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Featured A Stephen Weinberg on Understanding Quantum Mechanics

  1. Jan 4, 2017 #1

    jedishrfu

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  3. Jan 4, 2017 #2

    bhobba

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    Wienberg as always cuts through the BS.

    Excellent piece - I like his other summary as well from his article Einsteins mistakes:
    http://infoproc.blogspot.com.au/2013/02/weinberg-on-quantum-foundations.html
    Bohr's version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wavefunction (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from? Considerable progress has been made in recent years toward the resolution of the problem, which I cannot go into here. It is enough to say that neither Bohr nor Einstein had focused on the real problem with quantum mechanics. The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wavefunction, the Schrödinger equation, to observers and their apparatus. The difficulty is not that quantum mechanics is probabilistic—that is something we apparently just have to live with. The real difficulty is that it is also deterministic, or more precisely, that it combines a probabilistic interpretation with deterministic dynamics. ...

    We now know a lot more about QM than the founding fathers such as Bohr and Einstein. IMHO about the only early pioneer that got it 'right' was Dirac who basically just stuck with the math rather than delve into philosophy, much of which like Complementary I find vacuous mumbo jumbo - either that or I aren't bright enough to get it - either way it leaves me cold.

    I cant let this go without my personal view aired. Its as per Schlosshauer:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    He gives 3 key issues that must be solved. The first 2 are solved, basically, by decoherence. The third is the stickler - technically its how an improper mixed state becomes a proper one or colloquially - why we get any outcomes at all. I simply say - that's how nature is - what a cop out - but hey I can live with it.

    Thanks
    Bill
     
    Last edited: Jan 4, 2017
  4. Jan 4, 2017 #3
    I like that Weinberg associates instrumentalism with the Copenhagen interpretation. There's an all too common attitude among working physicists that interpretations are a matter of taste, and that what truly matters is that the equations give correct predictions. Such a stance is a subtle concession to Copenhagen's anti-realism, as Weinberg correctly points out. After all, let us not forget that David Mermin invented the catchphrase "Shut up and calculate!" as a pithy summary of the Copenhagen interpretation. I'm somewhat dismayed, however, that Weinberg's sole reason for rejecting MWI is that he finds the idea of multiple worlds disturbing--as if one's emotional reaction to a theory should have anything to do with its acceptance or rejection. Hopefully, someone will experimentally test GRW and other objective collapse theories--which is what Weinberg appears to be flirting with--and put that dog to rest
     
    Last edited: Jan 4, 2017
  5. Jan 4, 2017 #4

    bhobba

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    I am terrible; its my reason as well and scientifically its invalid. But I cant help it.

    Thanks
    Bill
     
  6. Jan 4, 2017 #5
    Tsk tsk ;-)
     
  7. Jan 8, 2017 #6
    Well in a situation such as ours with little evidence to go around, how you decide which interpretation to study is more of a religious question than anything else, and in such a circumstance emotion is as valid as reason.

    Of course if observations could be gathered and experiments performed, I would agree with your sentiment.
     
  8. Jan 8, 2017 #7
    Is that really the only reason? So you think the several attempts at deriving the Born rule either have ironclad assumptions or are at least good enough? And any other technical gripes?

    Because I personally find the "feeling" of WMI it's most attractive feature. What I especially like it about it is how it creates subjective uncertainty out of determinism. Since the universe has no outside source of randomness (by definition), creating randomness is a tall order and WMI does it quite elegantly.
     
  9. Jan 8, 2017 #8

    The universe has no outside source of anything as far as anyone can detect?

    It does not even make semantic sense to consider sources outside the universe.

    To be clear;

    Since the universe has no outside source of universe (by definition), creating universe is a tall order.
     
    Last edited: Jan 8, 2017
  10. Jan 8, 2017 #9

    Mentz114

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    I doubt if everyone here would agree with this extract. I've seen it stated in this Forum that 'decoherence does not decide which outcome happens' ( to paraphrase).
     
