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jeremyfiennes
Post  Post subject: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 12:06 pm

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QM declares reality to be inherently indeterminate. Another hypothesis is that it is in fact determinate, but due to measurement uncertainty appears to be indeterminate. Why did Bohr, Heisenberg & Co adopt the first hypothesis and reject the equally valid second, when the two are experimentally indistinguishable? Bell's theorem, etc, did not exist at that time.


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PhDemon
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 12:51 pm

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I think maybe you are misunderstanding what is meant by "quantum uncertainty". The early QM workers were aware of measurement errors and understood that they can be reduced by better equipment or statistical treatment of data. In the early days of QM "quantum indeterminacy" had nothing to do with measurement errors but was considered a fundamental part of reality as without introducing "hidden variables" they could find no way this quantum indeterminacy could be made deterministic. Later interpretations of QM have addressed this point but the Copenhagen Interpretation took as one of its axioms that there were no hidden variables.


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jeremyfiennes
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 1:21 pm

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Ok. Thanks. By "uncertainty" I mean for instance the velocity-position uncertainty, which applies to the quantum but not the classical domains due to the observation disturbing the observed in the first. But rather than transferring the uncertainty onto reality itself, creating all the conceptual problems associated with the notion of collapse, why didn't Bohr +Heisenberg simply say that quantum measurement, and hence our knowledge of quantum systems, are inherently uncertain?


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PhDemon
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 4:23 pm

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That is your misunderstanding, it is not a measurement/observation problem (despite many popsci articles explaining it as one!) The particle simply does not have a well-defined position or momentum.


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jeremyfiennes
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 6:42 pm

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Right. So there is no reality with determined properties. What I'm trying to understand is Bohr/Heisenberg's reasons for adopting this somewhat "esoteric" thesis, when a simpler alternative would have been to say that the particle does have a definite velocity and position, but we cannot know simultaneously both precisely.


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PhDemon
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 7:05 pm

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Simple answer is they started off with the idea that energy is quantised and followed the mathematical implications of that where it led...


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jeremyfiennes
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 8:02 pm

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Ok. Thanks. If you will excuse my reticence, but I wanted to first hear what you had to say in general: I read in the afterword to Griffiths' book on QM that the reason was that if one makes a second quantum measurement "immediately" after the first, one gets "exactly" the same result. My reaction was 1) I never heard anything like that before; 2) it doesn't make much sense to me; 3) that he doesn't defines either what he means by "immediately", nor by "exactly". So I am somewhat confused, since he is evidently an authority on the subject. Any comments on this specifically? Thanks.


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PhDemon
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 10:05 pm

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Without reading the book and getting the context I'm not sure if I can answer your question fully... It could be that he is referring to the fact all quantum particles of the same type behave the same way, for example in radioactive decay the half life of a radioactive isotope is constant so the proportion of particles will be the same whenever you make the measurement. As I say, without reading the book this is a guess... If you are interested in this and have the maths chops I'd recommend finding an undergrad textbook and working through the relevant chapters. I'd also recommend getting hold of a copy of Richard Feynman's book QED, it's a good read and IIRC explains parts of QM excellently without getting bogged down in the maths.


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jeremyfiennes
Post  Post subject: Re: Indeterminate quantum reality  |  Posted: Wed Nov 09, 2016 11:56 pm

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Thanks. I'll look for Feynman's book. Bye for now.


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