Is the many worlds interpretation of quantum mechanics philosophically untenable?

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I saw this:

http://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy

In this article the author raises a number of intriguing philosophical challenges against the so-called "many worlds" interpretations of quantum theory. In particular, the author appears to argue that it is "philosophically and logically incoherent", suggesting that it must contain one or more logically contradictory statements. However, is there actually such a statement? Looking over the objections, I'm not entirely sure I find them convincing, but would be curious to hear from philosophy experts as to exactly whether or not these hold any ground. I've examined quantum mechanics quite a bit, and am not sure that the argument given here properly characterizes it - but nonetheless, I also am open to being totally, and completely wrong, and that's why I want to ask about this. And I'm posting it on this forum because I'd like to hear it from a philosophy pov, not physics, since that's what the author is challenging it upon.

In particular, the points he raises against the many worlds theory are:

  1. The time when the "worlds" split apart is not well-defined - it relies on the same vague notion as Copenhagenism does for the collapse, namely "when a 'measurement' occurs" which is a concept from intuitive natural language, not formal, rigorous mathematics. However I'm not sure this is a fair characterization. The "splitting of worlds" is nothing else than the uncompromising application of the Schrodinger equation to everything, as much as possible. When a particle collides with a barrier in a tunneling event, there is a splitting - this requires no vagary, it can be seen right in a direct simulation. When these superposed waves collide with detectors that read which side the particle went through, then the detector system is predicted to form a superposition by the same equation applied to the combined particle + detector system as a whole. The "branches" come from that alone, the expansion of superposition due to the linear nature of the equation, not some ill-defined process. The wave is in a very high-dimensional space, so we have no real hope of simulating this on a computer, however. Granted that as it's a smooth evolution, there is not a precise instant where the branches form, but it, I'd believe, has the same qualitative character as the initial splitting of the particle.

  2. The formation of many worlds undermines the concept of "I", namely that the bifurcation of a human under the Schrodinger evolution renders it meaningless. How exactly it does this is not clear from the author's description. To me though, it seems, why can't we just imagine the different branches to be literally different people? If they diverged soon after one's birth, then they could be considered as similar to twins separated at birth, arguably rather different people altogether. But perhaps someone here more well-versed in this issue could actually elucidate this in a logically well-parsed-out manner.

  3. The interpretation has no way to prove or refute its truth, there is no way to detect the "parallel universes". However, this seems to be a problem that plagues all quantum interpretations - we do not, with the framework of existing quantum theory, have any way to tell them apart, because the actual predictive theory of quantum mechanics is the math, which gives one tool - the Born rule - to connect the wave functions to the results of experiments, and that is shared by essentially all interpretations. The one exception may be the "objective collapse" interpretations as these actually do in fact predict a slightly altered statistics (but then again one can also fudge them so as to make this undetectable in light of new data coming in - but I'd think that at some point that would have to necessarily degenerate to special pleading and we'd have to say it's less likely this interpretation is true.). Any actual resolution would seem contingent upon discovering something beyond quantum mechanics that sets one interpretation as the correct one (while also rendering quantum mechanics only an approximate theory to something deeper) - which is exactly what the advocates that the author dismisses argue.

There's others too, but I think I'll stop here. So I'm curious: do these problems pose an intractable philosophical challenge that renders MWI useless? The other thing I notice is that he seems to rely heavily on popular-culture and layman characterizations of the theory, instead of the actual theory itself.

If anything, to me the big problem with MWI is the one which inspired it: the problem of reconciling the unicity of our observations of the universe and experimental measurements with the inherent pluralism of the Schrodinger equation. In particular, given that it involves a bifurcation of the observer as per Schrodinger, it is not clear why our train of subjective experience must follow only one path, and furthermore, what determines which path it follows (e.g. why does this coincide with the Born rule statistics?).

