If you ask Google to search for “no quantum world,” you will get nearly 300 hits. They all give the following quotation (or recognizable corruptions of it): There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature. Over 90% of them attribute the statement to Niels Bohr, with phrases like “Bohr's dictum …,” “Bohr insisted that …,” “Bohr proclaimed …,” “Niels Bohr said, in a frequently quoted passage …,” “Niels Bohr wrote [my emphasis] …,” and even “Explain and evaluate Bohr's philosophy of quantum theory with reference to his assertion ….” Here is yet another example of the power of the internet to enrich our knowledge. There is only one problem. Bohr, who took writing very seriously indeed, never published such an assertion in any of his writings, although he repeatedly refined, reformulated, and often simply repeated his position on the philosophical foundations of the quantum theory. The statement actually comes from an essay by Aage Petersen, “The Philosophy of Niels Bohr” [1], which he published in the Bulletin of the Atomic Scientists shortly after Bohr's death. Petersen introduced the words with When asked whether the algorithm of quantum mechanics could be considered as somehow mirroring an underlying quantum world, Bohr would answer … So what may now be the most celebrated of all Bohr quotations on the nature of the quantum theory is at best an attempt by a close associate to characterize Bohr's general response to the highly problematic notion of a “quantum world,” written too late for Bohr to respond. When you ask Google to list only those pages that also mention the actual author of the words, Petersen, the number of hits drops from 286 to 18. The status of this “quotation” as hearsay is in danger of being lost. I'm particularly sensitive to this risk because in the 1980s I used to enjoy giving physics colloquia on Bell's theorem and its implications for our understanding of quantum mechanics. I was always fond of Petersen's recollection of what Bohr used to say, and I felt that his formulation captured something important about the situation revealed by Bell's theorem.
If you ask Google to search for “no quantum world,” you will get nearly 300 hits. They all give the following quotation (or recognizable corruptions of it): There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature. Over 90% of them attribute the statement to Niels Bohr, with phrases like “Bohr's dictum …,” “Bohr insisted that …,” “Bohr proclaimed …,” “Niels Bohr said, in a frequently quoted passage …,” “Niels Bohr wrote [my emphasis] …,” and even “Explain and evaluate Bohr's philosophy of quantum theory with reference to his assertion ….” Here is yet another example of the power of the internet to enrich our knowledge. There is only one problem. Bohr, who took writing very seriously indeed, never published such an assertion in any of his writings, although he repeatedly refined, reformulated, and often simply repeated his position on the philosophical foundations of the quantum theory. The statement actually comes from an essay by Aage Petersen, “The Philosophy of Niels Bohr” [1], which he published in the Bulletin of the Atomic Scientists shortly after Bohr's death. Petersen introduced the words with When asked whether the algorithm of quantum mechanics could be considered as somehow mirroring an underlying quantum world, Bohr would answer … So what may now be the most celebrated of all Bohr quotations on the nature of the quantum theory is at best an attempt by a close associate to characterize Bohr's general response to the highly problematic notion of a “quantum world,” written too late for Bohr to respond. When you ask Google to list only those pages that also mention the actual author of the words, Petersen, the number of hits drops from 286 to 18. The status of this “quotation” as hearsay is in danger of being lost. I'm particularly sensitive to this risk because in the 1980s I used to enjoy giving physics colloquia on Bell's theorem and its implications for our understanding of quantum mechanics. I was always fond of Petersen's recollection of what Bohr used to say, and I felt that his formulation captured something important about the situation revealed by Bell's theorem.
Once I tried to teach some quantum mechanics to a class of law students, philosophers, and art historians. As an advertisement for the course I put together the most sensational quotations I could collect from the most authoritative practitioners of the subject. Heisenberg was a goldmine: “The concept of the objective reality of the elementary particles has thus evaporated…”; “the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them … is impossible …” Feynman did his part too: “I think I can safely say that nobody understands quantum mechanics.” But I failed to turn up anything comparable in the writings of Bohr. Others attributed spectacular remarks to him, but he seemed to take pains to avoid any hint of the dramatic in his own writings. You don't pack them into your classroom with “The indivisibility of quantum phenomena finds its consequent expression in the circumstance that every definable subdivision would require a change of the experimental arrangement with the appearance of new individual phenomena,” or “the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description but outside its scope.” I was therefore on the lookout for nuggets when I sat down to review these three volumes – a reissue of Bohr's collected essays on the revolutionary epistemological character of the quantum theory and on the implications of that revolution for other scientific and non-scientific areas of endeavor (the originals first appeared in 1934, 1958, and 1963.)
Richard Feynman famously said that nobody understands quantum mechanics and cautioned against asking: "But how can it be like that?" Something about the conceptual foundations of the theory is profoundly puzzling, but just what is so disturbing is not easy to pin down. Three books by Olival Freire (The Quantum Dissidents), Adam Becker (What is Real?), and Philip Ball (Beyond Weird) are about rival attempts to say how it can be like that and, in the case of the Freire and Becker books, about the hostility between opposing camps, which sometimes had devastating consequences for the professional careers of the protagonists. I review the issues raised in the three books, and I discuss what Bohr could have meant by the statement (attributed to Bohr by Aage Petersen) that "there is no quantum world," along the lines of a neo-Bohrian, non-representational interpretation of quantum mechanics. (Originally published, with minor changes, as a review article "Interpreting the quantum world: old questions and new answers.")
The Copenhagen Interpretation of quantum mechanics has become the standard way of explaining the physical philosophical meaning of the mathematics of microscopic behavior. It developed from the thought of people who attended the Institute for Theoretical Physics of the University of Copenhagen during the 40 years that Niels Bohr directed it, from 1920 to 1962. Bohr took a large part in developing the Copenhagen Interpretation, he didn't escape all controversy over it (although the quote above is a bit of an exaggeration). Bohr did die before the experimental theoretical developments that have transposed the debate over the appropriateness adequacy of the Copenhagen Interpretation into a new key, he may well have been lucky to have escaped when he did. There is no authoritative codification of CI [the Copenhagen Interpretation], write T Gornitz C. E von Weizsacker of the Max Planck Institute in Starnberg, West Germany in' their formal presentation to the recent Loyola Conference on Mathematical Interpretational Problems in Relativistic Quantum Theory, and under the conditions of its origin there probably could not be one.... All prominent authors of the time who wrote on these questions, like Bohr, [Werner] Heisenberg, [Wolfgang] Pauli, J[ohn] v[on] Neumann on one side, [Albert] Einstein [Erwin] Schrodinger on the other, fell, so we feel, into some stammering when they tried to express their own positions. Nevertheless there is what might be called a Copenhagen attitude. It concentrates on observable phenomena, makes statistics probability central to its understanding, considers quantum mechanics primarily a theory of knowledge or information becomes a kind of physica negativa (analogous to a theologia negativa) in trying not to say too much about things that can't be seen or felt. For example, it doesn't much care whether the orbits of electrons in atoms are real or not and, in fact, tends to regard them as unreal. It appears in most textbooks as the standard way of looking at things.