  11. Jan 8, 2017 #10

    George Jones

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    Weinberg expresses his views more mathematically and with more technical prose in his grad-level book "Lectures on Quantum Mechanics". Examples of the prose in the fist edition (2013):

    "Page 82: In quantum mechanics the evolution of the state vector described by the time-dependent Schrödinger equation is deterministic. If the time-dependent Schrödinger equation described the measurement process, then whatever the details of the process, the end result would be some definite state, not a number of possibilities with different probabilities.

    This is clearly unsatisfactory. If quantum mechanics applies to everything, then it must apply to a physicist’s measurement apparatus, and to physicists themselves. On the other hand, if quantum mechanics does not apply to everything, then we need to know where to draw the boundary of its area of validity. Does it only apply to systems that are not too large? Does it apply if a measurement is made by some automatic apparatus, and no human reads the result?

    Page 88: There seems to be a widespread impression that decoherence solves all obstacles to the class of interpretations of quantum mechanics which take seriously the dynamical assumptions of quantum mechanics as applied to everything, including measurement. My own opinion is that these interpretations, like the Copenhagen interpretation, remain unsatisfactory. ...

    Statements of this sort about probabilities are predictions about how the state vectors evolve in time during measurements, so if measurement is really described by quantum mechanics, then we ought to be able to derive such formulas by applying the time-dependent Schrodinger equation to the case of repeated measurement. This not just a matter of intellectual tidiness, of wanting to reduce the postulates of physical theory to the minimum number needed. If the Born rule cannot be derived from the time-dependent Schrodinger equation, then something else is needed, something outside the scope of quantum mechanics, and the many worlds interpretation thus shares the inadequacies of the Copenhagen interpretation.16

    Page 95: There is nothing absurd or inconsistent about the decoherent histories approach in particular, or about the general idea that the state vector serves only as a predictor of probabilities, not as a complete description of a physical system. Nevertheless, it would be disappointing if we had to give up the “realist” goal of finding complete descriptions of physical systems, and of using this description to derive the Born rule, rather than just assuming it. We can live with the fact that the state of a physical system is given by a vector in Hilbert space rather than by numerical values of the positions and momenta of all the particles in the system, but it is hard to live with no description of physical states at all, only an algorithm for calculating probabilities. My own conclusion (not universally shared) is that today there is no interpretation of quantum mechanics that does not have serious flaws, and that we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is merely a good approximation.

    Page 336: There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas.

    16 For a strong expression of this view, see A. Kent, Int. J. Mod. Phys A 5, 1745 (1990)."
     
  12. Jan 8, 2017 #11

    bhobba

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    All interpretations are equally valid, scientifically that is. Choice is made purely on 'emotional grounds'

    You should study a number of interpretations - they all shed light on the formalism. I think MW is a crock of the proverbial - its totally emotional not scientific, but it didn't stop me from getting David Walllace's book on it and studying it:
    https://www.amazon.com/Emergent-Multiverse-Quantum-according-Interpretation/dp/0198707541

    I got a lot out of it and understand what its saying much better. I understand the standard formalism better as well. Did it change my mind? No. In fact it did the opposite - I now think its even more of the proverbial. Why? I now know the modern version is simply Decoherent Histories where instead of one history occurring they all occur at once in different worlds. Its seems an unnecessary added weirdness to me - but hey others view it differently. Its also mathematically and conceptually very beautiful - but so is Decoherent Histories. Why don't I subscribe to Decoherent Histories then? Beautiful it is, but for me its a bit contrived - I like the ignorance ensemble because it IMHO get to the heart of the matter. Its just a slight modification to the ensemble interpretation advocated by Ballentine and Einstein (yes despite what you may have read Einstein believed in QM and even had his own interpretation - he simply thought it incomplete). The slight modification is I only apply it to the outcome of decoherence - in practice there is no difference. Interestingly Ballentine doesn't believe decoherence has anything to do with interpretative issues. What would Einstein think? I believe he will still maintain his view its incomplete and put his finger on the exact issue - why do we get any outcomes at all - but we shall never know for sure.

