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Physics World

In defence of “measurement”

Ronald F. Peierls

Published Jan 1, 1991• 87 citations
Publisher

Abstract

In a stimulating article (Physics World August p33) the late John Bell professed dissatisfaction with the foundation of quantum mechanics as usually presented, particularly in connection with the so-called “collapse of the wavefunction” as a result of a measurement. He agreed that for all practical purposes the use of quantum mechanics by qualified practitioners leads to well defined answers which, where they can be checked, agree with experiment.

DOI
10.1088/2058-7058/4/1/19

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In defence of “measurement”

Ronald F. Peierls

In a stimulating article (Physics World August p33) the late John Bell professed dissatisfaction with the foundation of quantum mechanics as usually presented, particularly in connection with the so-called collapse of the wavefunction as a result of a measurement. He agreed that for all practical purposes the use of quantum mechanics by qualified practitioners leads to well defined answers which, where they can be checked, agree with experiment.

World Scientific series in 20th century physics•Apr 1, 1997•2 citations

Against ‘measurement’

John Marcus Bell

Surely, after 62 years, we should have an exact formulation of some serious part of quantum mechanics? By ‘exact’ I do not of course mean ‘exactly true’. I mean only that the theory should be fully formulated in mathematical terms, with nothing left to the discretion of the theoretical physicist...until workable approximations are needed in applications. By ‘serious’ I mean that some substantial fragment of physics should be covered. Nonrelativistic ‘particle’ quantum mechanics, perhaps with the inclusion of the electromagnetic field and cut-off interaction, is serious enough. For it covers ‘a large part of physics and the whole of chemistry’ (P A M Dirac 1929 Proc. R. Soc.A 123 714). I mean too, by ‘serious’, that ‘apparatus’ should not be separated off from the rest of the world into black boxes, as if it were not made of atoms and not ruled by quantum mechanics.

