Quantum Reality

Jim Baggott’s new book, Quantum Reality, is now out here in US, and I highly recommend it to anyone interested in the issues surrounding the interpretation of quantum mechanics. Starting next week I’ll be teaching a course on quantum mechanics for mathematicians (more about this in a few days when I have a better idea how it’s going to work). I’ll be lecturing about the formalism, and for the topic of how this connects to physical reality I’ll be referring the students to this new book (as well as Philip Ball’s Beyond Weird).

When I was first studying quantum mechanics in the early-mid 1970s, the main popular sources discussing interpretational issues were uniform triumphalist accounts of how physicists had struggled with these issues and finally ended up with the “Copenhagen interpretation” (which no one was sure exactly how to state, due to diversity of opinion among theorists and Bohr’s obscurity of expression). Everyone now says that the reigning ideology of the time was “shut up and calculate”, but that’s not exactly what I remember. The Standard Model had just appeared, offering up a huge advance and a long list of new questions with powerful methods to attack them. In this context it was was hard to justify spending time worrying about the subtleties of what Copenhagen might have gotten wrong.

In recent decades things have changed completely, with the question of what’s wrong with Copenhagen and how to do better getting a lot of attention. By now a huge and baffling literature about alternatives has accumulated, forming somewhat of a tower of Babel confronting anyone trying to learn more about the subject. Some popular accounts have dealt with this complexity by turning the subject into a morality play, with alternative interpretations portrayed as the Rebel Alliance fighting righteous battles against the Copenhagen Empire. Others accounts are pretty much propaganda for a particular alternative, be it Bohmian mechanics or a many-worlds interpretation.

Instead of something like this, Baggott provides a refreshingly sane and sensible survey of the subject, trying to get at the core of what is unsatisfying about the Copenhagen account, while explaining the high points of the many different alternatives that have been pursued. He doesn’t have an ax to grind, sees the subject more as a “Game of Theories” in which one must navigate carefully, avoiding Scylla, Charybdis, and various calls from the Sirens. One thing which is driving this whole subject is the advent of new technologies that allow the experimental study of quantum coherence and decoherence, with great attention being paid as possible quantum computing technology has become the hottest and best-funded topic around. Whatever you think about Copenhagen, what Bohr and others characterized as inaccessible to experiment is now anything but that.

While one of my least favorite aspects of discussions of this subject is the various ways the terms “real” and “reality” get used, I have realized that one has to get over that when trying to follow people’s arguments, since the terms have become standard sign-posts. What’s at issue here are fundamental questions about physical science and reality, including the question of what the words “real” and “reality” might mean. In Quantum Reality, Baggott provides a well-informed, reliable and enlightening tour of the increasingly complex and contentious terrain of arguments over what our best fundamental theory is telling us about what is physically “real”.

Update: For a much better and more detailed review of the book, Sabine Hossenfelder’s is here.

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37 Responses to Quantum Reality

  1. Matt Grayson says:

    I have just started reading Quantum Reality, and I’m grateful for the sensible definitions of “reality”, a term I’ve always hated, but one thing confuses me still.

    How can anyone contemplate Netwonian mechanics and see “smooth continuities and merciless certainties”? Imagine two balls colliding. At the moment of collision there is infinite acceleration. Not large, infinite. Make the balls spongy? Still infinite where they first touch. The only way I can see to avoid infinities is for the balls to interpenetrate – overlap – and then gradually, if very quickly, rebound.

    And yet we have no worries about the interpretation of Newtonian mechanics. We say “clockwork” as if that explains anything. It is truly a theory of “shut up and calculate”.

  2. Thomas says:

    I don’t know .. it seems to me that the added value of yet another popular book on the foundations of QM is approaching zero exponentially fast. Why not write a book on progress in quantum technology and engineering?

  3. Peter Woit says:

    Matt Grayson,
    It has always seemed to me that the classical picture of the world is far more incoherent, inconsistent, and hard to make sense of than the quantum one.

    I disagree, since while there have been lots of such books recently, mostly they have been of either little value, or seriously negative value, with the amount of attention they get often proportional to how misleading they are. Philip Ball’s is the only other recent one that I think gets things right, having another one written in a somewhat different way I think is quite valuable.

