Follow-up questions on the set-selection problem

Physics StackExchange user QuestionAnswers asked the question “Is the preferred basis problem solved?“, and I reproduced my “answer” (read: discussion) in a post last week.  He had some thoughtful follow-up questions, and (with his permission) I am going to answer them here. His questions are in bold, with minor punctuation changes.

How serious would you consider what you call the “Kent set-selection” problem?

If a set of CHs could be shown to be impossible to find, then this would break QM without necessarily telling us how to correct it. (Similar problems exist with the breakdown of gravity at the Planck scale.) Although I worry about this, I think it’s unlikely and most people think it’s very unlikely. If a set can be found, but no principle can be found to prefer it, I would consider QM to be correct but incomplete. It would kinda be like if big bang neucleosynthesis had not been discovered to explain the primordial frequency of elements.

And what did Zurek think of it, did he agree that it’s a substantial problem?

I think Wojciech believes a set of consistent histories (CHs) corresponding to the branch structure could be found, but that no one will find a satisfying beautiful principle within the CH framework which singles out the preferred set from the many, many other sets. He believes the concept of redundant records (see “quantum Darwinism”) is key, and that a set of CHs could be found after the fact, but that this is probably not important. I am actually leaving for NM on Friday to work with him on a joint paper exploring the connection between redundancy and histories.

[Added 2015-1-6: This paper is now available: arXiv:1312.0331.]

You mention Many Worlds and David Wallace. Does this basically mean that you lean towards Many Worlds being right but that there are still details to be sorted out, or are you saying that the problem might be too fundamental and K.O. MWI ?

In general, I don’t think it’s useful to argue about whether the other branches are “real”. However, if the set-selection problem could be solved, then I would say that a MWI description is consistent with our observations, internally self-consistent, and complete. (As I said, I do not consider Copenhagen-like interpretations to be complete.) Any competing interpretation would be either (a) incomplete or (b) observationally indistinguishable from MWI.

It seems to me that you (and Zurek) are quite sure that a set of CHs will be found, but you seem doubtful of the existence of the principle needed to call QM complete?

Zurek and most many-word’ers (of which Zurek is not one!) are quite sure that a set of CHs can be found. I am slightly less sure. I have a back-of-the-enveloped calculation that says that no satisfactory set can exist, but the calculation is suspect (and does not strongly change my a priori Bayesian likelihood). If a set could be found, but no nice principle was used to find it (i.e., if it was basically just intuition or maybe even just a non-constructive existence proof), then I would be very uncomfortable, but I wouldn’t say QM was incomplete. It would be similar to how I feel about simply postulating a low-entropy initial state of the universe without explanation (see Sean Carroll).

So quantum darwinism wont replace this principle, but can only be used to find a set in the first place?

Zurek has great hope for Quantum Darwinism as a fundamental principle, without CHs playing an important role.  I believe it is a very useful hint in the search for a principle or at least a CH set, but I am skeptical that Darwinism will play a fundamental role without being substantially modified.

Where does this leave MWI ? Obviously if MWI a la Wallace is correct then there’s no doubt that the other worlds are real. But if this is contingent upon finding a principle that singles out one set does that mean you doubt MWI is correct?

Although I hate when people dismiss discussion as being “just philosophy”—since the actual way we do science is built on an implicit acceptance of all sorts of non-trivial philosophical argument—answers to questions like this one can border on meaningless.  (Although, again, it’s not always trivial to know what distinctions are meaningless. See John Bell.) Hopefully I won’t say anything stupid.

As I understand it, Wallace’s claim is that the branches of the wavefunction are just as real as a rock because, like a rock being composed of many atoms which are constantly shed and acquired, the branch structure is “emergent” from more objective/real stuff.  If (1) the wavefunction itself is real and (2) an acceptable solution to the set-selection problem exists, then Wallace is probably correct.  Addressing these in reverse order,

(2) Suppose there were many totally inequivalent (but equally justified) ways of describing the atoms in the universe of which most did not admit a useful notion of “this rock”.  Then I would certainly not be so confident that rocks were “real”, and I would probably be more inclined to say they were purely subjective (i.e a world of solipsism).  But still, I’d at least know the atoms were real.  With the wavefunction, on the other hand…

(1) I think it’s a bad idea to say “the wavefunction itself is real in exactly the same way that atoms are real”.  The wavefunction is just too similar to a purely epistemic (i.e. subjective) probability distribution for anyone to be confident that the whole thing is just as real as the part we experience.  As stressed by my great friend and colleague Godfrey Miller, the only thing that separates the wavefunction from merely a probability distribution is Bell’s inequality.

