The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics

Longtime readers know that I’ve made a bit of an effort to help people understand, and perhaps even grow to respect, the Everett or Many-Worlds Interpretation of Quantum Mechanics (MWI) . I’ve even written papers about it. It’s a controversial idea and far from firmly established, but it’s a serious one, and deserves serious discussion.

Which is why I become sad when people continue to misunderstand it. And even sadder when they misunderstand it for what are — let’s face it — obviously wrong reasons. The particular objection I’m thinking of is:

MWI is not a good theory because it’s not testable.

It has appeared recently in this article by Philip Ball — an essay whose snidely aggressive tone is matched only by the consistency with which it is off-base. Worst of all, the piece actually quotes me, explaining why the objection is wrong. So clearly I am either being too obscure, or too polite.

I suspect that almost everyone who makes this objection doesn’t understand MWI at all. This is me trying to be generous, because that’s the only reason I can think of why one would make it. In particular, if you were under the impression that MWI postulated a huge number of unobservable worlds, then you would be perfectly in your rights to make that objection. So I have to think that the objectors actually are under that impression.

An impression that is completely incorrect. The MWI does not postulate a huge number of unobservable worlds, misleading name notwithstanding. (One reason many of us like to call it “Everettian Quantum Mechanics” instead of “Many-Worlds.”)

Now, MWI certainly does predict the existence of a huge number of unobservable worlds. But it doesn’t postulate them. It derives them, from what it does postulate. And the actual postulates of the theory are quite simple indeed:

  1. The world is described by a quantum state, which is an element of a kind of vector space known as Hilbert space.
  2. The quantum state evolves through time in accordance with the Schrödinger equation, with some particular Hamiltonian.

That is, as they say, it. Notice you don’t see anything about worlds in there. The worlds are there whether you like it or not, sitting in Hilbert space, waiting to see whether they become actualized in the course of the evolution. Notice, also, that these postulates are eminently testable — indeed, even falsifiable! And once you make them (and you accept an appropriate “past hypothesis,” just as in statistical mechanics, and are considering a sufficiently richly-interacting system), the worlds happen automatically.

Given that, you can see why the objection is dispiritingly wrong-headed. You don’t hold it against a theory if it makes some predictions that can’t be tested. Every theory does that. You don’t object to general relativity because you can’t be absolutely sure that Einstein’s equation was holding true at some particular event a billion light years away. This distinction between what is postulated (which should be testable) and everything that is derived (which clearly need not be) seems pretty straightforward to me, but is a favorite thing for people to get confused about.

Ah, but the MWI-naysayers say (as Ball actually does say), but every version of quantum mechanics has those two postulates or something like them, so testing them doesn’t really test MWI. So what? If you have a different version of QM (perhaps what Ted Bunn has called a “disappearing-world” interpretation), it must somehow differ from MWI, presumably by either changing the above postulates or adding to them. And in that case, if your theory is well-posed, we can very readily test those proposed changes. In a dynamical-collapse theory, for example, the wave function does not simply evolve according to the Schrödinger equation; it occasionally collapses (duh) in a nonlinear and possibly stochastic fashion. And we can absolutely look for experimental signatures of that deviation, thereby testing the relative adequacy of MWI vs. your collapse theory. Likewise in hidden-variable theories, one could actually experimentally determine the existence of the new variables. Now, it’s true, any such competitor to MWI probably has a limit in which the deviations are very hard to discern — it had better, because so far every experiment is completely compatible with the above two axioms. But that’s hardly the MWI’s fault; just the opposite.

The people who object to MWI because of all those unobservable worlds aren’t really objecting to MWI at all; they just don’t like and/or understand quantum mechanics. Hilbert space is big, regardless of one’s personal feelings on the matter.

