H. Dieter Zeh

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  • #2896
    H. Dieter Zeh
    Participant

    Dear Jiri,

    I think my answer to your modified QM is contained in my last sentence above.

    The empirically well established wave function that I am referring to is defined in configuration space, and hence nonlocal. If it is used consistently (as in decoherence theory), you may call it “real” (if you like). Indeed, individual states of isolated quantum systems (such as the He-atom – but also Bell states) are completely described by such nonlocal wave functions, while statistical properties occur only in connection with a (true or apparent) collapse. Von Neumann had replaced the kinematical dualism (wave/particle) by a dynamical one.

    In the here much discussed Bohmian mechanics, in particular, the wave function is also used consistently (and was hence regarded as real by John Bell). As well known to Bohmians, it therefore also leads to decoherence. Without the thus arising autonomous branches of the wave function, they could not consistently speak of “empty components”. The trajectory is then merely used to define “our quantum world” (a tiny component of the global wave function) as the “occupied” branch. The precise definition of this branch is a matter of convenience. However, since the trajectory is unobservable, this selection by means of a trajectory remains a model-dependent hypothesis; the “subjective selection” of a specific branch from many, many others can be done without using any trajectories. In this – but only in this – sense I agree with the Copenhageners. As the other components would nonetheless “exist” under these assumptions, you are back at Everett!

    Best, Dieter

    #2434
    H. Dieter Zeh
    Participant

    The irreversible “dislocalization of superpositions” over many degrees of freedom (decoherence) is an objective process that leads to dynamically automous “branches”. This does not require any bipartition. However, since this dislocalization does not eliminate the superposition globally, its importance for what we observe can only arise from the locality of observers (the bipartition between them and rest of the universe). Their “subjective individualization” in their locally defined component states is confirmed empirically, and thus avoids an objective collapse as a new dynamical law (which is conventionally located in the measurement apparatus). In the formalism of QT, this individualization (corresponding to a splitting observer in the formalism) can obviously not be explained as a mere increase of information.

    There is neither any decoherence nor branching in your simplistic e+p example. So I don´t understand what it has to do with Everett’s branches. The observer’s component states are sufficiently defined in the decoherent branches. In particular, they cannot observe any Schrödinger cats.

    Can we leave it that way (whatever else you may have in mind)?

    Regards, Dieter

    #2425
    H. Dieter Zeh
    Participant

    I tried to emphasize that – similar to Everett – I am assuming the existence of fundamental “observer” systems, which need only be vaguely known to be localized somewhere in the brain for this purpose. (It would be great if you could find out more about them!) All other subsystems are a matter of convenience, and usually chosen as local systems, too, since causal relations propagate in space.

    Best,
    Dieter

    #2422
    H. Dieter Zeh
    Participant

    Dear Miroljub,

    my point was that your arguments regarding definitions are correct but irrelevant for the measurement problem in terms of Everett. Nothing ever “branches” objectively in any definition. Instead of repeating my arguments let me invite you to read pp. 14-15 and pp. 18-19 of my first reference.

    We may simply be discussing different things.

    Best regards
    Dieter

    #2392
    H. Dieter Zeh
    Participant

    Dear Miroljub,

    you are certainly right that concepts like entanglement or decoherence depend on an arbitrary definition of systems (subsystems of the universe). Therefore, branches cannot (and need not) be exactly defined; these branches are merely a matter of convenience.

    What IS important is a concept of local observers that is compatible with the assumed global unitarity. If observers are assumed to be organized and strongly coupled subsystems of the universe, they must be pretty local (assuming local dynamics), but as a consequence of unitarity, and in a universe of growing entanglement between its local parts (an important cosmic initial condition!), they must then permanently split into dynamically separate components states (versions) – just like Schrödinger’s cat together with its environment. However, each component is coupled only to its own relative branch, and so he observes only one outcome of a quantum measurement.