Physics World•Aug 1, 1990•386 citations

Mermin habitually answers opinions, real and abstract

Philip Pearle

David Mermin cautions against taking our “most successful abstractions to be real properties of our world.” I think he has set up a straw man. To me, the tenable realist position is not that our concepts are real but rather that they have a correspondence to reality: There can be many such correspondences, each with its utility, none capturing reality—whatever that is.I am not surprised he uses that argument to bolster the position “that quantum states are calculational devices.” In the vein of the disagreement about reality he cites, between Bishop Berkeley and Samuel Johnson, I recall a similar disagreement, between Mermin and John Bell, concerning the foundations of quantum theory. It took place at a meeting in Erice, Italy, in August 1989. Bell had finished his now famous talk “Against ‘Measurement,’ “ in which he argued that there is something wrong with quantum theory, that its rules of application are ill-defined. 1 1. J. Bell, Sixty-Two Years of Uncertainty, A. Miller, ed., Plenum, New York (1990), p. 17, https://doi.org/10.1007/978-1-4684-8771-8_3 reproduced in Phys. World 3, 33 (August 1990). He introduced the acronym FAPP, meaning that quantum theory is good “for all practical purposes” but insufficient for a truly fundamental theory. In the question-and-answer session, Mermin gave an argument not unlike the one in his Reference Frame. When he was done, Bell replied, “FAPPtrap,” and that was that.Mermin says that if one takes his view, it “can diminish the motivation for theoretical or experimental searches for a ‘mechanism’ underlying … the ‘collapse of the wavefunction’—searches that make life harder than it needs to be.” But why would one want to diminish such motivation? Might not the deficiencies of quantum theory elucidated by Bell be a clue to some deeper theory? And is that not good for physics? Certainly, in this generation, in which we did not predict the acceleration of the universe nor know the nature of dark energy or dark matter, we should have the humility to think that perhaps we do not yet have the “final” quantum theory. For example, in standard quantum theory, how or why an event occurs is a complete mystery. Nature determines an outcome, but we are told it is impossible for us to understand that in terms of anything deeper. Things happen for no reason at all. Why should we follow Mermin’s advice and not try to do better?In my own work and that of Gian-Carlo Ghirardi, Alberto Rimini, and Tullio Weber, we have developed what is called a dynamical theory of wave-function collapse (continuous spontaneous localization, or CSL). 2 2. Two reviews are P. Pearle in Open Systems and Measurement in Relativistic Quantum Field Theory, H. P. Breuer, F. Petruccionne, eds., Springer, Berlin (1999), p. 195 and A. Bassi, G. C. Ghirardi, Phys. Rep. 379, 257 (2003). https://doi.org/10.1016/S0370-1573(03)00103-0 To Schrödinger’s equation, a term is added that describes the interaction of matter with a randomly fluctuating field, so that a state vector in a superposition of macroscopically different states rapidly evolves to one of them. The final state describes the world as we see it. Therefore, the state vector can be said to correspond to reality and not be merely a computational device. Bell regarded CSL as an example of a well-defined theory, which overcomes his objections to standard quantum theory. Moreover, it provides a mechanism, a description, for the occurrence of events. And it makes some predictions that differ from those of standard quantum theory, so it is experimentally testable. Experiments so far have shown that the coupling of random field to matter must be mass proportional, but otherwise they have neither confirmed CSL nor denied it. This is a good thing, and that such attempts should not be discouraged, as Mermin’s article does, was underlined by Richard Feynman, who said in 1964,We have to find a new view of the world that has to agree with everything that is known, but disagree in its predictions somewhere, otherwise it is not interesting. And in that disagreement it must agree with nature. If you can find any other view of the world which agrees over the entire range where things have already been observed, but disagrees somewhere else, you have made a great discovery. It is very nearly impossible, but not quite, to find any theory which agrees with experiments over the entire range in which all theories have been checked, and yet gives different consequences in some other range, even a theory whose different consequences do not turn out to agree with nature. 3 3. R. Feynman, in The Character of Physical Law, MIT Press, Cambridge, MA (1970), p. 171. REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. Bell, Sixty-Two Years of Uncertainty, A. Miller, ed., Plenum, New York (1990), p. 17, https://doi.org/10.1007/978-1-4684-8771-8_3 , Google ScholarCrossrefreproduced in Phys. World 3, 33 (August 1990). , Google Scholar2. Two reviews are P. Pearle in Open Systems and Measurement in Relativistic Quantum Field Theory, H. P. Breuer, F. Petruccionne, eds., Springer, Berlin (1999), p. 195 and , Google ScholarCrossref A. Bassi, G. C. Ghirardi, Phys. Rep. 379, 257 (2003). https://doi.org/10.1016/S0370-1573(03)00103-0 , , Google ScholarCrossref, ISI3. R. Feynman, in The Character of Physical Law, MIT Press, Cambridge, MA (1970), p. 171. Google Scholar© 2009 American Institute of Physics.

Physics Today•Sep 1, 2009•0 citations

Problems with “measurement”

Walter Whitham

Bohr is said to have remarked that anybody who is not shocked by quantum theory has not understood it. No physicist, however, challenges its practical usefulness in predicting experimental results and providing an explanatory framework for an enormous range of study. Nevertheless, more thoughtful physicists have been astonished and disturbed by what is implied when attempts are made to interpret its meaning. As a result a variety of ingenious, and sometimes bizarre, schemes have been devised since its inception in order to make these implications more acceptable to the scientific mind. Some of these schemes have resulted in proposals which appear stranger than the problems they have been designed to resolve. After some sixty years there is no consensus of opinion upon the solution of the epistemological problems posed. Indeed the debate keeps on resurfacing in the scientific literature and continues to generate library shelves of books. Some progress, however, has been made and the late John Bell himself and the experimental work of Alain Aspect have demonstrated that some epistemological positions cannot be sustained.