    I won’t disagree that a good new book on quantum technology would be great, and advances in this technology I think will ultimately help clarify interpretational issues. Baggott has some interesting material on this, in particular about implications of quantum computer designs for the preferred basis problem in many-worlds theory.

  4. Terry Jenkins says:

    Dear Peter,

    You write, “ Whatever you think about Copenhagen, what Bohr and others characterized as inaccessible to experiment is now anything but that.”

    Extraordinary claims require extraordinary evidence, but as you are quite busy, perhaps we could get just an example or two, or even a reference?

    Thanks in advance.

  5. Peter Woit says:

    Terry Jenkins,

    What I was referring to was for instance one of the main issues in developing a quantum computer: how do you keep qubits from decohering, while at the same time being able to manipulate them to do computations? This is currently under intensive experimental and theoretical study, though you’ll have to find someone more expert than me for good references on the state of the art.

    The fact that such study was impossible in the early days of QM I think had a lot to do with the early Copenhagen insistence that one needed to treat the classical and quantum in completely different ways.

  6. Mark says:

    I think this might be a good reference on one approach to quantum computing:

    Robust encoding of a qubit in a molecule
    Victor V. Albert,1,2 Jacob P. Covey,1 and John Preskill1,2
    Institute for Quantum Information and Matter1 and Walter Burke Institute for Theoretical Physics2 California Institute of Technology, Pasadena CA 91125, USA
    (Dated: November 20, 2019)


  7. Peter Woit says:

    That’s more of an alternate proposal for a quantum system to encode qubits, doesn’t really address the transition to classical question.

    For something closer to what I had in mind, see the Nobel citation for Haroche/Wineland
    and the textbook “Exploring the Quantum” by Haroche/Raimond.
    The Physics Today review of that book
    starts out

    In 1952 Erwin Schrödinger wrote in the British Journal of the Philosophy of Science , “We never experiment with just one electron or atom or (small) molecule. In thought-experiments we sometimes assume that we do; this invariably entails ridiculous consequences.”

    which shows the very different situation the first generation of quantum theorists faced when thinking about the measurement problem.

  8. Mark says:


    Thanks, I have read the book Exploring the Quantum, and highly recommend it. Forty years ago when I read Zurek & Wheelers: Quantum Theory and Measurement, I thought to myself that this will all be resolved in the next few decades with advancing technology. But that does not seem to have been the case. Not sure if the “measurement” problem is a real physics problem or simply one of those philosophical problems that never get resolved.

  9. Michael Weiss says:

    Peter and Matt,
    With regard to QM being more sensible than classical mechanics, you may remember the late Ed Purcell’s light-hearted remark when we were undergraduates that it made as much or more sense to speak of a “quantum certainty principle” than of the standard “uncertainty principle.” Ed was referring to the implausibility of classical mechanic’s infinite precision; he might also have had in mind Matt’s point about infinite accelerations in hard-sphere collisions. Ed’s quip reflected his always-deep insight. John Preskill was Ed’s TA in intro QM in 1975 (“Physics 143”) and might have more insight.

  10. Mark Hillery says:

    For something a lot more technical, and very recent, that summarizes several different approaches to obtaining a classical world out of quantum rules, I recommend “Roads to objectivity: Quantum Darwinism, Spectrum Broadcast Structures, and Strong quantum Darwinism” by J. K. Korbicz arXiv:2007.04276.

  11. Low Math, Meekly Interacting says:

    I’m probably getting this partially wrong…I donated most of my old pop-sci books a long time ago. Anyway, I think it was in “Dreams of a Final Theory”. Weinberg recounted a colleague’s rueful explanation for a promising student’s eventual failure: He tried to understand quantum mechanics. The larger point, I think, was Weinberg sympathized with this point of view. However, despite his distaste for philosophy, he saw the unavoidable need. Otherwise, one could never truly understand cosmology; and perhaps a “final theory” would remain forever out of reach until QM was, in fact, “understood”.