Which saddens me, as an MWI proponent, because I am very quick to admit that there are potentially quite good objections to MWI, and I would much rather spend my time discussing those, rather than the silly ones. Despite my efforts and those of others, it’s certainly possible that we don’t have the right understanding of probability in the theory, or why it’s a theory of probability at all. Similarly, despite the efforts of Zurek and others, we don’t have an absolutely airtight understanding of why we see apparent collapses into certain states and not others. Heck, you might be unconvinced that the above postulates really do lead to the existence of distinct worlds, despite the standard decoherence analysis; that would be great, I’d love to see the argument, it might lead to a productive scientific conversation. Should we be worried that decoherence is only an approximate process? How do we pick out quasi-classical realms and histories? Do we, in fact, need a bit more structure than the bare-bones axioms listed above, perhaps something that picks out a preferred set of observables?

All good questions to talk about! Maybe someday the public discourse about MWI will catch up with the discussion that experts have among themselves, evolve past self-congratulatory sneering about all those unobservable worlds, and share in the real pleasure of talking about the issues that matter.

This entry was posted in Science. Bookmark the permalink.

115 Responses to The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics

  1. Ray Gedaly says:

    Each time this topic comes up, I ask the same questions but never get a response back … at least not in my universe. Of course, even if I were to get a response, I wouldn’t know if I’m in the universe where I get a sincere answer or the one where I get a mischievous reply. But here goes …

    1. If MWI is correct, doesn’t this mean that at every instant in time, everything that can possibly happen, will happen?

    2. I look for my car keys. Most likely they’re on my desk where I left them. But there’s also a possibility that the car keys have rematerialized on Mars. Obviously the universe doesn’t split only into two, or I would have a 50% chance to find my keys on Mars. So does the universe split into a huge number of copies where in most my keys are still on my desk?

    3. A murder is committed and the police investigate. Doesn’t this create multiple universes where in one the murderer is me, and in another the murderer is you? Wouldn’t MWI require that there be universes where each of us has committed the most hideous crimes possible.

    4. MWI implies that it would be impossible to communicate across universes or we would already have evidence for it.

  2. Quentin says:

    @Ray Gedaly I think the answer is yes to all your questions, except the first part of question 3: an investigation doesn’t change the past (regarding 4: yes, by definition of a “world”).

  3. Reader297 says:

    I have a simple question about MWI.

    The physical system that is my own body is not described by a pure state evolving according to the Von Neumann equation, but by a reduced density matrix evolving according to some complicated and nonlinear equation that may be approximated by a linear completely-positive trace-preserving mapping, such as that determined by the Lindblad equation. This is because my body is not a perfectly closed system.

    If I enlarge the scope of my system to include all the air in the room around my body, then that’s even worse — that system has far more degrees of freedom and thus an even messier reduced density matrix, with even more nonlinear dynamics.

    I can enlarge to my building, my city, Earth, the solar system, the galaxy, etc., and each time I get a messier and messier reduced density matrix evolving more and more nonlinearly.

    So to my question: How do we know that this process ultimately terminates at some pure state evolving exactly unitarily?

    It may or it may not. To insist that it does is to postulate something untestable, because we certainly can’t see to the ends of the universe to find out, but it appears to be a crucial assumption of MWI, as usually formulated. By contrast, it’s not an assumption of shut-up-and-calculate, which just says to take whatever density matrix you have to whatever approximation is acceptable and just start computing probabilities and expectation values.

    I haven’t found a satisfying answer to this question from proponents of MWI. I’d be appreciative if you could supply one. Thanks!

  4. How should one think about the fact that on any given branch of the wave function, the amplitude is continuously decreasing at a drastic rate. So eg. on our branch of the wave function, the amplitude is a tiny fraction of what it was a second ago. I understand that we do not observe the overall amplitude of the wave function. But it seems strange metaphysically. Is there a helpful way to think about this?

  5. Mark F. says:

    Justin –

    You seem to be saying that evidence for textbook quantum mechanics is evidence for many-worlds. Do you not see why that would be unconvincing to people not already convinced?

  6. Marc Geddes says:

    The MWI has always seemed to me to be the most natural interpretation. The Copenhagen Interpretation is just logical positivism (denying that we can find a realist picture of reality – as such, CI not really scientific). Given that we accept ontological realism, *then* MWI naturally follows.

    However, I don’t think the case for MWI is a slam-dunk. The problem with QM as it stands at the moment is that it isn’t actually compatible with the general theory of relativity – QM is mainly used as a ‘black-box’ calculation tool-kit. There is a missing deeper picture here – there should be a way to explain how all the QM effects emerge from an underlying space-time.