The_Sympathizer

Posted 2018-02-18T09:32:09.200

Reputation: 297

3+1 For one philosophical approach to QM see Shimon Malin, "Nature Loves to Hide". He is a physicist using Whitehead and Plotinus as guides. – Frank Hubeny – 2018-02-18T13:55:08.780

1For me the problem is that MWI has no metaphysical benefit. It may help explain QM but it leaves metaphysics where it always was so cannot ever be a comprehensive interpretation but is a non-reductive stop-gap. I also think it is logically flawed but I'll leave that argument to the physicists. – None – 2018-02-18T14:06:10.743

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I think this is the link you are referring to: https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy

– Frank Hubeny – 2018-02-18T14:17:28.133

2Technically, the worlds never split apart, the gradual process is described by decoherence theory. Considering that many philosophers consider "I" to be a spurious notion of folk psychology, quite independently of quantum mechanics, undermining it is hardly is an "intractable" problem. Inflated ontology and lack of empirical access are often cited as disadvantages of the multiverse. But then the same can be said about mathematical platonism, and it is still around. – Conifold – 2018-02-18T23:30:13.607

2I think your criticisms of Ball's criticisms are spot on. From my reading, I don't think Ball knows what MWI is in the first place; he seems to be under the mistaken impression that MWI posits there are multiple parallel universes, and as some corollary down the line suggests that there's one wavefunction. This is both backwards and mistaken; MWI posits that there's a single wavefunction and that collapse isn't "real". The worlds just fall out as a consequence. This matches how you seem to be describing it, so I think you already know better than Ball. – H Walters – 2018-02-19T00:13:09.113

Ball made a good point when he quoted Sean Carroll's attempt to falsify MWI by falsifying QM itself. This was part of your point 3. The reason it is not easy to falsify these interpretations is they are talking about quantum reality when we are not able to look at it. As Malin mentioned in "Nature Loves to Hide" p 260: MWI "does not really resolve the ambiguities related to the collapse; it merely translates them into a new context." Think of these many worlds as actualized possibilities. There is still randomness for our world. To take a multiverse approach to randomness does not explain it. – Frank Hubeny – 2018-02-19T03:01:14.230

@FrankHubeny You mean this point?: "But most other interpretations of quantum theory assume them (at least) too" If so, not really. Ball fundamentally misunderstands MWI, and Sean's point. The "at least" parenthetical is proof that Carroll's comments went straight over Ball's head. MWI rejects, in particular, that the Born Rule is real (in Everett's paper "The Theory of the Universal Wavefunction", see the introduction; "Process 1" is what he calls this). ... – H Walters – 2018-02-19T04:47:02.130

@Frank Hubeny : Yes, that's kind of what I was getting at with my last point. MWI does not do too well to explain the unicity of our experience versus its pluralistic ontology. And I wonder whether it truly answers the question of the when-time of the nondeterministic behavior as well: when do we switch from one branch to another? Decoherence is not instantaneous, so what is happening while the system is decohering that is switching us to the "right" (but random!) branch? (cont'd) – The_Sympathizer – 2018-02-19T04:49:41.467

(cont'd) Arguably Bohmian mechanics does a better job in that regard, though one could argue it is "ontologically wasteful" (heard that somewhere else) in that the same unrealized worlds still exist, but at least it gives a selection mechanism and a mathematically well-defined when-time for the switch between them so explains the unicity of experience. But it's also rather incomplete in that it's unclear how to generalize it to Lorentzian (i.e. relativistic) QM (i.e. QFT). – The_Sympathizer – 2018-02-19T04:49:49.810

...in this thesis, Everett presents a paradox where there are two observers of a system S; A, and B. B observes the combined A+S system. The paradox is that A should apply the Born Rule to S, but B should apply The Schrodinger Equation to A+S. His proposal is the rejection of the BR, with alternative. If the BR is shown to be real, however, in the sense that a collapse can be explained in terms of BR but not in terms of SE, that counts as a violation of SE, and falsifies MWI; but it doesn't falsify any theory that includes both SE and the BR. – H Walters – 2018-02-19T04:51:27.737

1@PeterJ : I'm curious - can you point out an actual logical inconsistency (i.e. a contradiction) in the many worlds theory? What would it be? The thing I find grating about the posts I mentioned was what @H Walters and others have mentioned here about how that these critiques are not based on an informed understanding of what they are critiquing. I have no problem with critique, I have a problem with critique that doesn't properly represent that which it goes after. So I'm wondering if you might have something that's actually of better calibre that can be brought to bear here. – The_Sympathizer – 2018-02-19T04:51:44.040

...IOW, if a theory includes "at least" SE and superpositions, it might also include the BR. If it does, showing that the BR "really" collapses the wavefunction doesn't falsify that theory. Or, phrased another way, there's an entire world in that "at least" that flew over Ball's head. – H Walters – 2018-02-19T04:53:16.560

It is curious that you find Bohmian mechanics better than MWI. The two are not just empirically equivalent, in a sense they are metaphysically equivalent. There is a 1-1 translation of any Bohmian description of events into MWI description and vice versa. Bohmian mechanics has decorations in the form of particles but they only serve to label branches, and decoherence works exactly the same way in both. I think the splitting time anxieties illustrate what Wallace calls the fallacy of exactness, see Everett and Structure.

