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QM Speculation

  1. Jul 22, 2008 #1
    I see a lot of statements here which appear (best I can tell) to be based on one interpretation or another of QM. I’m surprised how often a wave function collapse is mentioned like something real. So please tell me what interpretation you like and why.

    I’m only starting to learn a little QM and personally, I prefer the “shut up and calculate” direction. I don’t know what interpretation that fits but I think QM is just a great way to predict experiment results and it says nothing about reality. All the rest sounds more like speculation to me.
     
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  3. Jul 22, 2008 #2


    :frown::uhh: Don't you think it would be infinitely more rational to suspend judgement on the interpretation of QM until you learn more about the various interpretations?
     
  4. Jul 22, 2008 #3
    While I'm sympathetic towards this attitude, if you believe that QM is a fundamental description of nature, then it doesn't make sense to say that it says nothing about reality. If, however, you believe that it may very well be an approximation to a more fundamental theory, like 'T hooft and Smolin, then this philosophy is entirely appropriate. Personally, I'm careful not to disregard this possibility, and thus I'm somewhat guarded about interpretations. If I had to choose, I think Zurek's decoherence approach is very promising, though in need of further development. I'm also following Valentini's work on Bohmian mechanics, and hold out hope for a hidden variables approach such as this. So in direct response to your question, Bohm and Zurek's approach are the two interpretation that intuitively intrigue me most.
     
  5. Jul 23, 2008 #4

    Demystifier

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    I will just say that I am a Bohmian. Some reasons for this preference are explained in a casual manner in my blog.
     
  6. Jul 23, 2008 #5
    First thing to remember about QM interpretations is that they are more about philosophical preferences than the science itself. If you like the "shut up and calculate" line of thinking; then you are clearly not interested in the philosophical implications of QM.

    But you are incorrect about your statement that QM tells us nothing about reality. It very much does, and this is why there are a plethora of interpretations. Each offers a slightly different view of how QM actually effects our physical reality.

    And frankly, if you have not understood why QM is a paradigm shift from classical/newtonian thinking then you have not understood QM :smile:
     
  7. Jul 23, 2008 #6
    This is not a fair statement. I like to "shut up and calculate" all day long, and I don't argue with other scientists whether their philosophical preferences suit my own. It still does not prevent me to read about philosophical considerations as an amateur at night. What's wrong with that ?

    QM interpretations are interesting but secondary with respect to being able to calculate, especially if you are just starting QM.
     
  8. Jul 23, 2008 #7

    vanesch

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    I'll second that. To the OP: by all means, keep your shut up and calculate attitude for learning QM! However, you might find out after a few years, that there are "bizarre" aspects to quantum theory, especially in the usual way of dealing with it, which don't just defy "common sense" (that wouldn't be a problem in itself), but which seem to be totally illogical. One stumbles on them when analyzing certain (gedanken) experiments: should I project ? Should I keep a wavefunction ? For instance, in quantum chemistry there is a priori a difficulty with chiral molecules: the ground state of these molecules is not a chiral state, but a superposition of the two chiral states. Indeed, a priori, the left-handed molecule is in a certain state, the right-handed molecule is in another state which is obtained by applying the parity operator to the state, which commutes with the (electromagnetic) hamiltonian. So or these two states are both degenerate ground states, or they are not energy eigenstates. It turns out that they are not energy eigenstates, but that their sum and difference is, one of which is the ground state, and one of which is a slightly excited state.
    So normally, one shouldn't have to be able to find molecules in a chiral state, but rather in their non-chiral superposition, which is the ground state.
    In fact, for small molecules, that is exactly what happens: the ammonia molecule (which isn't chiral, but which has a certain spatial orientation) doesn't occur in a specific "geometric" state, but rather in the ground state which is a superposition of two mirror geometrical states. One can even measure the transition to the first excited state which is the same superposition but with different sign: the ammonia maser is based upon that transition.
    But bigger molecules appear in chiral states, and not in energy eigenstates. How come ?
    Decoherence has an answer to that, but in order to even appreciate decoherence, one has to come awfully close to thinking exactly about what is this quantum-classical transition.