    Ultimately, the ontological picture I subscribe to is that reality is composed of mathematics aka. Tegmark (my position is mathematical Platonism). So my position is that physics itself is just a branch of geometry. Do you agree with this? I am not just saying that geometry is useful to *describe* physics, I am saying that geometry literally IS physics.

    If I am right that geometry and physics are one and the same, physics can broken down into three levels, consisting of 3 core ingredients:

    (1) Fields
    (2) Transforms
    (3) Symmetries

    These 3 levels should in principle encompass all of physics. The deepest level is the field level. Transforms are higher-level properties of the underlying fields. And symmetries are higher-level properties of transforms. Symmetries constitute ‘eddies of stability’ in the underlying fields and transforms.

    So what is missing from quantum mechanics as it stands, is a deeper geometrical picture of reality in terms of the fundamental symmetries, transforms and fields. This deeper picture would unify QM with general relativity and explain *why* the whole QM black-box tool-kit actually works. Until this is done, we cannot be absolutely confident that MWI is correct.

  7. Anonymous Coward says:

    I think my comment partially echoes that of Tom Maciukenas above, but I’ll proceed anyway.

    While I think the statement that “MWI is not a good theory because it’s not testable” is a little over-the-top, my understanding of MWI is something very close: “MWI is one of several interpretations of standard quantum theory, which are all currently scientifically indistinguishable.”

    My question to Sean would be: Do you agree? If not, then what experimental prediction of MWI is there that differs from Copenhagen?

  8. Mark F. says:

    Am I right in thinking that amplitudes can be assigned to histories and not just states? That’s my lay understanding of what Feynman’s sum-over-histories approach does, but I could be mistaken in so many ways.

    But assuming that is correct, then it is tempting to say that we live in one of the histories.

    And if we can say that, then we can believe or disbelieve in the real existence of the other histories as we see fit, depending on our intuitions about parsimony and the meaning of “real existence.”

    What I am proposing here is a kind of one-time extratemporal wave-function collapse, that picks one history out of all the many ones as the real one. I can see why you would reject this as a useless extra postulate, and argue that it is my proposal that founders on not being testably different from MWI. But (assuming it isn’t bogus on other grounds) you could also spin it as a way for people to, in effect, accept MWI as the most sensible scheme for building a quantum model of the universe, without having to make philosophical commitments they’re uncomfortable with. Because that last step of choosing one universe as the Real one would never be part of the physics that anybody cared about.

    But maybe none of this makes any sense at all. If it’s utter nonsense to assign an amplitude to a history then of course this falls apart, but if so I’d love to see an explanation of why that’s so, pitched at the level of Feynman’s little QED: The Strange Theory of Light and Matter. I bet it would be helpful to others too.

  9. Abalieno says:

    Huh? Above in the comments it is explained that a “world” might be just a convenient metaphor.

    I think that the only stuff that comes out of the theory is “more data”, that we define into “worlds” because that’s the kind of shape the data takes. I think it means that you can’t reach some of the data from the place where you are, and so the distinction in “worlds” as separate states.

    So a world simply implies a threshold, a boundary that limits the availability of that data.

  10. Ray Gedaly says:

    I’ve heard Quantum Theory used as an argument for free will. But it seems that MWI would be an argument against free will.

  11. Magnema says:

    @Tom Maciukenas: I understand the reluctance, but that doesn’t change reality.
    To quote Feynman, “Reality must take precedence over public relations, for nature cannot be fooled.” Or, alternatively, see “The Litany of Gendlin.”
    At the end of the day, you can hope one theory is right all you want; that does not make that theory right or wrong.
    Now, do I understand why one might want to reject many worlds on the philosophical grounds? Yes. However, that does not make it more or less likely to be true; likes and dislikes, suffice to say, do not qualify as Bayesian evidence for or against any particular theory.
    It is perfectly fine to hold out hope. However, a distaste for many worlds should not be an argument for the truth in this (or any) case.