    I don’t recall if Weinberg had the measurement problem in mind specifically, or something else. Anyway, that book is about 30 years old now. My sense is one could pick it up today and find an essentially contemporary summary of the outstanding issue(s), i.e. troubling and completely unresolved. Every new pop-sci article I read these days on “foundations”, a new experimental result is hyped as if it sheds some incredible new light. But you read past the click-bait and it turns out the results agree as precisely as one can measure with QM as currently formulated. The results don’t actually resolve any interpretational issues, despite sometimes weaselly, or even borderline dishonest language suggesting the contrary. It’s wonderful that these experiments may be pushing us ever closer to a veritable and practical quantum computer, etc., but I do scratch my head at the notion foundational progress is being made. To use a variant of the deadly word: Really? I’ll likely purchase this book, just because it’s been a while since I’ve read anything book-length for fun. But I bet I’ll still feel disappointed.

  12. Peter Woit says:

    Weinberg has continued to be concerned about whether interpretational problems mean we are missing something fundamental about QM, writing about this in 2017 in the New York Review of Books. He has pursued some ideas about introducing non-linearities in the theory, but I think concluded they don’t work.

    Unlike Weinberg, my point of view on this has always been that it’s not quantum mechanics we don’t understand, it’s classical mechanics (i.e. how our classical reality emerges from a fundamental quantum theory). The quantum mechanical formalism is simple, of great mathematical beauty, and completely successful experimentally. What’s hard to understand is not that story, but why the classical picture works as a way to describe our everyday experience of reality.

    I don’t think there’s the slightest reason to believe that new more powerful experimental techniques will see a violation of the laws of quantum mechanics. What they can do though is help understand the really difficult and complicated problem of classical emergence. The paper Mark Hillery points to is a good example showing the kinds of questions that one can now experimentally study to learn more about this, experiments of a kind that Bohr and his generation could not seriously consider.

  13. Matt Grayson says:

    “Everyday experience of reality” is, IMHO, the elephant in the room. I frequently compare our perception of Nature with a flatworm’s experience at the Opera (borrowed from Vernor Vinge). There’s no reason at all that Nature should be in the tiniest bit comprehensible to flatworms or to us. Nor need our experience meaningfully reflect it. This means giving up on the “thing-in-itself” problem – e.g., reality of the wave function or the aether – but it does not in any way mean that we should give up on science, nor fail to visit both metaphysical and empirical shores.

  14. WTW says:

    When you said
    “…the advent of new technologies that allow the experimental study of quantum coherence and decoherence, with great attention being paid as possible quantum computing technology has become the hottest and best-funded topic around. Whatever you think about Copenhagen, what Bohr and others characterized as inaccessible to experiment is now anything but that…”
    I thought that you were referring to the now commonplace practice in quantum experiments to infer the values of observables without causing the “collapse of the wave function” — e.g., preserving coherence of separated systems even during “weak” measurements.

    Similarly, “using single electrons [or photons] as observers” is precisely what recent experiments claim to be doing. Yet most philosophers of quantum mechanics seem to want to ignore such topics, and few theorists seem to want to deal directly with the results of those experiments.

    In terms of understanding “quantum reality”, Nature seems to want to continually throw up roadblocks in our path. See, for example, the issues related to “quantum pigeon hole” experiments,
    and see
    also at
    https://www.nature.com/articles/s41567-020-0990-x and

    Then there’s “Closing the superdeterminism loophole
    in Bell’s theorem” at
    all more grist for the mill/food for thought.

  15. Jim Baggott says:

    If you’re currently reading ‘Quantum Reality’ then you’ll be discovering – spoiler alert – that I’m sympathetic to the Kantian position of giving up on acquiring knowledge of ‘things-in-themselves’ beyond the basic assumption that, whatever ‘they’ are, they continue to exist when we’re not looking. This is why it’s so fundamentally important in my view to try as much as possible to distinguish our (metaphysical) ideas about reality from our (empirical) experience of it.
    But this is the nature of the Game we play: we blend our ideas and experience in a scientific theory and the extent of correspondence between theory and experience allows us to judge its relative success. After studying both anti-realist and realist interpretations of QM for 30 years or so, I’ve developed a very bad feeling about it.

