Lee Smolin

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  • #1787
    Lee SmolinLee Smolin
    Member

    Hi Ken,

    Do these interesting results imply that if one doesn’t want to admit retrocausality in quantum physics one must also deny time reversal symmetry? At what level must one give up time reversal symmetry? Must unitary time evolution or the Schroedinger equation be modified?

    Thanks,

    Lee

    #1762
    Lee SmolinLee Smolin
    Member

    Dear Dieter,

    Thank you for your remarks. I couldn’t disagree more, on substance as well as methodology. First, I don’t agree that the Everett interpretation is “inferred” rather than invented. Without an operational hypothesis as to how the mathematical symbols correspond to measured values a theory, any theory, is just empty mathematical formulas. Any assertion of a connection between mathematical expressions and measured values relies on metaphysical and ontological assumptions. You cannot pretend otherwise. When you do so you are just refusing to expose your own metaphysical assumptions or expose them to criticism and debate.

    Second, while I agree it is a fair question to ask whether Everett is cogent taken on the assumption the current formulation of QM is correct, I don’t agree that it is. To my understanding, the following criticism has never been successfully answered: there are an infinite number of branches where Born’s rule is not satisfied and an infinite number where it is. To assert we are in one of the latter requires we make additional assumptions, extraneous to the theory, thus negating the claim that the theory produces its own interpretation. Attempts by Saunders, Wallace etc to provide a convincing basis for this step in decision theory are impressive but don’t change the outcome, as they also rely on extraneous assumptions.

    Third, there is a whole suite of arguments that establish that QM as currently formulated cannot be applied to the universe as a whole. These go under the name of the cosmological fallacy. I won’t repeat these here as they are lengthy and they are the core of my last two books, Time Reborn and The Singular Universe and the Reality of Time. A summary is in my paper Temporal Naturalism, arXiv:1310.8539. We strongly welcome criticism of these arguments as we believe they are crucial for the future of physics.

    Finally, I would argue on historical and methodological grounds that if a theory has been on the table for more than 50 years and has not succeeded in answering objections posed to it or convincing critics, and has no empirical support (not shared with many other approaches) our presumption should be that it has failed. We should put it aside and encourage ourselves and especially young scientists to put forward new ideas and new hypotheses which have a better chance of succeeding.

    I would apply this both to Everett and dBB.

    Best wishes,

    Lee

    Many thanks,

    Lee

    #1720
    Lee SmolinLee Smolin
    Member

    Dear Dieter,

    I’m sorry, I misunderstood you; I thought by referring to the wavefunction, as an alternative to hidden variables, you were referring to dBB.

    Everett Many Worlds fails to answer any of the questions that puzzle me about quantum physics. I can elaborate my objection to it but others such as David Albert have already done a convincing job of this, in my view.

    I do hope that my program is ambitious; I believe we have to be ambitious because the measurement problem in my view cannot be solved without completing quantum mechanics, ie seeing QM as an approximation to a different theory. And I think its likely that that more correct theory will also address the issues of giving a quantum description of spacetime and cosmology.

    You can find summaries of my present research program in the later chapters of either Time Reborn or my new book with Roberto Mangabeira Unger. Those books argue for a perspective on cosmological theories that we argue must frame any attempt to resolve the measurement problem.

    Many thanks,

    Lee

    #1703
    Lee SmolinLee Smolin
    Member

    Dear Dieter,

    I certainly do want to avoid many worlds. For reasons we explain in our new book with Mangabeira Unger, I don’t think that the present framework of quantum theory can apply to the universe as a whole.

    I agree the wave function contains more than a probability distribution-its phase appears to be “ontic”. But I don’t like that the wavefunction influences the particle trajectories but not vise versa-that violates Einstein’s principle of reciprocity.

    One attempt to derive the wavefunction from a real ensemble within the single universe, in a way that satisfies reciprocity, is the real ensemble formulation.

    Thanks,

    Lee

    #1143
    Lee SmolinLee Smolin
    Member

    Dear Bob,

    Many thanks for your comments. My first response is that I don’t think that the REF has the issue with retrodiction you mention, but I’ll look into it.

    I haven’t thought through your consistent histories framework carefully in many years so I will take your invitation to do so again. At one time there was an argument by Dowker and Kent that consistent sets of histories did not distinguish the semiclassical domain from many other equally consistent sets of histories, was that ever resolved?

