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The general approach to Quantum Mechanics is that one first takes a classical system and then quantise to obtain a quantum mechanical system. This holds for QM itself, and QFT such as QED and QCD and also more esoteric theories such as string theory and LQG.

However, this strikes me as being a little backward. Surely, if quantum mechanics is a fundamental theory, and every indication suggests that it is, one ought to begin with a quantum mechanical system and then derive a classical world from it. That is we ought to have a Quantum Mechanics that is indigenously Quantum mechanical.

This seems to me a basic and fundamental question in physics, and I'm wondering what philosophically minded physicists, which of course, all good physicists should be (and have been in the past), have addressed this question in any substantial way (I do know of Causal Set Theory as one particular attempt) - or are they still busy obsessing (or spinning!) over 'so-called Quantum Supremacy?'

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    Quantum mechanics is not a fundamental theory, and physicists do not suggest that it is. Not even QFT is, both have to be reconciled with GR, and string theory and LQG are the proposals for that. But the "general approach" to QM does start from QM itself, and classical laws are derived in the limit. Quantization of classical systems is just a heuristic for guessing correct quantum models in specific cases, not a "general approach", and not "deriving" QM from CM. – Conifold Nov 4 '19 at 22:41
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    @conifold: Until QM mechanics IS reconciled its considered as a fundamental theory. This is why QM has been reconciled with SR and that is QFT. The heuristic of quantisation is exactly what physicists do to actually to find a quantum system, it's not covered by any abstract system of axioms. Tus my question. – Mozibur Ullah Nov 5 '19 at 14:34
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    @conifold: by the way, the term for a non-fundamental theory of physics is an effective theory, but I didn't bother to put that assuming that people would understand what I meant instead picking pedantic holes in it. – Mozibur Ullah Nov 5 '19 at 14:44
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    Physical modeling begins from a phenomenon, which is initially described in colloquial or observational terms, not even classical mechanical. Classical model to be quantized is a stepping stone. A path from phenomenon to a model, of which quantization is a part, is not up to a "fundamental theory", it is heuristic. Only once the model is formulated can it "derive the world" theoretically, and be tested. It functioned this way in CM, QM and other scientific theories, the method is hypothetico-deductive, and the hypothetical part isn't meant to be deductive. – Conifold Nov 5 '19 at 18:06
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    No, it is does not mean that. "Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed", Einstein. Measurements and observations are what the theory used to interpret them says they are. If it is QM they are "quantum". And before classical mechanics they were not "classical" either, as Aristotelian interpretations show. – Conifold Nov 7 '19 at 4:45
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The general approach to Quantum Mechanics is that one first takes a classical system and then quantise to obtain a quantum mechanical system.

Quantisation is not an algorithm. It consists of applying principles of quantum theory to coming up with a theory whose equations bear some faint resemblance to the equations of a classical theory and that gives similar predictions when the system decoheres. See

https://arxiv.org/abs/quant-ph/0304202

This seems to me a basic and fundamental question in physics, and I'm wondering what philosophically minded physicists, which of course, all good physicists should be (and have been in the past), have addressed this question in any substantial way (I do know of Causal Set Theory as one particular attempt) - or are they still busy obsessing over 'so-called Quantum Supremacy?'

There are obstacles to coming up with theories that start out as quantum mechanical, the main one is that physicists don't take quantum theory seriously as a description of how the world works,as David Deutsch explains here:

https://vimeo.com/5490979

Deutsch has tried to do some work on non-quantised quantum theories, such as qubit field theory:

https://arxiv.org/abs/quant-ph/0401024

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  • -1: for qubit dogmatism. – Mozibur Ullah Nov 7 '19 at 4:24
  • It would be -1000 for double-dutch and double-dutch dogmatism. – Mozibur Ullah Nov 7 '19 at 4:28
  • @MoziburUllah Your comments are rather cryptic. Could you explain or link to an explanation? – alanf Nov 7 '19 at 9:18
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Surely, if quantum mechanics is a fundamental theory, and every indication suggests that it is, one ought to begin with a quantum mechanical system and then derive a classical world from it.

[W]hat philosophically minded physicists … have addressed this … ?

Sean Carroll is a philosophically minded physicist, and he does take this approach. I am not familiar with (and not equipped to understand) his technical papers, but his newest book (for a broader audience), Something Deeply Hidden seems to take the approach you are describing. There is also has an episode of his podcast about Finding Gravity Within Quantum Mechanics.

It sounds like he is talking about exactly what you are talking about, though he connects this «QM first»-approach to an "Everettian" way of understanding QM. The following is from the podcast:

When we construct a quantum mechanical theory, whether it’s a theory of a single particle or the standard model of particle physics or whatever you wanna think about, we typically start with a classical theory and then we quantize it. … But if you are a good Everettian, you think that even that very first move is probably kind of wrong-headed because the world isn’t classical. Classical mechanics is not there at the foundation of reality, the world is fundamentally quantum.

The striking similarity with the way you formulate your question, and Carroll's explicit ambition to «derive spacetime itself from something purely quantum mechanical», makes me think that the technical papers he links to in Finding Gravity Within Quantum Mechanics might contribute to an answer.

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  • Although the papers are 'technical', Sean Carroll himself admits that 'All of this stuff is extremely speculative...'; it's always important to add this qualification in cases like this ... – Mozibur Ullah Nov 19 '19 at 12:31
  • @MoziburUllah Sure, but if the question is whether the relevant understanding of QM is possible, speculation is exactly what you need. Speculating in this way is asking whether it is possible. – Michael Amundsen Nov 19 '19 at 13:05
  • I'm not denying that, I'm pointing out when speculation is being indulged in, then speculation should be advertised as such - which by the way what exactly Sean Carroll did. – Mozibur Ullah Nov 19 '19 at 13:35

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