Furthermore, it’s well known that there are many observationally equivalent ways to describe the same physics (whether quantum or otherwise), some doubtlessly undiscovered, and I think it’s presumptuous and probably meaningless to declare a particular mathematical description “real”.  At the most, it’s the equivalence class of theories which are observationally indistinguishable and of equal a priori probability (because of beauty or whatever) that has the most claim to that title, if anything does.

What if one just said “all sets of CHs are real”, would that lead to a Many Many Worlds interpretation?

If the answer turns out to be meaningful, then this is a fascinating question. It is explicitly discussed by Dowker and Kent.Starting here, I previously wrote “In short: no.  Remember that a set of histories is essentially a catalog of all the worlds. According to the mere requirement of “consistency” (which is currently the only mathematically rigorous principle we have to single out sets) there are many, many consistent but mutually incompatible sets. If all the consistent sets are equally real, then this says there are way more worlds out there than the ones claimed by MWI.  So at the very least, MWI would be massively incomplete.” On 2014-8-22, I realized I missed the second “many” in the question, and so completely misunderstood it. So it was re-written. a   Yes, if you asserted that they were all real, then there would be many many worlds. But the consistency criterion is very weak, so the vast, vast majority of these worlds would have no physical interpretation. The would also have inconsistent arrows of time, etc. See also IshamC. J. Isham, Int.J.Theor.Phys. 36 (1997) 785-814 [arXiv:gr-qc/9607069.] b  .

[Minor edits on 2013-6-16, 2015-1-6, 2016-6-28.]

Footnotes

(↵ returns to text)

  1. Starting here, I previously wrote “In short: no.  Remember that a set of histories is essentially a catalog of all the worlds. According to the mere requirement of “consistency” (which is currently the only mathematically rigorous principle we have to single out sets) there are many, many consistent but mutually incompatible sets. If all the consistent sets are equally real, then this says there are way more worlds out there than the ones claimed by MWI.  So at the very least, MWI would be massively incomplete.” On 2014-8-22, I realized I missed the second “many” in the question, and so completely misunderstood it. So it was re-written.
  2. C. J. Isham, Int.J.Theor.Phys. 36 (1997) 785-814 [arXiv:gr-qc/9607069.]
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11 Comments

  1. QuestionAnswers

    Hello Riedel,

    You definitely have my permission.

    The first thing that stands out to me is that you say that Zurek is *not* a Many Worlder. How does that work given the views he hold? Do he believe in some sort of mysticism(indeterminism/antirealism) ?

    As for point one about the wavefunctions ontology, could you dumb that down yet another notch? Wallace claims something like the wavefunction only describing something he calls “Spacetime State Realism”. He seems to reject the purely “wavefunction realism” view as he states in this abstract: http://bjps.oxfordjournals.org/content/61/4/697.abstract

    I realize that the consequence of saying that CH’s being real would mean there are almost infinite worlds out there, but is there any good reason other than extravaganza to reject it? Like the probability of being in compatible sets or something?

    Last question: you don’t seem very optimistic about Many Worlds (if I have interpreted you correctly), but what other alternative seem more alluring to you?

    • > The first thing that stands out to me is that you say that Zurek is *not* a Many Worlder. How does that work given the views he hold? Do he believe in some sort of mysticism(indeterminism/antirealism) ?

      Wojciech has resisted being pinned down to any well-known interpretation, much to the chagrin of some of his colleagues. I’d guess that his flexibility here has been a positive contributor to his professional success. Section 6 of this (the “existential interpretation”) is about as explicit as he gets. He’s been strongly influenced by the views of his adviser John Wheeler, especially the philosophy of “it from bit”.

      (By the way, I would hardly categorize indeterminism as a type of mysticism!)

      >As for point one about the wavefunctions ontology, could you dumb that down yet another notch? Wallace claims something like the wavefunction only describing something he calls “Spacetime State Realism”. He seems to reject the purely “wavefunction realism” view

      Thanks for the pointer to this article. (PDF here.) It was an interesting read. I agree that, in light of how little we would trust a one-particle ontology over the N-particle ontology in classical mechanics, we should be very suspicious of naive wavefunction realism (whether over N-particle or wave configuration space). That said, I don’t find “Spacetime State Realism” at all attractive.

      But I actually don’t think this really matters for what I was discussing. Wallace is analyzing a distinction within the more general idea that “the wavefunction in some formulation is a real thing”, while I was considering alternatives like “the wavefunction is just a calculational or epistemic device, and the true ontology is something totally different”. For instance, some positivist physicists (especially those skeptical that these philosophical conversations are meaningful or useful) would say that the wavefunction is just a tool for calculating experimental predictions, and that an official ontology is totally unnecessary. Others, like Christ Fuchs, argue that the wavefunction is epistemic, in the same way that a Bayesian probability distribution describes the observer, not the universe. (“The map is not the territory”.)