    For me the measurement problem is just one of several reasons why I believe that none of our present theories can be successfully extended to the universe as a whole. (The other reasons are detailed in the books and papers mentioned.) So even were that resolved there would be additional reasons to presume QM is an approximation to a yet to be defined cosmological theory, suitable for small subsystems.

    This is to say that there is a need for careful foundational thinking of the kind that is so well demonstrated here in the cosmological domain.

    Many thanks,

    Lee

    #1118
    Lee SmolinLee Smolin
    Member

    Dear Shan,

    I agree with you strongly. But deciding that the wavefunction is ontic, while very important, is of course not the end of the discussion, it is the beginning. It opens up the floor for us to propose hypotheses as to what beables the wave function corresponds to. Quantum mechanics doesn’t tell us, because of the measurement problem, because what we observe is particles not the wavefunction, and because it doesn’t give a complete description of individual processes. To answer the question of what properties of reality the wavefunction corresponds to requires a completion of quantum mechanics.

    So, I would like to suggest your four questions pose a challenge to all of us. What hypothesis do we propose as to what the reality of the wavefunction corresponds?

    dBB offers such a hypothesis: the wavefunction is a beable, it is a potential which guides the particles, which are also beables. The real ensemble formulation (REF) offers another: the wavefunction corresponds to a real ensemble of similarly prepared systems present in the universe. These trade beables through nonlocal processes which reproduce the Schroedinger dynamics.  

    Since the wavefunction corresponds to an ensemble of real, existing systems, it can correspond to real objective properties of this ensemble. It turns out that the absolute value and the phase correspond to different aspects of the ensemble. The absolute value measures a relative frequency probability distribution within the real ensemble of a beable corresponding to configuration space. The phase corresponds to another beable.

    Both the dBB and REF predict regimes where the correspondence with QM fails, which opens the door to experiments which would distinguish them from QM. For the REF this regime consists of entangled states of systems complex enough to have no natural counterparts.

    But this is just one hypothesis. Can I close with the provocative statement that with the PBR and other arguments for the reality of the wavefunction, it is time for research in QF to move beyond the phase of “interpretations” of QM to the task of discovering its completion. In other words, to answer your question, the way to make sense of the wavefunction is to replace it with a more precise description of the reality it corresponds to.

    Many thanks for including me in this wonderful workshop.

    Lee

    #966
    Lee SmolinLee Smolin
    Member

    Dear Shan,

    Sure, as I wrote, the theory can be tested by doing exactly what you say: construct a multiparticle entangled state of many degrees of freedom that would be very improbable to have been created naturally. If nonetheless the Schroedinger equation is satisfied that is evidence against the real ensemble formulation. Or you could try to save it with a story about aliens trying to communicate non-locally with us. I personally don’t find such stories compelling but it would probably make a better movie than wormholes. Of course, it could be fun to work out a protocol for this kind of communication.

    Thanks,

    Lee

    #961
    Lee SmolinLee Smolin
    Member

    Dear Ken,
    Thanks so much for your response. Again, I’llgo point by point:

    “After reading through some of the PIRSA slides, I think I see the big-picture of what you’re attempting here… and I certainly appreciate that having Hamilton’s Principal Function as the phase of the wavefunction might invoke an interpretation where the distant past is directly influencing the future.”
    I’m afraid I don’t appreciate this point. The phase of the wavefunction does correspond to a beable in the real ensemble formulation, but there is no sense there in which the ‘distant past is directly influencing the future.’ There is such a sense in the principle of precedence but the phase of the wavefunction plays no role there. Perhaps you are able to do something I so far haven’t been able to, which is to combine them.

    “(Basically, noting the same strange feature of the H.P.F. that I criticize in my own paper here, and then tackling this strange implication head-on.)
    So I’d like to follow up on any connection you might see between the H.P.F. and the action principles that inspired it in the first place (leaving the final endpoint variable, and extremizing the action for every different possible outcome). Does the HPF survive, but the original action principle disappear?”
    I find what you are saying very interesting, but these are not things I’ve thought about. Perhaps I should.

    “In general, I’d like to better understand how action principles fit into the big picture that you’re promoting here. If the laws are evolving in time, then any rule that is extremizing solutions that span a range of time would seem to be a non-starter…?”

