Many-worlds interpretation

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The quantum-mechanical "Schrödinger's cat" paradox according to the many-worlds interpretation. In this interpretation, every event is a branch point; the cat is both alive and dead, even before the box is opened, but the "alive" and "dead" cats are in different branches of the universe, both of which are equally real, but which do not interact with each other.

The Many-worlds interpretation (also referred to as MWI, the relative state formulation, the Everett interpretation, the theory of the universal wavefunction, many-universes interpretation, or just many-worlds) is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction and denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In layman's terms, the hypothesis states there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.


  • MWI is not some crazy speculative idea that runs afoul of Occam’s razor. On the contrary, MWI really is just the “obvious, straightforward” reading of quantum mechanics itself, if you take quantum mechanics literally as a description of the whole universe, and assume nothing new will ever be discovered that changes the picture.
  • If the MWI were supported by some sound science, we would have to deal with it – and to do so with more seriousness than the merry invention of Doppelgängers to measure both quantum states of a photon. But it is not. It is grounded in a half-baked philosophical argument about a preference to simplify the axioms. Until Many Worlders can take seriously the philosophical implications of their vision, it’s not clear why their colleagues, or the rest of us, should demur from the judgment of the philosopher of science Robert Crease that the MWI is ‘one of the most implausible and unrealistic ideas in the history of science’.
  • The “many worlds interpretation” seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet…It may have something distinctive to say in connection with the “Einstein Podolsky Rosen puzzle,” and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world…which seems to be in some ways a highly improbable one.
    • John S. Bell, "Six possible worlds of quantum mechanics", Proceedings of the Nobel Symposium 65: Possible Worlds in Arts and Sciences. (1986)
  • Yes, I have strong feelings against it, but I have to qualify that by saying that in this particular Einstein-Podolsky-Rosen situation there is some merit in the many-universes interpretation, in tackling the problem of how something can apparently happen far away sooner than it could without faster-than-light signalling. If, in a sense, everything happens, all choices are realized (somewhere among all the parallel universes), and no selection is made between the possible results of the experiment until later (which is what one version of the many-universes hypothesis implies), then we get over this difficulty.
    • John S. Bell, interview in The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics (1986) edited by P. C. W. Davies and Julian R. Brown
  • It's extremely bizarre, and for me that would already be enough reason to dislike it. The idea that there are all those other universes which we can't see is hard to swallow. But there are also technical problems with it which people usually gloss over or don't even realize when they study it. The actual point at which a branching occurs is supposed to be the point at which a measurement is made. But the point at which the measurement is made is totally obscure. The experiments at CERN for example take months and months, and at which particular second on which particular day the measurement is made and the branching occurs is perfectly obscure. So I believe that the many-universes interpretation is a kind of heuristic, simplified theory, which people have done on the backs of envelopes but haven't really thought through. When you do try to think it through it is not coherent.
    • John S. Bell, interview in The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics (1986) edited by P. C. W. Davies and Julian R. Brown
  • He explains that the multiple worlds are not an add-on to the theory but instead are simply what happens naturally when you take the equations of the theory at face value. The standard (“Copenhagen”) interpretation is the one that needs to postulate an ad hoc extra rule. We should simply rename things:

    The “many-worlds interpretation of quantum mechanics” should henceforth be known as “quantum mechanics.”
    The Copenhagen interpretation should henceforth be known as “the disappearing-worlds interpretation of quantum mechanics.”