    In ‘Dreams of Final Theory’ Weinberg wrote: ‘A final theory will be final in only one sense – that it will bring an end to a certain sort of science, the ancient search for those principles that cannot be explained by deeper principles’. I understand this to mean that, for Weinberg, finality implies futility. With the deepest possible principles in hand, there’s little sense in seeking a deeper scientific understanding. We are then left with philosophical questions, for which good arguments can be formulated, this way and that, but for which lack of empirical evidence will mean that any kind of resolution or consensus on the answers will remain forever elusive. If indeed quantum mechanics is science’s final word on the physics of quantum objects, then foundational questions can only ever be philosophical questions. Just don’t expect physicists with more realist preconceptions to agree with this anytime soon.

    I barely touch on it in ‘Quantum Reality’, but anyone broadly comfortable with the notion that QM is no more than a convenient (but extraordinarily powerful) way of connecting our experiences of quantum physics will be interested to explore what this means for classical mechanics. The simple answer is of course that precisely the same interpretation can be given to classical mechanics. Instead of the debating the reality of the ‘wavefunction’ we debate the reality of classical concepts such as space, time, energy, mass, momentum. The fact that we can ‘see’ objects moving through space, in time shouldn’t prevent us from realising that there’s still a big difference between the ‘objects in reality’ and our scientific representations of them. There’s no arguing with the utility of assuming that something like mass is a real physical property of a real physical thing, but I think it helps if we don’t lose sight of the fact that this is still a (metaphysical) assumption.

  16. “I don’t think there’s the slightest reason to believe that new more powerful experimental techniques will see a violation of the laws of quantum mechanics. What they can do though is help understand the really difficult and complicated problem of classical emergence.”

    You’re sounding a lot like an Everettian there; those two sentences would be at home in David Wallace’s book The Emergent Multiverse. It makes me wonder if there’s really all that much difference between your positions. I guess it comes down to wave-function collapse: do you think that’s an actual, fundamental physical phenomenon, or some sort of emergent phenomenon?

  17. Alessandro Strumia says:

    The unsatisfactory aspect (at least to me) of Copenhagen QM is that it’s probabilistic. This is the root of all debates about wave-function ontology. Physical progress would be understanding how randomness arises. But we don’t have sensible deterministic theories that reproduce quantum randomness in some limit, so we don’t know where we should search. Maybe it’s some fast high-energy dynamics, and low-energy experiments are irrelevant. For sure, neither the papers by Weinberg nor experiments about decoherence address this issue.

  18. More Anonymous says:

    Hey Peter,

    “my point of view on this has always been that it’s not quantum mechanics we don’t understand, it’s classical mechanics (i.e. how our classical reality emerges from a fundamental quantum theory)” … It’s obvious to me that classical mechanics emerges in the limit of some parameter. Do we know what this parameter is though?
    I was under the impression that it would be the thermodynamic limit but I don’t know of any heursitic argument that suggests the same. Is there any in the literature?

  19. Peter Shor says:

    Kevin S Van Horn:

    I don’t see why you think Peter Woit’s belief that these issues are not going to be settled experimentally has anything to do with belief in the Everett interpretation.

    There are two possibilities here:
    • There is some way of predicting which of the outcomes that quantum theory says are random will be realized, at least with better accuracy than quantum mechanics gives.
    • There is no way of predicting these outcomes more accurately than quantum mechanics.

    If the first case holds, it means that the myriads of quantum mechanical experiments we have done just happened to have their experimental set-up completely orthogonal to the means of predicting the outcomes, as otherwise we would have observed probabilities that are not in accordance with quantum mechanics.

    And the second case holds if you believe in any of the standard interpretations of quantum mechanics — wave function collapse, pilot waves, or many-worlds.

    So I would agree with Peter that the first case is extremely unlikely.
    (Of course, we believe that QFT is incompatible with GR, so something may happen at very high energies. In this case, it may still be impossible to settle the situation experimentally, as we are not going to build particle accelerators the size of galaxies.)

  20. Peter Woit says:

    More Anonymous,
    It’s not at all obvious that classical mechanics emerges as a limit of a parameter. For an interesting read, see this blog posting by Jess Riedel
    One simple point he makes is that
    “The most glaring problem is that the state spaces of classical and quantum mechanics are completely different, so you can’t have a simple limiting procedure unless you describe how you’re going to map one onto the other. “

  21. Peter Woit says:

    Alessandro Strumia (and Kevin van Horn),
    Yes, I agree that’s the most unsatisfactory aspect of QM, especially in the Copenhagen interpretation where you make Born’s rule a fundamental axiom. I take Copenhagen to say that you can in principle analyze an experimental setup purely using Schrodinger’s equation, with no probability (except that in practice, your description of the initial state is always as a mixed state), but as you try and push your description to the point where it would capture our notion of “I measured X and got result Y”, at some point you’re going to have to give up on Schrodinger, introduce a “Copenhagen cut”, and use Born’s rule.

