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Under the traditional interpretation of quantum mechanics, there is no realism and no “definite” reality. However, arguably, there is also no locality, depending on how you understand the term. Of course, when it comes to whether you can send signals faster than light, it is considered local.

But there are nonlocal correlations without a supposed cause and hence there’s “brute” nonlocality. Particles at huge distances from each other that do not have definite states still somehow manage to be correlated without a supposed cause and are treated as one unit. As such, there’s also non realism in the sense that the world is not “definite” until you make measurements. To make matters even more complicated, the very concept of a measurement is itself ill defined and still not well understood.

So on the traditional interpretation of QM, you have to give up the notion of a definite world and give up the notion of a cause that creates non local correlations.

However, on a non local deterministic theory of QM, you now have a definite world so you don’t have to give up realism. You also don’t have to give up the notion of correlations at huge distances from each other happening without cause. Note that apart from this instance, there is no example in the world anywhere of strong, consistent correlations occurring without a cause, or common shared cause, explaining it.

Does this mean that a non local deterministic theory is most parsimonious? Given that this kind of theory makes the exact same kind of predictions as standard QM, it will have the same kind of explanatory power but will be simpler. Why isn’t this then preferred?

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    There is a difference between parsimony and indulging pet preconceptions. But they do claim parsimony advantage for just about every major interpretation of QM (fewer entities in Copenhagen, fewer postulates in MWI, fewer indeterminacies in objective collapse, etc.), you just pick your favorite and then define "parsimony" accordingly. This is what multicritereal optimization degenerates into, you get a bunch of incomparable Pareto optima. And "parsimony" is just a vague umbrella term for multiple conflicting criteria.
    – Conifold
    Nov 28, 2023 at 0:02
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    The comment by @confifold is spot-on. The idea of parsimony is hugely abused and cited by all kinds of confidence tricksters to justify why their pet theories, without a shred of any other evidence in their favour, are better than someone else's. It drives me effing nuts when I hear, for example, the claim that MWI is right because it is more parsimonious. Nov 28, 2023 at 9:51
  • @Conifold very thought provoking comment, and I think it's completely correct.
    – TKoL
    Nov 28, 2023 at 10:17
  • @MarcoOcram "the claim that MWI is right because it is more parsimonious" 😀 Isn't MVI the theory that everytime anything happens, billions of universes get magically created? How can that pretend to be parsimonious?
    – Olivier5
    Nov 28, 2023 at 10:25
  • @Olivier5 my point entirely; nevertheless, you will find people who claim it is parsimonious by focussing on a particular aspect of it. Nov 28, 2023 at 12:18

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There is a controversy that is commonly described as being about the interpretation of quantum theory.

These different interpretations are actually different accounts of what is happening in reality and in general they make different predictions. For example, spontaneous collapse theories have different implications than quantum theory without collapse:

https://arxiv.org/abs/2310.14969

As such, calling them interpretations of quantum theory is extremely misleading: they are alternatives to quantum theory.

In quantum theory without collapse, the Everett interpretation, the mechanism by which Bell correlations arise is known and it is entirely a result of local interactions. The correlations arise when information about the measurement results interact locally and not before. That information is carried in the form of locally inaccessible quantum information in decoherent channels that is only revealed by that interaction:

https://arxiv.org/abs/quant-ph/9906007

https://arxiv.org/abs/1109.6223

The Everett interpretation can be applied in any context in which quantum theory is currently applied, including quantum field theory. By contrast, the applicability of all of the other interpretations is unclear since the theory required to apply them hasn't been created:

https://arxiv.org/abs/2205.00568

So you can either have a local interpretation of quantum theory that explains the Bell correlations you claim are brute and can be applied to any context in which quantum theory is currently used. Alternatively, you can discard quantum theory in favour of other theories that provide no explanation of the Bell correlations and virtually all of the phenomena currently explained by quantum field theory, including apparently mundane issues why the sky is blue.

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In qm "a definite world" and "realism" are old concepts no matter how you make the interpretation. Bohmian mechanics replaces the "undecided position" of particles by attributing the states/positions in a field configuration in a way that particles are basically reduced to fields. I do not consider this theory more parsimonious.

Now, regarding the explanatory power, I do not see how considering particles as mathematical abstractions can shed light on the underlying ontology. To my view, the copenhagen interpretation when carefully examined has more clear concepts as to what is happening.

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Serious philosophers of science have generally considered "parsimony" to be only a suggestive criterion, as it is too vague a term to actually use to justify any conclusion. Karl Popper, when he has to replace "falsifiability" in his criteria of science based on Quine-Duhem's critiques, DID fall back on a variant of Occam's Razor. But rather than the vague "parsimony", he proposed "predictive power" measured as how many possibilities in the world are excluded by a theory as an effective replacement for "parsimony". If you want to use Occam to select between QM theories, then predictive power would be a better metric. And I don't think there is a predictive power advantage for Bohm over Copenhagen.

It is true that all QM theories have a degree of non-locality. For Copenhagen, it is a limited non-locality, as it only seems to matter with cases of entanglement, which is a phenomenon of only limited applicability to our world. With Bohm, in contrast, every event in our world is influenced by every other part of it. Non-locality is wholesale. And as much of our universe is unobservable by us, but does still influence Bohmian Mechanics, this wholesale influence serves as a serious limitation on our ability to actually perform Bohmian Mechanics predictions for -- anything. This lack of predictive confidence is a pretty serious downside for Bohm.

All QM theories are unreal. You claimed that Bohm IS real, but Bohm accepts the Heisenberg Uncertainty Principle, which holds that there IS no real term that describes particles below the uncertainty threshold -- So Bohm also is unreal.

The only advantage for Bohm over Copenhagen is that Bohm is deterministic, while Copenhagen is not. Your personal psychological preference for a deterministic universe is shared by a minority of physicists, and this psychological rationale is the major reason that Bohm is still considered seriously.

The tests between Bohm and Copenhagen have not been going well for Bohm, plus as I noted above, Bohm has serious difficulties in application. Here are links making these points: https://physics.stackexchange.com/questions/8817/what-is-wrong-with-the-de-broglie-bohm-theory-a-k-a-causal-interpretation-of-q https://www.quora.com/Does-Bohmian-mechanics-make-the-same-predictions-as-quantum-mechanics.

Meanwhile, physics is stochastic no matter whether one uses QM or BM, see: Deterministic or stochastic universe? Even the psychological preference you have for BM will not make our universe deterministic.

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  1. Quantum mechanics (QM), as long as speaking about undisturbed elementary particles, deals with a world of possibilities. Reality originates when we force the microworld by an experiment to create a phenomenon. In this sense I understand your opening remark about “no realism and no “definite” reality”.

  2. Each accepted scientific theory agrees with the principle of locality: There is no information transfer with a speed bigger than the velocity of light.

    Do your considerations take into account that the two correlated particles are entangled? There are not just two arbitrary, separate particles, but the two particles are linked (= entangled) and have to be considered a system.

  3. Deterministic theories have the problem of hidden variables. One formulation of Bell’s theorem from 1964: There are quantum phenomena which contradict every theory which satisfies locality and determinism, see The two Bell's theorems of John Bell, Theor. 7.

  4. Do you have a reference to a non-local deterministic theory which makes the same predictions like QM?

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  • 4. Bohmian mechanics
    – user62907
    Nov 28, 2023 at 10:53
  • @thinkingman Do you consider Bohmian mechanics - Schroedinger equation + guiding equation - more parsimonious than the probability interpretation of QM? plato.stanford.edu/entries/qm-bohm/#ObjeResp
    – Jo Wehler
    Nov 28, 2023 at 11:01

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