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Decoherence - the myth

  1. Jul 21, 2008 #1
    "decoherence" - the myth

    It's interesting when one hears supposedly fact-based scientists claim that decoherence has resolved the "measurement problem".

    Having been brought up on a staple of QM texts I was flabbergasted when i heard these kind of statements claiming decoherence was the answer to foundational problems of qm. So i decided to investigate these claims and allow myself to be convinced. Having now come to my conclusion that decoherence is nothing other than bunk; its made me wonder about the ethics of qm interpretational research.

    I'm not criticising decoherence as a practical framework for working with qm but i am severely critical of the extra weight which has been given to it as a solution to the "measurement problem". For anyone who wants a fair appraisal or what decoherence actually achieves and what it does not i believe the best paper available at the moment is:
    2005 updated version:

    http://arxiv.org/abs/quant-ph/0312059v4

    To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox. So what we basically have here is not the truth but a glorified FAPP type construct.

    But lets forget the majority for a moment. What about this minority who claim that decoherence resolves foundational issues such as the "measurement problem". If i as a layman can work out they are talking rubbish (re the real problem) then what does it say about their motives or their intellect that they feel they can sweep this incredible paradox under the rug with some made-up false construct?

    Frankly i am gobsmacked.
     
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  3. Jul 21, 2008 #2

    Hurkyl

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    Re: "decoherence" - the myth

    A criticism rejected as unjustified by the very author you cite....
     
  4. Jul 21, 2008 #3
    Re: "decoherence" - the myth

    The author of that paper says the same thing except he is far more diplomatic and "round the houses" in his manner of objecting to the false claims about "decoherence" solving the measurement problem. He is a phycists after all and i dont expect him to come out with guns blazing.

    He praises decoherence as a practical tool, as a way of seeing the wave collapse in a different manner. That would be like praising algebra as a useful tool but it tells us nothing about objctive reality.

    Its all about "appearances". Seriously if you want to fall into the FAPPTRAP be my guest but do not confuse a practical framework for a foundational truth. And its the foundational truth which is what we are after in concerns to the "measurement problem".

    Decoherence fails in providing any more knowledge about the meaning of qm for our reality.
     
    Last edited: Jul 21, 2008
  5. Jul 21, 2008 #4

    ZapperZ

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    Re: "decoherence" - the myth

    Can you point to me physics papers that actually claim that decoherence has resolved the measurement problem? It makes no sense, even among those who have studied decoherence, because we continue to study these things even today.

    There are certainly many papers that try to link decoherence as the "culprit", but I have never seen one that claim to have the definitive "smoking gun". In fact, there are other alternative approaches to decoherence published in major journals, such as this:

    https://www.physicsforums.com/showpost.php?p=1520644&postcount=59

    So this is ample evidence that no one is thinking that this is a done deal.

    Zz.
     
  6. Jul 21, 2008 #5
    Re: "decoherence" - the myth

    The point i am really trying to get at is the "politics" behind qm interpretations. I'm not knocking Zurek's work. It's a very clever way to explain the wave collapse mechanism simply as a leakage of a quantum state/superposition into the wider (collapsed) environment. And equally there is nothing wrong with the interpretation as a FAPP placeholder. You want students to shut up and calculate; give them the "decoherence" version of events and unless they had previously read various qm books they would be forever ignorant of the deeper implications of what it is telling us about the nature of reality.

    Problem is physics has finally encountered nature at a level which has significant implications about being, existence, and other ontological subjects. So far, it appears we have some of the physics community in a sort of self delusional state where they are so desperate to disprove the observers role in wave function collapse that they will believe almost anything else as long as it appears to resolve the "measurement problem".

    And its not just phycists - who at least have an excuse that they need to see the world in objective terms to do good science. Many modern philosophers are woefully ignorant even to the most basic tenets of qm and hence they develop ideas which appear grounded in classical physics! That is almost funny if not so sad.
     
  7. Jul 21, 2008 #6
    Re: "decoherence" - the myth

    ZapperZ,

    "Can you point to me physics papers that actually claim that decoherence has resolved the measurement problem? It makes no sense, even among those who have studied decoherence, because we continue to study these things even today."

    The paper i linked has various quotes from phycists implying if not explicitly stating that "decoherence" had solved the "measurment problem". Its in the first section where he gives the "for and against" quotes from various phycists.

    Yes i agree it is nonsense but it is often used as a disingenuous way to reject the observer's role.
     
