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Physics Today

Reviews of quantum foundations

L. E. Ballentine

Published Jun 1, 2020• 1 citation
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Abstract

I enjoyed the February 2019 issue of Physics Today on Reviews of Modern Physics at 90 but was disappointed with the article “Quantum foundations” by David DiVincenzo and Christopher Fuchs (page 50). The most useful part of that article was the reference list, which shows RMP’s diversity of papers on the subject. My 1970 article on the statistical-ensemble interpretation of quantum mechanics (QM),11. L. E. Ballentine, Rev. Mod. Phys. 42, 358 (1970). https://doi.org/10.1103/RevModPhys.42.358 which people tell me has encouraged them to continue research on quantum foundations (QF), was omitted from the list.Unfortunately, DiVincenzo and Fuchs continue to mystify measurement in QM, as if it were some deep philosophical concept that must be treated before QM has even been fully formulated. They assert that “physicists and philosophers are still debating what a ‘measurement’ really means.” What is important for QF is not the meaning of the word but an understanding of the physical process. The authors do not cite any of the published papers that provide such an understanding. And they give too much attention to two marginal interpretations: the many-worlds interpretation (MWI) and quantum Bayesianism (QBism).In QM, a measurement of an observable should yield an eigenvalue of the observable. If the initial state of the measured object is a superposition of eigenstates corresponding to different eigenvalues, then the interaction of the measurement apparatus with the object will lead to a final state of the whole system—measured object plus apparatus—that is a superposition of different measurement results. The squared amplitude of each term yields the probability of obtaining that result in an individual measurement. That statistical prediction, the Born rule, is common to the Copenhagen and statistical-ensemble interpretations. But the MWI takes a radically different turn. It postulates that the universe branches into several parallel worlds, with each term of the superposition corresponding to the unique result of the measurement in one branch world.The usual role of an interpretation of QM is to begin with the established mathematical formalism and provide an intuitively comprehensible idea of the physical process that the math describes. The MWI does not do that. Instead, it adds a mysterious process of world-splitting, a strange new cosmology that is alien to the mathematics of QM and not really an interpretation of QM at all. A typical QM measurement, such as that of a spin component in the Stern–Gerlach experiment, is a local and very low energy event. It is not credible that the measurement could have the huge cosmological effect of bifurcating the universe.When I first heard of the world-splitting assumed in the MWI, I went back to Hugh Everett’s paper22. H. Everett III, Rev. Mod. Phys. 29, 454 (1957). https://doi.org/10.1103/RevModPhys.29.454 to see if he had really said anything so absurd. I found that he had not said so explicitly, but he sometimes used words that could be interpreted in more than one way. The MWI is a possible interpretation of them, but not the most natural one, so I thought. And Everett’s framework still has value even without resorting to the MWI’s world-splitting. His concept of a “relative state” is useful, for instance, and he is correct in rejecting the notion of the quantum state “collapsing” after a measurement.QBism begins with the assumption that all kinds of probability can be regarded as subjective Bayesian probabilities. That assumption can be maintained only by ignoring the literature on interpretations of probability, from which it is clear that several different kinds—or interpretations—of probability exist. DiVincenzo and Fuchs may have ignored the classic philosophical writings on the subject because they were written by philosophers for philosophers and so do not address the needs of physicists.I have published a paper on the foundations of probability theory, written from the point of view of a quantum physicist.33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0 I classify the main kinds or interpretations of probability into three groups: inferential probability, of which Bayesian theory is an example; frequency or ensemble probability, commonly used in Gibbsian statistical mechanics and in QM; and propensity theory. Propensity, a degree of causality that is weaker than determinism, is not merely another interpretation of probability. Its mathematical theory must also differ from that of probability theory, as Paul Humphreys showed44. P. Humphreys, Philos. Rev. 94, 557 (1985). https://doi.org/10.2307/2185246 in 1985. Although the axioms of propensity33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0 differ from those of probability, the two axiom sets overlap. Both support the law of large numbers, so propensity theory is compatible with the most useful part of the frequency interpretation of probability.In general, QM states do not determine the results of a measurement, only the probabilities of the possible results. That a state’s influence on the results is not deterministic suggests strongly that the quantum probabilities given by the Born rule should be interpreted as propensities. They refer objectively to the physical system and its environment, not to any agent’s knowledge, so they are not naturally interpreted as subjective Bayesian probabilities.Interpretations of probability may differ not only in philosophy but also in substance. As I discuss in reference 33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0, John Bell’s theorem illustrates how local hidden-variable theories are incompatible with QM. E. T. Jaynes was a well-known supporter of the Bayesian theory of probability. In 1989 he repeated Bell’s derivation of inequality but carefully treated all instances of probability as Bayesian. He found that the derivation could not be completed without invoking an extra assumption that was not justifiable in the Bayesian theory. Bell’s theorem involves questions about causality, so it is natural to use propensity theory to treat it. That method is successful in deriving Bell’s inequality.33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0Not all probabilities occurring in QM can be treated as subjective Bayesian probabilities. That limitation disqualifies QBism, a Bayesian-based theory, as an interpretation of QM that can succeed in quantum foundations. The initial assumption of QBism is not valid.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. L. E. Ballentine, Rev. Mod. Phys. 42, 358 (1970). https://doi.org/10.1103/RevModPhys.42.358, Google ScholarCrossref2. H. Everett III, Rev. Mod. Phys. 29, 454 (1957). https://doi.org/10.1103/RevModPhys.29.454, Google ScholarCrossref3. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0, Google ScholarCrossref4. P. Humphreys, Philos. Rev. 94, 557 (1985). https://doi.org/10.2307/2185246, Google ScholarCrossref© 2020 American Institute of Physics.

