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.

115 Comments

115 thoughts on “The Wrong Objections to the Many-Worlds Interpretation of Quantum Mechanics”

  1. Dan,

    A standard example of the difference between God and Many Worlds is the cosmological horizon: we know that, according our current best physical models, there are galaxies far away from us we will never be able to reach or see light from, because of the expansion of the universe. These are whole worlds inaccessible to observation by us or by each other. As the universe expands further and further, the “number” of such causally separate worlds grows, as well.

    But nothing in our laws of physics says “there must be a conscious entity outside of the spacetime which knows everything and controls everything”.

  2. Dan,

    Re “Do the laws of general relativity imply the existence of a point of infinite density and spacetime curvature at the center of a black hole? No.”

    They imply something else, slightly less intuitive. They imply that, once you fall through the horizon, your lifetime is finite, because the spacetime itself ends after a few short moments. They also imply that your life in these last few moments will not be a pleasant one, you will be stretched, ripped and folded many times over (“spaghettified”).

    Odds are, some new unknown laws take over once the spacetime curvature reaches Planck scale, but of this we can only speculate.

  3. Richard Benish,

    Having dealt with proponents of various “unconventional and revolutionary ideas” for some years now, I feel sorry you have fallen into this trap. In my experience there is nothing I can say or link here that would make you change your mind, so I will not try.

    As for a tunnel toward/through the center of the earth, it would be an interesting undertaking (if not currently technologically feasible) for various reasons, but testing gravity is not one of them. We know how gravity behaves inside large massive bodies because it affects how stars shine, and we have a very good handle on this.

  4. As for the discussion on the distinction of God and MWI:

    To get MWI, you guess (based on evidence) the core rules of QM (which are agreed upon), without any collapse postulate, and get MWI. If you make the ADDITIONAL postulate of collapse, you eliminate other worlds. For God, however, you need to make the additional postulate that God exists, making the existence of God the “complication,” so to speak.

    To make explicitly a point on “simplest theories” which most people don’t understand (as this discussion illustrates).: “God causes things” is a more complicated hypothesis than it first appears. This is because, in order for this explanation to have any explanatory or predictive power, you need to enumerate exactly what you mean by “God,” which takes a lot more time to say. What does “God” cause to happen? Let’s suppose you additionally postulate that “God” causes particular things (physics as we see them) to occur, then the world would be simpler if one hypothesizes the same laws of physics without God (assuming the laws of physics are mathematical and not something like “the world behaves in such a way as to optimize happiness”; in the latter case, it might, in fact, be a plausible simplest belief to suppose an intelligent design).

    In MWI, however, one can get all of the subjective experience one finds purely from the two postulates Sean discusses. The additional postulate of collapse changes nothing about subjective experience; it just presupposes that only one of MWI’s “many worlds” exists (selected as MWI would select for subjective experience). This makes MWI the “simpler” hypothesis for QM in the following sense: it supposes fewer assumptions and gets to the same results. Sure, you can hold on to some sort of
    ontological distinction between reality as you experience it and the possible realities you could have experienced, but that doesn’t really add anything.

    The Copenhagen interpretation serves to render the subjective experience of MWI as absolute existence. It has more postulates than MWI. Thus, we can assume MWI, because it has fewer postulates (so is the better theory, assuming Occam’s razor) while resulting in the same subjective experience.

