Physics Today

Classical and quantum framing of the Now

Berge Tatian

Published Sep 1, 2014• 0 citations

Abstract

The trouble David Mermin is having with the concept of Now is similar to the problem that Plato, Aristotle, and Martin Heidegger had with the concept of being. Both have no meaning when considered as individual points; they are continuous entities and can only be understood as such. Heidegger pointed out that for most people, being and Now are self-evident. Apparently, it’s only physicists and philosophers who are embarrassed by their lack of understanding of such obvious concepts. According to Ockham’s razor, the simplest explanation is usually the best one. And Albert Einstein, who venerated simplicity, once said that the only reason time exists is “so that everything doesn’t happen all at once.” That statement may seem facetious, but it is a corollary of his equating of space and time.© 2014 American Institute of Physics.

DOI: 10.1063/pt.3.2493

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Although I have neither an objection to nor an endorsement of David Mermin’s major premise in his Quick Study, “There is no quantum measurement problem” (Physics Today, June 2022, page 62), I would like to make several contextual points.Mermin’s argument has an extensive history, along with a comparable history of rebuttal. Five decades ago Max Jammer wrote that “measurements in quantum mechanics are no more or less problematic than in classical physics, for the Hilbert space vector is only a purely formal device for relating the statistics associated with these arrangements to the physics of observations in classical physics.” That seems like a condensed version of the argument Mermin presents. On the following page, however, Jammer notes that a “problematic aspect whose serious implications were only gradually understood was the fact that as long as a quantum mechanical one-body or many-body system does not interact with macroscopic objects, as long as its motion is described by the deterministic Schrödinger time-dependent equation, no events could be considered to take place in the system.”11. M. Jammer, The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective, Wiley (1974), pp. 473, 474.Niels Bohr giving a lecture in Copenhagen in April 1929. (Photograph by Samuel Goudsmit, courtesy of the AIP Emilio Segrè Visual Archives, Goudsmit Collection.)PPT|High resolutionRegarding the quotations from Niels Bohr, Mermin’s point is entirely valid and represents Bohr’s position accurately, but Bohr’s thought is always subtle and difficult to capture completely. Another quotation from Bohr, which is consistent with everything else he wrote, is, “The decisive point is that in neither case does the appropriate widening of our conceptual framework imply any appeal to the observing subject, which would hinder unambiguous communication of experience.”22. N. Bohr, Essays 1958–1962 on Atomic Physics and Human Knowledge, Ox Bow Press (1987), p. 7. That quotation seems inconsistent with Mermin’s paragraph that includes the passage “Why should I insist that my interpretation of science, which I use to make sense of the world that I experience, should never make any mention of me?” Bohr always insisted that his framework was a rational “generalization” of classical objectivity.The last and most conceptually important point concerns the ontological status of probability. I don’t necessarily disagree with Mermin and Bruno de Finetti, but it is by no means obvious that they are correct either. David Hawkins, for example, begins 50 pages of rigorous analysis with a brief statement of a major premise.The obvious and natural interpretation of probability in this context is that it introduces, not something that has to do with the nature of the physical system, but something that has to do with the incompleteness of our knowledge….But this opinion turns out to be false. Ignorance of dynamical regularity may be a motive for resorting to probabilities, but it does not explain the probabilities to which we resort.33. D. Hawkins, The Language of Nature: An Essay in the Philosophy of Science, W. H. Freeman (1964), p. 135.Another example can be found in Jammer’s well-known historical account of quantum theory: “For Einstein the notion of probability … was the traditional conception of classical physics, a mathematical objectivization of the human deficiency of complete or exact knowledge but ultimately a creation of the human mind…. For Born probability, as far as it was related to the wave function, was not merely a mathematical fiction but something endowed with physical reality.”44. M. Jammer, The Conceptual Development of Quantum Mechanics, McGraw-Hill (1966), p. 286.Of course, such examples don’t mean that Mermin’s position is incorrect, but I do think that they demand a better argument than a comparison to “Fairies and Witches.” I am confident that Mermin has such an argument, and I look forward to reading it as I continue working toward a coherent position on the unresolved interpretational issues of quantum theory.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. M. Jammer, The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective, Wiley (1974), pp. 473, 474. Google Scholar2. N. Bohr, Essays 1958–1962 on Atomic Physics and Human Knowledge, Ox Bow Press (1987), p. 7. Google Scholar3. D. Hawkins, The Language of Nature: An Essay in the Philosophy of Science, W. H. Freeman (1964), p. 135. Google Scholar4. M. Jammer, The Conceptual Development of Quantum Mechanics, McGraw-Hill (1966), p. 286. Google Scholar© 2022 American Institute of Physics.