    Clearly, we do not know very much about the precise (conscious?) observer states in the brain. (There is much decoherence still going on within the brain.) Nonetheless, we can estimate what superpositions in the world can have coherent effects on such local observers. If they can not any more, their components must separately affect separate components of the observer. Precisely such a coherent effect on potential observers is eliminated by an irreversible decoherence process. Therefore, it is CONVENIENT to consider components separated by decoherence as forming an incoherent ensemble thereafter (an apparent collapse), regardless of whether an observer will ever enter the scene.

    I may have missed your essential point. Did I also overlook the definition of your letters DIS?

    Regards, Dieter Zeh

    #2257
    H. Dieter Zeh
    Participant

    Mark, I hope your proposal and its relation to the empirical world and to our generally quite successful concepts and theories (including quantum mechanics) will be further discussed and explained in the main workshop.

    Regarding its classification, my “hidden variables” were certainly meant to be understood in a causal picture (which you seem to be calling a prejudice), while my point 5 mentions the case of “no concepts of kinematics or dynamics any more”. This may fit better, although we may also explicitly invent a category of “acausal (and/or retrocausal) interpretations”. But I don’t want to dominate this debate about classification – so let me shut up for a while!

    #2252
    H. Dieter Zeh
    Participant

    One more re-Mark: If you presume the path integral, you certainly presume the wave function (or the SPP). But what is its interpretation if you also presume hidden variables?

    #2251
    H. Dieter Zeh
    Participant

    Mark, I don’t understand your model sufficiently (and it would not be my job) to define its meaning and intentions, but you have already indicated what you may have in mind. (That’s enough for a tentative classification.) So I am not yet ready for a discussion in detail, but I wonder how you describe or explain your “events” (and their probabilities) within the fundamental dynamics of your model. Are they (explicitly or tacitly) assumed to be determined by your hidden variables? Perhaps I will find out myself when I read more carefully.

    #2245
    H. Dieter Zeh
    Participant

    Thanks, Ruth – although I did not recognize a universal ontic wave function that explains definite measurement outcomes.

    #2241
    H. Dieter Zeh
    Participant

    Thanks, Ruth, your second paragraph is precisely what I tried to suggest and what hopefully will help us not to misunderstand each other. Can you recommend a paper (or part thereof) where the transactional model is generally defined (if possible without relying on pedagogical examples a la Wheeler-Feynman, that is, without using classical particles and fields – as you seem to indicate)? I am not yet sure that the model can be dynamically consistent – but that should not be discussed at this point.

    I have not sufficiently understood Mark Stuckey’s propal in order to criticize or suggest an improvement of his own classification (so I hope his may help us to better understand him). For all solutions belonging to 3b, I would, in particular, like to understand explicitly how they explain the general success of the superposition principle (including wave mechanics).

    Another model for which I woud like to know precisely where to put it in my taxonomy according to its proponents is ´´consistent histories´´.

    If you like, you may subdivide 1a (collapse theories) as
    a1) Pearle, GRW
    a2) gravitationally induced (Penrose, Diosi)
    a3) mind induced (Wigner)

    You may have noticed that I did not explicitly list decoherence, since imo it belongs to 1b if regarded as a solution of the measurement problem (although it has also to be taken into account as a partial but unavoidable and important step in other psi-ontic proposals).

    #2212
    H. Dieter Zeh
    Participant

    There is indeed considerable confusion about the most pressing problems of quantum theory and, depending on them, their possible solutions. It is even more unfortunate that this confusion seems to be accompanied by a certain amount of prejudice (for or against some kinds of proposals). Since it seems that statements starting with ”I believe that …” are not very helpful in this debate, let me suggest first to categorize different existing proposals. Everybody may then decide (and tell us) to which category his own proposal belongs (in many cases that may be obvious), or, alternatively, explain why he would dismiss some of these categories for being ”not at all promising” or even inconsistent. I think that every individual proposal that does not explicitly postulate the superposition principle (that is, the origin of the wave function in configuration space and beyond) as part of its kinematics should at least indicate how it would justify the well established general applicability (or possible limits) of this most important principle of quantum mechanics. (Or is this already a prejudice?). Similarly, it should indicate its position regarding non-locality if this is not obvious.