      > I realize that the consequence of saying that CH’s being real would mean there are almost infinite worlds out there, but is there any good reason other than extravaganza to reject it? Like the probability of being in compatible sets or something?

      Well, let me again emphasize the distinction between all the histories in one consistent set being “real” (which would be something like MWI) and all the histories in all the consistent sets being “real”. The latter is a much, much larger number of real worlds. More troubling: there would be multiple sets of histories which would be identical up to the current time but completely incompatible afterwards. Some of these would be “quasi-classical” (i.e. describing something like what we intuitively call “worlds” containing nice classical objects like you and me) but most would not be. If all consistent sets are on equal footing, we would then be left wondering why we continue to experience a classical world. You could make some sort of anthropic argument, but—in the absence of a mathematically precise definition of an observer—it would probably be so vague as to be useless and unconvincing. Dowker and Kent discuss this in detail.

      > Last question: you don’t seem very optimistic about Many Worlds (if I have interpreted you correctly), but what other alternative seem more alluring to you?

      I assign ~80% probability that something observationally indistinguishable from MWI is correct. (Conditional on that, I expect with probability ~50% that there is a satisfying set-selection principle that will be found, which would be a refinement/improvement on MWI.)

      I think it’s unlikely that MWI will be flat-out wrong without throwing out QM completely. I consider standard Bohmian mechanics (in “quantum equilibrium” to be an awkward but equivalent interpretation, while I consider Bohmian mechanics outside of quantum equilibrium and experimentally-verifiable objective collapse theories to be departures from QM; I also consider them extremely improbable as-is.

      I secretly hope that QM is wrong, but I have absolutely no idea what would replace it. As Scott Aaronson has argued, QM is a (bafflingly robust) island in theory space.

      • Damn, I had typed out a long reply and then I x’ed out the wrong window. Well here we go again:

        I must admit that I have never really grasped the “It from Bit” debate.
        To me it seems weird to imagine information as something fundamental before *something* itself.
        The 2013 FQXi essay contest is about whether It from Bit or Bit from It.
        Is Zurek submitting an entry?
        So basically Zurek’s view is that Many Worlds is true *unless* we have got the fundamentals of realism wrong and It from Bit is the way?

        I honestly find indeterminism appalling and bordering on antirealism. The idea that particles are randomly doing X, Y, Z but within the Born probability rule is just crazy talk to my ears. Either the Universe is 100% free (and so Born Rule would be non-existent) or 100% determined, that I think will be my dying opinion.

        You mention that you find Wallace’s proposal unattractive eventhough you agree with him that “pure WF realism” is pretty bad and unlikely. What is it about Wallace’s idea that is unconvincing to you?
        I am under the impression that most Everett adherents adhere to Wallace and the few who don’t usually hold pretty “far out” views of Everett worlds.

        As for Chris Fuchs, isn’t this view sort of anti-realism? Unless he postulates that there is a “hidden variable world” then how does he ground quantum mecanics in reality if the wavefunction is only a mathematical tool?

        Regarding the Many Many Worlds idea, I had written a long paragraph before I x’ed out the window, but suffice to say: I agree that it looks terrible and I guess it’s just wrong 🙂
        Are there anyone (that you know of) that actually take that serious and try to argue for it using some anthropical principle?

        Your ~80% probability for something Many World-esque was surprisingly high. What represents the 20% doubt?
        Most seem to put it at 50/50 in my experience on different forums etc. mainly because they don’t see a way for the Born Rule to ever be recovered in Many Worlds, does this mean you are much less worried about the Born Rule?

        I agree that QM definitely seems pass every test we put it through, but there are definitely some people who think the issues with General Relativity and Quantum Mechanics is enough evidence to doubt QM.

        Steven Weinberg , Anthony Leggett, Gerard ‘t Hooft are 3 great science giants that have expressed this belief in the later years.
        Gerard ‘t Hooft is perhaps the one who has been most vocal about it. He even participate on physics.stackexchange.com about it 😛
        You can read about his unorthodox ideas here: http://www.staff.science.uu.nl/~hooft101/

        So while QM definitely seems robust there is no solution to this problem yet, which seems to be at least weak evidence for the need of supplementation?

        • I’m not sure that It From Bit is anything like a “precise” philosophical claim. I don’t know Wojciech’s opinion except to say that he believes the quantum information *thinking* is important, i.e. it is very useful to approach the world from the point of view of correlations, entropy, and redundancy.

          > Is Zurek submitting an entry?

          I don’t know.

          > So basically Zurek’s view is that Many Worlds is true *unless* we have got the fundamentals of realism wrong and It from Bit is the way?