Physics World•May 1, 1991•0 citations

Recollections of John Bell

Michael Nauenberg

It is a pleasure to contribute to this anthology some of my recollections of John Bell. I first met him at SLAC, where we were visitors during 1964–5 when he was on leave from CERN, and I was on leave from the Columbia University Physics Department. Soon I found that we had a common interest in the foundations of quantum mechanics, and we had lively discussions on this subject. We were concerned with the “reduction of the wave packet” after an experiment in quantum mechanics was completed, and wrote a tongue-incheek article on this subject for a festschrift in honor of Viki Weisskopf [1], stating that We emphasize not only that our view is that of a minority but also that current interest in such questions is small. The typical physicist feels that they have long been answered, and that he will fully understand just how, if ever he can spare twenty minutes to think about it. Now, 50 years later, this topic, known as the measurement problem , has become the source of numerous articles representing many different viewpoints. At the time, however, I confess that I did not realize that the main flaw in John von Neumann's presentation of this problem was his assumption that a measuring device can be represented by a pointer with only two quantum states. Actually, a measuring device must be able to record the outcome of an experiment, and for this purpose it has to have an enormous number of quantum states, leading to an irreversible macroscopic transition in the device. A correct discussion of the measurement problemwas given, for example, by Nico vanKampen [2], but recently he told me that he was unable to persuade Bell, who continued to be concerned about this problem for the rest of his life. Indeed, shortly before he died, he wrote a diatribe [3] entitled “Against Measurement,” where he continued to argue that this problem constituted a fundamental flaw in quantum mechanics. Kurt Gottfried, who referred to this problem as Bell's second major theme, called his article “a fervent jeremiad summarizing a lifetime of reflection on what he saw as the fatal flaws of the orthodox theory” [4].

Cambridge University Press eBooks•Aug 31, 2016•1 citation

Editor's Foreword

Kip S. Thorne

The quantum theory of measurement has been widely regarded, since the 1940s, as an esoteric subject of little relevance to “real” physics. Quite the opposite was true in the 1920s and 1930s when Niels Bohr, John von Neumann, and others were struggling, by means of gedankin experiments, to develop an understanding of how measurements on single quantum objects were to be incorporated into quantum theory. However, the understanding they achieved turned out to be of little use in the quantum mechanical applications that commanded the attention of physicists between 1940 and 1980: the interaction of photons, atomic nuclei, and elementary particles, the theory of masers and lasers, the properties of matter (superfluidity, superconductivity, semiconductors), etc. In each of these applications, although the phenomena studied were quantum mechanical at heart, the measurements used to probe them were so far removed from the quantum domain that the only feature of the quantum theory of measurement which entered into the experiments was the probability interpretation of squared amplitudes. There was no need to invoke what soon came to be regarded as an esoteric, problematic, dubious “collapse (reduction) of the wave function.” Fundamentally, the reason for this irrelevance of the quantum theory of measurement was technological.

