Abstract
Christopher Fuchs and Rüdiger Schack have developed a way of understanding science, which, among other things, resolves many of the conceptual puzzles of quantum mechanics that have vexed people for the past nine decades. They call it QBism. I speculate on how John Bell might have reacted to QBism, and I explain the many ways in which QBism differs importantly from the orthodox ways of thinking about quantum mechanics associated with the term “Copenhagen interpretation.”
Dedicated to my friend and Cornell colleague Geoffrey Chester (1928–2014), who for over 50 years enjoyed my more controversial enthusiasms, while always insisting that I keep my feet firmly on the ground.
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Notes
- 1.
Selected Correspondence of Rudolf Peierls, v. 2, Sabine Lee [ed], World Sci., 2009. I have the impression (confirmed at the conference) that all three of these quotations are unfamiliar even to those who, like me, have devoured almost everything John Bell ever wrote about quantum foundations.
- 2.
C. A. Fuchs and R. Schack, Rev. Mod. Phys. 85, 1693–1714 (2013).
- 3.
When the QBist view of science is used to solve classical puzzles I have suggested calling it CBism; N. D. Mermin, Nature, 507, 421–423, March 27, 2014.
- 4.
For “my”, “me,” “I”, you can read appropriate versions of “each of us”; the singular personal pronoun is less awkward. But unadorned “our”, “us”, and “we” are dangerously ambiguous. In QBism the first person plural always means each of us individually; it never means all of us collectively, unless this is spelled out. Part of the 90-year confusion at the foundations of quantum mechanics can be attributed to the unacknowledged ambiguity of the first-person plural pronouns and the carelessness with which they are almost always used.
- 5.
H. C. von Baeyer, Scientific American 308, 46–51, June 2013; M. Chalmers, New Scientist, 32–35, May 10, 2014. I believe that in both cases these gross distortions were the fault of overly intrusive copy editors and headline writers, who did not understand the manuscripts they were trying to improve.
- 6.
I have more to say about \(p=1\) below.
- 7.
Christopher A. Fuchs, arXiv:1003.5182.
- 8.
Christopher A. Fuchs, arXiv:1405.2390, especially pp. 546–549.
- 9.
Generally named Alice.
- 10.
The Future of an Illusion, 1927, concluding paragraph.
- 11.
Nature and the Greeks, Science and Humanism, Cambridge (1996), p. 92. See also Mind and Matter and My View of the World.
- 12.
Schrödinger to Sommerfeld, 11 December, 1931, in Schödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon, Springer Verlag, 2011.
- 13.
Niels Bohr, 1929. In Atomic Theory and the Description of Nature, Cambridge (1934), p. 18.
- 14.
Niels Bohr, 1961. In Essays 1958–1962 on Atomic Physics and Human Knowledge, Ox Bow Press, Woodbridge, CT (1987), p. 10.
- 15.
I count this as progress. The four stages of acceptance of a radical new idea are: (1) It’s nonsense; (2) It’s well known; (3) It’s trivial; (4) I thought of it first. I’m encouraged to find that stage (2) is now well underway.
- 16.
Heisenberg and Peierls are quite clear about this. Bohr may well have believed it but never spelled it out as explicitly. Landau and Lifshitz, on the other hand, are so determined to eliminate any trace of humanity from the story that I suspect their flavor of Copenhagen might reject the view of quantum states as mathematical tools.
- 17.
In consistent histories, which has a Copenhagen tinge, its quantum state can be a true property of a system, but only relative to a “framework”.
- 18.
R. E. Peierls, Physics World January 1991, 19–20.
- 19.
See in this regard my remarks above about the dangers of the first-person plural.
- 20.
And unlike any other way of thinking about quantum mechanics.
- 21.
Translated into English by John Bell, who was therefore intimately acquainted with it.
- 22.
But Peierls identifies their positions, referring to “the view of Landau and Lifshitz (and therefore of Bohr)” in his Physics World article. He disagrees with all of them, saying that it is incorrect to require the apparatus to obey classical physics.
- 23.
John S. Bell, Physics World 3 (8), 3340 (1990).
- 24.
As John Bell may have heard the bell of the tea trolley. Hearing something, of course, is a personal experience.
- 25.
I shall stop adding the phrase “or any other interpretation”, but in many cases the reader should supply it.
- 26.
Bell used the word “information”, not “knowledge”, but his objection has the same force with either term.
- 27.
Peierls to Bell, 13/11/1980, Selected Correspondence of Rudolf Peierls, vol. 2, Sabine Lee [ed], World Scientific, 2009, p. 807.
- 28.
Indeed, the term “classical” has no fundamental role to play in the QBist understanding of quantum mechanics. It can be replaced by “experience”.
- 29.
A good example to keep in mind is my above mentioned assignment of probability 1 to my belief that you have personal experiences of your own that have for you the same immediate character that my experiences have for me.
- 30.
A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777–780 (1935).
- 31.
N. Bohr, Phys. Rev. 48, 696–702 (1935).
- 32.
Known as Dutch-book coherence. See the Fuchs-Schack Revs. Mod. Phys. article cited above.
- 33.
David Hume, An Enquiry concerning Human Understanding (1748).
- 34.
C. A. Fuchs, N. D. Mermin, and R. Schack, Am. J. Phys. 82, 749–754 (2014).
- 35.
I would have expected philosophers of science, with an interest in quantum mechanics, to have had some instructive things to say about this connection, but I’m still waiting.
- 36.
I have had no success finding any of them with Google. For example, there is no point in googling “Bell bicycle.” “ ‘John S. Bell’ bicycle” does no better. Even “ ‘John S. Bell’ bicycle quantum” fails to produce anything useful, because there is a brand of bicycle called “Quantum”, and Quantum bicycles have bells.
- 37.
Ninety years after the formulation of quantum mechanics, a resolution of the endless disagreements on the meaning of the theory has to be shocking, to account for why it was not discovered long, long ago.
Acknowledgments
I am grateful to Chris Fuchs and Rüdiger Schack for their patient willingness to continue our arguments about QBism, in spite of my inability to get their point for many years. And I thank them both for their comments on earlier versions of this text.
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Mermin, N.D. (2017). Why QBism Is Not the Copenhagen Interpretation and What John Bell Might Have Thought of It. In: Bertlmann, R., Zeilinger, A. (eds) Quantum [Un]Speakables II. The Frontiers Collection. Springer, Cham. https://doi.org/10.1007/978-3-319-38987-5_4
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