    I would like to mention to the OP also another "use" of interpretation issues. It is in fact my favorite way of thinking about interpretations. I've filled up a good chunk of the quantum physics forum discussing about that until I decided that I had said what I had to say at least a dozen times and so I stopped doing that.
    My preferred interpretation is the many worlds (class of) interpretations, but probably not for the reasons you imagine. In fact, I prefer that interpretation simply because it gives me a mental picture of the quantum mechanical machinery which is close to the actual mathematical formalism. I will bluntly agree that it is an extremely weird view, and that it stretches philosophical positions to the extreme. That's why many people can't accept it. But I don't see this interpretation as a "picture of reality", I rather see it as a mental aid to "visualize" the formal machinery of quantum mechanics. Probably a bit like, say, a computer scientist imagines his/her objects "talking" to eachother and "being there", although for sure he/she doesn't think it is a good representation of the reality of the physical processes inside of his/her computer, but it helps him/her to think about the program and get a "feeling" for what "is going on". In the same way, a many worlds interpretation helps me to get a "feeling for the machinery of quantum theory". It is something that is much more "picturesque" than the quantum-classical transition in more traditional views.
    Another quantum "interpretation" that is also satisfying in that way is Bohmian mechanics ; however, that view clashes with the "picture" behind relativity, which is why I don't prefer it.
     
  9. Jul 23, 2008 #8
    Nothing wrong with that. But for me "shut up and calculate" comes across as a way to prevent discussion on the philosophical implications of QM.

    Yes interpretations are secondary if all one wants to do is solve a QM problem; however they are primary if one wants to understand what QM tells us about the nature of reality.
     
  10. Jul 23, 2008 #9
    If you are unable to do the calculations, you will not be able to appreciate what QM can tell you about the "Nature of reality".
     
  11. Jul 23, 2008 #10
    I can't agree with that in the slightest. The calculations are for practical work and are completely unnecessary in order to understand what QM is telling us about the nature of reality from a philosophical perspective.

    Popular science achieves that very goal by educating a wider audience. If you insist that everyone learn qm maths in order to understand what its telling us about the unvierse then most of the world will remain forever ignorant.
     
  12. Jul 23, 2008 #11
    I'm not an expert in the field, but I was under the impression that current thinking in quantum decoherence was that wavefunction collapse never actually happens - it just happens that, statistically, when a particle becomes entangled with enough other particles, it will act as if the wavefunction collapsed.
     
  13. Jul 23, 2008 #12
    I do care a lot about communicating science too, and understand your point. If someone has no intention to dive into the formalism, they can still read and try to build their own "QM cultural understanding". However, they would only be able to have a second hand opinion, at most reach an "interactional expertise" in the sense presented in [thread=241388]this discussion[/thread]. There is nothing really wrong with that. However, the OP does intend to master the formalism. Once he has done that, he will be able to read more advanced books, such as the ones by Omnes and d'Espagnat, where he will fully appreciate all the technical details. Why bother and loose time reading approximate statements before being able to read the real discussions ? It is merely a matter of efficiency at this point.

    That being said, my previous post goes even beyond in making a strong claim. If there was a consensus about what QM has told us about reality, then maybe a book explaining fully what we have learnt philosophically in layman terms would be available. But it is not the case right now. Let me take an example : I do not think I can have a decent discussion about EPR in Penrose's twistor approach without any technical formalism. I tried it with engineer friends of mine, who have quite a decent scientific background and a true interest in QM. Maybe this was only my own failure. However, you must admit that using "Nature of reality", it does not seem innapropriate to raise Penrose's book :smile: IMHO, he rightly introduced a decent amount of mathematical background before discussing those matters.
     
