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Is Quantum mechanics internally inconsistent?

  1. Nov 20, 2014 #1
    Roger Penrose says QM is internally inconsistent. It is actually an issue of dynamics, when does the projection postulate dynamics take over from the Schrodinger dynamics? Penrose says the U process of quantum mechanics which is the Schrodinger evolution of the wave-function is entirely deterministic but he says randomness comes into the picture when someone makes a measurement of the QM which is the R process. Penrose says because of the contradiction of dynamics between the process U and R QM is both internally inconsistent and incomplete. Is QM internally inconsistent?
     
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  3. Nov 20, 2014 #2

    atyy

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    Quantum mechanics is not internally inconsistent. Having two different processes is not inconsistent, as it just reflects two different procedures you can carry out on the system. Having more than one procedure means there is someone external to the system who is manipulating it, whom we call an observer. Can we do without an observer in quantum mechanics? Quantum mechanics is formulated in an "abstract" space. In the Copenhagen Interpretation of quantum mechanics, it is only when an observer measures the system that something "real" happens. This is ok, since in all our experiments and observations, we are external to what we are observing. The abstract space of quantum mechanics is just a very good tool to help us predict the reality we see. In this approach, we cannot use quantum mechanics to describe the reality of the whole universe, since there are no external observers to the whole universe.

    But if quantum mechanics cannot describe the reality of the whole universe, what alternative theories do so? Approaches like Bohmian Mechanics try to answer this question.

    Or is there another interpretation of quantum mechanics that will make it produce reality without observers? The Many-Worlds approach tries to answer this question.
     
  4. Nov 20, 2014 #3
    I think Penrose is right when he says, "QM makes absolutely no sense what so ever".

    A General Argument Against the Universal Validity of the Superposition Principle

     
  5. Nov 20, 2014 #4

    atyy

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    If Bassi and Ghirardi are right, there should be at least one theory that reproduces all the successes of quantum mechanics, and that also differs from quantum mechanics outside the regime we have tested. So far, there is no theory that does that. However, Ghirardi and others have suggested various theories that reproduce some successes of quantum mechanics, but in which the superposition principle is only approximate, not exact. Can these alternatives be tested? It is hard, but there is some discussion on how one might try.

    http://arxiv.org/abs/1410.0270
    Nature Physics 10, 271 (2014)
    Testing the limits of quantum mechanical superpositions
    Markus Arndt, Klaus Hornberger
    Quantum physics has intrigued scientists and philosophers alike, because it challenges our notions of reality and locality--concepts that we have grown to rely on in our macroscopic world. It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain. Scientific progress over the last decades inspires hope that this debate may be decided by table-top experiments.
     
    Last edited: Nov 20, 2014
  6. Nov 20, 2014 #5

    MathematicalPhysicist

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    How do you know that there are no external observers to the whole universe?
    We don't even know what's the scope of this universe, let alone if there aren't no external observers.
    If you ask me, you should expect an infinite cases of observer and observee (one that is being observed), inside and outside our universe.
    Turtles all the way down...
     
  7. Nov 20, 2014 #6

    Nugatory

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    We don't know. But we don't need to, because there are interpretations of QM that avoid this endless regress of observers and the unanswerable "who observes the universe?" question. Raising these questions here only risks pushing this thread into the swamp of interpretational debate, where the only outcome will be an ignominious locking.

    The thread will stay open for now, but I am asking everyone to please focus on the material that atyy has introduced above, instead of rehashing the observer problem.
     
  8. Nov 20, 2014 #7

    bhobba

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    He is wrong.

    The projection postulate is another name for the collapse postulate. It's actually not part of QM, even though some textbooks will tell you it is - only some interpretations have it. Most of the time what is being observed is destroyed by the observation. The exceptions are so called filtering observations which these days is associated with a state preparation procedure. In modern times state preparation procedure and state are synonymous. All you have done is prepared a system in a different state.

    Basically the issue with most interpretations is the so called problem of outcomes or why do we get any outcomes at all, or how, after decoherence, does a particular outcome occur. In a couple of interpretations like MW or BM its trivial, but for most its a big question mark. Still the fact interpretations exist where its not an issue shows its not part of QM proper - there is no inconsistency.

    Thanks
    Bill
     
    Last edited: Nov 20, 2014
  9. Nov 20, 2014 #8

    bhobba

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    We don't.
    You expect science to have all the answers. That's why there is ongoing research.
    Fortunately nature gets by just fine without worrying about your philosophical musings.

    Thanks
    Bill
     
  10. Nov 20, 2014 #9

    bhobba

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    You mean you expect realms beyond everyday experience to conform to the intuition you developed in the everyday common-sense world?

    There is a difference between understandable which QM is, and making sense - which is a reaction you have. Nature doesn't care how you feel about it - or me for that matter.

    Hmmmm, the superposition principle follows from the vector space structure of pure states as implied by the Born rule. Challenging the Born rule is a pretty tall order, especially considering Gleason:
    http://en.wikipedia.org/wiki/Gleason's_theorem

    The so called strong superposition principle can be challenged, but I suspect that isn't what's being suggested.

    Thanks
    Bill
     
    Last edited: Nov 20, 2014
  11. Nov 22, 2014 #10
    I was under the impression that in modern physics, quantum field theory supplanted the Schrodinger equation?
     
  12. Nov 22, 2014 #11

    bhobba

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    Actually that's a deep issue.

    The situation is this - the POR and Galilean relativity implies Schroedingers equation - see chapter 3 - Ballentine - Quantum Mechanics - A Modern Development. Its origin is symmetry.

    However if you want Einsteinian relativity because Galilean relativity implies instantaneous action at a distance (see Landau - Mechanics where he carefully analyses that point - in his usual terse style of course) you can't do that. There are a number of paths forward such as the Dirac equation etc but the QFT approach is to have every thing local by parametrising both position and time. In standard QM position is an operator and time a parameter - but in relativity time and space need to be treated on the same footing so position is made a parameter like time - and you get a field. QFT is applying the rules of QM to a field.

    If you want the detail I am reading a good book right now - QFT For The Gifted Amateur:
    https://www.amazon.com/Quantum-Field-Theory-Gifted-Amateur/dp/019969933X

    Just like standard QM we find that symmetry considerations also constrain QFT theories which various types of symmetries leading to descriptions of particles - photons, electrons, quarks etc etc.

    As you would expect QFT reduces to QM in a certain limit - the so called dilute limit. The following carefully analyses this, as well as putting forward the view, one I agree with, that QFT resolves many of the issues with QM:
    https://www.amazon.com/An-Introduction-Realistic-Quantum-Physics/dp/9812381767

    At the beginner/populist level the following also uses that approach:
    https://www.amazon.com/Fields-Color-theory-escaped-Einstein/dp/0473179768

    Popular books are often so so - but the above book is pretty good.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  13. Nov 26, 2015 #12
    Uncertainty in quantum mechanics: faith or fantasy?

    That's not the point, the linearity of the Schrödinger's equation is in contradiction with the collapse of the wavefunction. There is something in the physical world that doesn't obey the Schrödinger's equation and hence causes the quantum system to behave differently when it encounters itself with the former.
     
  14. Nov 26, 2015 #13

    Nugatory

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    There is nothing inherently inconsistent in an interpretation that says that the wave function evolves according to Schrodinger's equation except at the discontinuities where it doesn't. It may not be aesthetically pleasing, but... I can't even get other people to accept my aesthetic prejudices, so I don't understand why we should expect the universe to pay much attention to them.

    I'm closing this thread. It's a year old, and we aren't saying anything that hasn't already been said.
     
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