  • The conclusion, therefore, is that multiple worlds automatically occur in quantum mechanics. They are an inevitable part of the formalism. The only remaining question is: what are you going to do about it? There are three popular strategies on the market: anger, denial, and acceptance.
    The “anger” strategy says “I hate the idea of multiple worlds with such a white-hot passion that I will change the rules of quantum mechanics in order to avoid them.” And people do this! […]
    The “denial” strategy says “The idea of multiple worlds is so profoundly upsetting to me that I will deny the existence of reality in order to escape having to think about it.” Advocates of this approach don’t actually put it that way, but I’m being polemical rather than conciliatory in this particular post. And I don’t think it’s an unfair characterization. This is the quantum Bayesianism approach, or more generally “psi-epistemic” approaches. The idea is to simply deny that the quantum state represents anything about reality; it is merely a way of keeping track of the probability of future measurement outcomes. […]
    The final strategy is acceptance. That is the Everettian approach. The formalism of quantum mechanics, in this view, consists of quantum states as described above and nothing more, which evolve according to the usual Schrödinger equation and nothing more. The formalism predicts that there are many worlds, so we choose to accept that. This means that the part of reality we experience is an indescribably thin slice of the entire picture, but so be it. Our job as scientists is to formulate the best possible description of the world as it is, not to force the world to bend to our pre-conceptions.
  • Fortunately, a minority of physicists, myself included, likewise side unequivocally with realism, by adopting Hugh Everett’s multiple-universes interpretation of quantum theory. According to this view, no particles exist where they have insufficient energy to be; it is simply that in some universes they have more energy than average, and in others, less. All alleged “paradoxes” of quantum theory are similarly resolved.

    So, while most accounts say that Bohr won the debate, my view is that Einstein, as usual, was seeking an explanation of reality, while his rivals were advocating nonsense. Everett’s interpretation doesn’t make Einstein a demigod. But it does make him right.