    As for the name: Note that this was not the name Everett gave it; it is the name that has been rendered to it since then. One can imagine the “many worlds” arising in a classical sense just as much as a quantum sense; imagine it as follows: a typical problem in classical mechanics assumes an initial condition and predicts (deterministically) the behavior after that. In some sense, you can think of this as a “path” through the “space of possible states” (called “phase space”). Suppose, however, that paths in phase space rapidly separate; suppose also we can’t perfectly measure our current position in phase space. Subjectively, then, it would appear probabilistic exactly which path we’re on. MWI reasons as follows:
    The path requires an initial condition to pick it out. Phase space doesn’t, it just requires the differential equation. Thus, if our path is typical (for a life-containing universe), we can say that “phase space exists” rather than that any particular path exists. Since the intelligent observer only finds himself on any particular path, the question being asked is which path (i.e., which world) he is on, not which one is “real.” These yield equivalent subjective experiences, but assuming phase space exists rather than only one path allows for fewer assumptions. (I note, however, that if we were on an atypical path for a life-containing universe, then the phase space hypothesis would have Bayesian evidence against it.)
    This is not exactly the same thing, but is somewhat analagous, and gives a feel for where “many worlds” comes in. It’s not that the world itself “splits”; it’s that previously indistinguishable worlds separate in phase space. (I note that this is a hidden-variables interpretation, so isn’t correct, but, like I said, suffices for an idea, I think.)

  5. Thanks Sean, you spelled out frustrations with Ball’s article many of us share.

    One question I’ve wondered about: is there any reason other than Occam’s Razor to favor MWI? That is, if we didn’t believe Occam’s Razor was a valid criterion for choosing between alternative explanations that fit the data, would there be any other reason to prefer MWI over say Copenhagen or Bohm?

    I suspect the answer is no, which is fine by me, since as far as I’m concerned choosing between explanations without Occam’s Razor is like choosing between paintings without eyes.

  6. Jacob Eliosoff: Occam’s razor is a rough guide to where to keep looking for further advances. MWI passes that criterion. In this post Sean has outlines multiple interesting questions which need to be answered.

    However, the ultimate test is still Popperian. Hopefully some day MWI can make a testable prediction beyond what collapse gives you. Until then Sean and others settle for deriving apparent collapse from more natural ideas, then using the Born rule for actual calculations.

  7. @Shmi Nux: “They imply that, once you fall through the horizon, your lifetime is finite, because the spacetime itself ends after a few short moments. They also imply that your life in these last few moments will not be a pleasant one, you will be stretched, ripped and folded many times over (“spaghettified”).”
    I have a suggestion for anyone who wants to do this experiment and avoid painful last moments. You probably know that spaghettification is proportional to 1/r^3. So choose a supermassive black hole with a very large event horizon. There will not be much spaghettification and your last moments will not be painful!!

  8. @kashyap vasavada, This is maybe taking us off-topic from Sean’s post, but having recently been somewhat confused on the issue myself, I owe it to those who corrected me to suggest that your “last moments” of consciousness will probably not be at the event horizon – and that between there and the singularity you may well have plenty of opportunity to experience the ultimate rack of spaghettification.

  9. Shmi Nux wrote:

    “As for a tunnel toward/through the center of the earth, it would be an interesting undertaking (if not currently technologically feasible) for various reasons, but testing gravity is not one of them. We know how gravity behaves inside large massive bodies because it affects how stars shine, and we have a very good handle on this.”

    Though partially true, the conclusions stated in both of these statements are incorrect, especially the latter one, because we have never observed how test objects move through the centers of larger massive bodies, whether stars or stones. You may of course join the status quo in pretending to know the result of Galileo’s experiment. But then you would be violating the ideals of science, according to which we are supposed to look, if possible, where we have not yet looked. We are supposed to verify our guesses based on extrapolations (from outside matter to inside matter; from static data on gravity to data on gravity-induced motion) with empirical data, not remain satisfied with our guesses.

    The well respected physicist, Herman Bondi warned of pretending to know the unknown based on the kind of extrapolation that you have made:

    “It is a dangerous habit of the human mind to generalize and to extrapolate without noticing that it is doing so. The physicist should therefore attempt to counter this habit by unceasing vigilance in order to detect any such extrapolation. Most of the great advances in physics have been concerned with showing up the fallacy of such extrapolations, which were supposed to be so self-evident that they were not considered hypotheses. These extrapolations constitute a far greater danger to the progress of physics than so-called speculation.”

    Whatever unconventional ideas I might have about gravity are completely irrelevant with regard to the large gap in our empirical knowledge of gravity, as it pertains to the insides of matter. Neither Newton’s nor Einstein’s theory have been put to the test proposed by Galileo 383 years ago.