    Thanks
    Bill
     
  13. Jan 8, 2017 #12

    bhobba

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    The Born rule derivation is valid. It hinges on a key theorem of the approach - the non contextuality theorem which emerges naturally in MW - all other approaches lead to absurdities. That is the key assumption of the Born rule as discovered by Gleason. They also have a decision theory approach - it also valid IMHO but opinions vary.

    Its purely an emotional choice.

    One thing that needs to be pointed out is MW avoids the why we get outcomes at all issue (technically its how does an improper mixture become a proper one) - Decoherent Histories doesn't. But is the cost worth it - the choice is not rational - just emotional. Same with non contextuality - it is pretty much inevitable in MW - but not Decoherent Histories where its merely natural.

    Thanks
    Bill
     
    Last edited: Jan 8, 2017
  14. Jan 8, 2017 #13

    bhobba

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    Spot on - it doesn't occur so often now but when I first started posting it seemed to occur a lot - people thinking decoherence solves the issues in QM - it doesn't.

    I agree, but since I now understand the precise issue as espoused by Schlosshauer (how an improper mixture becomes a proper one) it doesn't worry me that much:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    I simply accept it but of course would love to know why. Trouble is its debatable if there even has to be a why. Einstein would certainly think so - but like I say it's debatable and I am used to it. Which side of the debate do I come down on - well I don't think there has to be a why - but that would be a whole new discussion. Certainly Weinberg has but his finger on a key if not the key issue.

    Thanks
    Bill
     
  15. Jan 8, 2017 #14

    bhobba

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    Can you elaborate on why you think its an inconsistency?

    Technically decoherence converts a superposition to an improper mixed state. Only by assuming its a proper on is this issue solved.

    Thanks
    Bill
     
  16. Jan 8, 2017 #15
    This. QM, like all earlier physical theories, is most likely a provisional theory, which ultimately will be superseded. A non-linear extension of QM seems the most probable direction, given history of physics and the relative novelty of the non-linear perspective.

    It is interesting to note that others have made this exact same argument. A recent experiment however offers a counterintuitive unification of entanglement and classical chaos:
    https://phys.org/news/2016-07-blur-line-classical-quantum-physics.html
     
  17. Jan 8, 2017 #16

    George Jones

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    Years ago, Weinberg was a proponent of non-linear generalizations of quantum mechanics (I attended two talks that he gave on this), but then flaws were pointed out. From the same book that I quoted above:

    "Page 340 Any attempt to generalize quantum mechanics by allowing small nonlinearities in the evolution of state vectors risks the introduction of instantaneous communication between separated observes.3

    3 N. Gisin Helv. Phys. Acta 62 363 (1989); J. Polchinski, Phys. Rev. Lett. 66 397 (1991)."
     
  18. Jan 9, 2017 #17
    I was actually taking 'non-linear extensions' to encompass quite a bit more than merely adding small nonlinearities to the evolution of the state vector, i.e. pretty much any technique of generalisation available from nonlinear dynamical systems theory.
     
  19. Jan 9, 2017 #18

    DrDu

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    In solid state physics, already a small bar of metal is treated very successfully as an infinite system. We know that even for simple model systems e.g. an infinite system of spin 1/2 particles, it isn't possible to write down neither a wavefunction nor a Schroedinger equation. Rather, quantities like the average spin per particle or energy density arise as classical observables.
    The more illusoric it appears to describe observiers or even the whole universe in terms of a wavefunction. Bohr had understood this.
     
  20. Jan 9, 2017 #19

    Demystifier

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    There is at least one counterexample, namely non-linear quantum-like theory without instantaneous communication. It is classical mechanics itself:
    https://arxiv.org/abs/0707.2319
     
  21. Jan 9, 2017 #20
    Okay, that's interesting. From what you said above it seems that it gives you a "weird" feeling even if it's "mathematically and conceptually very beautiful".

    Personally I have trouble understanding this "weird" feeling in others and I wonder if I'm missing something. Does it have something to do with the multiverse aspect - that there are other "worlds" in this same universe, or other "worlds" at all?

    Let's say QM was clearly a single-world theory and decoherence just caused a single outcome to wipe out all the others - a kind of quantum darwinism monopoly. Would that still be weird?
     
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