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One of the interpretations of quantum mechanics is the Many Worlds Interpretation which basically states that the universe as a whole develops like an unobserved quantum system, and any observation effects ("collapse of the wave function") are illusions which are caused by the observer getting entangled with the observed system, which effectively causes a split of his world into many worlds, one for each measurement outcome.

Most physicists are against this theory because those other worlds are, by definition, unobservable, and thus violate the common positivist view. However I'd be interested in what philosophers (especially non-positivist ones) think about that interpretation. Is it philosophically sound, or does it have fundamental problems?

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    Since these other worlds are unobservable, the question of whether or not they exist is moot. A thing which exists but which cannot be observed has the same effects and influence as something which doesn't exist at all. Sep 24, 2012 at 23:23
  • I suspect if such many universes outcomes are possible, then each of the units (in terms of quanta, atoms, protons,neutrons, electrons, quarks etc.) of mass undergoing observation symmetry breaking, i.e where the state of the system diverges from superposition to simultaneous distinct outcomes, has a universe of its own. Bot I do not think this is true myself though. I used to a believer pf many universes too, but there is a flaw somewhere in it I think.
    – user2452
    Oct 6, 2012 at 6:09
  • Plenty of philosophies have dreamt of other worlds than jsut the world we live in, but they're not backed by such heavy mathematical/physical formalism. Which may or may not be an advantage depending on your point of view. Feb 16, 2013 at 17:06
  • The many worlds interpretation in my opinion fails to address the fact that we only are conscious of one alternative and not the whole multiverse of alternatives. This is a serious draback of the interpretation. In other words collapse seems real
    – Nikos M.
    Oct 31, 2021 at 15:08
  • Check "The fabric of reality" by David Deutch
    – Jencel
    Sep 17, 2022 at 14:40

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The intersection of metaphysics and quantum physics is the best place to be! There are so many interesting questions about the nature of reality that are posed in modern quantum physics. Just as you've stubbled on, one of this problems of philosophy of physics is the Many Worlds Interpretation.

My sense (I'm an undergrad, so take this for what you will) is that your general scientific realists and determinists (not mutually exclusive) will not be in favor of the Many Worlds Interpretation. I say this for two reasons: (1) quantum logic is a very new field and (2) the interpretation itself has some interesting metaphysical consequences.

To elaborate on the second point, to assume that the interpretation is true and that an infinite number of possible worlds are generated by each individual decisions that individuals make (ignoring the more complex problem of event occurrences and just focusing on choice). To allow for an interpretation that presents within itself a plurality of possibilities for the next 10 seconds, let alone the next 10 years, increases the importance of any individual choice. My decision to wear jeans instead of khakis may be a decision that is required to lead to World War III happening in 2050 rather than in 3000. Still you run into ex post facto bias.

This probably isn't the greatest explanation (as it lacks all specificity to appeal to a more general audience), but its just one undergrad's opinion :)

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Questions of the form 'What do philosophers think about x?', where x is a broad topic, are doubly difficult to answer comprehensively, because there are so many interpretations of x and so many philosophical opinions on each interpretation. Given that, I will confine my answer to some observations I find interesting and hope that you will too.

The original formulation of quantum mechanics, known loosely now as the Copenhagen interpretation, associated mathematical functions- called wave functions- with particles and systems of particles, and assumed that under certain circumstances wave functions would change instantly when a 'measurement' was performed on the particle or system associated with the wave function. This presented two obvious objections that are still not resolved to everybody's satisfaction today, namely:

How and why can the assumed change happen instantly everywhere?

More importantly, what makes a 'measurement' special, given that it is presumably just an interaction between the quantum particle being measures and whatever collection of particles forms the measuring device?

There have been many different approaches to resolving these objections, one of which has evolved into a range of ideas now known as MWI. I say a range of ideas because there isn't an authoritative single definition of MWI, and some of its incarnations are far more plausible than others (IMHO). The 'pop science' idea of there being literally multiple Universes each as real as the one you suppose you are in is a classic example of an extraordinary claim utterly lacking in extraordinary evidence, and I follow Hitchins' maxim in relation to it- what can be claimed without evidence can be dismissed without evidence.

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According to the PBR theorem, since the quantum wavefunction has been directly measured as a real physical object, the vision of the universe no longer can be considered psi-epimestic (merely information contained in the quantum wavefunction). The possible remained alternatives are psi-ontics (real objects outside our conciousness) or psi-ontic-epistemics (information with an objective underlying reality) Thus, the Copenhagen interpretation was shifted in favour of the Many Worlds Interpretation (MWI). In MWI you can imagine yourself living in infinite copies of yourself, one for each possible universe, with infinite outcomes. Therefore, if in this universe you are the Dalai Lama, then in another universe you are Adolf Hitler. In my humble opinion, that's one more reason to believe that quantum mechanics theory should not be extended to our common macro reality and our human beliefs, otherwise we risk to fall into an absolute relativism and our life could stick to just a random nonsense. That's why I blame quantum mysticism or, in general, theories which want to merge science and religion. Nevertheless, science is falsifiable, as stated by Karl Popper. Thus, even quantum religions should be forced to review their statements, and that is a nonsense too. That's why I do not trust too much New Age an other quantum woo beliefs.

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    There are at least some serious scientists who support MWI, the most public of whom is probably Sean Carroll. There is plenty of pseudo-science around the topic for sure, but I don't think you can simply label all MWI proponents as new age quantum woo salesmen. I doubt very much that Sean Carroll would agree with your characterisation that MWI means that somewhere there is a universe where you are the Dalai Lama and another where you are Hitler. Feb 20, 2022 at 9:32
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    @RisingMaverick: David Deutsch is another MWI advocate, & a foundational thinker in the field of quantum computing who really couldn't have a stronger academic pedigree (Cambridge Math Tripos pt 3, & leading Oxford professor)
    – CriglCragl
    Oct 3, 2022 at 20:51
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    I had an opportunity to ask Sean Carroll a question about MWI, and his answer did shake my own belief in the reality of it. I asked Carroll, "can you approximate the rate that the universe splits"? His answer was "Continuously." Not once a second or once a week but as a continuous flow. This adds a whole new dimension to the idea.
    – Seti Net
    Oct 7, 2022 at 16:12
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    "...otherwise we risk to fall into an absolute relativism and our life could stick to just a random nonsense." so you would potentially refuse to accept a scientific truth because you didn't like it's philosophical consequences? 2 days ago

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