    For all the resources that go into financing the Large Hadron Collider and the enterprise of Black Holes, gravitons, and Multiple Universes, a curious child still yearns to ascertain the truthfulness (or falsity) of the common assertion that the test object in Galileo’s experiment oscillates, as claimed. All that’s needed is a Small Low-Energy Non-Collider, such as could be built in an Earth-based laboratory (modified Cavendish balance) or launched into Earth orbit.

    As I see it, the spirit of Galileo, Michael Faraday and curious children has been trampled and squelched. Physics has become a glitzy entertainment industry. Gathering empirical evidence to back up the unverified predictions given by (Newtonian and Einsteinian) scripture is not in fashion. Rather sad, in my humble opinion.

  10. @Alan Cooper: I was just pointing out that you are lot safer with supermassive black hole in approaching the event horizon. But do we really know what happens inside? Are you assuming inside gravity varying linearly with distance? Do you have a reference for a serious model about inside gravity? I would like to know.

  11. kashyap vasavada: we do know rather well what happens inside, as long as the spacetime curvature is well below the Planck scale. Black holes hold many mysteries, but this is not one of them. Well, unless you believe in the horizon firewalls, but even for that you have to let them evaporate awhile.

  12. @Jacob: I think you are correct, that Occam’s razor is essentially the only reason to choose the multiverse. There is a way you could test this, or at least the existence of any multiverse (quantum suicide), albeit that it is an odd experiment because your results cannot convince anyone else. Another related argument would be anthropic: if our computations were to show that, given the current environment of the universe, in spite of its size, it would still be highly unlikely that life would have developed (e.g., it would have required very special initial conditions, say spontaneous self-assembly of a first cell by a bunch of particles randomly ending up in the correct arrangement), then one can reasonably conclude that there would be Bayesian evidence for a multiverse. (I note, however, that it’s not necessarily evidence for MWI specifically; the Copenhagen interpretation combined with an infinite-volume universe yields the same outcome, but for all practical intents and purposes that’s identical to MWI anyway.)

  13. Magnema, I’ve never bought the quantum suicide test. One’s subjective experience post-decoherence is exactly identical under Copenhagen and MWI. Even if I survived 100 rounds I could only conclude that I was lucky; I still couldn’t tell whether it was from 100 collapses going my way, or from getting the lucky branch.

    But anyway there are enough digressions already on this thread. Suicidal experiments aside, anyone else think there are reasons other than Occam’s Razor to prefer MWI (or agree that there aren’t)?

  14. Sean’s comments are an excellent summary of the wrong turns that debates take whenever the Everett relative state model of quantum theory is discussed. Having been involved in such debates myself, I can share my frustration about the kind of arguments made against Everett’s ground breaking work. In my opinion, it was Everett who finally broke through the cloud of obscurity erected by Bohr , an obscurity which had served to stunt the advance of our understanding of the measurement problem as well as given aid and comfort to all manner of new age woo-woo thinking. The Copenhagen interpretation involved too much philosophy and too little science, it took Everett to correct that.

  15. As an interested layperson, here’s how this debate looks to me. I’d be interested to hear if I have misunderstood.

    The starting point, which is not controversial, is the formal description of quantum events. When we observe a large number of events, the formalism describes their distribution with great precision. The problem is that the formalism gives us only a probability distribution, even as a description of individual events. This contrasts with macro-level descriptions, which apply probabilities only to distributions of large numbers of events or to unknown future events, but not to individual events that have actually taken place.

    There are several ways of resolving this problem:

    1. Just accept that our intuitions for macro level events don’t apply to quantum events. “A discrete event can’t be described in terms of probabilities” is one of those things that is true at large scales but not small ones. In other words, Feynman’s “shut up and calculate” position.

    2. Simply reject the problem of understanding individual quantum events. When we are dealing with arbitrarily large numbers of events, after all, there is no problem of interpretation, probabilities mean just what they do in normal life. And science is concerned with general laws, not individual cases. In other words, the ensemble interpretation.