DOI
10.1063/pt.3.4488

Related papers

Measured responses to quantum Bayesianism

Michael Nauenberg

Some time ago, John Bell and I wrote a tongue-in-cheek article on the interpretation of quantum mechanics;11. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270;reprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. in it we remarked that only a minority of physicists had any interest in the topic and that the typical physicist thought he would understand it “if ever he can spare twenty minutes to think about it.” According to David Mermin, however, that situation has changed dramatically, and “new interpretations [of quantum mechanics] appear every year.”Actually, most textbooks on the subject continue to be based on the well-established Born interpretation of quantum mechanics, which postulates that the absolute square of the Schrödinger wavefunction, projected onto a particular quantum state, gives the probability for the occurrence of that state after an appropriate measurement has been made. Mermin indicated that the problem with the Born interpretation involves a confusion, “prevalent among physicists,” regarding the nature of probability. He presented his preference for a “personalist Bayesian view,” which leads separate “agents,” like Eugene Wigner and his friend, to make different probability assignments based on their beliefs. But as Richard Feynman remarked, “Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.”22. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7.Indeed, as applied to experiments, probability simply corresponds to the relative frequency for the occurrence of a given final state after identical states are prepared initially and measurements are made by appropriate detectors—devices that, through an irreversible macroscopic process, provide a permanent record of the outcome. Hence, the results of an experiment are independent of any observers, whose only role is to set up appropriate measurement devices, calculate the probabilities from quantum theory, and then write papers based on the recorded results.As Max Born also emphasized, in classical mechanics the situation is analogous, when one takes into account that in practice initial conditions in an experiment are known only imprecisely.33. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Hence the observed evolution of the system is also determined by a probability function that satisfies the Liouville equation. But in that case, after a measurement is performed, no one talks about the “collapse” of this classical function, whereas in quantum mechanics it is common to use that controversial expression for the Schrödinger wavefunction.Mermin discussed a “shifty split” between the classical and the quantum domain, a split that supposedly “haunts quantum mechanics.” He claimed that “regardless of what is split from what, all versions of the shifty split are vague and ambiguous.” However, that is not always the case, as a simple example shows. Consider an initial quantum wavepacket for a particle under an attractive central force that varies inversely with the square of the radial distance—the classic problem of planetary motion from the viewpoint of quantum mechanics, for example, or an excited electron in a Rydberg atom. For a finite time, the wavepacket will follow a classical Keplerian elliptic orbit, but like the Liouville distribution for the analogous classical problem, from the outset it will disperse due to the spread in initial position and momentum.For the classical distribution to be in close correspondence with the quantum wavepacket, its initial spread is assumed to satisfy the uncertainty relation. Eventually, however, the head of the wavepacket, like in the classical distribution, will catch up with its tail. Afterwards, further spreading of the wavepacket leads to interference effects between the two components, and the classical–quantum correspondence ceases to be valid. The time when such interference effects first appear is of order nτ/3, where n is the mean principal quantum number of the wavepacket and τ is the Kepler period.44. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133 Experimentally, that transition has been observed to agree with theory in Rydberg atoms55. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44 for n = 100, τ = 152 picoseconds, and nothing shifty about this classical–quantum correspondence is found. For planetary orbits, however, n is of order 1070, and therefore the answer to Albert Einstein’s often repeated remark “Is the moon there when nobody looks?” (see David Mermin, Physics Today, April 1985, page 38) is yes.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270; Google Scholarreprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. Google Scholar2. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7. Google Scholar3. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Google Scholar4. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133, Google ScholarCrossref, CAS5. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44, Google ScholarCrossref© 2012 American Institute of Physics.

Physics Today•Nov 30, 2012•0 citations

Measured responses to quantum Bayesianism

N. David Mermin

Mermin replies: Robert Griffiths takes my joke about an undergraduate collapsing a wavepacket to be an attempt to solve the measurement problem. The actual QBist solution—versions of which predate QBism (see, for example, reference 11. J. B. Hartle, Am. J. Phys. 36, 704 (1968). https://doi.org/10.1119/1.1975096)—is that a quantum state is not an objective property of the world but a compendium of probabilities constructed by an agent. When an agent updates her state assignment, nothing changes in her external world. The only change is in the agent’s expectations for her subsequent experience of that world, and therefore there is no measurement problem. Consistent historians agree that collapse is not a physical process, but for entirely different reasons.QBism takes the “mysterious whatever-it-is” that Griffiths criticizes in quantum orthodoxy to be the experience induced in an agent by the world. That, and the fact that different agents may have different experiences, drains the swamp, slays the dragon, and quiets the maddening paradoxes.QBists hold that any agent can indeed apply quantum mechanics to her own external world, from quarks to quasars. Only an agent’s personal, directly perceived experience is boxed off from the domain of applicability. That exclusion disagrees with most versions of the many-worlds interpretation, but it does not limit the scope of science: Alice can apply quantum mechanics to the experience of another agent, Bob, as represented through his words, writings, or drawings, all of which do belong to her external world.The way in which Bob’s experience impinges on Alice resonates with Niels Bohr’s emphasis on the need to state results of experiments in ordinary language. It also addresses Ching Hung Woo’s question of how different agents can come to agreement on their state assignments.Griffiths correctly notes that QBism violates the first desideratum he quotes from my 15-year-old “Ithaca interpretation.” Like Barack Obama’s view of marriage, my thinking about quantum foundations has evolved. But Griffiths wrongly claims that QBism rejects the other desideratum he quotes: locality. Actually, QBism insists on it. The absurdity of quantum nonlocality provides an independent argument that quantum states cannot be objective internal properties of the systems they describe, whether or not one regards probabilities as subjective. Griffiths is probably right, however, that John Bell would not have been pleased with how QBists fix his shifty split.I would not call QBism an “antiquated approach.” It is younger than Griffiths’ consistent histories and, as I have remarked upon elsewhere,22. M. Schlosshauer, ed., Elegance and Enigma: The Quantum Interviews, Springer, Berlin (2011), pp. 177, 280. taking reality itself to be relative to what he calls a framework, is vastly more radical than taking quantum state assignments to be relative to an agent.But foreshadowings of QBism can indeed be found in antiquity. Since writing my commentary, I came across another striking anticipation in a 1931 letter from Erwin Schrödinger to Arnold Sommerfeld:33. K. von Meyenn, ed., Eine Entdeckung von ganz außerordentlicher Tragweite: Schrödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon, Springer, Berlin (2011), p. 490. “Quantum mechanics forbids statements about what really exists—statements about the object. It deals only with the object–subject relation. Although this holds, after all, for any description of nature, it appears to hold in a much more radical and far-reaching sense in quantum mechanics” (translated by N. D. Mermin and Ruediger Schack).When Bell and Michael Nauenberg wrote their delightful article in 1966, there was indeed much less interest in quantum foundations than there is today. Most textbooks have wisely steered clear of the many contemporary interpretive positions, since none of them are held by more than a fraction of those physicists with an interest in such issues.QBists do not reject the Born rule for calculating probabilities, but they do reject the objective frequentist interpretation of those probabilities presented by Nauenberg and held by most physicists. The frequentist view is notoriously circular. It defines probability using such notions as “equally probable,” “unlikely,” or (with Nauenberg) “identically prepared,” none of which make sense without a prior definition of probability. “Identically prepared” might seem safe from circularity, but two different preparations cannot be strictly identical. It would be more accurate to say they must differ in unimportant ways. Unimportant for what? For the probabilities of the outcomes.When I started to learn about subjective probability, I was surprised to discover that most of the books were not in Cornell University’s physics or mathematics libraries but in the business school library. In our recent election, Americans have been told that business experience is necessary for being president; I would suggest that it may be even more helpful for understanding quantum mechanics.I cannot tell from Nauenberg’s remarks about the Liouville equation whether he agrees with the view of QBists (and many others) that the collapse of the wavefunction is no more than the updating of a collection of probability distributions.Nauenberg’s example of the Rydberg atom does not contradict the QBist resolution of Bell’s shifty split. The Rydberg atom is part of the world external to the agent, and therefore on the quantum side of her split. There are indeed circumstances under which fully quantum mechanical behavior can look very classical.Art Hobson repeats the common view that only mixed states can be associated with subjective probabilities. QBism takes that to be the very mistake that got us lost for so long in the quantum swamp. Hobson also believes that wavefunctions were collapsing before there were physicists. Does he believe that probabilities were updating before there were statisticians?Hobson wrongly claims that Chris Fuchs “might dispute the notion that some kind of physical reality actually exists.” What he and other QBists do deny is that there are properties of the external world—“elements of reality”—that underlie quantum state assignments. (The argument of Matthew Pusey and coauthors that Hobson cites, implying that quantum states are physically real, relies on such hidden variables.) An agent’s state assignment rests entirely on her experience of the external world. It is neither solipsistic nor superstitious to maintain that any agent’s experience comes fully into being only at the moment it is experienced.Hobson’s treatment of Eugene Wigner and his friend relies on decoherence producing an objective mixed state, which both of them must agree on. But since quantum mechanics holds all the way out to infinity (as Griffiths puts it), even from an objective view of quantum states, that switch from superposition to either/or is at best a FAPP solution, to use Bell’s wonderfully sardonic abbreviation of “for all practical purposes.”QBism does not, as Hobson concludes, give up on a realist interpretation of nature. But it does warn us not to confuse nature with the abstractions we have ingeniously constructed to help any agent deal with the very real impact of nature on his or her own internal experience (see my Reference Frame, Physics Today, May 2009, page 8).I’m glad that Blake Stacey has called attention to some of the more applied spinoffs of QBism. I’m also pleased that he mentions Bruno de Finetti, one of the great 20th-century pioneers of subjective probability. Indeed, since there are objective as well as subjective Bayesians, if I had my way, the B in QBism would stand not for Thomas Bayes, but for Bruno de Finetti, who put the crucial point like this: “The abandonment of superstitious beliefs about the existence of Phlogiston, the Cosmic Ether, Absolute Space and Time … , or Fairies and Witches, was an essential step along the road to scientific thinking. Probability, too, if regarded as something endowed with some kind of objective existence, is no less a misleading misconception, an illusory attempt to exteriorize or materialize our [actual (vero)] probabilistic beliefs.”44. B. de Finetti, Theory of Probability, Interscience, New York (1990), preface.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. B. Hartle, Am. J. Phys. 36, 704 (1968). https://doi.org/10.1119/1.1975096, Google ScholarCrossref2. M. Schlosshauer, ed., Elegance and Enigma: The Quantum Interviews, Springer, Berlin (2011), pp. 177, 280. Google ScholarCrossref3. K. von Meyenn, ed., Eine Entdeckung von ganz außerordentlicher Tragweite: Schrödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon, Springer, Berlin (2011), p. 490. Google ScholarCrossref4. B. de Finetti, Theory of Probability, Interscience, New York (1990), preface. Google Scholar© 2012 American Institute of Physics.

