Hypnotized by Quantum Mechanics

It remains embarrassing that physicists haven’t settled on the best way of formulating quantum mechanics (or some improved successor to it). I’m partial to Many-Worlds, but there are other smart people out there who go in for alternative formulations: hidden variables, dynamical collapse, epistemic interpretations, or something else. And let no one say that I won’t let alternative voices be heard! (Unless you want to talk about propellantless space drives, which are just crap.)

So let me point you to this guest post by Anton Garrett that Peter Coles just posted at his blog:

Hidden Variables: Just a Little Shy?

It’s quite a nice explanation of how the state of play looks to someone who is sympathetic to a hidden-variables view. (Fans of Bell’s Theorem should remember that what Bell did was to show that such variables must be nonlocal, not that they are totally ruled out.)

As a dialogue, it shares a feature that has been common to that format since the days of Plato: there are two characters, and the character that sympathizes with the author is the one who gets all the good lines. In this case the interlocutors are a modern physicist Neo, and a smart recently-resurrected nineteenth-century physicist Nino. Trained in the miraculous successes of the Newtonian paradigm, Nino is very disappointed that physicists of the present era are so willing to simply accept a theory that can’t do better than predicting probabilistic outcomes for experiments. More in sorrow than in anger, he urges us to do better!

My own takeaway from this is that it’s not a good idea to take advice from nineteenth-century physicists. Of course we should try to do better, since we should alway try that. But we should also feel free to abandon features of our best previous theories when new data and ideas come along.

A nice feature of the dialogue between Nino and Neo is the way in which it illuminates the fact that much of one’s attitude toward formulations of quantum mechanics is driven by which basic assumptions about the world we are most happy to abandon, and which we prefer to cling to at any cost. That’s true for any of us — such is the case when there is legitimate ambiguity about the best way to move forward in science. It’s a feature, not a bug. The hope is that eventually we will be driven, by better data and theories, toward a common conclusion.

What I like about Many-Worlds is that it is perfectly realistic, deterministic, and ontologically minimal, and of course it fits the data perfectly. Equally importantly, it is a robust and flexible framework: you give me your favorite Hamiltonian, and we instantly know what the many-worlds formulation of the theory looks like. You don’t have to think anew and invent new variables for each physical situation, whether it’s a harmonic oscillator or quantum gravity.

Of course, one gives something up: in Many-Worlds, while the underlying theory is deterministic, the experiences of individual observers are not predictable. (In that sense, I would say, it’s a nice compromise between our preferences and our experience.) It’s neither manifestly local nor Lorentz-invariant; those properties should emerge in appropriate situations, as often happens in physics. Of course there are all those worlds, but that doesn’t bother me in the slightest. For Many-Worlds, it’s the technical problems that bother me, not the philosophical ones — deriving classicality, recovering the Born Rule, and so on. One tends to think that technical problems can be solved by hard work, while metaphysical ones might prove intractable, which is why I come down the way I do on this particular question.

But the hidden-variables possibility is still definitely alive and well. And the general program of “trying to invent a better theory than quantum mechanics which would make all these distasteful philosophical implications go away” is certainly a worthwhile one. If anyone wants to suggest their favorite defenses of epistemic or dynamical-collapse approaches, feel free to leave them in comments.

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102 Responses to Hypnotized by Quantum Mechanics

  1. Mike W. says:

    Agreed, except that until we have a satisfactory way of reconciling the Born rule with the underlying MW picture, it’s a bit of an overstatement to say “it fits the data perfectly”. The various claims about deriving the Born rule are all of the form “if the Born rule weren’t true, then no reasonable stable probabilities would emerge”. True, but not sufficient. Imagine a classical physicist saying “If black-body radiation is not finite, then all hell breaks loose. So our theory must give finite black-body radiation.” That’s basically how people proceeded for a while, but of course it wasn’t right.
    There are some interesting ideas about how the Born rule might emerge. Jacques Mallah has some thoughts on how to count relative numbers of different mental experiences given a quantum state which includes some background quantum noise. And of course, the Many Interacting Worlds, crew has another idea, involving a seriously modified theory.

  2. vmarko says:

    Hi Sean,

    Regarding MWI, my fear is that the problems of classicality and the Born rule may end up being more than merely technical, but rather to require additional postulates (equivalent in power to the collapse postulate and the Born rule postulate from standard von Neumann’s interpretation of QM). And if that turns out to be true, adding those new postulates will destroy the “ontological minimality” of MWI, i.e. make it less parsimonious. Such a situation would make MWI less preferable than it appears now, compared to other interpretations.

    That said, I must say I am very pleased to see you let alternative voices be heard on your blog. It is very important not to get indoctrinated in one’s preferred QM interpretation, and help the audience to keep an open mind. Commendable post!

    Best, 🙂

  3. Sean Carroll says:

    Just so nobody forgets, I think there is more than one perfectly good way of deriving the Born Rule in MWI. Emergence of classical worlds seems to me to be a much more pressing problem, given the current state of the art.