    The following is meant as a rough suggestion to begin with, and it can, of course, be subject to change or an introduction of subcategories (but I would prefer not to go into too much detail at this point):

    1. Ontic wave function postulated as the exclusive kinematics (psi-complete)
    a) collapse of the wave function (modified dynamics)
    b) Everett (universal unitarity)
    2. Classical kinematics (ontic particles and fields) under novel (non-local) dynamics
    a) pilot wave (psi-ontic but not psi-complete)
    b) other novel (such as stochastic) dynamics
    3. New kinematics (”hidden variables”)
    a) including the wave function (psi-ontic but not psi-complete again, but not just classical variables as in 2a)
    b) excluding the wave function (psi-epistemic: psi to be explained by ensembles or else)
    b1) nonlocal dynamics
    b2) nonlocal kinematics other than wave function
    b3) both nonlocal
    4. Denial of any underlying microscopic reality (pragmatic approach only, that is, ”shut up and calculate”?)
    5. Anything else (such as no concepts of kinematics and dynamics any more)

    It may be too simple yet (or incomplete), but that question may become part of the debate. For the historians among us it may also be fun (and probably not always trivial) to classify great quantum physicists of the past (or their versions at various times) according to this or a similar scheme. However, it seems that we cannot decide between all these possibilities without any novel empirical evidence. Are they, therefore, presently no more than a matter of taste?

    #1763
    H. Dieter Zeh
    Participant

    Dear Lee,

    thanks for your response, which may help at least to locate the source of our disagreement a bit better. So here are a few further remarks in reply.

    You may call it a hypothesis that I am using an ontic interpretation for the wave function (superposition) – very similar as in collapse theories, for example. However, I think that there are excellent arguments for this physical (ontic) interpretation of this general superposition principle, and it is the reason why I do not need any ´´operational hypothesis´´: all physical operations must be described by the dynamics of the wave function. What I do need, though, is a new definition (based on empirical knowledge) of subjective observers as parts of the physical world in terms of the wave function (von Neumann’s psycho-physical parallelism). [I do not want to explcitly ask you to read my papers and books, but a look at my contribution for this workshop might already answer some of your questions, including those discussed below.]

    Your objection about an infinite number of branches (most of them incompatible with the Born rule) is a very old hat – but nonetheless quite inappropriate. These branches are not objectively and not precisely defined, they are not infinite (although even Everett thought so), and there is absolutely no reason to regard them as ´´equally probable´´. You need weights for them in a very specific sense (that I have often discussed – for example in my contribution to this forum). I do not know any interpretation of QM that does NOT postulate Born’s rule in some sense. (Indeed, all fundamental elements of a physical theory must be empirical.) I have never referred to decision theory in this context, since (1) I do not understand the meaning of the ´´agent´´, and (2) I do not see any problem in postulating my subjective probabilities. (They have no objective meaning, since objectively there is only ONE quantum world – one superposition – according to Everett.)

    I do not know what you call the ´´cosmological fallacy´´, but I may understand your traditional reservations for applying quantum theory to the universe. However, what does your assumption of nonlocal ´´ontic phases´´ then mean, and what limits their validity in a chain of unitary interactions, which would otherwise lead to a superposition of many worlds? Extrapolating the known basic laws and concepts is the most conservative or the only way to construct ´´physical cosmology´´. The alternative is simply to shut up. Classical cosmology is certainly ruled out, in particular after the success of decoherence in deriving classical concepts in wave mechanical terms (including the spacetime metric – see Claus Kiefer’s contributions in the literature), while I would regard any cosmology that is based on new and not yet confirmed physical theories (such as strings) as no more than ´´educated science fiction´´. It is a shame that such things are nowadays taught students as ´´science´´ (even if they may form interesting mathematics). You may well suggest novel theories that are intended even to explain the wave function, but you cannot expect us to accept them before their novel consequences or their empirical foundation have been confirmed.