– Conifold – 2018-02-19T05:30:54.210

@Conifold : I was saying it was better in one regard - in particular, that it gives an explanation for the unicity through the use of a "preferred" set of particles that tracks a certain series of wave function branches, with the assumption that these particles are the ones that correspond to the actual world we are experiencing. But then I also mentioned it had a problem in that the wave function consists then of many "empty waves" that are essentially the other worlds of MWI, but denied reality despite their involvement in the mathematical governance of the motion of the particles of – The_Sympathizer – 2018-02-19T05:36:38.573

the "real" world. And this is what is called as "ontological wastefulness" because it "wastes" all those branches, positing them as part of the formalism but saying they have no use to our actual reality. And I would also think that thinking about it that perhaps the when-time questions are, as you say, less relevant for MWI/Bohmian interpretations as they have a smooth dynamics, as they are to collapse interpretations that posit discontinuous changes in wave functions. – The_Sympathizer – 2018-02-19T05:36:46.880

You might as well dispense with the particles and declare one of the branches "actual". Or, better yet, actual for the "I" that inhabits it. Then every "I" will have the experience of "unicity". Equivalently, you can pour in enough particles to inhabit every branch. As I said, there is always a translation, but decoherence produces only approximate preferred basis on any interpretation. Objective collapse theories are the only ones where timing matters, but they are revisions rather than interpretations. And given relativity of time in QFT this is a dubious advantage. – Conifold – 2018-02-19T06:06:01.880

@The_Sympathizer - I don't think I can make the argument for exactly the reason you mention, that it needs to made by a physicist. My point was mainly that MWI is a metaphysical theory and (in metaphysics) a useless one. If someone can show it is a scientific theory then this objection would be met. It serves a purpose in physics but in metaphysics it is of no value since here the world-as-a-whole includes all universes. . . – None – 2018-02-19T11:15:15.957

Answers

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The numbered objections given on your question don't take account of the actual literature on quantum mechanics without collapse.

In the MWI systems exist in different versions that can interact without one another in interference and entanglement effects. Those effects are the evidence that refutes single universe explanations, see "The Fabric of Reality" and The Beginning of Infinity" by David Deutsch.

Interference can no longer take place between different versions of a system when information has been copied out of those different versions. This does not require measurement, just an interaction that copies information. For example, if a photon is reflected from an object that may be sufficient to prevent interference even if nobody sees that photon and no instrument records it. See

https://arxiv.org/abs/1212.3245.

Branches in the multiverse are defined in terms of the flow of information:

https://arxiv.org/abs/quant-ph/0104033.

Information can flow between a version of system 1 in branch A and a version of system 2 in branch A, or between a version of system 1 in branch B and a version of system 2 in branch B. But information can't flow between a version of system 1 in branch A and a version of system 2 in branch B. Nor can it flow between system 1 in branch B and a version of system 2 in branch A.

Thoughts and sensations are patterns of information processing taking place in your brain. Information can't flow between different versions of you, so you have no way to experience the thoughts or sensations of a version of you in another branch. So the MWI refutes objection 2 and the objection at the end of the question.

On point 3, the interpretations fall into two categories. (1) Alternatives to quantum mechanics that produce different predictions. (2) Taking quantum mechanics seriously as a description of how the world works: the MWI. (3) Attempts to evade the implications of quantum mechanics through vagueness and handwaving about what the theory sez about reality, or even the denial that any such description is possible. The pilot wave and GRW theories are in category 1 and so are distinguishable from the MWI. The Copenhagen interpretation, the statistical interpretation and a few related theories are in category 3. Objecting to the MWI on the grounds that it can't be emprically distinguished from the Copenhagen interpretation is like objecting to evolution on the grounds that god could have made the world the way it is 6000 years ago, or five minutes or five seconds ago. The theories in category 3 are not candidates for experimental testing because they don't rise to the bar of making specific claims about reality.

There are also specific explanations of how the MWI could be tested:

https://arxiv.org/abs/1508.02048.

alanf

Posted 2018-02-18T09:32:09.200

Reputation: 6 821