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Cambridge University Press eBooks

Letters to Bob Griffiths

Christopher A. Fuchs

Published Jan 6, 2011• 0 citations
Publisher

Abstract

10 February 1997, “Sunday Afternoons and Consistent Histories” I've finally completed to my satisfaction your challenge of taking a plunge into “consistent histories.” And I've had great fun! In particular, I read all or most of each of the following: “Consistent Histories and the Interpretation of Quantum Mechanics,” “Quantum Interpretation Using Consistent Histories,” “Correlations in separated quantum systems: A consistent history analysis of the EPR problem,” “The Consistency of Consistent Histories: A Reply to d'Espagnat,” “A Consistent History Approach to the Logic of Quantum Mechanics,” “Consistent Histories and Quantum Reasoning.” I don't think I've read everything, but I'll trust that I haven't missed anything too significant – at least for a novice education in the subject. I say I had great fun because, before this, my only introduction to consistent histories had been through various talks by Gell-Mann, Hartle, and Zurek. And, I can't say I ever found any of those satisfactory – they all made it a very confusing muddle to me. I never could figure out the point, i.e., what actually was being proposed as a “solution” to the quantum interpretation problem. Consequently, I often just fell asleep or thought about other things or both. In contrast, I found all your papers logically crisp.

DOI
10.1017/cbo9780511762789.018

Related papers

Decohering histories and open quantum systems

Eric D. Chisolm

I briefly review the "decohering histories" or "consistent histories" formulation of quantum theory, due to Griffiths, Omnès, and Gell-Mann and Hartle (and the subject of my graduate work with George Sudarshan). I also sift through the many meanings that have been attached to decohering histories, with an emphasis on the most basic one: Decoherence of appropriate histories is needed to establish that quantum mechanics has the correct classical limit. Then I will describe efforts to find physical mechanisms that do this. Since most work has focused on density matrix versions of decoherence, I'll consider the relation between the two formulations, which historically has not been straightforward. Finally, I'll suggest a line of research that would use recent results by Sudarshan to illuminate this aspect of the classical limit of quantum theory.

arXiv:quant-ph/0611293•Journal of Physics Conference Series•Nov 1, 2009•0 citations

Consistent treatments of quantum mechanics

Yakir Aharonov, Sandu Popescu, Jeff Tollaksen

Aharonov, Popescu, and Tollaksen reply: We are grateful to David Mermin for stating loud and clear, in his unmistakable style, that “the authors are correct.” As for the rest of his letter, a lesson on clear writing from a great master of the subject is always welcome. We did strive for clarity in our article, but there is always space for improvement, and Freudian slips of Newspeak often creep in—that’s their very nature.However, as Mermin certainly knows, analyzing sentences in isolation without the many clues found in context doesn’t do them justice. The initial setup is indeed elementary, and we put in all the details, so it is hard to see how it could be misinterpreted. The situation becomes far more interesting and surprising, though, when weak measurements are taken into account; to ignore them, as Michael Nauenberg and Art Hobson did, is to completely miss the point. We believe that no true understanding of quantum mechanics can be obtained without taking weak measurements seriously.The letter by Robert Griffiths is mainly concerned with the merits of the consistent histories approach to quantum mechanics. It is an interesting subject but one that has very little to do with our article. We presented a number of quantum effects that arise in the context of pre- and postselection, first and foremost the effects that appear when weak measurements are involved. And no, there is no error in reasoning when we claim that in a situation like that described in our article, spin measurements may, in a consistent way, indicate that spin-1⁄2 particles have value 100; that is a statement of fact about results of measurements, which incidentally have been confirmed experimentally. One is free to disagree with the meaning we attach to those facts but cannot deny the facts themselves. Of course, since our effects are purely quantum mechanical, they can be analyzed using any interpretation of quantum mechanics, including consistent histories. However, before having such a discussion, it helps if the effects are first discovered! Their existence is our main message.© 2011 American Institute of Physics.

Physics Today•Oct 1, 2011•6 citations

Review of R. Omnes, THE INTERPRETATION OF QUANTUM MECHANICS

Robert B. Griffiths

Omnes' interpretation of quantum mechanics is summarized, and compared with other consistent-history approaches by Gell-Mann and Hartle, and by Griffiths.

arXiv:quant-ph/9505008•May 16, 1995•2 citations

The Consistent Histories Interpretation of Quantum Mechanics

Edward M. MacKinnon

The consistent histories reformulation of quantum mechanics was developed by Robert Griffiths, given a formal logical systematization by Roland Omn\`{e}s, and under the label `decoherent histories', was independently developed by Murray Gell-Mann and James Hartle and extended to quantum cosmology. Criticisms of CH involve issues of meaning, truth, objectivity, and coherence, a mixture of philosophy and physics. We will briefly consider the original formulation of CH and some basic objections. The reply to these objections, like the objections themselves, involves a mixture of physics and philosophy. These replies support an evaluation of the CH formulation as a replacement for the measurement, or orthodox, interpretation.