  • In fact the physicists have no good point of view. Somebody mumbled something about a many-world picture, and that many-world picture says that the wave function ψ is what's real, and damn the torpedos if there are so many variables, NR. All these different worlds and every arrangement of configurations are all there just like our arrangement of configurations, we just happen to be sitting in this one. It's possible, but I'm not very happy with it.
  • There is, I think, no sense at all to be made of the splitting of worlds-plus-agents in many worlds. Of course, one can repeat the words over and over until one becomes deaf to the nonsense, but it remains nonsense nevertheless. Curiously, those who favor this interpretation concentrate their defense on dealing with some obvious technical issues: preferred basis, getting the right probabilities via “measures of existence” (or the like), questions of identity and individuation across worlds, and so on. But the fundamental question is just to explain what it means to talk of splitting worlds, and why we should not just write it off, à la Wittgenstein, as language on holiday. (Einstein once described the writings of Hegel as “word-music.” Perhaps that would be a gentler way of dismissing many worlds.)
    • Arthur Fine, in M. Schlosshauer (ed.), Elegance and Enigma, The Quantum Interviews (2011)
  • The greatest danger I see in the many-worlds/one-Hilbert-space point of view (beside the ridiculous silliness of it all) is the degree to which it is a dead end. The degree to which it is morally bankrupt. Charlie, by thinking that he has taken some of the anthropocentrism out of the picture, has actually emptied the world of all content.
    Beyond that though, I think, many-worlds empties the world of content in a way that’s even worse than classical determinism. Let me explain. In my mind, both completely deterministic ontologies and completely indeterministic ones are equally unpalatable. This is because, in both, all our consciousnesses, all our great works of literature, everything that we know, even the coffee maker in my kitchen, are but dangling appendages, illusions. In the first case, the only truth is the Great Initial Condition. In the second, it is the great “I Am That I Am.” But many-worlds compounds that trouble in a far worse fashion by stripping away even those small corners of mystery. It is a world in which anything goes, and everything does. What could be more empty than that?
    • Christopher A. Fuchs, Letters to Herb Bernstein, “Epiphenomena Chez Dyer”, 02 August 1999, published in Coming of Age with Quantum Information: Notes on a Paulian Idea (2011)
  • It is true that the MWI, in this realist form, avoids some of the paradoxes of QM. The so-called “measurement problem,” for example, is no longer a problem because whenever a measurement occurs, there is no “collapse of the wave function” (or rotation of the state vector in a different terminology). All possible outcomes take place. Schrödinger’s notorious cat is never in a mixed state of alive and dead. It lives in one universe, dies in another. But what a fantastic price is paid for these seeming simplicities! It is hard to imagine a more radical violation of Occam’s razor, the law of parsimony which urges scientists to keep entities to a minimum.
  • The many-worlds theory is incoherent for reasons which have been often pointed out: since there are no frequencies in the theory there is nothing for the numerical predictions of quantum theory to mean. This fact is often disguised by the choice of fortuitous examples. A typical Schrödinger-cat apparatus is designed to yield a 50 percent probability for each of two results, so the “splitting” of the universe in two seems to correspond to the probabilities. But the device could equally be designed to yield a 99 percent probability of one result and 1 percent probability of the other. Again the world “splits” in two; wherein lies the difference between this case and the last?
    Defenders of the theory sometimes try to alleviate this difficulty by demonstrating that in the long run (in the limit as one repeats experiments an infinite number of times) the quantum probability assigned to branches in which the observed frequencies match the quantum predictions approaches unity. But this is a manifest petitio principii. If the connection between frequency and quantum “probability” has not already been made, the fact that the assigned “probability” approaches unity cannot be interpreted as approach to certainty of an outcome. All of the branches in which the observed frequency diverges from the quantum predictions still exist, indeed they are certain to exist. It is not highly likely that I will experience one of the frequencies rather than another, it is rather certain that for each possible frequency some descendants of me (descendants through world-splitting) will see it. And in no sense will “more” of my descendants see the right frequency rather than the wrong one: just the opposite is true. So approach of some number to unity cannot help unless the number already has the right interpretation. It is also hard to see how such limiting cases help us: we never get to one since we always live in the short run. If the short-run case can be solved, the theorems about limits are unnecessary; if they can’t be then the theorems are irrelevant.
    • Tim Maudlin, Quantum Non-Locality and Relativity (3rd ed., 2011), Introduction
  • I regard this last issue as a problem in the interpretation of quantum mechanics, even though I do not believe that consciousness (as a physical phenomenon) collapses (as a physical process) the wave packet (as an objective physical entity). But because I do believe that physics is a tool to help us find powerful and concise expressions of correlations among features of our experience, it makes no sense to apply quantum mechanics (or any other form of physics) to our very awareness of that experience. Adherents of the many-worlds interpretation make this mistake. So do those who believe that conscious awareness can ultimately be reduced to physics, unless they believe that the reduction will be to a novel form of physics that transcends our current understanding, in which case, as Rudolf Peierls remarked, whether such an explanation should count as “physical” is just a matter of terminology.
    • N. David Mermin, in M. Schlosshauer (ed.), Elegance and Enigma, The Quantum Interviews (2011)