    The only substantial Copenhagen/Everettian difference I see is that the Everettian’s try to get rid of the Copenhagen cut, finding a way of understanding “I measured X and got Y” in a context purely governed by the Schrodinger equation. I’m an Everettian to the extent I think it’s reasonable to expect this is possible. But what I see happening when people go down this path is one of two things
    1. Serious, very interesting and difficult research about the emergence of classicality and well-defined outcomes (see for example the paper Mark Hillery pointed to).
    2. Just ignoring the problem or waving it away as essentially solved, and claiming silly things like invoking a multiverse solves the problem. This is the usual multiverse situation: theorists without a viable theory claiming that an empty idea solves their problems.

    I’m sorry to see that 2. gets a lot more public attention than 1.

  22. Peter Shor:

    This is the relevant part: “the really difficult and complicated problem of classical emergence.” Wallace’s book is all about investigating how the classical world can emerge from QM, without assuming the problematic wave-function collapse.

  23. Thomas says:

    In the mean time I did read (most of) Baggott’s book, and I have not really changed my mind regarding its merits. Some years ago I read Becker’s book, which is indeed annoying in its emphasis on the struggle between the quantum dissidents against the evil Copenhagen empire. However, the book was interesting because it contained many historical tidbits that are not well known in the physics community. Bagott’s book may be more sensible, but there is just absolutely nothing new or interesting here. And regarding him being more sensible, how about this about the Everett interpretation: “The ship of science is hurtling towards .. a dangerous whirlpool of metaphysical nonsense about the nature of reality” (this is about Everett, DeWitt and Wheeler, not Everett channeled by Tegmark).

  24. André says:

    @ Peter Woit: “… my point of view on this has always been that it’s not quantum mechanics we don’t understand, it’s classical mechanics…”

    I think this distinction is a bit mute. We understand both just fine, it is the connection of the two that is the problem. Would you say, for instance, that Boltzmann’s contribution to understand the emergence of thermodynamics from statistical mechanics improved upon our understanding of thermodynamics or of (stochastic) classical mechanics? I would argue that it was neither.

    The relevant question is, if the emergence of classicality can be understood on the basis of only quantum mechanics as it stands today (as in the case of thermodynamics from stochastic mechanics) or if there is some ingredient missing, something one would need to modify about quantum mechanics.

    I understand your preference for the first possibility, although I don’t think that the solution can come out of nonrelativistic (Schrödinger equation) quantum mechanics. But maybe there is a way to keep all of the nice mathematical structures (e.g. connections with group theory) and still understand the emergence of classicality, for instance, once gravity is properly included.

    If, however, you opt for the second possibility, then it should be out of question that the missing ingredient, the required change must be something on the quantum theory side. I don’t see a way how you can explain away the difficulties in the foundations of quantum mechanics by somehow modifying classical mechanics (in a way that would still be compatible with observations).

    Hence, in this sense I would argue against your point, and would say that the problem with our understanding is with quantum physics and not with the properties of its classical limit. But my impression is, what you actually have in mind is a derivation in a similar sense as Boltzmann’s work?

    @ Alessandro Strumia: “But we don’t have sensible deterministic theories that reproduce quantum randomness in some limit, so we don’t know where we should search.”

    Although I am not a big fan of Bohmian mechanics for other reasons, I would argue that it does give a satisfactory explanation on this issue, with the Born rule following as a corollary and the randomness coming entirely from the initial conditions, just like in stochastic classical mechanics.

    I agree with your feeling, that this is the most unsatisfactory aspect, and I hold a strong belief that the solution must be somewhere along those lines, that the stochasticity ultimately stems from the initial state of the universe, in some sense.