    3. Postulate that a fuller understanding of quantum events will reveal that they are determinate. The fact that are formalism describes even individual events only in terms of probabilities is a sign that it is incomplete. In other words, hidden variables interpretations.

    4. Postulate that a fuller understanding of quantum events will reveal that they are not discrete. To preserve out intuition that probabilities must describe frequencies, we should assume that each observed quantum event really is a draw from a larger universe of events. In this way, probabilities can be interpreted as they are in everyday life. In other words, the MWI/Everett interpretation.

    I have to admit, I don’t quite understand why it is so important to preserve the everyday understanding of probability, which seems to be the motivation of both hidden-variables and Copenhagen interpretations. Why can’t we just say “In normal life, it does not make sense to talk about a discrete event that has taken place in terms of a probability distribution, but at quantum scales it does”? Lots of other things are different at different scales, why not this?

  16. Quantum Mechanics (QM) must be all wrong because it depends on Heisenberg’s Uncertainty Principle (UP), which is completely wrong. Heisenberg himself said if UP is wrong then QM will also be wrong.

    UP is wrong for many reasons. I think people should read the proof of UP from Heisenberg’s own book published in 1930. It has an English translation.
    Here are some reasons why UP is wrong. (1) The proof starts with an assumption that position and momentum are related by Fourier Transform (FT). If you make such an assumption then you will automatically get that their variations also will be related, which is the UP inequality. (2) FT uses infinity and infinity is not their in nature. No large number can be an approximation of infinity. If you replace infinity by a finite number then FT will completely change and uncertainty will go away. (3) Nature cannot make any assumption and therefore all of math and physics must be wrong. UP cannot be tested by any engineering experiment, because we cannot wait for infinite time.

    Engineering uses objects of nature and interacts with nature. Therefore all engineering experiments will automatically eliminate all of assumptions of math and physics. Therefore no math can be tested by any experiment. For more details please look at Chapter-1 on Truth and also the chapter on QM in the blog site on Soul Theory at https://theoryofsouls.wordpress.com/

  17. As a complete novice at this I sort of get what you are saying. But apply this to Schrodinger’s cat. My question is what did the cat die of if you found it dead? Then what is the relationship to that cause? Doesn’t that imply that there are more possibilities instead of a cat alive or dead? There has to be thousands or millions of dead cats with all the various reasons for why the cat is dead.

    So how does the theory resolve this issue?

  18. I can’t resist rising to this tired old fallacy:

    “I will give you the simplest hypothesis: ” God designed all phenomena using his infinite wisdom!” ”

    An incomprehensible being of unknown origin and unknown means of producing its effects is only simple in that it explains nothing and predicts nothing. In fact, such an entity seems infinitely complex to me – than which anything else, no matter how complicated and difficult to understand, would be more simple.

    I guess it depends on viewpoint. From my viewpoint, Dr. Carroll’s explanation and those of his supporters in the comments seem more clear than the objections. To me the essential point Dr. Carroll makes as that QM operates in Hilbert Space, which (unless I am confused which as a layman is quite likely) is a conceptual mathematical space of possibilities, in which, once an event has occurred, new possibilities fan out from it to occupy parts of HS which heretofore were empty. (I think also parts of the fan may recombine in their evolution and interfere or amplify.) That is, I see the MWI as a conceptual framework for understanding the experimental effects of QM (and a much simpler one than the “god ate my homework” hypothesis).

    My speculation (worth only what you paid for it, as usual) is that HS is large but not infinite. I think Zeno turned out to be right: continuous motion is an illusion, as is every other physical quantity which we approximate in math as continuous. The Real Line, with its infinite density of rational and irrational points is just a mathematical concept. Everything is composed of (very tiny) discontinuous steps. So there are not an infinite number of possible worlds in HS, just too big a number for anyone to write down in his or her lifetime. In fact, I very much doubt that HS includes the two cases in which car keys are either on the table or on Mars, because I think there is a minimum unit of probability which exceeds that.