    This is an excellent question, I can see how in a causal set picture of the kind Marina Cortes and I introduced the amplitudes for elementary events can evolve in time, but I agree its not clear what this does to the classical limit.

    “Also, I’m curious (after reading your response to Bob) why you don’t see any net benefit of putting the explanation of all the observed macrosocpic CPT-asymmetries onto a single boundary constraint on the universe. Isn’t that *some* explanatory improvement over CPT-asymmetric laws? (You implied that such special boundaries are themselves time-asymmetric, but that’s a tricky question; if one reverses the universe around the boundary itself, it might look roughly the same in both directions. CPT-asymmetric laws on the other hand, are asymmetric no matter how you slice them.”

    I would put it the other way. For a variety of reasons I’ve come to believe that there is an objective, physical distinction between the past, present and future. Part of this is that there is an objective asymmetry between what we can know or give truth values to in the past, as opposed to the future. One reason out of several I am convinced of this is that the attempts to explain the observed time asymmetry of nature in terms of time asymmetric initial conditions imposed on time symmetric laws appears to fail because the very special initial conditions required are highly improbable or fine tuned, in the space of possible initial conditions.

    Once one admits that there is an objective distinction between the past and the future an opportunity opens up, which is that this asymmetry is fundamental and directly explains the several very manifest arrows of time we observe in nature.

    “I like to note Einstein’s 1905 warning about using an ontology that introduces “asymmetries which do not appear to be inherent in the phenomena”, and apply it to CPT-symmetry.”

    But time asymmetry is all around us, it is absolutely inherent in the phenomena. One arrow of time I dwell on is the electromagnetic arrow: that light brings us coherent images from the past and never from the future. That is, there are only retarded potentials, and no advanced potentials. How is this drastic asymmetry to be explained? As Steve Weinstein has emphasized, this requires drastically time asymmetric initial conditions. My approach is to simply say nature is like that. The real theory must be a time asymmetric extension of Maxwell which only has retarded solutions.
    “I take this as a warning about introducing time-asymmetries right from the start of theory-building. Since you’re starting off with a time-asymmetric account, I gather you think we have sufficient empirical justification for such a step?”

    From my point of view what is surprising is how long Boltzmann’s story of time asymmetric initial conditions imposed on time symmetric laws has held up dispite the fact that it is inadequate to explain why the universe is still out of equilibrium 13.7 billion years later.

    Best,

    Lee

    #926
    Lee SmolinLee Smolin
    Member

    Dear Bob,

    Thanks enormously for your interest. Let me answer your questions in sequence:

    “Can you say a bit more about what is driving your ideas? Obviously there is
some discontent with quantum mechanics as currently formulated, but what is it
that you think you can improve on?”

    I am driven by the now more than 40 year failure to go beyond the standard models of particle physics and cosmology, as well as the problems of quantum gravity, and the measurement problem in quantum foundations.

    In addition, I am deeply curious about two questions:

    What chose the laws of physics?

    What chose the cosmological boundary conditions?

    To me these are all part of the same package because they have the same source: that we are at the point where we have to go beyond a methodology suited to modeling small subsystems of the universe and invent a new methodology appropriate to doing science at the level of the universe as a whole.

    With Roberto Mangabeira Unger we have done a careful analysis which leads us to propose a starting point for discovering how to extend the laws of physics to cosmology. The full argument is in our forthcoming CUP book:

    http://www.cambridge.org/us/academic/subjects/physics/history-philosophy-and-foundations-physics/singular-universe-and-reality-time-proposal-natural-philosophy.

    A summary is my essay Temporal Naturalism: arXiv:1310.8539. The popular version of the argument is in my book, Time Reborn.

    “Many of us consider the measurement problem of quantum foundations, the inability to describe the measuring process in fully quantum terms, a real scandal.”

    Of course, I agree. But from my present point of view the measurement problem has to be seen as part of the larger problem of finding a proper framework for cosmological theory. Quantum theory as presently formulated can only make sense as a theory of small subsystems of the universe and the puzzles it raises can only be resolved by the construction of an adequate cosmological theory (by which I mean a theory that can also explain the origin of the laws and initial conditions.). Note that this is not a casual assertion, the full argument is a detailed case for this conclusion.

    “…but you seem to be multiplying the scandal by multiplying the number of measurements without explaining how they work.”