  • As a philosopher, I am fascinated by the appearance of the Many-Worlds Interpretation as a cultural phenomenon. This is so similar to what we have seen over and over again in the history of metaphysics! A well-known poet (Derek Walcott) once riddled, "What is the difference between a philosopher and a ruler?" The answer was a pun: "A ruler will only stretch to one foot, but a philosopher will go to any length." But the pun contains a deep observation; it is part of our philosophical tradition that at least one kind of philosopher will go to any length to preserve what he regards as a central metaphysical principle, a principle that is "necessary" in the peculiar philosophical sense of "necessary." What is startling is to observe a metaphysical system as daring as any being born in the unexpected locus of a discussion among physicists about how to understand the deepest and the most accurate physical theory we possess.
  • The philosophical moral behind my question is this: once you give up the distinction between actuality and possibility—as the Many Worlds interpretation in effect does, by postulating that all the quantum mechanical possibilities are actualized, each in its own physical universe—once you say that all possible outcomes are, ontologically speaking, equally actual— the notion of ‘probability’ loses all meaning. ‘No collapse and no hidden variables’ is incoherent.
  • Some very good theorists seem to be happy with an interpretation of quantum mechanics in which the wavefunction only serves to allow us to calculate the results of measurements. But the measuring apparatus and the physicist are presumably also governed by quantum mechanics, so ultimately we need interpretive postulates that do not distinguish apparatus or physicists from the rest of the world, and from which the usual postulates like the Born rule can be deduced. This effort seems to lead to something like a "many worlds" interpretation, which I find repellent. Alternatively, one can try to modify quantum mechanics so that the wavefunction does describe reality, and collapses stochastically and nonlinearly, but this seems to open up the possibility of instantaneous communication. I work on the interpretation of quantum mechanics from time to time, but have gotten nowhere.
  • Another aspect of the realist approach, which some physicists find implausible, is that it seems to lead inevitably to the “many-worlds interpretation” of quantum mechanics, presented originally in the 1957 Princeton Ph.D. thesis of Hugh Everett (1930–1982). In this approach, the state vector does not collapse; it continues to be governed by the deterministic time-dependent Schrödinger equation, but different components of the state vector of the system studied become associated with different components of the state vector of the measuring apparatus and observer, so that the history of the world effectively splits into different paths, each characterized by different results of the measurement.
    • Steven Weinberg, Lectures on Quantum Mechanics (2nd ed. 2015), Chap. 3 : General Principles of Quantum Mechanics
  • In addition to its other problems, the realist approach faces the challenge of deriving the Born rule. If measurement is really described by quantum mechanics, then we ought to be able to derive such formulas by applying the time-dependent Schrödinger equation to the case of repeated measurement. This is not just a matter of intellectual tidiness, of wanting to reduce the postulates of physical theory to the minimum number needed. If the Born rule cannot be derived from the time-dependent Schrödinger equation, then something else is needed, something outside the scope of quantum mechanics, and in this respect the many-worlds interpretation would share the inadequacies of the instrumentalist and Copenhagen interpretations.
    • Steven Weinberg, Lectures on Quantum Mechanics (2nd ed. 2015), Chap. 3 : General Principles of Quantum Mechanics
  • When a physicist measures the spin of an electron, say in the north direction, the wave function of the electron and the measuring apparatus and the physicist are supposed, in the realist approach, to evolve deterministically, as dictated by the Schrödinger equation; but in consequence of their interaction during the measurement, the wave function becomes a superposition of two terms, in one of which the electron spin is positive and everyone in the world who looks into it thinks it is positive, and in the other the spin is negative and everyone thinks it is negative. Since in each term of the wave function everyone shares a belief that the spin has one definite sign, the existence of the superposition is undetectable. In effect the history of the world has split into two streams, uncorrelated with each other. … This is strange enough, but the fission of history would not only occur when someone measures a spin. In the realist approach the history of the world is endlessly splitting; it does so every time a macroscopic body becomes tied in with a choice of quantum states.
  • There is another thing that is unsatisfactory about the realist approach, beyond our parochial preferences. In this approach the wave function of the multiverse evolves deterministically. We can still talk of probabilities as the fractions of the time that various possible results are found when measurements are performed many times in any one history; but the rules that govern what probabilities are observed would have to follow from the deterministic evolution of the whole multiverse. If this were not the case, to predict probabilities we would need to make some additional assumption about what happens when humans make measurements, and we would be back with the shortcomings of the instrumentalist approach. Several attempts following the realist approach have come close to deducing rules like the Born rule that we know work well experimentally, but I think without final success.
  • So let me state then, very clearly, on behalf of any and all physicists out there who dare not say it themselves: Many-worlds wins outright given our current state of evidence. There is no more reason to postulate a single Earth, than there is to postulate that two colliding top quarks would decay in a way that violates conservation of energy. It takes more than an unknown fundamental law; it takes magic.
    The debate should already be over. It should have been over fifty years ago. The state of evidence is too lopsided to justify further argument. There is no balance in this issue. There is no rational controversy to teach. The laws of probability theory are laws, not suggestions; there is no flexibility in the best guess given this evidence. Our children will look back at the fact that we were STILL ARGUING about this in the early 21st-century, and correctly deduce that we were nuts.
    We have embarrassed our Earth long enough by failing to see the obvious. So for the honor of my Earth, I write as if the existence of many-worlds were an established fact, because it is. The only question now is how long it will take for the people of this world to update.

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