          No, I don’t think so. I don’t even know what that would mean. It’s not useful to think of some list of N mutually exclusive interpretations, with each physicist having to subscribe to one or the other.

          >What is it about Wallace’s idea that is unconvincing to you?

          It just looks horrendously ad hoc. Now the density matrices are the real things? If you want to keep all the branches of the wavefunction, I don’t see how concentrating on local density matrices makes anything less ridiculous.

          > I am under the impression that most Everett adherents adhere to Wallace and the few who don’t usually hold pretty “far out” views of Everett worlds.

          I think you’re under the wrong impression. This paper of Wallace’s was from 2009. Of the few hundred (a thousand?) physicists worldwide who one could say work in quantum foundations, you really think they all dramatically changed their beliefs in the past 4 years? Hardly. 95% aren’t even aware of the paper’s existence.

          You should think of the physicists as coming to their own opinions, and then only maybe 5% of them as caring at all about what some philosopher like Wallace thinks. Of those 5%, most probably just point to a few of his pieces (not that man himself) as a rough approximations to their thinking. Wallace is a smart guy, but hardly the standard bearer.

          And if you’re just concentrating on the handful of philosophers of physics out there, well, then I don’t really care what their majority opinion is. If they don’t have a strong physics training (like Wallace), they are empirically unlikely to say something I would find insightful about quantum mechanics.

          Maybe another thing I should say, at the risk of sounding condescending (which I don’t mean to be): when I read your questions, I get the sense that you take the words people have said about quantum mechanics much too seriously. For example: just because some philosophers made a distinction between two interpretation, does not mean that this distinction is necessarily meaningful, especially when there is so much disagreement. The map is not the territory, etc.

          There is a reason that most physicists look down upon quantum foundations: most of the stuff people have said is meaningless BS.

          >As for Chris Fuchs, isn’t this view sort of anti-realism?

          I don’t know. I have never tried to sit down and understand Fuch’s view.

          > Are there anyone (that you know of) that actually take that serious and try to argue for it using some anthropical principle?

          I believe the “Many Minds” interpretation may something about that. Not sure.

          > Your ~80% probability for something Many World-esque was surprisingly high. What represents the 20% doubt?

          Gut feeling. Even after learning about it, I don’t like it and I feel confused. The 20% does not represent anything concrete. My inability to formulate a coherent alternative is extremely weak evidence that none exists. The inability for the rest of humanity to do so it only mildly stronger evidence.

          > Most seem to put it at 50/50 in my experience on different forums etc.

          You are reading *way* to much into these numbers if you think the difference between 50/50 and 80/20 is meaningful. Those numbers are going to be determined more by what people had for breakfast that morning than anything substantive. We’re debating extremely fuzzy and ill-defined metaphysical claims, after all.

          > mainly because they don’t see a way for the Born Rule to ever be recovered in Many Worlds, does this mean you are much less worried about the Born Rule?

          Yes

          > So while QM definitely seems robust there is no solution to this problem yet, which seems to be at least weak evidence for the need of supplementation?

          I think most people have problems with QM because of QM. Then, they go looking for places where QM could maybe be wrong because they already want it to be wrong. And gravity is one sexy place to do this. But I don’t think anyone actually said “you know, I thought QM was complete and solid, but then I heard about gravity and realized that maybe the interpretation of QM is connected to that.”

          So basically, there are two problems: (1) the interpretation of QM and (2) gravity. Beyond the fact that both exist, the only connection between them that I can think of off the top of my head involves thermo and the arrow of time. (Closed time-like curves in particular have some very interesting properties in QM that touch on the measurement problem.) I suppose this is weak evidence for a connection, but it’s pretty weak.

          • QuestionAnswers

            Thanks once again.

            The reason I have this impression of Zurek is a clip from CloserToTruth where he basically seems to be 99% for Many Worlds, but then mentions at the end that It from Bit might be worth thinking about as can be seen [url=http://www.closertotruth.com/video-profile/What-is-a-Theory-of-Everything-Wojciech-Zurek-/1617] here [/url]

            As for Wallace’s idea: it seems to have won some skeptics over. I’m not going to waste a lot of your time by asking the same questions over and over, but is there anything specific about the idea that density matrixes are real that puts you off? You call it “ridiculous”, but what exactly is ridiculous about it ?

