the term "physically real". That's part of what makes discussions of QM interpretations difficult: one is trying to go beyond the basic model of QM, which is expressed in math and has precise definitions, to interpretations that use ordinary language, where words do not have precise definitions.
  6. Jan 10, 2018 #5

    Nice article that shows a facet of epistemic (#1)/ontological (#2) duality. My understanding (and thus my point of view) is that the observer (humain being, measurements apparatus) interact with something that resist to us ("real in itself"), but can only capture the interaction effects and not the causes.

    To buid a rational and complet interpretation, it seems to me that we need to dissect how we construct our knowledge from the effects we capture up to ours objectifications and taking in acount all the humain process from ours first-person experience. Moreover It could be relevant to be aware of our blind spot when we made ours objectivations/reifications.

    Best regards,
    Last edited: Jan 10, 2018
  7. Jan 10, 2018 #6
    Ok, and do you think that term, "physically real" refers only to physical quantities as position, momentum, spin components, etc, or even to something else? For example could I pretend that "physically real" is the "setting of the experiment" if it's prepared in a specific and reproducible way?
    To which of the 2 "paradigms" you describes belongs this vision?

  8. Jan 10, 2018 #7
    What are the Interpretations of Classical Mechanics ?
  9. Jan 10, 2018 #8

    A. Neumaier

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    The book by Laurence Sklar, Physics and Chance, discusses the philosophy of classical mechanics. But the subject has fallen out of fashion since it is known that classical mechanics fails completely in the subatomic domain.
    Last edited: Jan 14, 2018
  10. Jan 10, 2018 #9


    Staff: Mentor

    Please read what I said in the article: I said that viewpoint #2 says that the quantum state is "physically real". In other words, the wave function/state vector/whatever you want to call it, a particular well-defined thing in the math, represents something "physically real". The quantum state is not position, momentum, spin components, etc. It's the particular well-defined thing in the math.
  11. Jan 10, 2018 #10
    In my opinion, part of the reason there is such scope for interpretations is that nobody actually KNOWS what Ψ means. Either there is an actual wave of there is not, and here we have the first room for debate. If there is, how come nobody can find it, and if there is not, how come a stream of particles reproduce a diffraction pattern in the two slit experiment? No matter which option you try, somewhere there is a dead rat to swallow. As it happens, I have my own interpretation which differs from others in two ways after you assume there is an actual wave. The first, the phase exp(2πiS/h) becomes real when S = h (or h/2) - from Euler. This is why electrons pair in an energy well, despite repelling each other. Since it becomes real at the antinode, I add the premise that the expectation values of variables can be obtained there. The second is that if there is a wave, the wave front has to arrive at the two slits about the same time as the particle. If so, the wave must transmit energy (which waves generally do, but the dead rat here is where is this extra energy? However, it is better than Bohm's quantum potential because it has a specific value.) The Uncertainty Principle and Exclusion Principle follow readily, as does why the electron does not radiate its way to the nucleus. The value in this, from my point of view, is it makes the calculation of things like the chemical bond so much easier - the hydrogen molecule becomes almost mental arithmetic, although things get more complicated as the number of electrons increase. Nevertheless, the equations for Sb2 gave an energy within about 2 kJ/mol, which is not bad.
  12. Jan 10, 2018 #11

    Stephen Tashi

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    True, but your article inherits an imprecise meaning directly from imprecision of "physically real". It would be unreasonable to expect any commentary on interpretations of QM to be free of all ambiguity, but in order to get the "general drift" of what you are saying it would be helpful to have examples of how , in your opinion, different concepts of the reality of the wave function lead to well-known interpretations of QM.

    Even if we cannot precisely define "physically real" to the extent of proposing a physical test for it or a mathematical definition, it may be possible to agree on certain properties of "physically real" things. For example, thinking in terms of mathematics, if I grant that X and Y are physically real aspects of something then should I also say that any function f(X,Y) is also physically real? One would suspect the answer is "No". A tricky mathematician would have us consider the constant function f(X,Y) = 13. So perhaps the mathematical property of a physically real f(X,Y) should be that we can reconstruct the values of X and Y given the value of f(X,Y) or , more generally that we can reconstruct the values of X and Y from a given value f(X,Y) and values of some other functions of X and Y.

