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by gus_massa
4016 days ago
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It's very difficult to read "Quantum Mechanics as Quantum Information, Mostly", in particular because it mixes the equations with unrelated comments like: > A grain of sand falls into the shell of an oyster and the result is a pearl. The oyster's sensitivity to the touch is the source of a beautiful gem. > Last year, I watched my two-year old learn things at a fantastic rate, and though there were untold lessons for her, there were a sprinkling for me too. A better reading material is the solution of a simple exercise, that explains the difference between the usual approach and the QB approach. (Is there any differences in the results?) Someone has suggested the double slit experiment, because it's nice and easy to explain with words, but the continuous distribution makes the calculations difficult. I prefer the three Stern-Gerlach experiments because it's discrete and the math is easier. I think I read that experiment in a Feynman book, but I don't remember the exact citation. (The SG in the middle is the equivalent to the double slit.) I just found this PDF that explain clearly the situation: http://docslide.us/documents/spin-and-quantum-measurement-da... . It's the "Experiment 4" (subsection 1.2.4, page 10). Can you explain the differences between the usual and the QB approach in this experiment? |
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In particular, things like collapse of the wave-function (for example) present some difficulty for (b) -- not for (a) -- which is what QBism is trying to address. It's also why you don't get problems to solve here, and why I recommended "Quantum Mechanics as Quantum Information, Mostly". Yes, it's written casually (have a look at Fuchs' and others' publications on the arXiv that made it to scientific journals if you want fewer asides about children) but the casual nature is because this is about how we view the problems in the first place, and why we make the calculations we do, not how to carry out the specific calculations.
Consider this (example stolen from Fuchs, somewhere): We knew the correct equations of special relativity years before Einstein came along -- that's why it's called the Lorentz transform, not the Einstein transform. But Einstein's genius was to boil things down to two laws (within an inertial reference frame, typical laws of motion hold, and the speed of light is the same in all reference frames). From there we moved from simple calculations that we already knew how to do to a much deeper understanding of the subject. That's what Fuchs' and others working on interpretations of QM are trying to do -- not change the way we make calculations, but understand why the laws are the way they are in the hopes of extracting something new and different from that knowledge.