            And I don’t mean that *that* specific paper is Wallace’s most influential one. But when you browse through the most cited papers on quantum foundations (especially realist ones) in the last decade, then Wallace seems to be the most cited (after Everett) on Many Worlds. Especially his work on the preferred basis problem, which he claims to have solved through “emergence”, is brought up again and again by Everett proponents.
            He has also published 2 big books on the Many Worlds interpretation so I think everyone looks to him as “the” authority.
            As for his specific 2009 proposal that he seems to still be sticking with, I have yet to find anyone critizing it (except you :))

            Most people object to Many Worlds and state problems with the Born Rule as their main reason. If you’ve spent any time over at LessWrong or OvercomingBias you will know that even people who are fairly certain that Many Worlds is correct (Robin Hanson, Eliezer Yudkowsky) accept that the Born Rule is the biggest problem. Hanson has attempted a solution, but I don’t think it’s very promising.
            The Decision-Theoretic approach by Deutsch-Wallace-Saunders has been hailed as “the solution” in popsci magazines and seems to have won over quite a few skeptics too. But most still remain very skeptical. Huw Price, David Albert, Adrian Kent, Peter J. Lewis, Richard Dawid, Alastair Rae and the list goes on and on, these people have all written extensively against the decision theoretic “solution” and I think I agree with them. I find the idea of “measure” of someone to be ridiculous.
            So what position do you land on here? Do you buy the decision theory programme or do you have a different approach?
            Ultimately I think the Born Rule will be the deciding factor for most people so hearing as many differing opinion as possible is illuminating.

            I flat out admit that I am definitely influenced by brighter and more informed minds than mine. But I always try to flesh out a persons opinion. I am not a big fan of just accepting a persons opinion without trying my best to actually understand it. Which is why I ask you these “mundane” questions as I try my best to grasp the core of your opinion on this subject.

            Re: Many Minds interpretation. I actually had the pleasure of asking M Donald about it a few years ago and it seemed to me that his biggest objection to Many Worlds and thus motivation for Many Minds is the preferred basis / failure of ontology.

            As for reading too much into the difference between 20/80 and 50/50, I want to back that up with a distinction.
            You say your 20% estimate is basically pure gut feeling and not something you can back up with criticism of the Many Worlds intepretation itself. Those who say 50/50 usually have very precise criticism of the interpretation, like the Born Rule. Someone like Adrian Kent consider Many Worlds highly unlikely because A) It cannot account for the Born rule and B) Set selection problem. So he would probably give Many Worlds less than 5% of being correct and those 5% would represent the sober attitude that perhaps something revolutionary that we cannot even imagine right now will make it work. So I do think there is a important difference.

            As for your last point: I think you are partially correct, I do believe a lot of people are really puzzled by QM and therefore go seeking elsewhere for a solution. But people like Gerard ‘t Hooft comes in from the other end. He states that his motivation comes from blackholes basically. Nima Arkani-Hamed has suggested that quantum mechanics may emerge together with space-time just recently too and he has never expressed any problems with quantum mechanics. So I do think quite a few are very sincerely motivated by the issue of gravity and space-time itself.

          • Hey, sorry it took so long to reply. It’s been very busy on this end.

            > As for Wallace’s idea: it seems to have won some skeptics over.

            Who? Most physicists have barely read Wallace, if they’ve heard of him at all. Even among the peopel who say they work on foundations (which already selects for non-mainstream), I bet half haven’t heard of him. So are you talking about philosophers?

            > is there anything specific about the idea that density matrixes are real that puts you off? You call it “ridiculous”, but what exactly is ridiculous about it ?

            Off the top of my head, I’d say because of the original density matrix’s instrumental origins. It is essentially defined as the mathematical object which contains all information necessary to predict experiments run by a local observer. It would be very convenient that this turns out to be the form of the underlying reality.

            Moreover, the particular construction of Wallace is even weirder than this. Wallace’s ontological density matrix is just an object constructed by tracing out everything further away than some (totally arbitrary) radius. This isn’t anything that’s at all accessible to a realistic observer, since observers will be confined to branches while the density matrix described by Wallace has contributions from all the branches. And these various space-local (but not branch-local) density matrices interact in a way that is *much* more natural from the standpoint of wavefunction realism.

            It doesn’t feel like we “discovered” these density matrices in the satisfying way that I would demand from philosophy (in order for the philosophy to justify itself as more than just words stretched over physics). It just feels like what’s left over if you decide you really don’t like the non-locality of wave-function realism and decide to start tracing out far away parts of the universe to fix that.

            > And I don’t mean that *that* specific paper is Wallace’s most influential one. But when you browse through the most cited papers on quantum foundations (especially realist ones) in the last decade, then Wallace seems to be the most cited (after Everett) on Many Worlds.

            Right. Wallace is the best-known philosopher of quantum physics, a fact I have never disputed. That doesn’t mean this particular proposal is right *or* accepted by anyone. (To stress: I’m not just challenging the idea that it’s his most influential proposal. I’m challenging the idea that anyone else besides him and close collaborators subscribe to it.)

            > Especially his work on the preferred basis problem, which he claims to have solved through “emergence”, is brought up again and again by Everett proponents.