  8. Jul 21, 2008 #7
    Re: "decoherence" - the myth

    ZapperZ,

    There are certainly many papers that try to link decoherence as the "culprit", but I have never seen one that claim to have the definitive "smoking gun". In fact, there are other alternative approaches to decoherence published in major journals, such as this:

    https://www.physicsforums.com/showpos...4&postcount=59

    So this is ample evidence that no one is thinking that this is a done deal.


    Yes the above theory kind of makes my original point. Why is everyone so desperate to eject the observer's role? Its like a religious movement to prove that the world is flat after all! Get used to it folks, QM tells us something peculiar about the nature of our reality and we have a distinct role in defining the world around us.

    No extra interpretational work is necessary until we understand what constitutes an observer. When we understand that we may be closer to understanding what is the actual causal factor in wave collapse.
     
  9. Jul 21, 2008 #8

    ZapperZ

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    Re: "decoherence" - the myth

    You have not answered my question. Please point to me evidence that the physics community has accepted decoherence as the definitive reason for the measurement problem. Point to me reputable, peer-reviewed papers that make such claims.

    If not, your starting premise of your complaint is faulty. You want us to explain to you a non-existent scenario, which would be a waste of time for everyone involved. So it is you who need to examine how you were able to draw up such a conclusion in the first place.

    Zz.
     
  10. Jul 21, 2008 #9

    ZapperZ

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    Re: "decoherence" - the myth

    Where does it say that? Read the final paragraph:

    Where does it say that this is settled?

    Zz.
     
  11. Jul 21, 2008 #10
    Re: "decoherence" - the myth

    ZapperZ,

    What's funny is you are now behaving defensively like i once did when i was told that decoherence removes the observer's role. :approve:

    "Please point to me evidence that the physics community has accepted decoherence as the definitive reason for the measurement problem. Point to me reputable, peer-reviewed papers that make such claims."

    Excuse me; I've always said the "majority" agree it doesn't resolve the "measurement problem". You are putting words in my mouth. My previous post was clear on this point:

    "To sum it up; the findings suggest and appear to be supported by the majority of qm phycists that decoherence does not even touch the "measurement problem" except as a practical approach to interpreting qm without the typical observer related paradox."

    Where does it say that this is settled?

    Never said it was. I'm arguing the other way - if you had not yet noticed :smile:

    By the way the quote i was suggesting to you from that paper is:

    "In his monumental book on the foundations of quantum
    mechanics (QM), Auletta (2000, p. 791) concludes that
    the Measurement theory could be part of the interpretation
    of QM only to the extent that it
    would still be an open problem, and we think
    that this is largely no longer the case.
    This is mainly so because, according to Auletta (2000,
    p. 289),
    decoherence is able to solve practically all the
    problems of Measurement which have been discussed
    in the previous chapters
    "

    my emphasis.

    By the way, there are lots of these tricky statements which suggest decoherence means we no longer need to worry about the "measurement problem". They are out there..look them up.

    However i agree that most mainstream phycists now accept that decoherence has not resolved the "measurment problem". ie. observer controversy - what it should really be called.
     
  12. Jul 21, 2008 #11

    ZapperZ

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    Re: "decoherence" - the myth

    Er... defensively? I have no idea what this is about, actually because I haven't made any decision on whether decoherence actually plays any significant role in the measurement problem.

    Then I may have interpreted your intention incorrectly.

    However, I still do not see who you are criticizing here. Are you up-in-arms against the MWI adopters, who, btw, have bigger fish to fry with their model than the "measurement problem"? They have their hands full already addressing the sharp criticism form Leggett that MWI is nothing more than "window dressing". What brought about this thread in attacking decoherence as a "myth"?

    Zz.
     
  13. Jul 21, 2008 #12
    I have seen an exercise essay written by an undergraduate student, which claimed that the Schrödinger's cat paradox is basically solved with the decoherence explanation. Okey, this is not a published paper, like what ZapperZ was asking for, but notice: It was not an undergraduate's own personal project. It was a supervised exercise, and the student got credit for it when approaching his graduation.

    Perhaps there're researchers out there, who are more interested in teaching their beliefs to the students, than publishing them in peer-reviewed journals? :biggrin: :rolleyes:

    There should be surveys carried out, to figure out what kind of beliefs really are dominant. I understood from the discussion in this thread, that the majority of physicists are not believing that decoherence is here to solve the measurement problem, but I would still be curious to know some percentage numbers. The evolution of beliefs should be observed in this business.
     
