5

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?).

  • 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 Feb 18 '18 at 13:55
  • 1
    For 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. – PeterJ Feb 18 '18 at 14:06
  • 1
    I think this is the link you are referring to: aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy – Frank Hubeny Feb 18 '18 at 14:17
  • 2
    Technically, 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 Feb 18 '18 at 23:30
  • 2
    I 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 Feb 19 '18 at 0:13
1

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.