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I take it that quantum interpretations can be roughly classified into realist and instrumentalist interpretations. In the former category we have the De Broglie-Bohm interpretation, Everettian interpretation and objective collapse interpretations. Interpretations with as least some instrumentalist content are the Copenhagen and Statistical Interpretations.

Proponents of the Everettian interpretation are required to demonstrate that assuming reality of the wavefunction gives rise (for example through decoherence) to the emergence of a multiplicity of classical-like worlds of which a typical one is the one we experience. This is no doubt a difficult exercise, however once done for one quantum theory (say non-relativistic N-body quantum mechanics) the results easily carry over to other quantum theories which are all based around the same standard quantum postulate of superposition within a Hilbert space of states (e.g. relativistic quantum field theory, string theory and other quantum gravity theories).

It seems the Everettian interpretation is unique in this claim, so for those unsatisfied that physics can do no more that provide an instrumentalist account of experimental data, isn't this interpretation currently the best we have?

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closed as primarily opinion-based by Kyle Kanos, ZeroTheHero, Jon Custer, user191954, John Rennie Sep 14 '18 at 6:34

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Don't forget the TIQM. $\endgroup$ – PM 2Ring Sep 13 '18 at 19:03
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    $\begingroup$ I am not convinced by your usage of the word "theory" here. The Everettian interpretation is, well, an interpretation, of the single QM theory we have. I do not see what "all existing quantum theories" really means. $\endgroup$ – Stéphane Rollandin Sep 13 '18 at 19:25
  • $\begingroup$ Along with actually saying what a world is, Many Worlds needs to explain where the Born rule comes from. But with respect to these two questions, Many Worlds advocates cannot even agree that they need to do this, they disagree with each other on how to answer, and the specific proposals they have do not work. $\endgroup$ – Mitchell Porter Sep 13 '18 at 21:59
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    $\begingroup$ There is active research going on about the relativistic formulation of realistic theories, also on collapse models and Bohmian mechanics, see e.g. here bohmian-mechanics.net/research_papers.html#relativity or this review: arxiv.org/abs/1804.08853 $\endgroup$ – Luke Sep 20 '18 at 15:21
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The only interpretation that has been worked out for all existing quantum theories is the one which says, observable X has value x with probability |<X=x|Psi>|^2. If you don't even have that, you have something less than a quantum theory; and if you do have that, you have a way to interpret it as making statements about some kind of reality. But yes, something is lacking; it doesn't tell you which observables are the ones that actually get to exist. You the user of quantum mechanics, are free to choose which observables you want to calculate.

The Everett interpretation proposes to obtain a more objective picture by saying, "the wavefunction" is what actually exists, and it contains multiple "worlds", only one of which is the world we observe. I should emphasize that in the only truly universal interpretation of quantum mechanics, the Copenhagen interpretation as originally understood, the wavefunction is not a physical object, it is just part of a calculation. In Copenhagen, the observables, not the wavefunctions, are what's real; even if later generations of physicists have got into the habit of regarding the wavefunction as a physical entity.

You suggest that the Everett idea of parallel worlds has been demonstrated for the nonrelativistic case, and can then painlessly be adapted to more advanced quantum theories, like relativistic quantum field theory. This is not true.

First of all, there is nothing like a consensus on exactly what worlds are hiding in a given wavefunction. You suggest that multiple worlds exist once there is decoherence. But decoherence is a matter of degree. Are you saying there is a particular amount of decoherence at which one world suddenly becomes many? If so, how much decoherence? If not, are you saying there is no objective fact about when one world becomes many? That wouldn't be very "realist".

Second, uplifting this to the relativistic case introduces extra problems. In relativistic QFT, wavefunctions are defined on spacelike surfaces. Are you going to say that a particular set of spacelike surfaces define an objectively preferred notion of simultaneity? That's against relativity.

There is a histories formalism (called decoherent histories or consistent histories) which is relativistic. But it doesn't ascribe reality to wavefunctions. Instead, like Copenhagen, it ascribes reality to observables. The observables can be attached to individual space-time points, which means that you don't need an objectively preferred reference frame.

But like Copenhagen, the histories formalism doesn't tell us which observables, at which space-time points, are the ones that take actual values. The only constraint is that the histories (each of which is a different ensemble of specific observables taking specific values) must all mutually decohere, as calculated by a so-called decoherence functional. An individual history could in principle contain just a single observable. People who want the histories formalism to provide a picture of reality, have suggested that the actual histories are as dense with observables as possible (as dense as is possible while preserving mutual decoherence), but while a reasonable criterion, this does not remotely pick out a unique set of histories as the objectively existing multiverse.

Then there's the problem of how to interpret probability in this framework. The decoherence functional also assigns an apriori probability to each history, and then the conditional probabilities of ordinary quantum mechanics can be derived from these. But what do these apriori probabilities associated with entire histories of the universe, actually mean? One way to interpret it would be, there was only ever going to be one universe, the specific universe was somehow selected at the start of time with that probability, and that's it.

But if you want an Everett-style multiverse interpretation of the histories formalism, then all the histories exist, but some of them have to count for more than others. The only way I can see to justify that, is to say that there are multiple copies. If the "probability of universe A" is twice that of universe B, there must be two copies of A and one copy of B. So you'd better hope that all your probabilities are rational numbers...

I've gone on about this at length, in order to convey some of the difficulties with the Everett concept, even in the form that I find most workable. And I haven't even got into the more technical problems of quantum gravity, where you can no longer take space-time points for granted, as an anchor for your observables.

Other "realist interpretations" have their own problems. Bohmian mechanics does not naturally become relativistic. Retrocausal theories like the transactional interpretation exist at a level of handwaving comparable to the many worlds interpretation. Objective collapse theories also have messy problems to solve once you want to make them special- or general-relativistic. There is no realist theory that has been worked out for all quantum theories.

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    $\begingroup$ In my opinion Wallace is a case study in how not to make Everett work. Wallace explicitly says that there is no particular amount of worlds, just many of them; and he adopts Deutsch's back-to-front methodology of motivating probabilities from game-theoretic rationality. They are sophisticated apologists for completely untenable positions. $\endgroup$ – Mitchell Porter Sep 14 '18 at 6:18
  • $\begingroup$ @Bruce Greetham Are you and your existence a "cloud in the sky", ie something so vague you can't even tell if there's one of you? And I am talking about THIS you, not hypothetical almost-duplicates in other worlds. The singularity of your own existence should be the final proof that worlds cannot be vague as you suggest, because you live in one. $\endgroup$ – Mitchell Porter Sep 21 '18 at 21:43

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