    I don't understand that passage. For example, is the fact that a person is a resident of the state of Texas a physically real property of that person? Isn't belonging to the ensemble of Texans a real property of that single person? Would we define a physically real property of a person to be sufficient information to distinguish a unique person?
  13. Jan 10, 2018 #12


    Staff: Mentor

    I thought cases 1 and 2 in the article already described that, but I'll give it another shot.

    Case 1 says the state is not real; it's just a description of our knowledge of the system, in the same sense that, for example, saying that a coin has a 50-50 chance of coming up heads or tails describes our knowledge of the system--the coin itself isn't a 50-50 mixture of anything, nor is what happens when we flip it, it's just that we don't know--we can't predict--how it is going to land, we can only describe probabilities.

    Case 2 says the state is real, in the same sense that, for example, a 3-vector describing the position of an object in Newtonian physics is real: it describes the actual position of the actual object.
  14. Jan 10, 2018 #13
    I focus more on predictions than "interpretations."

    One could have differing interpretations of Lagrangian, Hamiltonian, and Newtonian Mechanics (and I've met physicists who do argue one is right and the other two are "wrong"). However, all the predictions are the same.

    The only interesting cases are where differing interpretations make different testable predictions. This is real science. Then we can perform an experiment to distinguish between them.
  15. Jan 10, 2018 #14


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    This is also my position, and in particular to understand how the choice of interpretation"implies" a certain research direction in the open questions, or even WHICH the open question are, such as unification of interactions. Then if a certain interpretations shows to provide a more fruitful "angle" to making progress, then that would be my "preferred" interpretation.

    Lets note that this is a DIFFERENT selection strategy than those that think we need no modification to current theories, and that the preferred interpretation thus is some kind of "minimalist one". I fully agree that the minimalist selection principle makes sense if the interpretation served only the purposes of decreasing angst over understanding or not understanding the foundations..

  16. Jan 10, 2018 #15


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    I often feel that the descriptor "real", would be better replaced in these discussions with "objective", or observer invariant.

    Thus real would mean, that different observers should arrive at a consistent descriptions of the same system. And then it begs the question of explaining how does one make this "consistency check"? This is a bit like trying to compare angles of vectors living at different tangent planes or so. You need some kind fo "parallell transport".

    As I see it, the consistency check is that the two systems making the inferences must physically interact/communicate. If they can do this without distorting the opinion of the other party, then they are consistent, and they have reached a consensus. And in many cases where there is an "apparent" disagreement, this is often identifed as an interaction term or force between the observers! And accounting for this, one can add some new interactions and recover a new elevated level of objectivity. This is also typically the situation that works fine in classical mechanics. And while we have observers also in classical mechanics, the fact that they typically easily reach a consensus of observations, is why the observations rarely are emphasised as of fundamental importance.

    So unless we can define the interaction that constitutes the consistency check, the notion of "real" is not only slightly ambigous, it seems too undefined to be recommended.

  17. Jan 10, 2018 #16

    Stephen Tashi

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    What I'm suggesting is that your give examples of the thesis of your article - that the two cases naturally divide the various interpretations- e.g. perhaps Case 2 leads to the Many Worlds or the Bohmian interpretation etc.

    Parsing that definition seems to hinge on whether probabilities are "real". In your definition, must a physically real state be sufficient to produce a deterministic outcome ?

    What aspect of that example is significant?

    One feature of that example is that if point particle_1 has the same 3-vector as point particle_2 then particle_1 and particle_2 are the same particle. By contrast, if we are only given that particle_1 and particle_2 are both in a laboratory in Texas then, from that description, the names might refer to different particles.

    Another aspect of that example is that we can measure the 3-vector associated with a particle. However, we could also determine whether a particle is in a laboratory in Texas, so the ability to measure the 3-vector seems not to be the outstanding feature of the example.
  18. Jan 10, 2018 #17


    Staff: Mentor

    Perhaps, but that would fail to acknowledge the fundamental difference in interpretations which was the main topic of the article. :wink: I think all interpretations agree that the quantum state/wave function is observer invariant; that's not where the difference lies.
  19. Jan 10, 2018 #18


    Staff: Mentor

    Ah, ok. If you want a quick categorization of some common interpretations, here it is:

    Case 1: Copenhagen, ensemble

    Case 2: Many worlds, Bohmian*

    * - the Bohmian interpretation is kind of a special case, because it contains nonlocal hidden variables: the actual particle positions. The standard Bohmian interpretation considers these to be in principle unobservable, but it also considers the wave function--the "quantum potential"--to be real, which is why I put it in case 2.