    Anyway, thanks for another interesting post.

  19. I go back to the old double-slit experiment. MWI offers a possible explanation of what happens to a photon (or other particle) when you observe (measure) it, but it doesn’t appear to answer the more interesting question of why the outcome of the double-slit experiment is different depending on whether or not you make the observation (measurement).

  20. QM is useful, but one does not have to turn it into dogma – its usefulness undoubtedly covers only some range of physical behaviour – like for instance General Relativity.

    Indeed, with both GR and QM no one has found an experiment to show that either theory only has limited scope. But for some reason collapse is not viewed as a weakness of QM, where things like singularities are viewed as a problem for GR.

  21. @ JimV
    The statement: “I will give you the simplest hypothesis: “God designed all phenomena using his infinite wisdom!” ”. was of course meant as a joke to emphasize that one should watch out before accepting seemingly simpler explanations. MWI may look simpler but it has lot of baggage, depending on the meaning of the word (many) “worlds”. Shut up and calculate philosophy has worked for 90 years. Agreement has been achieved to an amazing 1 part in 10^12 or so. MWI does not add anything to that. Taking worlds as Hilbert space does not add anything to our mental picture. Nobody has seen Hilbert space. There are also theoretical problems with this splitting, as Sean has admitted. Taking worlds as the kind of world we see around is metaphysical!
    By the way the statement “in which, once an event has occurred, new possibilities fan out from it to occupy parts of HS which heretofore were empty.” is totally wrong. Please consult a book on QM. Once an electron wave function is given (prepared in lab) e.g. (0.6 (spin up) + 0.8 (spin down)) the final result (theoretical) result is sealed even before you start measurement If you do experiment with say 1 million electrons, you will get 360,000 electrons with spin up and 640,000 electrons with spin down according to the Born rule. No interpretation is going to change that! Result of each spin determination is randomly up or down. What you do not know is whether next measurement will be up or down!

  22. @Jacob: If you don’t accept suicide-like arguments, then the Copenhagen interpretation and the MWI (I believe, although I haven’t studied the details enough to be certain) can be formulated equivalently, I think (arguably, they are even the same theory, under a reductionist framework of equivalent theories).

    I think, however, that if you don’t accept quantum-suicide-like arguments, then you can’t accept anthropic arguments on essentially the same grounds, and there are some kinds of anthropic argument which we would really like to have, e.g., to explain why we live on a planet uniquely suited for life (from which we could reasonably extrapolate to guess at the existence of other planets).

    Essentially, the axioms of the argument are:
    1. P(I feel like I survived)=P(some world has me surviving). [This is the controversial postulate, I would guess.]
    2. In Copenhagen, this probability is small; in MWI, this probability is 1
    3. There is no special reason why, if there were one branch of the wavefunction, that it would be the one where I survive.
    Anthropic arguments typically work as follows:
    1. P(some intelligent life feels like it exists)=P(some planet has intelligent life) [the equivalent of the first one, above; note this one feels natural.]
    2. The latter probability is much higher if there are multiple planets than if there are fewer worlds.
    3. There is no special reason why, if there were one planet, it should have intelligent life.

    Note the isomorphism of the arguments. The first postulate even follows on similar grounds. You could make some argument about how there is a “past you” in the first case which does not exist in the second case, but I don’t think this should matter – the experience of “future you” does not actually care that a past you existed. In fact, to take this a step further, “future you’s experience” (i.e., the experience of having survived the quantum suicide experiment) is an expected experience under the MWI.

  23. Magnema, I’m familiar with some of those anthropic arguments and I do find them interesting. I’m just skeptical that they’re any more consistent with MWI than with Copenhagen. I suppose if I read or thought more I might be convinced.

    I definitely disagree with your statement that Copenhagen and MWI could ever “be formulated equivalently” or “arguably be the same theory”. Under Copenhagen collapse happens; under MWI it doesn’t. These are conflicting premises about objective reality, even if no subjective observer can distinguish them. That’s Occam’s Razor role: to help us choose between theories when both fit the facts.

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