    @Ray: Yes, your imagination is essentially correct (at least in cases 1-3, I won’t address 4, as I don’t think I can do so adequately). All of those worlds would exist (assuming they are possible). However, even under the Copenhagen interpretation, there is some probability of those worlds being the real world. The distinction is that, in MWI, these other possibilities are also considered to exist; in Copenhagen, you only get one shot, so to speak.
    However, there’s an important distinction here about universes “splitting.” Take case 2, for example. Yes, there is (in essence) a large number of universes where your keys remain on the table. However, this is because the following occurred: there were many universes where the keys started out on your table. However, in a few of these, your keys tunneled to Mars. In other words, all of the worlds existed beforehand, but in some of them different things happened.
    (Or at least, as I understand all of these things; this is, of course, pending correction by a commenter who might know things better than I.)

  12. Jaime T. says:

    I am a layperson but I am extremely interested in these matters. Regarding a hypothetical experiment say the Schrodinger cat in a box, are these many worlds actual realities prior to opening the box, or are they simply probable outcomes/realities? Also, I think that to answer this question would necessitate our thorough understanding of what really happens when a wave function collapses. Are we actualizing probabilities or choosing from existing ones? What is the “mechanism” in play in a wave function collapse?

  13. Roger says:

    There is still no answer to the question — what does belief in those extra universes buy you?

    It allows you to drop the postulate that eliminates the extra universes. Okay, but is there ever a situation where that helps? Compare that to the many common examples where it is convenient to collapse the wave function.

    What do you mean by saying that there is currently so satisfactory psi-epistemic model? We have that in the textbook quantum mechanics of 1930, on up to QBism today. Exactly what is unsatisfactory (that is solved by MWI)?

  14. Super Funky says:

    My problem with many-worlds is the ontological excess.

  15. phayes says:

    Now, of course you are welcome to invent a theory (a “psi-epistemic” model) in which the wave function isn’t the reality, but just a black box we use to calculate probabilities. Good luck — it turns out to be hard, and as far as we know there isn’t currently a satisfactory model. The Everettian says, Why work that hard when the theory we already have is extremely streamlined and provides a perfect fit to the data? (Answer: because people are made uncomfortable by the existence of all those universes, which is not a good reason at all.)

    And the non-Everettian might say that QBism or RQM is just as (un)satisfactory as EQM and ask, why lazily ‘ontologise’ when what we already have is a natural generalisation of probability theory? (Answer [apparently]: because people are made uncomfortable by the idea of probability theory without ensembles, which is not a good reason at all.)

  16. robert Cattle says:

    Greatly support the MWI of QM . The experimental test although sounding as mad in some ways could come out of a OUIGI board if I spell it correctly.
    Accepting the MWI reality does not follow standard mechanics and accepted theory, the acceptance of time and space goes out of the window, meaning that the MW’s are contactable in zero time. This means that some form of test should be possible from our reality to the other. ie through what can be called “no physical means.”
    Since we, in person are the only real reality observers, it would seem that the experiment can only come through our perception. Oliver lodge started possiby unknowingly this experiment in London. 1800?? see his work, as you may only know its truth at your demise, unless you experiment paranormally now. Sounds totally daft from a normal scientist, but thats where it is leading us. The unknown unknown!!

  17. Justin says:

    Mark F. – I think the Everettian view is precisely that textbook QM generically predicts MWI. So if you accept QM as correct, MWI should be taken seriously.

    I understand how this may be unconvincing, but I believe it’s because people get hung-up on the unfortunate nomenclature about “worlds” and the associate (incorrect) imagery of new realities popping into existence. MWI is just saying that the superpositions of QM can be big.

  18. Thomas Kelly says:


    I would find the Everettian view to be philosophically dubious in that regard.

    There definitely seems to be a logical step between accepting textbook QM, and interpreting ψ as an ontic description of reality, as opposed to a tool for tracking observable properties.

    That step can certainly be taken, but I wouldn’t see the MWI as a self-evident consequence of textbook QM.