            Well, I think my advisor is the one who solved the preferred basis problem, if any one person can be said to have done so. (Zeh is probably #2.) Wallace simply argued that this solution is all that should be philisophically demanded in order to consider quantum mechanics complete. (Hence the comparison to tigers as another emergent structure, etc.)

            > He has also published 2 big books on the Many Worlds interpretation so I think everyone looks to him as “the” authority.

            Nope. People certainly look to him as a smart guy, and probably the best philisophical thinker on the subject. But you may be greatly overestimating the degree to which philosophers agree, and the degree to which physicists have even *heard* of him. There’s just nothing like an authority on these matters.

            > As for his specific 2009 proposal that he seems to still be sticking with, I have yet to find anyone critizing it (except you :))

            Who cites it? I count only 27 cites on Google Scholar in 4 years. How many are critical? My PRL has had 14 cites in 3 years, and it’s hardly known by anyone at all!

            Philosophy (and academics in general) is a cacaphony of ideas and papers. Most papers that attract little criticism do so because no one knows they exist. You certainly cannot presume acceptance when observing lack of criticism.

            > Most people object to Many Worlds and state problems with the Born Rule as their main reason.

            Who are these people? This is not the consensus in the physics community. I’m suspicious that the philosophers have decided that the Born Rule is the main worry, considering that philosophers are not nearly as mathematically handicapped on it as they are on the preferred basis problem.

            > If you’ve spent any time over at LessWrong or OvercomingBias you will know that even people who are fairly certain that Many Worlds is correct (Robin Hanson, Eliezer Yudkowsky) accept that the Born Rule is the biggest problem. Hanson has attempted a solution, but I don’t think it’s very promising.

            I greatly respect Yudkowsky and Hanson, but these are not people you should be trusting to give you expert advice on physics. For instance, Hanson’s attempt at deriving Born Rule from the counting measure was cute (really, it was neat!), but it also revealed that he is completely out of his element. It was based fundamentally on the mistaken assumption that branching is *discrete*. (This is necessary for the counting measure to be employed.) In fact, most branching is continuous, and discreteness *cannot* be salvaged by appealing to a hypothetical discretization of space.

            Note also: if the preferred basis problem really *were* the biggest problem with Many Worlds, and the Born Rule were just a side problem that existed independent of Many Worlds, then you would expect people who accepted Many Worlds to predominantly think the preferred basis problem had been solved and the Born rule was the only problem left. But that’s just a selection effect. Consider: Does anyone accept that the preferred basis problem is solved, but *reject* Many Worlds because of the Born Rule? I know of no such person. Mostly it’s people who already accept Many Worlds, and who think they can finally complete QM by deriving the Born Rule.

            > The Decision-Theoretic approach by Deutsch-Wallace-Saunders has been hailed as “the solution” in popsci magazines and seems to have won over quite a few skeptics too. But most still remain very skeptical. Huw Price, David Albert, Adrian Kent, Peter J. Lewis, Richard Dawid, Alastair Rae and the list goes on and on, these people have all written extensively against the decision theoretic “solution” and I think I agree with them. I find the idea of “measure” of someone to be ridiculous.
            So what position do you land on here? Do you buy the decision theory programme or do you have a different approach?

            I think Adrien Kent’s damning contribution to the book “Many Words?” is spot on. The fact that a titan like Deutsch and very smart guys like Wallace and Saunders are so wrong should give you pause before putting too much faith in these kind of authorities. We are all lost and wandering in the desert.

            > I flat out admit that I am definitely influenced by brighter and more informed minds than mine. But I always try to flesh out a persons opinion. I am not a big fan of just accepting a persons opinion without trying my best to actually understand it. Which is why I ask you these “mundane” questions as I try my best to grasp the core of your opinion on this subject.

            Great! We’re in agreement. And it’s not mundane at all!

            > As for reading too much into the difference between 20/80 and 50/50, I want to back that up with a distinction.
            You say your 20% estimate is basically pure gut feeling and not something you can back up with criticism of the Many Worlds intepretation itself. Those who say 50/50 usually have very precise criticism of the interpretation, like the Born Rule. Someone like Adrian Kent consider Many Worlds highly unlikely because A) It cannot account for the Born rule and B) Set selection problem. So he would probably give Many Worlds less than 5% of being correct and those 5% would represent the sober attitude that perhaps something revolutionary that we cannot even imagine right now will make it work. So I do think there is a important difference.

            I guess. In the sense that Adrian Kent is a smarter guy than me (he is) and has spent more time thinking about this than I have (he has), then yes, please do weigh his opinion more. But keep in mind that, *conditional* on most hand-wringing about the Born Rule in fact being pointless BS, the people who have participated extensively in such hand-wringing are *more* likely to be confused than those people who read the hand-wringing and dismissed it.