•Apr 1, 2009•5 citations

Towards the consistent histories approach to quantum mechanics

M R Sarkardei

We introduce and review the consistent (decoherent) histories approach to quantum mechanics due to Griffiths, to Gell-Mann and Hartle, and to Omnes. We will not attempt an in depth look at this approach as it would be impossible to treat it in such a short review. The emphasis is on understanding the broader meaning of the consistency and decoherence. The consistent history approach to quantum mechanics provides a precise conceptual framework for describing how a closed quantum system develops in time. The approach also provides a framework from which we may observe the emergence of an approximately classical domain for macroscopic systems, together with the conventional Copenhagen quantum mechanics for microscopic subsystems. We will study the formalism of decoherence and look at several approaches to decoherence. In examining a system of quantum Brownian motion and using the methods of Feynman and Vernon, we derive an equation of motion for the density matrix, the master equation of the system. This leads us to a brief overview of decoherence. PACS Nos.: 03.65.Ta, 03.65.–w, 03.65.Yz, 04.60.–m, 05.30.–d

Canadian Journal of Physics•Jan 1, 2004•6 citations

Time-symmetric quantum mechanics questioned and defended

Robert B. Griffiths

Readers of the article by Yakir Aharonov, Sandu Popescu, and Jeff Tollaksen might be interested in an alternative time-symmetric formulation of quantum mechanics, known as “consistent histories,” that was developed over roughly the same time period as Aharonov’s work (see the article by Robert Griffiths and Roland Omnès, PHYSICS TODAY, August 1999, page 26). Closely related is the “decoherent histories” approach of Murray Gell-Mann and James Hartle,11. M. Gell-Mann, J. B. Hartle, in Complexity, Entropy and the Physics of InformationW. H. Zurek,ed.,Addison-Wesley, Redwood City, CA (1990), p. 425. but as that is not usually formulated in a way that is transparently time-symmetric, the following remarks refer to the consistent histories approach; see reference 22. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. for an up-to-date formulation.Both the consistent histories approach and that of Aharonov and coworkers pay attention to events at several different times, are formulated in a way that is time-symmetrical, and address a number of quantum paradoxes. Both are consistent with the calculational procedures taught to students in a typical quantum mechanics course, so they are “standard quantum mechanics,” without the additional variables of de Broglie–Bohm or the additional collapses of Ghirardi-Rimini-Weber. And both approaches do not accept the “shut up and calculate” mentality that alas continues to dominate much classroom instruction. So far as I can tell, all the results mentioned by Aharonov and coauthors and in the earlier work they cite are fairly readily translated into the language of consistent histories, though the reverse is not true (see below); therefore, the consistent histories view is more general.In the treatment by Aharonov and coauthors, measurement, as in textbook quantum theory, remains a black box: It collapses the wavefunction, but nothing more can be said. And for good reason: The textbook approach of introducing probabilities by reference to measurement yields what appear to be insoluble difficulties if one attempts to apply quantum theory to the measurement process itself—that is, to actual apparatus constructed out of entities that are quantum mechanical. In the consistent histories approach, that difficulty does not arise, because it treats quantum dynamics as fundamentally probabilistic, not deterministic, and the same rules apply to measurements as to all other physical processes. Speaking metaphorically, the probabilistic approach used in consistent histories allows one to open the black measurement box and watch the quantum gears turn.The other major difference between the two approaches is their treatment of quantum paradoxes. We owe many of the most striking and delightful paradoxes of quantum theory to Aharonov and his coworkers, and he and Daniel Rohrlich have written a book on the topic.33. Y. Aharonov, D. Rohrlich, Quantum Paradoxes: Quantum Theory for the Perplexed, Wiley, Weinheim, Germany (2005) . But he leaves the paradoxes largely unresolved; the reader is encouraged to study but not unravel them. The consistent histories approach is exactly opposite: Paradoxes should be—and a large number of them have been—resolved by the correct application of well-formulated and fully consistent quantum principles (see reference 22. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. , chapters 19–25).Students new to quantum theory are often confused and deserve reasoned responses to their queries. Although paradoxes are valuable illustrations of how the quantum world differs from our everyday experience, I prefer to provide students with the conceptual tools needed to resolve and make sense of them. In particular, students benefit from learning a fully consistent approach to probabilities in the quantum domain, one not based on measurements but on general quantum principles. A colleague and I have just finished using that approach in teaching the first term of our introductory graduate quantum mechanics course. Although it requires extra time and effort to learn how to think about quantum processes rather than just do calculations, the reward comes in a deeper understanding of how the real (quantum) world works.REFERENCESSection:ChooseTop of pageREFERENCES <<1. M. Gell-Mann, J. B. Hartle, in Complexity, Entropy and the Physics of InformationW. H. Zurek,ed.,Addison-Wesley, Redwood City, CA (1990), p. 425. Google Scholar2. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. Google Scholar3. Y. Aharonov, D. Rohrlich, Quantum Paradoxes: Quantum Theory for the Perplexed, Wiley, Weinheim, Germany (2005) . Google ScholarCrossref© 2011 American Institute of Physics.

