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Scott Aaronson has an insightful essay on this in chapter 9 of his excellent book, "Quantum Computing Since Democritus". The TL;DR is that quantum mechanics ca

by lisper 3y ago

Scott Aaronson has an insightful essay on this in chapter 9 of his excellent book, "Quantum Computing Since Democritus". The TL;DR is that quantum mechanics can be derived as a generalization of probability theory using the 2-norm instead of the 1-norm. In Aaronson's words: "Quantum mechanics is what you would inevitably come up with if you started with probability theory, and then said, let's try to generalize it so that the numbers we used to call 'probabilities' can be negative numbers. As such, the theory could have been invented by mathematicians in the nineteenth century without any input from experiment. It wasn't, but it could have been." Well worth a read. In fact, I'd say it's worth buying the book just for this one chapter.

weinzierl 3y ago
I was thinking about Aaronson when I added the CHSH game to the comment, because one of his essays (which I unfortunately could not find anymore) made that click for me.
eigenket 3y ago
This page has lecture notes by Aaronson on exactly that topic https://www.scottaaronson.com/democritus/lec9.html https://www.scottaaronson.com/democritus/lec9.html
prof-dr-ir 3y ago
To counter your enthusiasm I must say that I rather disliked his reasoning. My problem is essentially that what Aaronson's calls "the theory" is a somewhat bastardized version of quantum mechanics that might suffice for quantum computing but, in my opinion, not for physics. I discussed the difference earlier: https://news.ycombinator.com/item?id=38255476 https://news.ycombinator.com/item?id=38255476
- nabla9 3y ago
  To counter your unenthusiasm, think about it from a mathematician's perspective. Mathematics of QM does not live in some separate corner created to do physics. The need for QM created short-lived confusion, now it's all embedded into a much larger coherent mathematical structure. For a pure mathematician, quantum mechanics is a lovely introduction to Hilbert Spaces.
  - ernesto95 3y ago
    True, but from a mathematician's point of view, the theory quickly becomes complicated (and in some sense limited) if you really want to do things rigorously when working with continuous systems, something that does not happen with finite dimensional systems as the parent comment probably alludes to.
  - abdullahkhalids 3y ago
    That's only kind of true. Standard continuous-variable quantum mechanics has all sorts of consistency problems, and the only way to get reasonable predictions out is to paper over infinities and pretend they don't exist. I know there are what are called C*-algebras, which help solve some of these issues, but I don't know anything about them. I do know that the Hilbert Space approach is not sufficient.
- eigenket 3y ago
  I think this is a very common view, and a somewhat mistaken one. Quantum mechanics is used in very different ways by people in quantum information, condensed matter, many body physics, quantum field theory, nuclear physics and may other (sub)fields. Of course it will be difficult for a quantum information theorist if they try to apply what they know directly to a hydrogen atom, but (speaking from experience) it will also be quite difficult for someone trained in what you call Hermitian quantum mechanics to directly apply what they know to quantum field theory, or quantum information or any other subfield that uses different language. I strongly disagree with your summary "if you know Hermitian quantum mechanics then unitary quantum mechanics is conceptually straightforward. If you know unitary quantum mechanics then you will have a lot of new concepts and mathematics to learn before you understand hermitian quantum mechanics". I challenge anyone trained in Hermitian quantum mechanics to make progress on (for example) proving or disproving the generalised quantum Stein's lemma, or any of the unsolved problems here https://arxiv.org/abs/2002.03233 https://arxiv.org/abs/2002.03233 using those methods.
  - prof-dr-ir 3y ago
    With conceptually straightforward I meant that the concepts are easy to pick up. For example, the paper you cite is entirely understandable for anyone with some training in hermitian QM. In contrast, good luck trying to understand elementary concepts like the spectrum of the hydrogen atom or interference of matter waves from unitary QM. Of course the field of quantum info has progressed enormously and has its own interesting challenges, for which hermitian QM is all but useless.
    - eigenket 3y ago
      I think maybe we have some difference in how we're talking about things. Concepts like the spectrum of the hydrogen atom or interference phenomena aren't particularly difficult to understand conceptually: the Hamiltonian has some eigenvectors and eigenvalues, you use the Dirac equation and work them out. The "matter waves" interfere essentially in the same way that waves on the surface of a pond do. The things that you're calling conceptual understanding I guess must be different to this: maybe something like detailed calculations of the structure of the spectrum?