    Everett’s theory has been SUCCESSFUL for 50 years or more, since it never required or assumed any new physical hypotheses. It has always predicted that there are no novel phenomena that go beyond von Neumann´s pragmatic collapse (now as a ´´collapse by convention´´ in accordance with decoherence – but without a collapse as a dynamical process). So what is missing in principle with respect to quantum mechanics and perhaps even quantum field theory (save the usual technical problems for interacting quantum fields)?

    Best wishes,
    Dieter

    In case you do want to read more non-technical details about my interpretation of QM, here is a link:
    http://www.rzuser.uni-heidelberg.de/~as3/ParticlesOrWaves.pdf

    #1733
    H. Dieter Zeh
    Participant

    Dear Lee,
    could it be that you are just asking the wrong questions about quantum physics? Unfortunately you did not provide us with an example. As far as I know David Albert’s papers, he has mainly reformulated the traditional prejudice against Many Worlds.

    Regarding Everett, let me suggest a variation of an often quoted remark by John Bell:
    ´´It is important to note that´´ [Everett’s Many Worlds interpretation] ´´is not assumed but inferred´´ [from the Schrödinger equation]. ´´It is remarkably difficult to get this point across.´´

    Of course, nobody may be expected to believe in the validity of this global extrapolation of unitarity (we are all still searching for the truth), but everybody should be able to agree that Everett would be its (consistent) consequence if nothing else is added to the theory – and thus it appears worth being seriously discussed as a possibility. I don’t presently see any other foundation for cosmology, in particular. Everett could some day be falsified, but that would require some novel (and probably quite surprising) empirical evidence. ´´Notoriously vague´´ and speculative hypotheses, just to exclude unwanted consequences, do not appear very helpful in answering this issue.

    All the best for 2015
    Dieter

    #1717
    H. Dieter Zeh
    Participant

    There seems still to be some confusion about terminology (such as the meaning of nonlocality and reality). So I also hope that the authors will clarify terms for their purpose. However, the situation apears completely clear in the Everett picture if based on decoherence, while the concept of relative states makes sense only with respect to individual (subjective) observers:

    Since the pointer states A and B are assumed to immediately decohere after measurement, there are objectively four ´´autonomous´´ Everett branches after both measurements. Only in the parallel case, two pairs of them are identical (In each branch there would then be a second measurement with predetermined outcome). According to Everett (or global unitarity), all four (or two) branches ´´exist´´. The branching becomes relevant to an observer only if and when he receives a (classical) message about the outcome. Only then will he ´´split´´, too. First into two and then (possibly) into four versions – with subjective(!) probabilities according to the empirically known (Born’s) weights. Measured spinors and pointer positions are according to this picture in their ´´relative states´´ with respect to these different versions of the observer, where they may be objectivized between different observers in one branch by means of their entanglement in the global Everett state.

    If this observer is a pragmatist, he will assume that the wave function had already collapsed (twice) into one single branch at the time of the first irreversible decoherence of each measurement result. This is the pragmatic ´´collapse by convention´´.

    I have precisley discussed this example on pp. 16/17 of http://www.rzuser.uni-heidelberg.de/~as3/ParticlesOrWaves.pdf (in case you would like to have a look).

    #1704
    H. Dieter Zeh
    Participant

    Dear Lee,

    that looks like a very ambitious program. I shall try to have a closer look (even though I am not very optimistic). Perhaps you have a brief outline of its basic ideas somewhere?

    In your second paragraph you seem to be referring to Bohm. In Everett’s Many Worlds there are no trajectories.

    Best wishes,
    Dieter

    #1702
    H. Dieter Zeh
    Participant

    I agree – but why then do we need non-local hidden variables instead of the wave function itself? Or do you think you can avoid Many Worlds?