  4. Anton Garrett says:

    Thanks for the link and comments Sean. Given your advocacy of many-worlds it would be rude of me to repeat my arguments against that view here, but I can assure your readers that the subject is treated in my essay at Peter’s blog, to which you link. Ultimately all I am doing is demanding that it is OK, and is the task of physicists, to ask the question: Which detector will go off the next time a particle passes through my double Stern-Gerlach setup? Surely a modest request…

  5. It is always advisable (IMHO) to remember (or to warn readers) that there is that view saying it is consciousness that is central to the quantum mechanics mysteries and workings, as I think the authors held in the book: “Quantum Enigma: Physics Encounters Consciousness, by Bruce Rosenblum and Fred Kuttner (2011).”

  6. I’m eager to hear your thoughts in this context on the recent work on non-linear mechanical 2D “bouncers” and “walkers” that seem to emulate many quantum features, including the double slit and bound states, through mechanical systems (something akin to pilot waves). Do these systems offer any real insight into quantum mechanics or is it just a nice analogy? For example, see the work of John Bush et al. at MIT:




    P.S. A personal aside:
    What I have been telling colleagues is that, yes, mechanical analogies are helpful. Also, these are neat systems. It is very cool that bizarre behavior previously thought to be unique to the quantum world do have a mechanical analog. However, with these systems, if taken too literally as a quantum alternative, are often trading one mystery (the weirdness of quantum mechanics) for another (non-local hidden variables pilot-like waves). The latter being non-linear and usually analytically intractable, the former being linear and (frequently) tractable. One is still left with the question “what is waving and why are the mechanical properties of this wave so weird.” Also, pilot-waves and other non-local hidden variable theories don’t seem to offer any new predictions and involve much more analytical overhead than quantum theory. Even something fairly simple like the double slit needs to be modeled numerically. There aren’t straightforward ways to introduce extra internal quantum numbers like spin, color, flavor, etc. While they appeal to the intuitive mechanical “push and pull on point particles” way of thinking, they also seem to introduce unintended consequences and implications that are even weirder than the usual quantum weirdness. For example, if you introduce charge particles that follow non-linear classical trajectories, why don’t they radiate? It reminds me a bit of the Ether where, yes, you can twist and turn intellectually to rationalize this wacky mechanical material with a laundry list of paradoxes and crazy properties contorted to fit the experiments, but isn’t relativity so much more elegant?

  7. Sean Carroll says:

    Anton– Thanks. (And I fixed your comment.) Yes, of course the subject is treated in your post! And of course you are welcome to ask such a question — but you are not allowed to demand that Nature provides a unique answer.

  8. walt says:


    I’m glad you continue to believe in and advocate a worldview based on
    an infinite number of worlds that we cannot detect and which doesn’t
    make any unique predictions. You’re religious now, yes?

  9. David Thiessen (SE Asia SciAm Bright Horizons 19) says:

    OFF QM TOPIC: Congrats Sean on winning the FFRF Emperor Award. I thoroughly enjoyed the transcript of your talk which has added to my trove of ammunition against my heaven bound extended family which they anticipate to be a place of eternal happiness — sounds boring to me. I’m looking forward to the Carroll, Novella, Alexander Moody Intelligence^2 on Death is Not Final on YouTube.

    ON QM TOPIC: ” “trying to invent a better theory than quantum mechanics which would make all these distasteful philosophical implications go away” is certainly a worthwhile one.” What is like about QM is that every prediction made by QM that has been testable by experiment has supported the theory. That is the sign of a very good theory. What I love about science that it is self-correcting over time in the light of new evidence and if new evidence points to a better theory than QM, scientists will embrace it in far less time than it took the Catholic Church to embrace helio-centrism ( >200 yrs). Ha!

    I enjoy your Blog Sean and those of your Guests — I always learn something (even when I don’t completely understand — can’t unring a bell)

  10. Since you are on the subject of alternate interpretations of QM, Dr. Carroll, I’ve been interested in learning more about Two-State Vector Formalism for a while, now, as I find the idea of retrocausality to be extremely interesting; unfortunately, I’ve had a great deal of difficulty finding any information on TSVF geared toward laypersons.

    Are you familiar with TSVF? Would you happen to know of any introductory material which would be appropriate for those of us with only a rudimentary understanding of the Schroedinger equation?


  11. Sean Carroll says:

    Don’t know much about the TSVF, but it strikes me as a way to calculate things, not really a competing formulation of QM in the usual sense.

  12. Jesse says:


    Is the mwi testable, in principle?

  13. Sean Carroll says:

    Of course. It’s tested every time you do an interference experiment, or look for evidence of dynamical collapse, or look for the influence of hidden variables.


  14. Jesse says:


    Can’t the same be true of other interpretations? If not, then why is there still a discussion about the right way to think about quantum mechanics? Why isn’t everyone on board?

  15. Ray Gedaly says:

    Two possible explanations of the apparent non-locality (spooky action at a distance) of two entangled particles:
    1. The particles remain localized in an extra spatial dimension like that posited in string theory.
    2. The two particles retain proximity on the lower-dimensional surface of the speculated holographic universe.