Physics Today•Nov 30, 2012•6 citations

More on the quantum measurement problem

Gregory N. Derry

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.

Physics Today•Nov 1, 2022•2 citations

Many Worlds?: Everett, Quantum Theory, and Reality

Robert M. Wald

Few, if any, issues in physics have engendered as much discussion as the measurement problem in quantum mechanics. It is generally agreed that the `normal' dynamical evolution of the state vector in quantum mechanics is given by a unitary map. The linearity of this map implies that the state vector will, in general, be found in a superposition of eigenstates of a given observable (or, similarly, that the density matrix describing a subsystem will not correspond to a definite value of this observable). However, when we make a measurement of an observable, we always obtain a define value—although it is impossible to predict with certainty which value will be obtained. The traditional response to this issue is to postulate that when a measurement is made, the wavefunction `collapses' to an eigenstate of the observable being measured, perhaps due to the inherent classicality of the measuring apparatus (Bohr), or to the consciousness of the observer (Wigner), or possibly to some modification of quantum dynamics that occurs even when observations are not being made. The main motivation for the Everett (`many worlds') interpretation is to avoid introducing any such collapse postulate. This volume commemorates the 50th anniversary of the publication of Everett's paper in 1957 and contains 20 original articles as well as the transcripts of several discussions that took place at meetings devoted to the Everett interpretation at Oxford University and the Perimeter Institute. The attractiveness of the Everett interpretation is very succinctly summarized by a sentence from Vaidman's contribution (p 582): `The collapse, with its randomness, non-locality and the lack of a well-defined moment of occurrence, is such an ugly scar on quantum theory, that I, along with many others, am ready to follow Everett and deny its existence.' But the main drawback of the interpretation is then equally succinctly stated in the next sentence: `The price is the many worlds interpretation, i.e., the existence of numerous parallel worlds.' In other words, if the state vector evolves to have a non-zero amplitude for two different values of an observable that I `measure' (i.e., interact with in an appropriate way), each of these alternatives remains equally `real', i.e., there is a `branch' of the wavefunction in which the observable takes the first value and I believe that it has this first value and a branch of the wavefunction where it takes the second value and I believe that it has this second value. Each of these `me's' sees a definite value for the observable, thus nicely accounting for what I believe I experience without invoking collapse. But then, it seems that I have to believe that there are many other `me's' around—even if they are very difficult for this particular me to access—and no way I can predict which `me' I will be in the future. Indeed, Saunders writes on p 192, `If Alice is a person (of course she is) then we must say, even prior to branching at tj that there are many Alices present, atom for atom duplicates...Alice should be uncertain after all— each Alice should be uncertain—for each as of tj does not (and as a matter of principle, cannot) know which of these branching persons she is.' Thus, if taken seriously, the Everett viewpoint leads directly to questions like `What determines which ``me'' I am now and/or will feel that I am in the future?' The response `You are all of them' somehow doesn't seem satisfying, and further pursuit of these issues would seem to lead to questions as fruitless as questions like `Why am I ``me'' and not ``you''?' I believe that this accounts for why the Everett interpretation does not have universal appeal. But enough of `me' for now (more later); let's discuss the book. Unlike many edited volumes and conference proceedings in physics—which often consist of hastily slapped-together re-hashes of previous papers—this volume meets very high standards in the choice of topics and authors and in the quality of the writing. Although each of the articles is closely tied in with the Everett interpretation, there are many articles of considerable value and interest in their own right, such as the discussion of quasiclassical realms by Hartle and and the discussion of the de Broglie–Bohm interpretation by Valentini. There is also an excellent introduction by Saunders to the quantum measurement problem and the main issues that arise in the Everett interpretation. A central issue in the Everett interpretation is the status of the `Born rule', which asserts that, for state ψ, the probability of obtaining a particular outcome of a measurement is ||Pψ||2, where P is the projection operator onto the eigensubspace associated with the measurement outcome. In traditional interpretations, the Born rule is simply postulated as part of the collapse hypothesis. In the Everett interpretation, it is far from obvious that the Born rule even has any meaning—if all outcomes occur, how can one talk about the probability of a particular outcome? Given the importance of this issue, it is highly appropriate that four chapters of the book (by Saunders, Papineau, Wallace, and Greaves and Myrvold) are devoted to addressing probability and the Born rule from the Everett viewpoint, and three chapters (by Kent, Albert, and Price) are devoted to criticising these views. As indicated by the above quote from Saunders' article, uncertainty and probability can enter the Everett interpretation via each Alice being uncertain as to which Alice she is or will be. However, in view of the difficulties to which this kind of argumentation will lead, it is perhaps not surprising that in the 123 pages of combined discussion by Saunders, Papineau, Wallace, and Greaves and Myrvold, there are only fleeting references to the use of probability along these lines. Rather, the approach taken is reminiscent of that of adept politicians: if you can't answer a question, then answer a different question, and answer it with conviction and detail. Instead of considering probabilities of outcomes—a highly problematical notion when all of the possible outcomes actually occur—consider only decision theory issues. In particular, Wallace gives a precise and detailed mathematical proof that, under certain hypotheses, the only rational strategy that a quantum observer can follow in making decisions is to weight the possible outcomes by Born rule probabilities. To illustrate this more graphically, suppose tomorrow's weather were determined by some quantum mechanical, Stern–Gerlach-like process. I would like to know whether it is likely to rain tomorrow. However, when I ask an Everett follower (who knows the state vector and has done the Born rule calculation), all she will tell me in direct response to my question is that `it will rain and it won't rain.' Nevertheless, she is willing tell me whether or not it is rational for me to take an umbrella. I suppose that if she tells me that it would be highly irrational for me to take an umbrella, I can take the hint and deduce that it is very unlikely to rain. However, there does seem something wrong with the fact that she is not allowed to say this directly. In any case, if the conclusion of a mathematically correct argument is that rational decision strategies require the Born rule, then there must be quite a bit lying in the assumptions. The articles by Kent, Albert, and Price do a good job of fleshing out these assumptions and pointing out the weaknesses and flaws in the probability and decision theory discussions within the Everett framework. I hope it is clear from what I have written above that this is an outstanding volume, which should be of great interest to anyone interested in the foundations of quantum theory. Indeed, I feel quite fortunate to be the particular `me' that happens to exist in the same branch of the wavefunction of the universe in which this book was published.