  19. kashyap vasavada says:

    @robert Cattle: It is interesting to see that you support MWI on metaphysical grounds. I do not mind, since my religion ( Hindu religion) is full of metaphysics. But you should realize that Sean and his atheist friends launch tirades against religion and metaphysics and at the same time are pushing MWI which sounds very much metaphysical to me!!

  20. Luke Somers says:

    Metaphysics is a topic, not a position or a style of thought. The atheist argument is that one ought to have minimalist opinion in metaphysics. I see no contradiction here.

  21. Thomas Kelly says:

    By “minimalist opinion”, do you mean we should try to minimise the number of metaphysical commitments we make? Surely that would lead to some hyper-skepticism. I assume, for example, the existence of a mind-independent reality.

    If the actual issues with MWI that Sean Carroll mentioned in the above blog were sorted, I think MWI would become a very compelling position despite the fact that its ontological features might not be testable.

  22. Diogenes says:

    Kashyap: “But you should realize that Sean and his atheist friends launch tirades against religion and metaphysics and at the same time are pushing MWI which sounds very much metaphysical to me!!”

    No. This is a typical flattening argument used against atheists, ‘Why you atheists believe things on faith, so atheism is just like a religion!’ No. It’s a South Parkian flattening argument: e.g. those who oppose bullies are themselves bullies, those who oppose religion are themselves religious, those who oppose fascism are themselves fascist, etc. etc. It’s about as sensible as saying “Those who don’t like mayonnaise do so because deep down inside, they really like mayonnaise.” No we don’t. An argument against mayonnaise is not mayonnaise-based.

    “Sean and his atheist friends” are not pushing MWI as a form of metaphysics, or for metaphysical reasons. There is a philosophical, or to be more precise, an epistemological reason to prefer MWI, namely the belief that the scientific method by definition should prefer the simpler hypothesis among those that are equally good at predicting and explaining the observations.

    And “simpler” here is not defined by counting the number of entities, especially not by counting the number of entities deducible as conclusions from a hypothesis. E.g. If I find termite damage in my house, I don’t say, “There must be ONE TERMITE in my house– because ONE TERMITE is a simpler hypothesis than 10,000! termites” No, neither just one termite, nor 50 million, is the simplest hypothesis; and zero termites would be ridiculous.

    Rather, to form the simplest hypothesis we would ask, what is the typical size of a termite infestation in a house? And any deviation from that number, higher or lower, is a less simple hypothesis. Thus, hypothesizing one termite is a more complex hypothesis, and hypothesizing ZERO termites is ridiculous!
    To be precise: in general, an unparsimonious hypothesis is assessed by counting the number of its extraordinary claims that are not supported by extraordinary evidence; and by ‘extraordinary claim’ I mean any assigning of a property to a hypothetical entity which is improbable for most entities of that type.

    This is a philosophical argument in the same sense that any restriction to the scientific method is epistemological, e.g. we’re rejecting conclusions that go against the scientific method, which we try to apply consistently.

    Moreover, as an epistemological matter, we’re including deductions from scientific hypotheses as being “scientific”, and part of science, even if the deductions are not themselves testable, so long as the premises upon which the deductions are based are testable. For example, if I were to ask, “Do your car keys still exist when no one is observing them?” the scientific answer is “Yes, they still exist when we’re not watching them” because that follows as a deduction from the testable premises “They existed a minute ago” and “matter rarely disappears.”

    To say, “My car keys disappear when I’m not watching them” may be ontologically simpler, because the number of hypothetical entities is smaller, but it is NOT the simpler hypothesis, because you’re hypothesizing extraordinary properties, namely that matter can disappear and re-appear higgled-piggledy. That’s extraordinary and that makes it complex.

    The anti-MWI position, which should correctly be called “Disappearing Worlds” or perhaps even better, “Disappearing Twins” (because they believe Schrodinger’s equation is invisibly violated to make all the variant versions of them disappear), pretends to define science in such a way that it excludes untestable deductions as not “scientific”, but it is hypocritical because it only defines “scientific” this way to make your invisible twins disappear, but not at any other times where this definition of “science” would lead to unpopular conclusions. If the “Disappearing Twins” proponents were consistent in their definition of “science”, i.e. rejecting untestable but natural deductions, they would have to call it “unscientific” to believe the far side of the moon exists when we’re not looking at it; or it’s unscientific to believe Comet Shoemaker-Levy collided with Jupiter, because it hit the far side where we couldn’t see the impact; etc. etc.