  16. Jesse says:


    Are there other interpretations besides the Copenhagen Interpretation that agrees with your statement? What’s wrong with the postulates of the Copenhagen interpretation? Must nature obey our aesthetics? What’s wrong with just saying that the purpose of physics is to find the correct mathematical description of the universe or the correct rules. Interpreting the mathematics is our doing and is not relevant. What if the universe actually does work according to Copenhagen? That also agrees with experiments.

    How do you distinguish between the two experimentally? It seems taking on an infinity of worlds is a big price to pay for mathematical simplicity, or economy of postulates.

  17. Sean Carroll says:

    Deleted some off-topic nonsense. If it continues I’ll ban the offenders.

  18. Robin Herbert says:

    Does Everettian Quantum Mechanics imply that there are infinitely many multiverses, if eternal inflation is also the case?

    When people talk of other universes, I am confused whether these other universes are so far away that information from them will never reach us, or so close we can just reach out and borrow them to help us with some calculations.

    Or are there both kinds?

  19. Robin Herbert says:

    David Thiessen wrote: “What I love about science that it is self-correcting over time in the light of new evidence and if new evidence points to a better theory than QM, scientists will embrace it in far less time than it took the Catholic Church to embrace helio-centrism ( >200 yrs). Ha!”

    Actually the Catholic Church embraced an infinite universe where the earth moved and was not the centre of the universe about 50 years before Copernicus was born. I speak, of course, of Nicolas of Cusa who was lionised by the Pope for his ideas and he enjoyed high reputation and status in the church. Of course he was just mainly guessing based on the astronomy of the time. He pointed out that it would seem to beings on any world that they were the centre and that the other stars and planets moved and also that, in an infinite universe, every world would be, relatively, the centre.

    Unfortunately, two centuries later and a collaboration of Aristotleans and scriptural literalists were in charge, many of whom had it in for Galileo.

    But Cusa’s reputation was never damaged and his books were never banned. In fact even Copernicus’ book survived in a lightly edited form when all of Galileo’s books (even the ones not about heliocentrism) remained banned in any place that the Church could control.

    In fact geocentrism was beginning to crumble as far back as Buridan and his pupils Oresme and Albert of Saxony (although from their wording it was obvious they had to take care about talking about how gravity would work in the case of two worlds). But to say that it took the Catholic Church 200 years to reject geocentrism is a gross simplification of a complex period in the history of ideas.

  20. Sean Carroll says:

    Robin– There are both kinds. The cosmological multiverse (far-away regions, implied by eternal inflation) is logically distinct from the many worlds of quantum mechanics (created by local quantum processes). But the concepts may be related to each other.


  21. Hansel Vudenmirsk says:

    The hidden variable interpretation is a bit of a cop out. It sort of states the obvious and then stops there. Unobtainable computation would be more accurate since we could exactly predict what quantum mechanics would do with enough time and given initial conditions; but the amount of time and complexity is just too great. I like it; I’m just critical of the ideas that I like because they are a valuable starting point. I also wouldn’t say it’s hidden if it’s simply a matter of never thinking to look in a certain area. Arrogance? would that be a good term? I don’t think so, since it implies a level of narcissism… on second thought. I think hidden variables doesn’t really qualify as an interpretation of quantum mechanics as much as it’s just a criticism of it. Many Worlds would be a great interpretation since it tells us something about this particular level of reality. Hidden variables simply says that this isn’t the last level of reality; which I think everyone already agrees with.

    Hidden variables is more of a philosophy and reminds me of artists, in that you can always take it a step further and your art is never really considered finished, you just have to decide when you think you can be OK with stopping. When do we know as much as we need to know? It would be incredible if this was the question we were asking ourselves one day. And as much as I seriously hate admitting it, it’s why philosophy will be with us until the very end.

  22. Bill James says:

    Entanglement is not any kind of action-at-a-distance. You can’t communicate faster than light or violate causality. Hidden variables is a dead horse that people continue to prop up and ride because it hasn’t been completely ruled out. But if you have a clear mind about QM frankly there are no “mysteries,” nothing to solve or to contemplate or to do. It’s done and it stands as the underlying theory of reality. The problems in regards to interpretations and so forth are really just problems of the humans and their thinking. QM remains untouched by this stuff.

  23. Anton Garrett says:


    You wrote: “Hidden variables is more of a philosophy and reminds me of artists, in that you can always take it a step further and your art is never really considered finished”

    So you think that science, or at least, physics, is finished at quantum mechanics, and that presumably all that remains is to explore its consequences?

    I agree, by the way, that advocacy of hidden variables is more a philosophy. Specifically, it is meta-physics.

  24. Anton Garrett says:

    Dear Bill James

    That is untrue. The predictions of QM violate causality in some situations and have been verified experimentally.

  25. allan says:

    Robin, I suspect the catholic church may have allowed some leeway in straying from the bible – if it was done in a low key way and with sufficient piety. That’s not being open to un-biblical ideas. There was only a grudging acceptance of evolution by the pope in 1950 – while still insisting on an Adam, original sin etc.