Classical and Quantum Gravity•Apr 21, 2011•24 citations

Measured responses to quantum Bayesianism

Art Hobson

Because it’s important that physicists discuss the problems of quantum foundations, I was delighted to see David Mermin’s commentary on “fixing the shifty split.” But I must disagree with my friend’s view.There are multiple ways of interpreting probabilities.11. A. Hobson, Concepts in Statistical Mechanics, Gordon and Breach, New York (1971), p. 27. In quantum physics, the Bayesian “degree of belief” interpretation can be appropriate for mixed states, but only the frequentist or “ensemble” interpretation applies to the pure states that Mermin discusses.A recent paper by Matthew Pusey and coauthors22. M. F. Pusey, J. Barrett, T. Rudolph, http://arxiv.org/abs/1111.3328v1. concludes that if two different pure quantum states are assigned to a single physical situation, one of the states is objectively wrong. The proof’s main assumption is that any quantum system has some set of real physical properties, labeled λ. Pusey and coauthors show that two different pure states cannot represent the same λ. So, although their paper implies but does not assume that quantum states are physically real, it does assume that each specific physical situation has behind it some kind of physical reality. Chris Fuchs, quoted favorably by Mermin, might dispute the notion that some kind of physical reality actually exists, but most physicists are probably sufficiently realist to grant that notion. Without such a notion, science lies somewhere between solipsism and superstition.Whenever I hear subjective interpretations of quantum physics, I wonder about such questions as the one Mermin quotes from Albert Einstein: Can a mouse collapse a wavefunction? Mermin’s Bayesian response, that a mouse cannot but a physics student can, doesn’t reassure me. Were wavefunctions not collapsing before there were physicists?The “Wigner’s friend” paradox, as described in Mermin’s commentary, doesn’t need a subjective interpretation to find a solution. In fact, Eugene Wigner abandoned his interpretation around 1970 once he became aware of the ideas underlying quantum decoherence.33. M. Schlosshauer, Decoherence and the Quantum-to-Classical Transition, Springer, Berlin (2007), p. 364. Suppose Wigner’s friend makes a measurement, the outcome of which is known to the friend but not to Wigner. Regardless of whether the outcome is known, and in fact regardless of the presence of the friend, the measurement device must decohere the quantum system that’s being measured, putting the system into a mixed state of the either/or form, with no interferences, just like a coin that’s been flipped. A definite outcome has thus occurred, and Wigner and his friend will both agree on this. Neither the friend nor Wigner affects the outcome. She has simply “looked” and now knows the outcome, while Wigner doesn’t know. That is not paradoxical and does not require a revision of our understanding of pure quantum states.There’s no reason to give up on a realist interpretation of quantum physics, much less a realist interpretation of nature itself.REFERENCESSection:ChooseTop of pageREFERENCES <<1. A. Hobson, Concepts in Statistical Mechanics, Gordon and Breach, New York (1971), p. 27. Google Scholar2. M. F. Pusey, J. Barrett, T. Rudolph, http://arxiv.org/abs/1111.3328v1. Google Scholar3. M. Schlosshauer, Decoherence and the Quantum-to-Classical Transition, Springer, Berlin (2007), p. 364. Google Scholar© 2012 American Institute of Physics.

Physics Today•Nov 30, 2012•1 citation

Quantum Philosophy

Christopher C. Gerry, Kimberley M. Bruno

Standard quantum theory is highly successful in all its applications, from photons and atoms, to semiconductors, superconductors, quarks, and so on. Yet the need to understand what the theory is really trying to tell us about the nature of the world beyond specific predictions forces us into the realm of philosophy, or, as some have dubbed it, quantum metaphysics: the study of the interpretation of quantum mechanics. There is a wide range of opinions regarding the philosophical foundations of quantum mechanics. This chapter focuses on two of the most prominent interpretations: the Copenhagen interpretation, and one alternative interpretation that has been gaining some support in recent years, the many-worlds interpretation (MWI), also known as the many universes interpretation. The central issues of contention are: firstly, that the measurement of some attribute of a quantum system generally involves the probabilistic collapse, or reduction, of the state vector where the probabilistic nature of the process is inherent—that is, not reducible to a more fundamental, deterministic explanation; and, secondly, the problem of bringing a measurement to a completion—a problem generally known as the measurement paradox.