  14. Jul 21, 2008 #13
    Re: "decoherence" - the myth

    ZapperZ,

    "However, I still do not see who you are criticizing here. Are you up-in-arms against the MWI adopters, who, btw, have bigger fish to fry with their model than the "measurement problem"? They have their hands full already addressing the sharp criticism form Leggett that MWI is nothing more than "window dressing". What brought about this thread in attacking decoherence as a "myth"?"

    Yes i constantly attack MWI on various fronts as well - my critcism of interpretations is not restricted to decoherence :smile:

    I'm for Copenhagen and even a literal Copenhagen taking the observer into expilicit account as the causation for wave function collapse. Experiments have repeatedly demonstrated that a human observer has a special, and as far as we know *unique*, relationship with matter/energy at its most reductive scale. How and why we dont know but that is the raw fact of the matter.

    Each of these other interpretations ignore the obvious and demonstrated causal agent in a wave function collapse. What they all appear to be attemtpting to do is falsify the essentiality of an observer. We dont need a different framework FAPP because qm is still the same beast mathematically. So whats the point of these alternatives?
     
  15. Jul 21, 2008 #14
    Re: "decoherence" - the myth

    Yes i agree. Its a bit like the argument about Darwinism vs Creationism in teaching. Of course its in no way as serious as that issue but I feel that neglecting to inform physics students of the real enigma, or observer paradox in qm, is a "bit" like refusing to teach Darwinism. Interpretations such as decoherence and even MWI allow a different interpretation to be taught which if taken on face value, would be mis-leading to the students about the deeper meaning of qm.
     
  16. Jul 21, 2008 #15
    Re: "decoherence" - the myth




    "It would seem that the theory [quantum mechanics] is exclusively concerned about "results of measurement", and has nothing to say about anything else. What exactly qualifies some physical systems to play the role of "measurer"? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system ... with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less "measurement-like" processes are going on more or less all the time, more or less everywhere. Do we not have jumping then all the time?"

    - J.S. BELL
    Against "Measurement"
     
  17. Jul 22, 2008 #16
    Re: "decoherence" - the myth

    Maaneli,

    I love the way Bell wrote - a real poet type phycist! He would have made learning physics a pleasure with the way he attacked the BIG issues.

    I was kind of thinking about this yesterday about my cat. For instance, its odd that cats cannot see themselves in a mirror. They clearly see something but their perception cannot stretch to the concept that they may be looking at themselves.

    What if each level of organism can only observe certain physical realities? Hence from a qm perspective they can only collapse certain wave functions which are accessible to them. If this is the case then one can imagine that before humans appeared the world may have looked very different. One could use that same scaling to go all the way back to the first primitive organism.
     
  18. Jul 22, 2008 #17
    Re: "decoherence" - the myth

    Actually the idea about scaling the level of biological/observer awareness into a correlation with physical reality is less crazy than it sounds.

    For instance, this grapgh represents (roughly) the growing awareness/consciousness of biology as it has evolved on earth:

    humans.............................................................................................
    monkeys..............................................................
    rats.....................................................
    cats.....................................
    reptiles....................
    vegetation........
    first microbes.

    So this represents a sort of horizon of reality. Each new evolutionary jump adds a little bit more reality to the universe based on what that organism can observe. For instance each biological predecessor kind of prepares the universe for the next evolutionary jump in perception. Of course, as we know with Wheeler's delayed choice experiment, our choices, and i would assume the same for simple organisms though their choices are far more limited, have a retro-causal effect on history. So saying that the universe existed as it does today even when there were only microbes existing is a paradox in itself because all the extra definiton our minds demand from the universe was not existent then.

    Is it not plausible that each new biological evolution which makes our minds more complex adds increasingly more detail to our universe?

    Again i refer to animals that cannot see themselves in a mirror. Its got nothing to do with their eyes, but all to do with their ability to perceive and self awareness. So the extent of an organism's consciousness/awareness could be what controls "scalable" wave function collapse.

    Further more i would argue that based on the above theory; Schrodinger's cat is a valid observer and would collapse the wave function before anyone looked to see if it was alive or dead. The wave function would collapse for the simple reason we have set up the experiment in the first place. The cat has the ability to perceive a gunshot, or anything else which is harmful to it, so there is no paradox.

    Am i missing something?
     
    Last edited: Jul 22, 2008
  19. Jul 24, 2008 #18

    vanesch

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    Re: "decoherence" - the myth

    Ok, try me :cool:

    MWI is my preferred *interpretation* of quantum mechanics. But you will be disappointed: I'm not claiming that nature IS that way. I'm claiming that MWI is a good way to *picture* the way the quantum formalism is working. I keep open the possibility that against all odds, quantum mechanics might be a "true" description of nature, but I don't really think that we can say so until we have a better understanding about the interplay of gravity and quantum mechanics.