Cambridge University Press eBooks•Sep 24, 1992•0 citations
In a stimulating article (Physics World August p33) the late John Bell professed dissatisfaction with the foundation of quantum mechanics as usually presented, particularly in connection with the so-called “collapse of the wavefunction” as a result of a measurement. He agreed that for all practical purposes the use of quantum mechanics by qualified practitioners leads to well defined answers which, where they can be checked, agree with experiment.
Surely, after 62 years, we should have an exact formulation of some serious part of quantum mechanics? By ‘exact’ I do not of course mean ‘exactly true’. I mean only that the theory should be fully formulated in mathematical terms, with nothing left to the discretion of the theoretical physicist...until workable approximations are needed in applications. By ‘serious’ I mean that some substantial fragment of physics should be covered. Nonrelativistic ‘particle’ quantum mechanics, perhaps with the inclusion of the electromagnetic field and cut-off interaction, is serious enough. For it covers ‘a large part of physics and the whole of chemistry’ (P A M Dirac 1929 Proc. R. Soc.A 123 714). I mean too, by ‘serious’, that ‘apparatus’ should not be separated off from the rest of the world into black boxes, as if it were not made of atoms and not ruled by quantum mechanics.
David Mermin cautions against taking our “most successful abstractions to be real properties of our world.” I think he has set up a straw man. To me, the tenable realist position is not that our concepts are real but rather that they have a correspondence to reality: There can be many such correspondences, each with its utility, none capturing reality—whatever that is.I am not surprised he uses that argument to bolster the position “that quantum states are calculational devices.” In the vein of the disagreement about reality he cites, between Bishop Berkeley and Samuel Johnson, I recall a similar disagreement, between Mermin and John Bell, concerning the foundations of quantum theory. It took place at a meeting in Erice, Italy, in August 1989. Bell had finished his now famous talk “Against ‘Measurement,’ “ in which he argued that there is something wrong with quantum theory, that its rules of application are ill-defined. 1 1. J. Bell, Sixty-Two Years of Uncertainty, A. Miller, ed., Plenum, New York (1990), p. 17, https://doi.org/10.1007/978-1-4684-8771-8_3 reproduced in Phys. World 3, 33 (August 1990). He introduced the acronym FAPP, meaning that quantum theory is good “for all practical purposes” but insufficient for a truly fundamental theory. In the question-and-answer session, Mermin gave an argument not unlike the one in his Reference Frame. When he was done, Bell replied, “FAPPtrap,” and that was that.Mermin says that if one takes his view, it “can diminish the motivation for theoretical or experimental searches for a ‘mechanism’ underlying … the ‘collapse of the wavefunction’—searches that make life harder than it needs to be.” But why would one want to diminish such motivation? Might not the deficiencies of quantum theory elucidated by Bell be a clue to some deeper theory? And is that not good for physics? Certainly, in this generation, in which we did not predict the acceleration of the universe nor know the nature of dark energy or dark matter, we should have the humility to think that perhaps we do not yet have the “final” quantum theory. For example, in standard quantum theory, how or why an event occurs is a complete mystery. Nature determines an outcome, but we are told it is impossible for us to understand that in terms of anything deeper. Things happen for no reason at all. Why should we follow Mermin’s advice and not try to do better?In my own work and that of Gian-Carlo Ghirardi, Alberto Rimini, and Tullio Weber, we have developed what is called a dynamical theory of wave-function collapse (continuous spontaneous localization, or CSL). 2 2. Two reviews are P. Pearle in Open Systems and Measurement in Relativistic Quantum Field Theory, H. P. Breuer, F. Petruccionne, eds., Springer, Berlin (1999), p. 195 and A. Bassi, G. C. Ghirardi, Phys. Rep. 379, 257 (2003). https://doi.org/10.1016/S0370-1573(03)00103-0 To Schrödinger’s equation, a term is added that describes the interaction of matter with a randomly fluctuating field, so that a state vector in a superposition of macroscopically different states rapidly evolves to one of them. The final state describes the world as we see it. Therefore, the state vector can be said to correspond to reality and not be merely a computational device. Bell regarded CSL as an example of a well-defined theory, which overcomes his objections to standard quantum theory. Moreover, it provides a mechanism, a description, for the occurrence of events. And it makes some predictions that differ from those of standard quantum theory, so it is experimentally testable. Experiments so far have shown that the coupling of random field to matter must be mass proportional, but otherwise they have neither confirmed CSL nor denied it. This is a good thing, and that such attempts should not be discouraged, as Mermin’s article does, was underlined by Richard Feynman, who said in 1964,We have to find a new view of the world that has to agree with everything that is known, but disagree in its predictions somewhere, otherwise it is not interesting. And in that disagreement it must agree with nature. If you can find any other view of the world which agrees over the entire range where things have already been observed, but disagrees somewhere else, you have made a great discovery. It is very nearly impossible, but not quite, to find any theory which agrees with experiments over the entire range in which all theories have been checked, and yet gives different consequences in some other range, even a theory whose different consequences do not turn out to agree with nature. 