Oxford University Press eBooks•Feb 21, 2013•0 citations
I enjoyed the February 2019 issue of Physics Today on Reviews of Modern Physics at 90 but was disappointed with the article “Quantum foundations” by David DiVincenzo and Christopher Fuchs (page 50). The most useful part of that article was the reference list, which shows RMP’s diversity of papers on the subject. My 1970 article on the statistical-ensemble interpretation of quantum mechanics (QM),11. L. E. Ballentine, Rev. Mod. Phys. 42, 358 (1970). https://doi.org/10.1103/RevModPhys.42.358 which people tell me has encouraged them to continue research on quantum foundations (QF), was omitted from the list.Unfortunately, DiVincenzo and Fuchs continue to mystify measurement in QM, as if it were some deep philosophical concept that must be treated before QM has even been fully formulated. They assert that “physicists and philosophers are still debating what a ‘measurement’ really means.” What is important for QF is not the meaning of the word but an understanding of the physical process. The authors do not cite any of the published papers that provide such an understanding. And they give too much attention to two marginal interpretations: the many-worlds interpretation (MWI) and quantum Bayesianism (QBism).In QM, a measurement of an observable should yield an eigenvalue of the observable. If the initial state of the measured object is a superposition of eigenstates corresponding to different eigenvalues, then the interaction of the measurement apparatus with the object will lead to a final state of the whole system—measured object plus apparatus—that is a superposition of different measurement results. The squared amplitude of each term yields the probability of obtaining that result in an individual measurement. That statistical prediction, the Born rule, is common to the Copenhagen and statistical-ensemble interpretations. But the MWI takes a radically different turn. It postulates that the universe branches into several parallel worlds, with each term of the superposition corresponding to the unique result of the measurement in one branch world.The usual role of an interpretation of QM is to begin with the established mathematical formalism and provide an intuitively comprehensible idea of the physical process that the math describes. The MWI does not do that. Instead, it adds a mysterious process of world-splitting, a strange new cosmology that is alien to the mathematics of QM and not really an interpretation of QM at all. A typical QM measurement, such as that of a spin component in the Stern–Gerlach experiment, is a local and very low energy event. It is not credible that the measurement could have the huge cosmological effect of bifurcating the universe.When I first heard of the world-splitting assumed in the MWI, I went back to Hugh Everett’s paper22. H. Everett III, Rev. Mod. Phys. 29, 454 (1957). https://doi.org/10.1103/RevModPhys.29.454 to see if he had really said anything so absurd. I found that he had not said so explicitly, but he sometimes used words that could be interpreted in more than one way. The MWI is a possible interpretation of them, but not the most natural one, so I thought. And Everett’s framework still has value even without resorting to the MWI’s world-splitting. His concept of a “relative state” is useful, for instance, and he is correct in rejecting the notion of the quantum state “collapsing” after a measurement.QBism begins with the assumption that all kinds of probability can be regarded as subjective Bayesian probabilities. That assumption can be maintained only by ignoring the literature on interpretations of probability, from which it is clear that several different kinds—or interpretations—of probability exist. DiVincenzo and Fuchs may have ignored the classic philosophical writings on the subject because they were written by philosophers for philosophers and so do not address the needs of physicists.I have published a paper on the foundations of probability theory, written from the point of view of a quantum physicist.33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0 I classify the main kinds or interpretations of probability into three groups: inferential probability, of which Bayesian theory is an example; frequency or ensemble probability, commonly used in Gibbsian statistical mechanics and in QM; and propensity theory. Propensity, a degree of causality that is weaker than determinism, is not merely another interpretation of probability. Its mathematical theory must also differ from that of probability theory, as Paul Humphreys showed44. P. Humphreys, Philos. Rev. 94, 557 (1985). https://doi.org/10.2307/2185246 in 1985. Although the axioms of propensity33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0 differ from those of probability, the two axiom sets overlap. Both support the law of large numbers, so propensity theory is compatible with the most useful part of the frequency interpretation of probability.In general, QM states do not determine the results of a measurement, only the probabilities of the possible results. That a state’s influence on the results is not deterministic suggests strongly that the quantum probabilities given by the Born rule should be interpreted as propensities. They refer objectively to the physical system and its environment, not to any agent’s knowledge, so they are not naturally interpreted as subjective Bayesian probabilities.Interpretations of probability may differ not only in philosophy but also in substance. As I discuss in reference 33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0, John Bell’s theorem illustrates how local hidden-variable theories are incompatible with QM. E. T. Jaynes was a well-known supporter of the Bayesian theory of probability. In 1989 he repeated Bell’s derivation of inequality but carefully treated all instances of probability as Bayesian. He found that the derivation could not be completed without invoking an extra assumption that was not justifiable in the Bayesian theory. Bell’s theorem involves questions about causality, so it is natural to use propensity theory to treat it. That method is successful in deriving Bell’s inequality.33. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0Not all probabilities occurring in QM can be treated as subjective Bayesian probabilities. That limitation disqualifies QBism, a Bayesian-based theory, as an interpretation of QM that can succeed in quantum foundations. The initial assumption of QBism is not valid.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. L. E. Ballentine, Rev. Mod. Phys. 42, 358 (1970). https://doi.org/10.1103/RevModPhys.42.358, Google ScholarCrossref2. H. Everett III, Rev. Mod. Phys. 29, 454 (1957). https://doi.org/10.1103/RevModPhys.29.454, Google ScholarCrossref3. L. E. Ballentine, Found. Phys. 46, 973 (2016). https://doi.org/10.1007/s10701-016-9991-0, Google ScholarCrossref4. P. Humphreys, Philos. Rev. 94, 557 (1985). https://doi.org/10.2307/2185246, Google ScholarCrossref© 2020 American Institute of Physics.
Some time ago, John Bell and I wrote a tongue-in-cheek article on the interpretation of quantum mechanics;11. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270;reprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. in it we remarked that only a minority of physicists had any interest in the topic and that the typical physicist thought he would understand it “if ever he can spare twenty minutes to think about it.” According to David Mermin, however, that situation has changed dramatically, and “new interpretations [of quantum mechanics] appear every year.”Actually, most textbooks on the subject continue to be based on the well-established Born interpretation of quantum mechanics, which postulates that the absolute square of the Schrödinger wavefunction, projected onto a particular quantum state, gives the probability for the occurrence of that state after an appropriate measurement has been made. Mermin indicated that the problem with the Born interpretation involves a confusion, “prevalent among physicists,” regarding the nature of probability. He presented his preference for a “personalist Bayesian view,” which leads separate “agents,” like Eugene Wigner and his friend, to make different probability assignments based on their beliefs. But as Richard Feynman remarked, “Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.”22. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7.Indeed, as applied to experiments, probability simply corresponds to the relative frequency for the occurrence of a given final state after identical states are prepared initially and measurements are made by appropriate detectors—devices that, through an irreversible macroscopic process, provide a permanent record of the outcome. Hence, the results of an experiment are independent of any observers, whose only role is to set up appropriate measurement devices, calculate the probabilities from quantum theory, and then write papers based on the recorded results.As Max Born also emphasized, in classical mechanics the situation is analogous, when one takes into account that in practice initial conditions in an experiment are known only imprecisely.33. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Hence the observed evolution of the system is also determined by a probability function that satisfies the Liouville equation. But in that case, after a measurement is performed, no one talks about the “collapse” of this classical function, whereas in quantum mechanics it is common to use that controversial expression for the Schrödinger wavefunction.Mermin discussed a “shifty split” between the classical and the quantum domain, a split that supposedly “haunts quantum mechanics.” He claimed that “regardless of what is split from what, all versions of the shifty split are vague and ambiguous.” However, that is not always the case, as a simple example shows. Consider an initial quantum wavepacket for a particle under an attractive central force that varies inversely with the square of the radial distance—the classic problem of planetary motion from the viewpoint of quantum mechanics, for example, or an excited electron in a Rydberg atom. For a finite time, the wavepacket will follow a classical Keplerian elliptic orbit, but like the Liouville distribution for the analogous classical problem, from the outset it will disperse due to the spread in initial position and momentum.For the classical distribution to be in close correspondence with the quantum wavepacket, its initial spread is assumed to satisfy the uncertainty relation. Eventually, however, the head of the wavepacket, like in the classical distribution, will catch up with its tail. Afterwards, further spreading of the wavepacket leads to interference effects between the two components, and the classical–quantum correspondence ceases to be valid. The time when such interference effects first appear is of order nτ/3, where n is the mean principal quantum number of the wavepacket and τ is the Kepler period.44. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133 Experimentally, that transition has been observed to agree with theory in Rydberg atoms55. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44 for n = 100, τ = 152 picoseconds, and nothing shifty about this classical–quantum correspondence is found. For planetary orbits, however, n is of order 1070, and therefore the answer to Albert Einstein’s often repeated remark “Is the moon there when nobody looks?” (see David Mermin, Physics Today, April 1985, page 38) is yes.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270; Google Scholarreprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. Google Scholar2. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7. Google Scholar3. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Google Scholar4. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133, Google ScholarCrossref, CAS5. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44, Google ScholarCrossref© 2012 American Institute of Physics.
Mermin replies: Robert Griffiths takes my joke about an undergraduate collapsing a wavepacket to be an attempt to solve the measurement problem. The actual QBist solution—versions of which predate QBism (see, for example, reference 11. J. B. Hartle, Am. J. Phys. 36, 704 (1968). https://doi.org/10.1119/1.1975096)—is that a quantum state is not an objective property of the world but a compendium of probabilities constructed by an agent. When an agent updates her state assignment, nothing changes in her external world. The only change is in the agent’s expectations for her subsequent experience of that world, and therefore there is no measurement problem. Consistent historians agree that collapse is not a physical process, but for entirely different reasons.QBism takes the “mysterious whatever-it-is” that Griffiths criticizes in quantum orthodoxy to be the experience induced in an agent by the world. That, and the fact that different agents may have different experiences, drains the swamp, slays the dragon, and quiets the maddening paradoxes.QBists hold that any agent can indeed apply quantum mechanics to her own external world, from quarks to quasars. Only an agent’s personal, directly perceived experience is boxed off from the domain of applicability. That exclusion disagrees with most versions of the many-worlds interpretation, but it does not limit the scope of science: Alice can apply quantum mechanics to the experience of another agent, Bob, as represented through his words, writings, or drawings, all of which do belong to her external world.The way in which Bob’s experience impinges on Alice resonates with Niels Bohr’s emphasis on the need to state results of experiments in ordinary language. It also addresses Ching Hung Woo’s question of how different agents can come to agreement on their state assignments.Griffiths correctly notes that QBism violates the first desideratum he quotes from my 15-year-old “Ithaca interpretation.” Like Barack Obama’s view of marriage, my thinking about quantum foundations has evolved. But Griffiths wrongly claims that QBism rejects the other desideratum he quotes: locality. Actually, QBism insists on it. The absurdity of quantum nonlocality provides an independent argument that quantum states cannot be objective internal properties of the systems they describe, whether or not one regards probabilities as subjective. Griffiths is probably right, however, that John Bell would not have been pleased with how QBists fix his shifty split.I would not call QBism an “antiquated approach.” It is younger than Griffiths’ consistent histories and, as I have remarked upon elsewhere,22. M. Schlosshauer, ed., Elegance and Enigma: The Quantum Interviews, Springer, Berlin (2011), pp. 177, 280. taking reality itself to be relative to what he calls a framework, is vastly more radical than taking quantum state assignments to be relative to an agent.But foreshadowings of QBism can indeed be found in antiquity. Since writing my commentary, I came across another striking anticipation in a 1931 letter from Erwin Schrödinger to Arnold Sommerfeld:33. K. von Meyenn, ed., Eine Entdeckung von ganz außerordentlicher Tragweite: Schrödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon, Springer, Berlin (2011), p. 490. “Quantum mechanics forbids statements about what really exists—statements about the object. It deals only with the object–subject relation. Although this holds, after all, for any description of nature, it appears to hold in a much more radical and far-reaching sense in quantum mechanics” (translated by N. D. Mermin and Ruediger Schack).When Bell and Michael Nauenberg wrote their delightful article in 1966, there was indeed much less interest in quantum foundations than there is today. Most textbooks have wisely steered clear of the many contemporary interpretive positions, since none of them are held by more than a fraction of those physicists with an interest in such issues.QBists do not reject the Born rule for calculating probabilities, but they do reject the objective frequentist interpretation of those probabilities presented by Nauenberg and held by most physicists. The frequentist view is notoriously circular. It defines probability using such notions as “equally probable,” “unlikely,” or (with Nauenberg) “identically prepared,” none of which make sense without a prior definition of probability. “Identically prepared” might seem safe from circularity, but two different preparations cannot be strictly identical. It would be more accurate to say they must differ in unimportant ways. Unimportant for what? For the probabilities of the outcomes.When I started to learn about subjective probability, I