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Classical and quantum framing of the Now

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Mermin replies: Understanding the Now was not the main purpose of my March 2014 commentary. My primary point was that a new way of resolving the puzzles and paradoxes of quantum mechanics, called QBism by Christopher Fuchs and Rüdiger Schack, was also able to settle that longstanding problem in classical physics. After my earlier commentary on QBism (Physics Today, July 2012, page 8), I was concerned that many of the letters about it (December 2012, page 8) had sounded the same theme: that the letter writer’s own way of looking at quantum mechanics was already perfectly satisfactory. I hoped that by applying QBist thinking to a strictly classical problem, I could disengage those readers from their favorite interpretation of quantum mechanics and help them think about QBism on its own merits. I recently emphasized in Nature11. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a that QBism sheds light on classical physics, too, but there I discussed quantum applications of QBism as well as classical applications (“CBism”).The letters here all address only the problem of the Now but not the fact that I deal with it through a classical application of QBism. While I’m disappointed that they say nothing about QBism or CBism, I’m pleased that they all agree that the problem of the Now is indeed a problem. Not everybody does.Berge Tatian and B. K. Ridley both criticize me for taking the Now to be a point. But I don’t. I call the Now an event “whose duration and location are restricted enough that it can usefully be represented as a point in space and time.” I say that “the events we experience are complex, extended entities” and that “to represent our actual experiences as a collection of mathematical points … is a brilliant strategic simplification, but we ought not to confuse a cartoon … with the experience itself.”Ridley’s comment “Nor could one person’s Now be exactly the same as another’s” suggests that I say it could be. What I do say is “The commonality of my Now and your Now whenever we are together requires that our Nows must coincide at each of two consecutive meetings.” “Commonality” or “coincide” mean only that our two private Now experiences happen at a common place and time, not that they are identical. Indeed, the personal experiences of different people are incomparable, except through the imperfect medium of language.I agree with James Hartle on much in the paper he cites, but we have important differences. He takes spacetime to be objective and fundamental; I take it to be an abstract tool used by an agent to organize her experience. He uses the notion of a point in spacetime uncritically; I regard it as an approximate representation of an agent’s spatially and temporally extended experience. He takes an agent’s experience to be an objective property of the agent, like the contents of a register. I take an agent’s experience to be private and self-evident to that agent and to be the fundamental basis for her inference of an external world; the experience of each agent plays a special role for that—and only that—agent, analogous to the special role played by “the classical domain” in the quantum mechanics of Lev Landau and Evgeny Lifshitz.Rudolf Peierls wrote to John Bell in 1980, “In my view, a description of the laws of physics consists in giving us a set of correlations between successive observations. By observations I mean … what our senses can experience. That we have senses and can experience such sensations is an empirical fact, which has not been deduced (and in my opinion cannot be deduced) from current physics.”22. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. If “us” is expanded to “each of us,” then nobody has ever put QBism and CBism more concisely than that.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. N. D. Mermin, Nature 507, 421 (2014). https://doi.org/10.1038/507421a, Google ScholarCrossref, ISI2. S. Lee, ed., Sir Rudolf Peierls: Selected Private and Scientific Correspondence, vol. 2, World Scientific, River Edge, NJ (2009), p. 807. Google ScholarCrossref© 2014 American Institute of Physics.

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Mermin habitually answers opinions, real and abstract

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