  25. peter hoffman says:

    Question for Peter Woit:
    Does your phrase “Others’ accounts are pretty much propaganda for … a many-worlds interpretation” apply to David Wallace’s book? Or similarly does Baggott’s “… a dangerous whirlpool of metaphysical nonsense about the nature of reality” ?
    If not, indeed in any case, it would be interesting to hear more concrete criticisms of that book now, so we needn’t wait for when your notes on this general question become available.
    That book is at least far more (mathematically) detailed than anything either Baggott or Ball has ever written as far as I know. So it is open for no end of specific criticism.

  26. Peter Woit says:

    peter hoffman,
    My reference to “propaganda” was to “popular accounts” (of which Wallace is not one), and for more about what I had in mind (and lots more discussion of “many-worlds” and Wallace) see here

    I don’t want to here start up again a serious discussion of Wallace and many-worlds, that’s both a different topic than Baggott’s book and I don’t think renewing the old discussion is going to lead to anything new. If you don’t want to read all the old stuff, the short version of my view of Wallace’s book and its attempt to argue based not on physics but on decision theory, is that it seems to me to not deal at all with the real problem (the difficult physics of the emergence of classicality). Put more bluntly, I don’t believe at all that it’s possible to solve deep problems about physical reality using the methods of social science.

  27. Nick Herbert says:

    In 1984, a book on quantum foundations with the same name as Baggott’s was published by Doubleday, sold close to 100,000 copies and is still in print. I know this because I am still receiving royalties from that book.

  28. Bob says:

    Similarly, there are many books out there titled “Quantum Mechanics”, “Quantum Field Theory” or “General Relativity”, etc.

  29. On the strength of this review, I bought a copy of Jim Baggott’s book. Once I started reading it, I found it almost impossible to put down. Very clear, well-organised, wide-ranging, balanced, exactly what a well-educated non-expert requires. It steers me back towards Copenhagen, balancing my tendency to take too much notice of Einstein.

  30. Will Sawin says:

    @Peter Woit: In the quest to understand how the classical emerges from the quantum, one could begin by setting some ground rules. Presumably some of the most fundamental questions are?

    (1) Does the quantum wave function exist? (in some objectively real sense)
    (2) Does anything except for the quantum wave function exist? (except by supervening on the quantum wave function)
    (3) Does the quantum wave function evolve under a souped-up, generalized form of Schrödinger’s equation? (and not some different evolution law.)

    I think it’s reasonable to answer “yes” to all these questions and proceed to understand how classical theories can emerge from that.

    Subjective theories of the wave function would be the prototypical rejectors of (1) (in addition to QM denialists, I guess), Bohmian mechanics is the prototypical rejector of (2), and objective collapse theories are the prototypical rejectors of (3).

    If you answer “yes” to all three, the existence of many worlds is the first, most basic, corollary. It’s immediate that you can only hope to derive a theory of multiple, branching classical worlds, with a probability measure assigned to each.

    I agree that it is silly to take this trivial observation and then claim that you have completely solved the difficult problem of classical emergence.

    I also think it is strange to accept the premises and rush off into attacking this difficult problem using lots of math without acknowledging the elementary point that there are many worlds.

  31. Peter Woit says:

    Hi Will,
    My problem is with 1, I think it’s meaningless and does nothing but obfuscate the fundamental problem.

    Personally I’m not rushing off trying to solve the problem of classical emergence,I can see why it’s extremely difficult and not something I have the energy or competence to make any progress on. If someone has a promising idea, that’s great and should be encouraged. As for those who think the reality of multiple worlds is some sort of insight needed to move forward, this proposal is now as ancient as I am (I was born in 1957, same year as Everett’s publication), and in a long career I’ve seen little worthwhile coming from it. Worse, in the last two decades I’ve seen a huge amount of damage to the field of fundamental physical theory from bogus claims that a many worlds ontology solves one important problem or another. I’ve seen such claims do a good job of selling books and convincing people not to work on important problems, seen none that they further any real science.

  32. Will Sawin says:

    Hi Peter,

    Thanks for allowing us to discuss quantum interpretations in your comments section, even though it can get silly and repetitive.

    I don’t think many worlds is necessarily an insight needed to move forward. It’s like, the fact that rocks on the moon are grey in color is not really a big insight into lunar geology, and is not really needed to move forward in any research direction for studying the moon. But it would still be strange to study moon rocks and steadfastly refuse to ever let slip that they are grey.

