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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. – Andrew Lambert Sep 24 '12 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 '12 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. – Mozibur Ullah Feb 16 '13 at 17:06
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Many worlds only violates positivism if you accept that there is some meaning to the statement that the other worlds "exist", beyond the statement that quantum mechanics is exact. This is not what the many-worlds business is about, it isn't about ontology, it's about the best framework within which to frame quantum mechanics and the map from quantum mechanics to experience.

Many advocates of many-worlds are positivist to the core, and don't care one whit if the other worlds exist or not. They just want to use this ontology (you can use any ontology you like consistent with experience) to make certain statements which are otherwise counterintutive manifestly obvious:

  • Quantum mechanics requires exponential resources to simulate efficiently, the amount of resources required to simulate quantum mechanics on a large system is equal to the resources required to run a parallel machine, what CS people call a "nondeterministic" or "NP" machine, with some extra communication between the processes.
  • Quantum computers can provide exponential speed-ups to certain problems, like prime factoring. Current models of quantum computers cannot exponentially speed up NP complete problems.
  • Quantum mechanics can perform measurements of Vaidman counterfactuals--- these are questions of the form "if I were to shine a photon on this mirror, would the mirror measure the photon or bounce it off without measurement?" You can answer a Vaidman counterfactual without actually bouncing off a photon off a mirror.

These properties of QM are counterintuitive without many worlds, and I consider these three things (and any more things that people might dream up) to be the positivist statement that "Quantum mechanics is a many-worlds theory". The ontology is just there to make these things intuitive, rather than surprising. These are surprising things in other points of view.

The philosopher's reaction to many worlds is first to co-opt it's major new idea:

  • Experience can be subjectively random, even if the underlying theory is deterministic.

This is explained in my answer here: Is there anything that is totally random? . This insight is original to Everett, and has been taken up by Daniel Dennett, possibly through Douglass Hofstadter, in the early 1980s, in a series of lectures and articles beginning with "Where Am I?". These served to popularize this point of view, but it is due to Everett, and he should be given credit for this, since in the centuries of history of thinking about determinism, nobody noticed before him. This is largely due to the decoupling of experience and physical law which is taken for granted in physics philosophy.

There is another idea in many worlds:

  • The states of experience of mind are such that a superposition is percieved as a random outcome. The measure of randomness is imposed by the map of physics to experience, rather than by the physics itself, which does not have a non-qualia notion of randomness.

This idea is not usually considered respectable among those philosophers that study the philosophy of quantum mechanics. This despite the fact that it is a sound position. It is derived from positivism--- Everett identifies the mental state of the observer with certain superpositions of basis states in the quantum theory, explicitly using a computational theory of the mind, using a computer, and he uses positivism and the attendant computational theory of mind to identify the experiences of the computer with our experience.

Since modern philosophy, past 1970, is built on the deranged rejection of postivism, this type of thing does not get much traction.

It is not 100% certain that quantum mechanics is true at the level of quantum computers, but if it is so, then many worlds is as right as anything else, in the postivist sense. The uncertainty comes from the fact that while we have observed Vaidman counterfactuals, we have not observed quantum computation, so we are not sure how far the counterfactual states can persist without some new source of fundamental decoherence turning them into probabilistic superpositions as opposed to quantum superpositions. Probabilistic superpositions are not philosophically thorny. There is no known source of fundamental decoherence in modern physics, but you can't be 100% sure it isn't there until you build a working quantum computer.

Among those that say it a quantum computer has a chance of failing are Gerard 'tHooft. This is the main positivist question of many worlds, and it will be resolved by experiment, not by philosophizing.

  • Would you explain your use of "the deranged rejection of positivism"? I assume you're referring to a variety other than logical positivism? – labreuer Oct 20 '13 at 6:45
  • I am referring to old Wittgenstein, and to other philosophers, who just decided to get rid of logical positivism (and illogical positivism) in the 1960s and 1970s, for ridiculous made up political reasons. – Ron Maimon Dec 4 '13 at 14:10
  • Would you care to offer an answer to Logical positivism today? As far as I am aware, the line of demarcation between meaningful and meaningless statements was an inescapable self-refuting one, but perhaps you know of a way it isn't, or a different formulation that maintains much of positivism without the self-refuting bits? – labreuer Dec 4 '13 at 17:36
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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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