3 3. R. Feynman, in The Character of Physical Law, MIT Press, Cambridge, MA (1970), p. 171. REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. Bell, Sixty-Two Years of Uncertainty, A. Miller, ed., Plenum, New York (1990), p. 17, https://doi.org/10.1007/978-1-4684-8771-8_3 , Google ScholarCrossrefreproduced in Phys. World 3, 33 (August 1990). , Google Scholar2. Two reviews are P. Pearle in Open Systems and Measurement in Relativistic Quantum Field Theory, H. P. Breuer, F. Petruccionne, eds., Springer, Berlin (1999), p. 195 and , Google ScholarCrossref A. Bassi, G. C. Ghirardi, Phys. Rep. 379, 257 (2003). https://doi.org/10.1016/S0370-1573(03)00103-0 , , Google ScholarCrossref, ISI3. R. Feynman, in The Character of Physical Law, MIT Press, Cambridge, MA (1970), p. 171. Google Scholar© 2009 American Institute of Physics.
Bohr is said to have remarked that anybody who is not shocked by quantum theory has not understood it. No physicist, however, challenges its practical usefulness in predicting experimental results and providing an explanatory framework for an enormous range of study. Nevertheless, more thoughtful physicists have been astonished and disturbed by what is implied when attempts are made to interpret its meaning. As a result a variety of ingenious, and sometimes bizarre, schemes have been devised since its inception in order to make these implications more acceptable to the scientific mind. Some of these schemes have resulted in proposals which appear stranger than the problems they have been designed to resolve. After some sixty years there is no consensus of opinion upon the solution of the epistemological problems posed. Indeed the debate keeps on resurfacing in the scientific literature and continues to generate library shelves of books. Some progress, however, has been made and the late John Bell himself and the experimental work of Alain Aspect have demonstrated that some epistemological positions cannot be sustained.
It is a pleasure to contribute to this anthology some of my recollections of John Bell. I first met him at SLAC, where we were visitors during 1964–5 when he was on leave from CERN, and I was on leave from the Columbia University Physics Department. Soon I found that we had a common interest in the foundations of quantum mechanics, and we had lively discussions on this subject. We were concerned with the “reduction of the wave packet” after an experiment in quantum mechanics was completed, and wrote a tongue-incheek article on this subject for a festschrift in honor of Viki Weisskopf [1], stating that We emphasize not only that our view is that of a minority but also that current interest in such questions is small. The typical physicist feels that they have long been answered, and that he will fully understand just how, if ever he can spare twenty minutes to think about it. Now, 50 years later, this topic, known as the measurement problem , has become the source of numerous articles representing many different viewpoints. At the time, however, I confess that I did not realize that the main flaw in John von Neumann's presentation of this problem was his assumption that a measuring device can be represented by a pointer with only two quantum states. Actually, a measuring device must be able to record the outcome of an experiment, and for this purpose it has to have an enormous number of quantum states, leading to an irreversible macroscopic transition in the device. A correct discussion of the measurement problemwas given, for example, by Nico vanKampen [2], but recently he told me that he was unable to persuade Bell, who continued to be concerned about this problem for the rest of his life. Indeed, shortly before he died, he wrote a diatribe [3] entitled “Against Measurement,” where he continued to argue that this problem constituted a fundamental flaw in quantum mechanics. Kurt Gottfried, who referred to this problem as Bell's second major theme, called his article “a fervent jeremiad summarizing a lifetime of reflection on what he saw as the fatal flaws of the orthodox theory” [4].
The quantum theory of measurement has been widely regarded, since the 1940s, as an esoteric subject of little relevance to “real” physics. Quite the opposite was true in the 1920s and 1930s when Niels Bohr, John von Neumann, and others were struggling, by means of gedankin experiments, to develop an understanding of how measurements on single quantum objects were to be incorporated into quantum theory. However, the understanding they achieved turned out to be of little use in the quantum mechanical applications that commanded the attention of physicists between 1940 and 1980: the interaction of photons, atomic nuclei, and elementary particles, the theory of masers and lasers, the properties of matter (superfluidity, superconductivity, semiconductors), etc. In each of these applications, although the phenomena studied were quantum mechanical at heart, the measurements used to probe them were so far removed from the quantum domain that the only feature of the quantum theory of measurement which entered into the experiments was the probability interpretation of squared amplitudes. There was no need to invoke what soon came to be regarded as an esoteric, problematic, dubious “collapse (reduction) of the wave function.” Fundamentally, the reason for this irrelevance of the quantum theory of measurement was technological.