    I admit, though, that it’s possible that saying that there exist multiple worlds has a poisonous effect on many people’s minds that impedes scientific progress. You would certainly know more about this than me.

    How is anyone going to derive the classical from the quantum if they don’t first accept that the quantum state exists? Is the plan to describe how something that exists emerges from something that doesn’t exist?

    To me, the blog post you linked, and the paper that Mark linked, and the other paper you linked in a previous discussion from this, all look like they are starting from the assumption that a quantum state, possibly though not necessarily a pure quantum state, exists as a real physical object, and then describing why that object would “appear” in some sense to have classical behavior. If to you they look like an alternative to doing that, then probably that points the way to a more fundamental disagreement.

  33. Peter Woit says:

    My problem here is that I don’t believe the statements “the quantum state exists” or “many-worlds exist” have any substantive meaning. As such, in practice such statements shed no light on anything, and, when coupled with bogus claims that such statements solve significant problems (see e.g. Sean Carroll’s recent book) can be seriously counter-productive.

    I’m perhaps over-sensitive to this because of the huge damage done to HEP theory by similar claims about a supposed cosmological multiverse. I wrote specifically about that in the short essay “Theorists without a Theory”, but much the same argument applies to the QM case. People are claiming to have solved a deep problem with an untestable idea, and if you look into it you realize that the source of the untestability is that there is no actual theory there. Their idea provides nothing substantive that addresses the problem at hand.

  34. John Baez says:

    Will Sawin wrote:

    (1) Does the quantum wave function exist? (in some objectively real sense)
    (2) Does anything except for the quantum wave function exist? (except by supervening on the quantum wave function)
    (3) Does the quantum wave function evolve under a souped-up, generalized form of Schrödinger’s equation? (and not some different evolution law.)

    I think it’s reasonable to answer “yes” to all these questions and proceed to understand how classical theories can emerge from that.

    From the rest of what you said, I believe you think it’s reasonable to answer “no” to question (2). Or did I misunderstand?

    If someone answers “yes” to question (2), my followup question is “what?”

  35. Will Sawin says:


    If you’re not prepared to assert the quantum state exists, what does exist? Do electrons exist? Do Higgs bosons exist? Do I exist?

    You can deny that something like, say, the sun, exists, but believe that every other observable phenomenon behaves as if there were a miasma of incandescent plasma in the sky illuminating everything half the time. If so, then someone telling you that the sun exists wouldn’t really shed any light on anything. But, I mean, the sun exists, and the sun is made mainly of quarks, so quarks exist, and quarks are excitations in the state of certain quantum fields, so that state exists.

    To me it’s simpler to take the best, most mathematically elegant theory of the universe, and accept that whatever structures it describes exist, no matter how silly they sound.

    I really don’t think the analogy between many world and the multiverse is that good. One proposal has the problem that there is no good notion of measure, the other we know roughly which notion of measure to use. One has the problem that it makes no predictions, except sometimes you can get it to make predictions that contradict our observations, while the other predicts our observations beautifully, with the only drawback that the exact same predictions are also made by a different, worse, theory that was discovered earlier because of a failure of bravery and imagination (and maybe an attachment to subjectivity).

  36. Peter Woit says:

    I don’t disagree with your comments about the use of the word “exist”, I just think they’re irrelevant to the problem. The situation seems not that different to me than arguing that whether or not prime numbers “exist” is relevant and important to understanding the problem posed by the Riemann hypothesis.

  37. Will Sawin says:


    I mean, the Riemann hypothesis is a very interesting problem, but it’s a problem of mathematics, not physics. If prime numbers existed in the physical world in some clear sense, then the Riemann hypothesis might be considered a physical problem.

    If this problem of “how does the classical emerge from the quantum” is supposed to be considered a physics problem, then presumably some component of the problem has to be something that really physically exists or a reasonable approximation of something that exists. I think one can argue that if anything in the setup exists, it has to be the quantum state, and that any attempt to argue that the quantum state is an approximation to something else that doesn’t lead to “many worlds” conclusions is untenable.

    So I don’t think you need to claim that quantum states exist to solve the problem. But at some point you need to justify why you are working on this problem and not, say, relaxing on the beach. I think “I am trying to understand what everything in the universe is made of” is a compelling answer to that.

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