    Let me go in your direction. I think that solipsism is then even better. I think that there is actually only ONE important observer in nature, and that's me. I think that before I was born, the classical world didn't exist, and I think that after I'm dead, it won't exist anymore. All the other human beings, animals and rocks are not true conscious beings, some only act behaviorally that way. I'm the universal wavefunction collapser. Not you. Just me. Now you again.
     
  20. Jul 24, 2008 #19
    Re: "decoherence" - the myth

    Vanesch,

    "I'm claiming that MWI is a good way to *picture* the way the quantum formalism is working. I keep open the possibility that against all odds, quantum mechanics might be a "true" description of nature, but I don't really think that we can say so until we have a better understanding about the interplay of gravity and quantum mechanics"

    Yes i would not argue with any of that - sorry to dissapoint :smile:

    You are distinguishing a way to "picture" qm and you have your favoured way to see things. I also understand why MWI appears a good interpretation because it allows all probabilities to actually exist instead of them all dissapearing into thin air once a measurement or decoherence occurs. I also think MWI seems more reasonable from the Schrodinger's cat paradox, if its a genuine paradox. However as i've said before i think Schrodinger's cat is not the paradox it appears to be and the cat will collapse the wave function before any observer looks in the box.

    By the way has a (non-lethal) Schrodinger's cat experiment ever actually been done? Surely its pretty simple to test using a water gun or something that wont kill the cat?

    "Let me go in your direction. I think that solipsism is then even better. I think that there is actually only ONE important observer in nature, and that's me. I think that before I was born, the classical world didn't exist, and I think that after I'm dead, it won't exist anymore. All the other human beings, animals and rocks are not true conscious beings, some only act behaviorally that way. I'm the universal wavefunction collapser. Not you. Just me. Now you again."

    I think thats going too far. I do actually believe that we are all capable of collapsing wave functions, but on a scalable system which means that animals can only collapse a wave function of which they can percieve because of their limited sensory devices. So humans can work with atomic matter because we can set up the double slit. A cat could not because they dont have the consciousness or self-awareness, nor the sensory devices to force a photon to behave with particle/wave duality.
     
  21. Jul 24, 2008 #20

    vanesch

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    Re: "decoherence" - the myth

    The Schroedinger cat paradox is a paradox for the quantum formalism - but surely not "for real". The Schroedinger cat paradox illustrates that IF we accept a unitary time evolution all the time, and IF we accept that everything is described by quantum mechanics, then it follows that the "cat state" must have a live and a dead component. It is a dramatic way to illustrate what it really means to have unitary time evolution and the superposition principle: it means you cannot get rid of the extra terms the way you do using a projection.

    In other words, the Schroedinger cat paradox pushes your nose on a FORMAL problem: it is impossible for a complicated physical process, described nevertheless by unitary dynamics, to bring about genuine state projection.

    No problem, you say. Make time evolution slightly non-unitary (non-linear). There are people who do this. However, we now run into a second difficulty. If we modify the time evolution in such a way as to make projection a physical process, then it becomes impossible to make that Lorentz-invariant. In other words, a physical process that implements a true projection will require an absolute ether frame. This is nothing else but Bell's theorem (with a few additional assumptions, such as no "gods book" correlations, and no superdeterminism).

    And here we have a REAL paradox, at least in the idealized gedanken experiment of EPR.

    If we can build a real EPR experiment, we will have data that will - independently of any quantum theory - give us a genuine paradoxial situation, unless we give up on relativity. If we insist on keeping the spirit of relativity (we live on a 4-dim spacetime manifold, with no preferred slicing) then true EPR data (as predicted by quantum theory) present a real headache. Funnily, unitary quantum theory can describe it perfectly, and respect relativity.

    So, if we keep relativity, and if we keep strict unitarity, then we have the formal problem of the Schroedinger's cat.
    If we give up on unitarity and introduce genuine collapse, then, through EPR, we have a problem with relativity. Even without quantum mechanics, the pure data of the EPR paradox would collide with relativity, were it not that unitary quantum theory can explain it and is compatible with relativity.

    In other words, if we accept that the cat is both live and dead, we can keep unitarity, we can explain EPR, and we can keep relativity. But we have "parallel worlds" of which we cannot get rid.

    What would be the observable effect ? You would open the box, and observe in 50% of the cases, a wet, angry cat, and in 50% of the cases a purring, dry cat.
     
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