«Quantum physics works. But why?»
QBism: Quantum theory as a hero's handbook. C. A. Fuchs and B. C. Stacey in Proceedings of the International School of Physics "Enrico Fermi" 🕮Foundations of Quantum Theory 197:133 (2016). What the paper says!?
This is a long essay that delves into some details of QBism as understood by its main proponent, Christopher Fuchs.
There is some comments on the etymology of Bism in QBism, which is not Bayesian anymore, but "bettabilitarian".
There is a list and comments on interpretations of quantum mechanics, seen as «campfire stories told to give meaning to the mathematics and say what it is “all about.”»:
- Copenhagen Interpretations – Bohr, Heisenberg, Pauli, von Weizsäcker, Peierls, Wheeler (they’re actually all different!)
- Nonlocal Hidden Variables – de Broglie, Bohm, Bub (early on), Maudlin, Goldstein, Valentini, Norsen, Hardy
- Stochastic Mechanics – Nelson, Smolin (Lee)
- Modal Interpretations – Kochen, Dieks, van Fraassen, Bub (later on), Healey (for a time), Spekkens & Sipe (briefly)
- Quantum Logics – Birkhoff & von Neumann, Mackey, Jauch, Piron, Finkelstein, Putnam
- Consciousness-Induced Collapse Theories – Wigner, von Neumann
- Objective Collapse Models – Ghirardi-Rimini-Weber, Pearl, Penrose, Frigg, Gisin, Tumulka, Albert
- Consistent Histories – Griffiths, Omnès
- Transactional Interpretation – Cramer, Kastner
- Relational Interpretations – Rovelli, Spekkens (TBA)
- Ensemble Interpretation – Einstein (?), Ballentine
- Informational Interpretations – Zeilinger, Brukner, Bub (presently)
- Superdeterminism – Bell (as a joke), ’t Hooft (in full seriousness)
- Many-Worlds Interpretations – Everett, Albert & Loewer, Barbour, Coleman, Deutsch (early version), Deutsch (later version), DeWitt, Gell-Mann & Hartle, Geroch, Graham, Greaves, Lockwood, Papineau, Saunders, Smolin (John), Tegmark, Vaidman, Wallace, Zurek...
All the various interpretations that result see quantum states as physical entities, like a blob of ψ-flavored gelatin
It has thought provoking observations on no-cloning theorem (not being intrinsically quantum and how Wigner almost got it):
The fact that both teleportation and no-cloning arise in classical statistical theories has implications for the project of reconstructing quantum theory. In our search for deep principles, should we try to rederive quantum physics from the postulate that “quantum information can be teleported,” or that “quantum information cannot be cloned”? These phenomena being not at all fundamentally quantum makes them feel like poor candidates for the seed from which quantum theory grows. It would be far better to seek that essential DNA in the answer to the question, Information about what?
It also highlights various important ideas, including Spekkens's toy model.[1], Maudlin's What Bell did[2]
See Refs. [16-24] on the problem of deriving probabilities from the Everettian interpretation:
Or take the Everettians. Their world purports to have no observers, but then it has no probabilities either. What are we then to do with the Born rule for calculating quantum probabilities? Throw it away and say it never mattered? It is true that quite an effort has been made by the Everettians to rederive the rule by one means or another. But these attempts may have re-imported at least as much vagueness as they claim to eliminate [16-20]. To many in the outside world, it looks like the success of these derivations depends upon where they are assessed—for instance, whether in Oxford [21, 22] or Cambridge [23, 24].
This is a serious, deep text for people willing to delve into the details of what QBism is, with some formalism in the second part on subtle, if not phisolophical, aspects of the theory, that requires full dedicated attention.
Excerpts of interest:
On one hand, QBism is a way of investing meaning in the abstract structure of quantum theory: It tells us that quantum theory is fundamentally about agency, actions, consequences and expectations. On the other, QBism points out the virtue of reconstructing quantum theory from deep, physical principles.
Each of these strategies gives rise to its own set of tough-to-swallow ideas.
Whose information? “Mine!” Information about what? “The consequences (for me) of my actions upon the physical system!” It’s all “I-I-me-me mine,” as the Beatles sang.
quantum states look, act, and feel like information in the technical sense of the word—the sense provided by probability theory and Shannon’s information theory.
Whose information? and Information about what?
“historical” pedagogy is almost inevitably a caricature.
The description of relativity as failing to detect movement is disappointing.