  14. Jul 23, 2008 #13
    humanino,

    "If there was a consensus about what QM has told us about reality, then maybe a book explaining fully what we have learnt philosophically in layman terms would be available. But it is not the case right now."

    Yes there is no consensus at the moment and considering most interpretations are not falsifiable I doubt there will be one for quite a while. However all the current batch of interpretations have been released to the public in one form or another. I dont think we need to wait for the consensus interpretation to emerge in order to discuss the various possibilties.

    Of course those are philosophical discussions and not ones really which have any relevance on the mathematical language used to describe, measure and manipulate matter and energy at its most reductive scale.
     
  15. Jul 23, 2008 #14

    vanesch

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    Although there is of course a big part of technicalities which are indeed not necessary to know for a philosophical debate on QM, I do think that a fair deal of mathematics must be mastered. Of course, you don't need to know how to solve certain differential equations with special functions. But you do need to know a lot about Hilbert spaces, linear operators and all that. For instance, in order to even understand the basic difficulty of the measurement problem (namely, that a unitary time evolution operator can never "project out" a single state from a superposition, no matter how complicated it gets, and hence that there is no hope that "complicated quantum dynamics" is equivalent to a measurement projection) one needs to be comfortable with these notions. Otherwise one misses the issue totally.

    Uh, most of the world IS forever ignorant.
     
  16. Jul 23, 2008 #15

    vanesch

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    First of all, such a book isn't there yet simply because I didn't have time yet to finish writing it :rofl:

    That said, I'm also fond of Penrose's book, but I think it is an illusion: you can only read it, I think, if you already know quite a lot about the material he presents. Not that he presents it badly, on the contrary. But I don't think that Joe Average has the necessary mathematical "maturity" to understand and appreciate what Penrose writes down. Maybe I'm wrong.
     
  17. Jul 23, 2008 #16
    What I've learned is that which interpretation you chose is almost like a religion, whatever your teacher/mentor taught you.
    If selflearned/selfthinking: whatever the **** is appealing to you.

    SCI FI lovers go for MWI usually.
    Realists tend to go with Bohmian interpretation.
    Positivists go with CI
    People who just don't care about philosophy go with STFU n calculate.


    as a realist I go with Bohmian.
     
  18. Jul 23, 2008 #17
    I think this is an unfair assessment of the situation. Though it is not my interpretation of choice, to say only SCI FI lovers are the only MWI(or related interpretations such as consistent histories) advocates is disingenuous. It is a direct interpretation arising from the language of the quantum formalism, without adding additional postualtes such as wavefunction collapse. Postivists are not the only CI adherants, and it appeals to those who do not want to make metaphysical judgements of the nature of reality beyond what they can measure. I think this is a very superficial way of looking at interpretations, and you should be careful not to underestimate their value in potentially guiding scientific progress.
     
  19. Jul 23, 2008 #18
    Please correct me if I'm wrong, but I think you could simply make sure amplitude squares add classically in a large entangled ensemble, and that would produce the same macroscopic measurable effects as "wavefunction collapse", except there never is any projection and the states remain superposed.
     
  20. Jul 24, 2008 #19
    Vanesch,

    Actually my suggested reading for any layman who wants to understand the "measurement problem" and the philosophical implications is "Quantum Enigma" by Kuttner & Rosenblum. Previously i had read maybe 20-30 popular qm titles but i dont think any of them explained the observer paradox quite as well.

    Kuttner & Rosenblum: "Many years ago, when Rosenblum was a graduate student, he and a fellow student got to spend an evening with Albert Einstein, who tried to discuss the enigma of quantum mechanics with them. But they were ill-prepared.

    "Our advanced courses in quantum mechanics taught us how to calculate, but avoided the mystery. Our ignorance of it disappointed Einstein,"
    Rosenblum said.
     
  21. Jul 24, 2008 #20
    I also agree with jms5631 that if we downplay the value of the correct interpretation - whatever that might be - we may fail at extending our knowledge of how qm can be harnessed further.
     