Physics Today•May 1, 2011•1 citation
10 February 1997, “Sunday Afternoons and Consistent Histories” I've finally completed to my satisfaction your challenge of taking a plunge into “consistent histories.” And I've had great fun! In particular, I read all or most of each of the following: “Consistent Histories and the Interpretation of Quantum Mechanics,” “Quantum Interpretation Using Consistent Histories,” “Correlations in separated quantum systems: A consistent history analysis of the EPR problem,” “The Consistency of Consistent Histories: A Reply to d'Espagnat,” “A Consistent History Approach to the Logic of Quantum Mechanics,” “Consistent Histories and Quantum Reasoning.” I don't think I've read everything, but I'll trust that I haven't missed anything too significant – at least for a novice education in the subject. I say I had great fun because, before this, my only introduction to consistent histories had been through various talks by Gell-Mann, Hartle, and Zurek. And, I can't say I ever found any of those satisfactory – they all made it a very confusing muddle to me. I never could figure out the point, i.e., what actually was being proposed as a “solution” to the quantum interpretation problem. Consequently, I often just fell asleep or thought about other things or both. In contrast, I found all your papers logically crisp.
I briefly review the "decohering histories" or "consistent histories" formulation of quantum theory, due to Griffiths, Omnès, and Gell-Mann and Hartle (and the subject of my graduate work with George Sudarshan). I also sift through the many meanings that have been attached to decohering histories, with an emphasis on the most basic one: Decoherence of appropriate histories is needed to establish that quantum mechanics has the correct classical limit. Then I will describe efforts to find physical mechanisms that do this. Since most work has focused on density matrix versions of decoherence, I'll consider the relation between the two formulations, which historically has not been straightforward. Finally, I'll suggest a line of research that would use recent results by Sudarshan to illuminate this aspect of the classical limit of quantum theory.
Aharonov, Popescu, and Tollaksen reply: We are grateful to David Mermin for stating loud and clear, in his unmistakable style, that “the authors are correct.” As for the rest of his letter, a lesson on clear writing from a great master of the subject is always welcome. We did strive for clarity in our article, but there is always space for improvement, and Freudian slips of Newspeak often creep in—that’s their very nature.However, as Mermin certainly knows, analyzing sentences in isolation without the many clues found in context doesn’t do them justice. The initial setup is indeed elementary, and we put in all the details, so it is hard to see how it could be misinterpreted. The situation becomes far more interesting and surprising, though, when weak measurements are taken into account; to ignore them, as Michael Nauenberg and Art Hobson did, is to completely miss the point. We believe that no true understanding of quantum mechanics can be obtained without taking weak measurements seriously.The letter by Robert Griffiths is mainly concerned with the merits of the consistent histories approach to quantum mechanics. It is an interesting subject but one that has very little to do with our article. We presented a number of quantum effects that arise in the context of pre- and postselection, first and foremost the effects that appear when weak measurements are involved. And no, there is no error in reasoning when we claim that in a situation like that described in our article, spin measurements may, in a consistent way, indicate that spin-1⁄2 particles have value 100; that is a statement of fact about results of measurements, which incidentally have been confirmed experimentally. One is free to disagree with the meaning we attach to those facts but cannot deny the facts themselves. Of course, since our effects are purely quantum mechanical, they can be analyzed using any interpretation of quantum mechanics, including consistent histories. However, before having such a discussion, it helps if the effects are first discovered! Their existence is our main message.© 2011 American Institute of Physics.
The consistent histories reformulation of quantum mechanics was developed by Robert Griffiths, given a formal logical systematization by Roland Omn\`{e}s, and under the label `decoherent histories', was independently developed by Murray Gell-Mann and James Hartle and extended to quantum cosmology. Criticisms of CH involve issues of meaning, truth, objectivity, and coherence, a mixture of philosophy and physics. We will briefly consider the original formulation of CH and some basic objections. The reply to these objections, like the objections themselves, involves a mixture of physics and philosophy. These replies support an evaluation of the CH formulation as a replacement for the measurement, or orthodox, interpretation.