    But no, the anti-MWI “Disappearing Twins” proponents do not apply their strict definition of “science” to exclude deductions where they would be unpopular, or bad for their PR for them to do so. So the anti-MWI crew will never say, “Your car keys don’t exist when you’re not looking at them!” They hypocritically redefine “science” this way, that way, and the other way as necessary for PR reasons, rejecting untestable “Alternate Me’s” one minute, but accepting untestable deductions like “My car keys still exist even when I’m not looking at them” the next minute.

    So yes, MWI does have a philosophical, or to be more precise an epistemological motivation: we’re trying to define “science” consistently, and they’re not. They’re into PR and emotional gratifications.

  23. Diogenes says:

    In order to explain the anti-MWI, or as I like to call it, the “Disappearing Twins” interpretation, perhaps a parable will help.

    The Parable of the Many Twins.

    You have a set of twin sibling. You’re quintuplets or sextuplets or something. Your siblings are all different from you: one’s disabled, one’s a billionaire. One had trouble in embryonic development and is of the opposite gender from you.

    Now NASA starts a space program to send people to distant star systems in hypersleep, but they can’t come back and we can’t communicate with them. Your siblings all sign up and off they go, each to a different planet. You can’t see them anymore and you can’t communicate with them.

    So now you announce that your siblings no longer exist. Not because they met with an accident, or their spaceships blew up en route; rather, you assert that they all vanished in outer space.

    Your justification for this is philosophical: it deflates your ego, and diminishes your value, to imagine there are other yous out there doing things that you’re not. Thus, you claim that, to use Phillip Ball’s stupid phrase, it is “ontologically incoherent” if anyone claims your twins still exist out there in outer space.

    That’s not enough. You now claim that, if anyone dares to say, “Well I can’t see ’em, and they could I guess be dead, but I’m pretty sure your twins didn’t disappear, and they or their remains are still out there somewhere,” you announce that this position is unscientific because it is untestable. You say that the simpler hypothesis is to think there’s only one you– it’s ontologically “extravagant”, you claim, to imagine your siblings exist, when the simplest hypothesis is that no siblings exist.

    Along these lines, you are forced to hypothesize that there are magic, but invisible and unseen, puffs of smoke in outer space that swallow up your twins and make them disappear. For each and every twin that you called “ontologically extravagant” and wished to deny, you are forced to hypothesize an equal number of unseen entities– the puffs of smoke– that are equally unobserved and ontologically extravagant, and that additionally violate Schrodinger’s equation and other laws in ways that are untestable and undefined.

    We complain, surely the continued existence of your twins follows as a deduction from known principles. You respond: “Oh, but if I have many twins and they’re all different, then you can’t define what it means to be ‘me.’ How different must a twin be before it’s a threat to ‘me’? If you can’t define ‘me’, then your ‘Many Twins Interpretation’ is ontologically incoherent, and therefore unscientific.”

    To this we might reply: “Wait a minute, you’re rejecting these deductions on the basis that they’re not you. You accept that you exist; but you reject the existence of twins entirely on the basis that they’re not you. So it’s your job to define what ‘you’ means, because that is your basis for rejecting natural deductions.”

    Your only response: “No, that’s your job. Call me when you can define ‘me.’ Until then, poof!

  24. peterpan says:

    *The worlds are there whether you like it or not, sitting in Hilbert space, waiting to see whether they become actualized in the course of the evolution*

    The Hilbert space is then the *womb* with immense potential from which the *potential* actualise.
    This happens automatically. But although a huge amount of worlds actualise, it does not mean the total potential actualise as worlds.
    Israel the potential infinite?
    Maybe I should reserve comment because I lack deep insight.

  25. DanielS says:

    MWI it’s not a theory. It’s only an interpretation of a theory. So the philosophical debate it’s only for fun. Please Sean, choose only the funny part of the debate. At least until MWI become a theory. Probable not very sun. Have a nice day!