was surprised to discover that most of the books were not in Cornell University’s physics or mathematics libraries but in the business school library. In our recent election, Americans have been told that business experience is necessary for being president; I would suggest that it may be even more helpful for understanding quantum mechanics.I cannot tell from Nauenberg’s remarks about the Liouville equation whether he agrees with the view of QBists (and many others) that the collapse of the wavefunction is no more than the updating of a collection of probability distributions.Nauenberg’s example of the Rydberg atom does not contradict the QBist resolution of Bell’s shifty split. The Rydberg atom is part of the world external to the agent, and therefore on the quantum side of her split. There are indeed circumstances under which fully quantum mechanical behavior can look very classical.Art Hobson repeats the common view that only mixed states can be associated with subjective probabilities. QBism takes that to be the very mistake that got us lost for so long in the quantum swamp. Hobson also believes that wavefunctions were collapsing before there were physicists. Does he believe that probabilities were updating before there were statisticians?Hobson wrongly claims that Chris Fuchs “might dispute the notion that some kind of physical reality actually exists.” What he and other QBists do deny is that there are properties of the external world—“elements of reality”—that underlie quantum state assignments. (The argument of Matthew Pusey and coauthors that Hobson cites, implying that quantum states are physically real, relies on such hidden variables.) An agent’s state assignment rests entirely on her experience of the external world. It is neither solipsistic nor superstitious to maintain that any agent’s experience comes fully into being only at the moment it is experienced.Hobson’s treatment of Eugene Wigner and his friend relies on decoherence producing an objective mixed state, which both of them must agree on. But since quantum mechanics holds all the way out to infinity (as Griffiths puts it), even from an objective view of quantum states, that switch from superposition to either/or is at best a FAPP solution, to use Bell’s wonderfully sardonic abbreviation of “for all practical purposes.”QBism does not, as Hobson concludes, give up on a realist interpretation of nature. But it does warn us not to confuse nature with the abstractions we have ingeniously constructed to help any agent deal with the very real impact of nature on his or her own internal experience (see my Reference Frame, Physics Today, May 2009, page 8).I’m glad that Blake Stacey has called attention to some of the more applied spinoffs of QBism. I’m also pleased that he mentions Bruno de Finetti, one of the great 20th-century pioneers of subjective probability. Indeed, since there are objective as well as subjective Bayesians, if I had my way, the B in QBism would stand not for Thomas Bayes, but for Bruno de Finetti, who put the crucial point like this: “The abandonment of superstitious beliefs about the existence of Phlogiston, the Cosmic Ether, Absolute Space and Time … , or Fairies and Witches, was an essential step along the road to scientific thinking. Probability, too, if regarded as something endowed with some kind of objective existence, is no less a misleading misconception, an illusory attempt to exteriorize or materialize our [actual (vero)] probabilistic beliefs.”44. B. de Finetti, Theory of Probability, Interscience, New York (1990), preface.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. B. Hartle, Am. J. Phys. 36, 704 (1968). https://doi.org/10.1119/1.1975096, Google ScholarCrossref2. M. Schlosshauer, ed., Elegance and Enigma: The Quantum Interviews, Springer, Berlin (2011), pp. 177, 280. Google ScholarCrossref3. K. von Meyenn, ed., Eine Entdeckung von ganz außerordentlicher Tragweite: Schrödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon, Springer, Berlin (2011), p. 490. Google ScholarCrossref4. B. de Finetti, Theory of Probability, Interscience, New York (1990), preface. Google Scholar© 2012 American Institute of Physics.
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.
Few, if any, issues in physics have engendered as much discussion as the measurement problem in quantum mechanics. It is generally agreed that the `normal' dynamical evolution of the state vector in quantum mechanics is given by a unitary map. The linearity of this map implies that the state vector will, in general, be found in a superposition of eigenstates of a given observable (or, similarly, that the density matrix describing a subsystem will not correspond to a definite value of this observable). However, when we make a measurement of an observable, we always obtain a define value—although it is impossible to predict with certainty which value will be obtained. The traditional response to this issue is to postulate that when a measurement is made, the wavefunction `collapses' to an eigenstate of the observable being measured, perhaps due to the inherent classicality of the measuring apparatus (Bohr), or to the consciousness of the observer (Wigner), or possibly to some modification of quantum dynamics that occurs even when observations are not being made. The main motivation for the Everett (`many worlds') interpretation is to avoid introducing any such collapse postulate. This volume commemorates the 50th anniversary of the publication of Everett's paper in 1957 and contains 20 original articles as well as the transcripts of several discussions that took place at meetings devoted to the Everett interpretation at Oxford University and the Perimeter Institute. The attractiveness of the Everett interpretation is very succinctly summarized by a sentence from Vaidman's contribution (p 582): `The collapse, with its randomness, non-locality and the lack of a well-defined moment of occurrence, is such an ugly scar on quantum theory, that I, along with many others, am ready to follow Everett and deny its existence.' But the main drawback of the interpretation is then equally succinctly stated in the next sentence: `The price is the many worlds interpretation, i.e., the existence of numerous parallel worlds.' In other words, if the state vector evolves to have a non-zero amplitude for two different values of an observable that I `measure' (i.e., interact with in an appropriate way), each of these alternatives remains equally `real', i.e., there is a `branch' of the wavefunction in which the observable takes the first value and I believe that it has this first value and a branch of the wavefunction where it takes the second value and I believe that it has this second value. Each of these `me's' sees a definite value for the observable, thus nicely accounting for what I believe I experience without invoking collapse. But then, it seems that I have to believe that there are many other `me's' around—even if they are very difficult for this particular me to access—and no way I can predict which `me' I will be in the future. Indeed, Saunders writes on p 192, `If Alice is a person (of course she is) then we must say, even prior to branching at tj that there are many Alices present, atom for atom duplicates...Alice should be uncertain after all— each Alice should be uncertain—for each as of tj does not (and as a matter of principle, cannot) know which of these branching persons she is.' Thus, if taken seriously, the Everett viewpoint leads directly to questions like `What determines which ``me'' I am now and/or will feel that I am in the future?' The response `You are all of them' somehow doesn't seem satisfying, and further pursuit of these issues would seem to lead to questions as fruitless as questions like `Why am I ``me'' and not ``you''?' I believe that this accounts for why the Everett interpretation does not have universal appeal. But enough of `me' for now (more later); let's discuss the book. Unlike many edited volumes and conference proceedings in physics—which often consist of hastily slapped-together re-hashes of previous papers—this volume meets very high standards in the choice of topics and authors and in the quality of the writing. Although each of the articles is closely tied in with the Everett interpretation, there are many articles of considerable value and interest in their own