            > As for your last point: I think you are partially correct, I do believe a lot of people are really puzzled by QM and therefore go seeking elsewhere for a solution. But people like Gerard ‘t Hooft comes in from the other end. He states that his motivation comes from blackholes basically. Nima Arkani-Hamed has suggested that quantum mechanics may emerge together with space-time just recently too and he has never expressed any problems with quantum mechanics. So I do think quite a few are very sincerely motivated by the issue of gravity and space-time itself.

            I don’t really believe ‘t Hooft. His worries about blackholes may or may not be right, but I was pretty unconvinced by his argument that these worries point to something wrong with QM. Then again, I’ve only quickly read through a couple of his papers on the subject. Maybe I missed something.

            I don’t know what Nima has written lately, but I’d be very surprised if it’s actually a departure of QM in any meaningful sense.

          • There is at least one skeptic that Wallace seems to have “won over” after being decades-long opponent of the Everettian view: Peter J. Lewis
            As you can read in his review of David Wallace’s “The Emergent Multiverse” book here (ctrl + f : chapter 8) http://ndpr.nd.edu/news/38878-the-emergent-multiverse-quantum-theory-according-to-the-everett-interpretation/

            Though Lewis still reserves that he thinks that the Born Rule problem might still be a lethal problem for MWI, it seems that he is now pretty content with the ontology and emergence that Wallce has brought forth compared to what he was pre-this proposal.

            The reason I tend to put philosophers of physics a notch above the actual physicists when it comes to the measurement problem debate is simply that it is a very philosophical problem.
            Most physicists just use the math of copenhagen and can do their job just fine and don’t particularly care about whether or not there are deeper mechanisms or some sort of many worlds structure.

            As for philosophers/physicist who reject MWI eventhough they consider the Preferred Basis issue solved? I guess Huw Price, Alastair Rae, Adrian Kent, Peter J. Lewis (at least he still considers it *the* reason MWI can’t be said to be right) because of the Born Rule?
            I could go on with quite a few names, but at least each of these has written several papers on why they reject the proposed solutions to the Born Rule issue.

            But let’s say Wallace is completely wrong about Space State Realism. Where does that leave MWI?
            Doesn’t the question about non-locality pop right back into the Everettian QM, and not just in the sense of “non-separability” as in Wallace’s proposal, but genuine relativistic-problematic non-locality?
            Also the Born Rule: unsolved and attempts like the decision theory “solution” is fallicious, Robin Hansons “mangled worlds” proposal is definitely a dead end and so on.
            I honestly don’t think that a “final theory” of any kind can just postulate something as important as the Born Rule.

          • Comment box is getting too thin. I’ve replied at a top-level comment here: http://blog.jessriedel.com/follow-up-questions-on-the-set-selection-problem/#comment-528

  2. QuestionAnswers

    I forgot to ask about your “back of envelope” calculation.
    Have you confronted anyone with it and got any responses and what is it really based on?

    • Yes, I’ve discussed it with Jim Hartle, Scott Aarronson, Adrien Kent, and Charlie Bennett in varying degrees of depth. As is unfortunately typical for these kinds of conversations, it generally ended with something equivalent to “That’s a very interesting claim, but I don’t understand the basics enough to critique it. All I can say now is that it isn’t obviously wrong, but at the same time I haven’t much shifted my priors. Tell me when you write up the formal paper.”

      It would take much too long to write it up in detail here, but this is the basic idea. We all seem to agree that some “branching events” (e.g. one run of a Stern-Gerlach experiment) are objective. Further, these objective branching events must correspond to the appropriate histories within the CH framework and, in particular, all branches must be orthogonal. The Hilbert space of the universe is effectively (if not formally) finite dimensional. Thus, we can only run a finite number of Stern-Gerlach experiments (or whatever). There either must be some point in the future where classicality dissolves or something really dramatic must save it, e.g. an infinite heirarchy of new environments, or a departure from QM. I think this time might be surprising short.

      Scott Aarronson had discovered this basic idea independently of me several years prior, but I think it’s safe to say I’ve fleshed it out much more than he has.

      The most intriguing response was from a joint conversations with Adrien and Charlie, where it was suggested that the time when you run out of space would be exactly the same time that the universe thermalizes. This would have been in conflict with my back-of-the envelope calculation, but we were all much too confused to root out where the two arguments diverge. If it’s true, it would be extraordinarily interesting in its own right.

      EDIT: Also, I’ve now discussed it was Graeme Smith. No new breakthroughs…

  3. [In reply to this. Posting here as a top-level comment because the nested reply box was getting too skinny.]