  22. Jul 24, 2008 #21

    Fra

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    Yes, this is my motivation for this discussion too.

    My "interpretation" reflects my view of how I think my current understanding could best be brought further. When I first took QM classes, during that time I found the CI interpretation to be the minimal and most comfortable one. Also in the middle of a course, when you are busy learning a tool, it is not the best time to question it.

    My deeper questioning of the QM formalism has come afterwards, where time for reflection is in place. I think the QM formalism itself will be modified, and that we will understand the current one as an effective formalism in special cases.

    I adhere to some minor subjective self-organising information theoretic interpretation, where the basic drive is self-preservation, and interactions may appear as a result of negotiating inconsistencis due to the subjective views. This view goes back to the foundations and philosophy of inductive logic and probability theory.

    To me the foundational issues of QM, goes hand in hand with the missing link to gravity. Clearly stability, inertia and mass and the flow of time seems related. And maybe we still have a long way to a reasonable understanding.

    But that has no impact on the effectiveness of current theory, from an engineering perspective. They do not contradict.

    /Fredrik
     
  23. Jul 24, 2008 #22
    Fra,

    "I adhere to some minor subjective self-organising information theoretic interpretation, where the basic drive is self-preservation, and interactions may appear as a result of negotiating inconsistencis due to the subjective views. This view goes back to the foundations and philosophy of inductive logic and probability theory."

    Thats very interesting. Would love to hear more about the "self-preservation" aspect you mention. I also have been wondering whether there can be a correlation between Darwinism and quantum mechanics. I've been trying to build a model of Darwinism which incorporates biological observership as an evolutionary driver for "better" observers. By "better" i mean better sensory development in order to observe more and more detail and defintion in the universe. What did the universe look like when the first primitive microbe self-evolved? It must have looked/felt very different to that microbe.

    Just to clarify; Im not challenging evolution theory - i just think its missing some major factor. It may have nothing to do with qm but i suspect it does because of what we have learned about qm's observer paradox. If biology are the only capable observers then i think from a logical perspective evolution and qm may be linked.

    Your idea about "negotiating inconsistencis due to the subjective views" is also very interesting. It makes alot of sense that if we do have an observer defined reality then there would be a need for such a mechanism as you have descibed. Perhaps qm is some sort of reality normalisation methodology. "Entanglement" is also perhaps a suspect in normalising reality far quicker than our eyes can notice it occuring?

    "To me the foundational issues of QM, goes hand in hand with the missing link to gravity. Clearly stability, inertia and mass and the flow of time seems related. And maybe we still have a long way to a reasonable understanding."

    Yes i think we many be missing something vital in our understanding of how QM and GR relate to eachother. My personal hunch is that QM is far more fundamental than GR, and gravity, mass, time are by-products or emergent properties of QM. But of course this is why the correct qm interpretation is so important for future advances. If we are using an erroenous interpretation it may cause us to miss the link.
     
  24. Jul 24, 2008 #23
  25. Jul 24, 2008 #24

    vanesch

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    I don't know if plants are using quantum mechanics (which is a theory or a model of nature). That's a bit like saying that black holes are using general relativity to grow or to attract stuff...
    :uhh:
     
  26. Jul 24, 2008 #25

    vanesch

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    Yes, that's decoherence. If you entangle "enough" with a sufficiently large (read: with enough degrees of freedom) system (the environment), chances that you will ever again have two full terms in the wavefunction "align" (not be perpendicular for at least one degree of freedom) will be essentially zero, for ever. That means that you will never again have any interference, and that you will never again have to add individual amplitudes. If that's the case, each term now lives its life totally independent of any other term, even when we do measurements.

    But, as you say, they still all exist in the wavefunction, but now lead their individual lives. This is the basis of the many worlds view: each term now behaves as a classical world all by itself and will never hear again of the others.
     
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