    But one could easily envision an extended Bohmian interpretation in which they were observable. This would amount to a new theory extending QM and making testable predictions that could distinguish it from standard QM, which would take it out of the topic area discussed in the article. Similar remarks apply to things like the GRW model, which adds an additional "objective collapse" process that makes different testable predictions from standard QM.
  20. Jan 10, 2018 #19


    Staff: Mentor

    Sure, that's why I said we're dealing with vague ordinary language. I'm not trying to put a specific definition on the word "real". I'm just trying to say that, as far as I can tell, people who espouse Case 1 interpretations consider the quantum state to be like probabilities, and in common ordinary language usage probabilities do not describe the physically real state of anything; they just describe our knowledge (or the limitations thereof). If it helps, substitute "case 1 says the quantum state is like probabilities" for "case 1 says the quantum state is not real".

    Case 2 interpretations of QM do not say a physically real state produces a deterministic outcome (that would contradict standard QM); they just say the quantum state is physically real. So I don't see how this helps to clarify anything relevant to this discussion.

    Just what I said: case 2 interpretations treat the quantum state vector similarly to the way Newtonian physics treats the position 3-vector of a particle: it describes the physically real state of something.

    I think you're making this more difficult that it needs to be. I'm not trying to be abstruse.
    Last edited: Jan 11, 2018
  21. Jan 10, 2018 #20
    I am somewhat surprised this thread is allowed at all, since interpretations of QM is purely a philosophy of science/physics topic. This side discussion about the imprecision of language is a red herring, since there are already certain words in philosophy which have specified meanings to make clear distinctions, for example, physical is such a word. As was said before in post 5, the main distinction between interpretations of QM is precisely whether or not the state is itself ontic or epistemic; everything else (e.g. whether or not it is determistic or stochastic) is independent with regard to the ontic/epistemic distinction.

    Moreover, some 'interpretations' of QM, in particular the collapse ones, are clearly not interpretations but incomplete extensions to or revisions of QM, i.e. alternative theories awaiting mathematical completion. It is here most clearly that philosophers of physics, i.e. physicists and other scientists focusing on these philosophical themes of their discipline by extending theories and placing extensions and alternative hypotheses in proper context, have contributed more to this aspect of physics than regular physicists have done so far.

    This is an essential aspect of science which does not nearly get enough attention, mostly because in the practice of physics we don’t often explicitly run into such difficulties (implicitly is a whole different story) and therefore don't see the need for philosophical expertise. When we do however run into these difficulties, this kind of philosophical reevaluation of some theory is the correct method to take that actually can point the way forward. It is somewhat a matter of luck that in the early 20th, both Einstein and the founders of QM acknowledged this and so could end up discarding and reformulating core traditional principles of physics and so end up creating the correct mathematical formulations thereof, which we today refer to as relativity and QM.

    Today we are somewhat priviliged that the philosophers of physics have already simplified and classified the different interpretations and also given instructions on how to proceed. It is tragic that not many physicists have been willing to listen. In any case, the only way physicists can do more in advancing our understanding of QM deeper than what the philosophers of physics have done so far, is by actually creating new mathematical theories based on or incorporating those ideas from first principle, with hopefully one of the resulting mathematical theories being self-consistent and simultaneously not being trivially equivalent to QM itself.

    The collapse theories tend to be of this variety, but their dynamical formulations to this day remain mathematically incomplete. Somewhat unfortunate is that the domain of physics concerned with critical phenomenon, of which the mathematical theory is very much a theory of principle, has become somewhat directly associated with condensed matter physics, which is itself a collection of constructive theories. This is unfortunate because the mathematical methods required by the theoreticians to derive these theories from first principles are precisely the mathematical methods taught to condensed matter physicists except in a very different context and with a completely different purpose.
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