    > The reason I tend to put philosophers of physics a notch above the actual physicists when it comes to the measurement problem debate is simply that it is a very philosophical problem.
    Most physicists just use the math of copenhagen and can do their job just fine and don’t particularly care about whether or not there are deeper mechanisms or some sort of many worlds structure.

    I agree that the reason that most physicists have not contributed to progress is because they are not good enough at philosophy. But likewise, most philosophers have also not contributed because they are not good at physics. But there is difference. First: most physicists, by virtue of being human, have at least some rudimentery philisophical skills. But most philosophers have essentially zero physics skills. Second: It’s an empirical fact that it’s easier to start in a technical field and transition to a non-technical one, than vice versa. This is why the best philosophers of physics (e.g. David Wallace) started as physicists and only later became philosophers.

    Progress is going to come from people with training in both. You say you put philosophers above physicists in part because most physicists just blindly use copenhagen, but why don’t you put physicists above philosophers because most philosophers concentrate on ethics or epistemology rather than worry about the Born Rule? Only a small subset of each field concentrates on this interdiciplinary topic. You should be looking to physicists who take quantum foundations seriously (e.g. the folks at Perimeter Institute), just as much as you ignore philosophers who say BS about postmodernism.

    Incidentally, Robin Hanson’s Born Rule derivation is a good example of an idea that could never have been proposed by a philosopher, because it is too technical. And yet it turns out that this idea is a non-starter for reasons that Hanson would not have known without still further training. If Hanson’s idea turned out to be right, *there would be no way for it to every be discovered without physics training*. There’s just no substitute.

    > As for philosophers/physicist who reject MWI eventhough they consider the Preferred Basis issue solved? I guess Huw Price, Alastair Rae, Adrian Kent, Peter J. Lewis (at least he still considers it *the* reason MWI can’t be said to be right) because of the Born Rule?

    The only name on that list I know of is Adrien Kent, and as I explained in my blog post he does *not* think the preferred basis problem has been solved. Presumably, the others are philosophers who (as I alluded to in my previous comment) simply do not understand enough physics to actually understand the preferred basis problem. Instead, they choose to concentrate on the topic they think they can contribute to. [Just went back and checked after writing this, and Alastair Rae is the only other physicist. So you were wrong about Kent, but you have identified one person who potentially understands enough about physics to understand the preferred basis problem and declare it solved, while also declaring Born’s Rule unsolved. You’ll forgive me for not being too influenced by this one guy I haven’t heard of.]

    > I could go on with quite a few names, but at least each of these has written several papers on why they reject the proposed solutions to the Born Rule issue.

    If you know more names of physicists off the top of your head who consider Preferred Basis solved but reject MWI because of Born’s Rule, I’d be very interested. But I certainly don’t mean to give you homework, and I won’t consider you to have conceded this point if you choose not to waste your time.

    > But let’s say Wallace is completely wrong about Space State Realism. Where does that leave MWI?

    If the prefered basis problem is solved, then (as you might guess) I personally don’t see too much more to worry about. Debating over what’s real sounds like debating over whether the charges or the fields (or both) are real in *classical* E&M. In that case, there is no canonical answer. You can choose to “integrate out” either of them, and talk about only the other. But that doesn’t seem to convince me of much! If this simple classical case has no good answer, why would we think we can make progress on the quantum case where there are many more places to be confused?

    > Doesn’t the question about non-locality pop right back into the Everettian QM, and not just in the sense of “non-separability” as in Wallace’s proposal, but genuine relativistic-problematic non-locality?

    Nope. Relativistic causality is a classical concept (or rather, it’s based in a world where there’s only one branch). You can extend this to a mathematical restriction in the case of a universal wavefunction. The known workings of quantum mechanics easily satisfy it, and no one disputes this. It’s just that, with a branching universes, relativistic causality simply has a more complicated interpretation than it does in a classical universe. The apparent violation of causailty from EPR pair experiments is rooted in demanding that there is no branching, i.e. there is objective collapse with definite outcomes. If you eliminate this demand by accepting a universal wavefunction, then the issue with causality evaporates beautifully. (In fact, this beautiful evaporation—or maybe dissolution, to use Yudkowsky’s terminology—was one of the things that convinced me that thinking about things in terms of a universal wavefunction was the right choice.)

    > I honestly don’t think that a “final theory” of any kind can just postulate something as important as the Born Rule.

    Well, I think careful reflection will show that your real intuitive objection to the Born rule is actually rooted in either (a) your objections about probability in general (which are reasonable and have been shared by other philosophers for centuries, but which have little hope for progress) or (b) treating the wavefunction as real in the same way that a rock is real (which I have cautioned against). There isn’t really an efficient way to explain the relevant arguments in this box, so email me if you want to set up a time to discuss it by gchat: jessriedel at gmail dot com.

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