by lisper 3y ago

weinzierl 3y ago

I was thinking about Aaronson when I added the CHSH game to the comment, because one of his essays (which I unfortunately could not find anymore) made that click for me.

eigenket 3y ago

This page has lecture notes by Aaronson on exactly that topic https://www.scottaaronson.com/democritus/lec9.html https://www.scottaaronson.com/democritus/lec9.html

prof-dr-ir 3y ago

To counter your enthusiasm I must say that I rather disliked his reasoning. My problem is essentially that what Aaronson's calls "the theory" is a somewhat bastardized version of quantum mechanics that might suffice for quantum computing but, in my opinion, not for physics. I discussed the difference earlier: https://news.ycombinator.com/item?id=38255476 https://news.ycombinator.com/item?id=38255476

nabla9 3y ago
To counter your unenthusiasm, think about it from a mathematician's perspective. Mathematics of QM does not live in some separate corner created to do physics. The need for QM created short-lived confusion, now it's all embedded into a much larger coherent mathematical structure. For a pure mathematician, quantum mechanics is a lovely introduction to Hilbert Spaces.
- ernesto95 3y ago
  True, but from a mathematician's point of view, the theory quickly becomes complicated (and in some sense limited) if you really want to do things rigorously when working with continuous systems, something that does not happen with finite dimensional systems as the parent comment probably alludes to.
- abdullahkhalids 3y ago
  That's only kind of true. Standard continuous-variable quantum mechanics has all sorts of consistency problems, and the only way to get reasonable predictions out is to paper over infinities and pretend they don't exist. I know there are what are called C*-algebras, which help solve some of these issues, but I don't know anything about them. I do know that the Hilbert Space approach is not sufficient.
eigenket 3y ago
I think this is a very common view, and a somewhat mistaken one. Quantum mechanics is used in very different ways by people in quantum information, condensed matter, many body physics, quantum field theory, nuclear physics and may other (sub)fields. Of course it will be difficult for a quantum information theorist if they try to apply what they know directly to a hydrogen atom, but (speaking from experience) it will also be quite difficult for someone trained in what you call Hermitian quantum mechanics to directly apply what they know to quantum field theory, or quantum information or any other subfield that uses different language. I strongly disagree with your summary "if you know Hermitian quantum mechanics then unitary quantum mechanics is conceptually straightforward. If you know unitary quantum mechanics then you will have a lot of new concepts and mathematics to learn before you understand hermitian quantum mechanics". I challenge anyone trained in Hermitian quantum mechanics to make progress on (for example) proving or disproving the generalised quantum Stein's lemma, or any of the unsolved problems here https://arxiv.org/abs/2002.03233 https://arxiv.org/abs/2002.03233 using those methods.
- prof-dr-ir 3y ago
  With conceptually straightforward I meant that the concepts are easy to pick up. For example, the paper you cite is entirely understandable for anyone with some training in hermitian QM. In contrast, good luck trying to understand elementary concepts like the spectrum of the hydrogen atom or interference of matter waves from unitary QM. Of course the field of quantum info has progressed enormously and has its own interesting challenges, for which hermitian QM is all but useless.
  - eigenket 3y ago
    I think maybe we have some difference in how we're talking about things. Concepts like the spectrum of the hydrogen atom or interference phenomena aren't particularly difficult to understand conceptually: the Hamiltonian has some eigenvectors and eigenvalues, you use the Dirac equation and work them out. The "matter waves" interfere essentially in the same way that waves on the surface of a pond do. The things that you're calling conceptual understanding I guess must be different to this: maybe something like detailed calculations of the structure of the spectrum?