We introduce and review the consistent (decoherent) histories approach to quantum mechanics due to Griffiths, to Gell-Mann and Hartle, and to Omnes. We will not attempt an in depth look at this approach as it would be impossible to treat it in such a short review. The emphasis is on understanding the broader meaning of the consistency and decoherence. The consistent history approach to quantum mechanics provides a precise conceptual framework for describing how a closed quantum system develops in time. The approach also provides a framework from which we may observe the emergence of an approximately classical domain for macroscopic systems, together with the conventional Copenhagen quantum mechanics for microscopic subsystems. We will study the formalism of decoherence and look at several approaches to decoherence. In examining a system of quantum Brownian motion and using the methods of Feynman and Vernon, we derive an equation of motion for the density matrix, the master equation of the system. This leads us to a brief overview of decoherence. PACS Nos.: 03.65.Ta, 03.65.–w, 03.65.Yz, 04.60.–m, 05.30.–d
Readers of the article by Yakir Aharonov, Sandu Popescu, and Jeff Tollaksen might be interested in an alternative time-symmetric formulation of quantum mechanics, known as “consistent histories,” that was developed over roughly the same time period as Aharonov’s work (see the article by Robert Griffiths and Roland Omnès, PHYSICS TODAY, August 1999, page 26). Closely related is the “decoherent histories” approach of Murray Gell-Mann and James Hartle,11. M. Gell-Mann, J. B. Hartle, in Complexity, Entropy and the Physics of InformationW. H. Zurek,ed.,Addison-Wesley, Redwood City, CA (1990), p. 425. but as that is not usually formulated in a way that is transparently time-symmetric, the following remarks refer to the consistent histories approach; see reference 22. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. for an up-to-date formulation.Both the consistent histories approach and that of Aharonov and coworkers pay attention to events at several different times, are formulated in a way that is time-symmetrical, and address a number of quantum paradoxes. Both are consistent with the calculational procedures taught to students in a typical quantum mechanics course, so they are “standard quantum mechanics,” without the additional variables of de Broglie–Bohm or the additional collapses of Ghirardi-Rimini-Weber. And both approaches do not accept the “shut up and calculate” mentality that alas continues to dominate much classroom instruction. So far as I can tell, all the results mentioned by Aharonov and coauthors and in the earlier work they cite are fairly readily translated into the language of consistent histories, though the reverse is not true (see below); therefore, the consistent histories view is more general.In the treatment by Aharonov and coauthors, measurement, as in textbook quantum theory, remains a black box: It collapses the wavefunction, but nothing more can be said. And for good reason: The textbook approach of introducing probabilities by reference to measurement yields what appear to be insoluble difficulties if one attempts to apply quantum theory to the measurement process itself—that is, to actual apparatus constructed out of entities that are quantum mechanical. In the consistent histories approach, that difficulty does not arise, because it treats quantum dynamics as fundamentally probabilistic, not deterministic, and the same rules apply to measurements as to all other physical processes. Speaking metaphorically, the probabilistic approach used in consistent histories allows one to open the black measurement box and watch the quantum gears turn.The other major difference between the two approaches is their treatment of quantum paradoxes. We owe many of the most striking and delightful paradoxes of quantum theory to Aharonov and his coworkers, and he and Daniel Rohrlich have written a book on the topic.33. Y. Aharonov, D. Rohrlich, Quantum Paradoxes: Quantum Theory for the Perplexed, Wiley, Weinheim, Germany (2005) . But he leaves the paradoxes largely unresolved; the reader is encouraged to study but not unravel them. The consistent histories approach is exactly opposite: Paradoxes should be—and a large number of them have been—resolved by the correct application of well-formulated and fully consistent quantum principles (see reference 22. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. , chapters 19–25).Students new to quantum theory are often confused and deserve reasoned responses to their queries. Although paradoxes are valuable illustrations of how the quantum world differs from our everyday experience, I prefer to provide students with the conceptual tools needed to resolve and make sense of them. In particular, students benefit from learning a fully consistent approach to probabilities in the quantum domain, one not based on measurements but on general quantum principles. A colleague and I have just finished using that approach in teaching the first term of our introductory graduate quantum mechanics course. Although it requires extra time and effort to learn how to think about quantum processes rather than just do calculations, the reward comes in a deeper understanding of how the real (quantum) world works.REFERENCESSection:ChooseTop of pageREFERENCES <<1. M. Gell-Mann, J. B. Hartle, in Complexity, Entropy and the Physics of InformationW. H. Zurek,ed.,Addison-Wesley, Redwood City, CA (1990), p. 425. Google Scholar2. R. B. Griffiths, Consistent Quantum Theory, Cambridge U. Press, New York (2002); available online at http://quantum.phys.cmu.edu/CQT. Google Scholar3. Y. Aharonov, D. Rohrlich, Quantum Paradoxes: Quantum Theory for the Perplexed, Wiley, Weinheim, Germany (2005) . Google ScholarCrossref© 2011 American Institute of Physics.