    Please rephrase, I don’t understand this question.

    “Nonlocality is bothersome to me, and I think it was to Einstein, because how
can you do decent physics without concentrating on some small system you hope 
you can understand without worrying about the rest of the universe?”

    Indeed the conclusion of our argument is that we have to worry about the rest of the universe; we’ve gone as far as we can without taking that on.
    “I gather that you are either content with nonlocality or consider it a blessing,”

    Yes, indeed, non-locality is also natural in a quantum gravity world where spacetime and locality are emergent from more fundamental causal structures. (As in causal set models, spin foam models, etc.)

    “…. which seems surprising in that this at once raises questions of how to reconcile
things with special relativity. “

    Indeed a big part of the story is how to reconcile non-locality with the strong experimental evidence for predictions of special and general relativity. Part of this story is a new formulation of GR called shape dynamics.

    “(I should not conceal the fact that I am an advocate of consistent/decoherent histories, which I consider a local interpretation of QM, and in this sense much to be preferred to Bohmian mechanics.)”

    As I believe QM as presently formulated must be restricted to small subsystems, so must all interpretations of it. The various formulations of QM have their advantages and disadvantages, but I am interested in something different, which is how to discover the right completion of an incomplete theory.

    “If you don’t want a deterministic universe with time invariant laws, then the easiest way to move back a bit from classical mechanics is to assume a probabilistic dynamics with time invariance of the laws of probability, and this is how I understand quantum mechanics–do you have any objections to such an approach, and doesn’t it agree with everything we know at present from experiments?”

    These are very interesting issues for me, because of arguments we recently developed with Marina Cortes about how the standard paradigm of physics must break down for elementary events-ie for the smallest as well as the largest scales. See
    Marina Cortês, Lee Smolin, The Universe as a Process of Unique Events, arXiv:1307.6167 [gr-qc]. in press at Physical Review D.

    “To a very good approximation the microscopic laws of physics as we now understand them are invariant under reversing the sense of time, and we can invoke initial conditions to explain macroscopic irreversibility, but you seem to be throwing all of this away.”

    I don’t think the appeal to time asymmetric initial conditions explains anything because that itself requires explanation. I follow Roger Penrose who in 1979 proposed that the time asymmetric initial conditions are to be explained by the dominance in the very early universe of time asymmetric fundamental laws, from which the time symmetric laws we know emerge as approximate descriptions of small subsystems. My current work develops this theme. For example, with Henrique Gomes and Marina Cortes we are studying a time asymmetric extension of GR he recently discovered.

    The main reason I am convinced the world is fundamentally time asymmetric is because the only way to explain the origin of the laws and initial conditions in a way that implies testible predictions is if the laws evolve in time. The philosopher Charles Sanders Peirce understood this clearly in 1893, it is a lesson we have been slow to learn.

    “So I am perplexed where your motivation lies.”

    I am very grateful for you giving me the opportunity to try to explain my motivation.

    Lee

    #920
    Lee SmolinLee Smolin
    Member

    Dear Shan,

    Thanks for your interest and question. You raise an important issue which is the definition of the ensembles corresponding to the quantum state. These are defined to consist of similarly prepared systems with similar external environments and the same constituents. The crucial issue, which I do address in section 6 of the paper as well as the talk, is what is the meaning of “similar” in this definition.

    Clearly “similar” cannot mean exactly identical, or there would be only ensembles with one element. Each atomic system is in a slightly different external environment. (Because the world is complex enough to distinguish different events by their environments.) So I am relying on a notion of approximate similarity of environments. However, I don’t specify this notion, and hence leave it informal. Why? Because the assumption is that QM is itself an approximation to another theory, suitable for descriptions of small subsystems of the universe. Notions such as “similar environments” can be expected to be quantified in that cosmological theory QM approximates.

    However, we can safely assume that two atoms can be in similar environments even if one is in a room with an experimenter and the other is in some natural environment, far from living things. So this theory does not in any way require any assumption about aliens.

    Thanks, Lee

    #894
    Lee SmolinLee Smolin
    Member

    I am having difficulty uploading the pdf of the talk, but interested participants can download the pdf of a similar talk from http://pirsa.org/13050072/

    #893
    Lee SmolinLee Smolin
    Member

    The file of the talk is attached here:

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