right, such as the discussion of quasiclassical realms by Hartle and and the discussion of the de Broglie–Bohm interpretation by Valentini. There is also an excellent introduction by Saunders to the quantum measurement problem and the main issues that arise in the Everett interpretation. A central issue in the Everett interpretation is the status of the `Born rule', which asserts that, for state ψ, the probability of obtaining a particular outcome of a measurement is ||Pψ||2, where P is the projection operator onto the eigensubspace associated with the measurement outcome. In traditional interpretations, the Born rule is simply postulated as part of the collapse hypothesis. In the Everett interpretation, it is far from obvious that the Born rule even has any meaning—if all outcomes occur, how can one talk about the probability of a particular outcome? Given the importance of this issue, it is highly appropriate that four chapters of the book (by Saunders, Papineau, Wallace, and Greaves and Myrvold) are devoted to addressing probability and the Born rule from the Everett viewpoint, and three chapters (by Kent, Albert, and Price) are devoted to criticising these views. As indicated by the above quote from Saunders' article, uncertainty and probability can enter the Everett interpretation via each Alice being uncertain as to which Alice she is or will be. However, in view of the difficulties to which this kind of argumentation will lead, it is perhaps not surprising that in the 123 pages of combined discussion by Saunders, Papineau, Wallace, and Greaves and Myrvold, there are only fleeting references to the use of probability along these lines. Rather, the approach taken is reminiscent of that of adept politicians: if you can't answer a question, then answer a different question, and answer it with conviction and detail. Instead of considering probabilities of outcomes—a highly problematical notion when all of the possible outcomes actually occur—consider only decision theory issues. In particular, Wallace gives a precise and detailed mathematical proof that, under certain hypotheses, the only rational strategy that a quantum observer can follow in making decisions is to weight the possible outcomes by Born rule probabilities. To illustrate this more graphically, suppose tomorrow's weather were determined by some quantum mechanical, Stern–Gerlach-like process. I would like to know whether it is likely to rain tomorrow. However, when I ask an Everett follower (who knows the state vector and has done the Born rule calculation), all she will tell me in direct response to my question is that `it will rain and it won't rain.' Nevertheless, she is willing tell me whether or not it is rational for me to take an umbrella. I suppose that if she tells me that it would be highly irrational for me to take an umbrella, I can take the hint and deduce that it is very unlikely to rain. However, there does seem something wrong with the fact that she is not allowed to say this directly. In any case, if the conclusion of a mathematically correct argument is that rational decision strategies require the Born rule, then there must be quite a bit lying in the assumptions. The articles by Kent, Albert, and Price do a good job of fleshing out these assumptions and pointing out the weaknesses and flaws in the probability and decision theory discussions within the Everett framework. I hope it is clear from what I have written above that this is an outstanding volume, which should be of great interest to anyone interested in the foundations of quantum theory. Indeed, I feel quite fortunate to be the particular `me' that happens to exist in the same branch of the wavefunction of the universe in which this book was published.
Because it’s important that physicists discuss the problems of quantum foundations, I was delighted to see David Mermin’s commentary on “fixing the shifty split.” But I must disagree with my friend’s view.There are multiple ways of interpreting probabilities.11. A. Hobson, Concepts in Statistical Mechanics, Gordon and Breach, New York (1971), p. 27. In quantum physics, the Bayesian “degree of belief” interpretation can be appropriate for mixed states, but only the frequentist or “ensemble” interpretation applies to the pure states that Mermin discusses.A recent paper by Matthew Pusey and coauthors22. M. F. Pusey, J. Barrett, T. Rudolph, http://arxiv.org/abs/1111.3328v1. concludes that if two different pure quantum states are assigned to a single physical situation, one of the states is objectively wrong. The proof’s main assumption is that any quantum system has some set of real physical properties, labeled λ. Pusey and coauthors show that two different pure states cannot represent the same λ. So, although their paper implies but does not assume that quantum states are physically real, it does assume that each specific physical situation has behind it some kind of physical reality. Chris Fuchs, quoted favorably by Mermin, might dispute the notion that some kind of physical reality actually exists, but most physicists are probably sufficiently realist to grant that notion. Without such a notion, science lies somewhere between solipsism and superstition.Whenever I hear subjective interpretations of quantum physics, I wonder about such questions as the one Mermin quotes from Albert Einstein: Can a mouse collapse a wavefunction? Mermin’s Bayesian response, that a mouse cannot but a physics student can, doesn’t reassure me. Were wavefunctions not collapsing before there were physicists?The “Wigner’s friend” paradox, as described in Mermin’s commentary, doesn’t need a subjective interpretation to find a solution. In fact, Eugene Wigner abandoned his interpretation around 1970 once he became aware of the ideas underlying quantum decoherence.33. M. Schlosshauer, Decoherence and the Quantum-to-Classical Transition, Springer, Berlin (2007), p. 364. Suppose Wigner’s friend makes a measurement, the outcome of which is known to the friend but not to Wigner. Regardless of whether the outcome is known, and in fact regardless of the presence of the friend, the measurement device must decohere the quantum system that’s being measured, putting the system into a mixed state of the either/or form, with no interferences, just like a coin that’s been flipped. A definite outcome has thus occurred, and Wigner and his friend will both agree on this. Neither the friend nor Wigner affects the outcome. She has simply “looked” and now knows the outcome, while Wigner doesn’t know. That is not paradoxical and does not require a revision of our understanding of pure quantum states.There’s no reason to give up on a realist interpretation of quantum physics, much less a realist interpretation of nature itself.REFERENCESSection:ChooseTop of pageREFERENCES <<1. A. Hobson, Concepts in Statistical Mechanics, Gordon and Breach, New York (1971), p. 27. Google Scholar2. M. F. Pusey, J. Barrett, T. Rudolph, http://arxiv.org/abs/1111.3328v1. Google Scholar3. M. Schlosshauer, Decoherence and the Quantum-to-Classical Transition, Springer, Berlin (2007), p. 364. Google Scholar© 2012 American Institute of Physics.
Standard quantum theory is highly successful in all its applications, from photons and atoms, to semiconductors, superconductors, quarks, and so on. Yet the need to understand what the theory is really trying to tell us about the nature of the world beyond specific predictions forces us into the realm of philosophy, or, as some have dubbed it, quantum metaphysics: the study of the interpretation of quantum mechanics. There is a wide range of opinions regarding the philosophical foundations of quantum mechanics. This chapter focuses on two of the most prominent interpretations: the Copenhagen interpretation, and one alternative interpretation that has been gaining some support in recent years, the many-worlds interpretation (MWI), also known as the many universes interpretation. The central issues of contention are: firstly, that the measurement of some attribute of a quantum system generally involves the probabilistic collapse, or reduction, of the state vector where the probabilistic nature of the process is inherent—that is, not reducible to a more fundamental, deterministic explanation; and, secondly, the problem of bringing a measurement to a completion—a problem generally known as the measurement paradox.