    #1701
    H. Dieter Zeh
    Participant

    Ruth, I don’t intend here to discuss the transactional interpretation, but the ´´result of a collapse´´ in the usual sense is a new global wave function, and hence still kinematically non-local. Only certain macroscopic aspects of this state may be approximately local, and even this is so just by construction: the collapse is usually defined to mimic what can well be understood by decoherence. Or do you know any example of an apparent quantum jump (into an apparent ensemble of final states) that cannot be understood by decoherence in principle?

    Surely, decoherence requires a drastic non-equilibrium – like many other arrows of time. This requires a very special (such as symmetric or homogeneous) initial condition at the big bang, for example. (I have written a whole book about that.) But what is ´´circular´´ about such an explanation (as you say in the abstract of your cited paper)? The evolution of an individual Everett branch is clearly not deterministic, and many of its properties are ´´created by chance´´ during its evolution, often related to symmetry breaking. So the resulting structure cannot be presumed.

    #1674
    H. Dieter Zeh
    Participant

    It is often amazing to see how far definitions of the same term may differ when used by different physicists. Roderich Tumulka offers four definitions of reality, and he argues that the Many Words interpretation is in conflict already with the weakest of them (his R4). Accordingly, it would violate the reality assumption most strongly. I completely disagree. (Note that his definition refers to local properties already, and thus may presume the locality of ´´beables´´!) Everett clearly assumed the wave function to be a complete and ontic concept.

    Everett’s Many Worlds interpretation is ´´realistic´´ in the sense that – in contrast to complementarity or QBism, for example – it is using a unique and consistent kinematical and dynamical conception to describe an observer-independent empirical world (that is, all observations performed by observers who can communicate and can in principle be described as parts of this world). There is no mention of space or locality yet. In fact, Everett’s specific reality is defined by a wave function in configuration space (including spin variables etc.), which is thus nonlocal in three-dimensional space. (This holds even for collapse theories.)

    The concept of locality is here used only but essentially dynamically (sometimes called ´´relativistic causality´´), which is still possible if this Hilbert space does have a local basis. This dynamical locality has the consequence that reasonable observer systems have to be local systems, which can have definite states in the quantum formalism only in Everett’s autonomous branches (defined by means of decoherence).

    We may all have very different opinions about quantum theory at this stage, but we should not decide between them just by constructing our own definitions. In my opinion, the wave function, if universal and taken seriously (which leads to Everett), tells us that the world may be assumed to be real but has to be nonlocal. So it is remarkable that Tumulka and I come to the same verbal conclusion about locality being the culprit and being ruled out in spite of using quite different definitions of ´´realism´´.

    #1621
    H. Dieter Zeh
    Participant

    If my link does not work, please use http://www.rzuser.uni-heidelberg.de/~as3/nonlocality.html (or insert the missing slash before nonlocality in the replaced website by hand)!
    Sorry for the inconvenience!

    #1620
    H. Dieter Zeh
    Participant

    Much of Christopher Timpson’s paper is ´´philological´´ or historical and about the different or changing use of concepts and words. This is quite intersting and potentially important, but not what I am mainly interested in. However, in the context of Everett, the concept of (non)locality seems to be quite clear to me. If you accept the ontic nature of the wave function or of a general superposition, this evidently defines a ´´kinematical nonlocality´´. (I assume this is what people usually call ´´non-separibility´´, when they are not necessarily regarding the wave function as ontic.) However, if you also assume the existence of a fundamental local basis for this Hilbert space (such as the configuration space for particle positions and/or spatial fields), you may still define local dynamics – no action at a distance – by means of local interaction Hamiltonians and relativistic propagators (see http://www.zeh-hd.de/nonlocality.html ).

    This definition is obviously model-dependent, but I do not know any (explicitly defined) proposal for how possibly to replace the successful wave function. (This is indeed my basic point.) So it all depends on whether or not we need an additional collapse dynamics, which in my opinion is an illusion – in agreement with Everett (see Sect. 4 of my contribution to the book).

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