Lettre de Cramer sur la réception de la TIQM : Différence entre versions

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Version actuelle en date du 1 mars 2013 à 13:00

Dear Dr. Towler,

Let me try to answer your question about why, in the 10 or so years since it was published, the transactional interpretation has not received more attention from those who write books about the interpretation(s) of quantum mechanics.

First I would have to say that I do not understand it either. Perhaps it's my fault for not putting more effort into promoting the transactional interpretation once it was in print and for not sending complaining letters to authors of review articles and books who have not given it enough (or any) attention. It did, by the way, receive considerable attention in a book by John Casti and in a recent biography of Erwin Schroedinger.

The interpretation of quantum mechanics is not my principal area of research. In fact, as an experimentalist I find quantum interpretation to be a very frustrating area because of its insulation from experimental tests. Therefore, once the ideas behind the TI were fully elucidated, I did not feel it necessary to continue with further publications on the subject. I feel, perhaps naively, that sufficiently good ideas should be able stand on their own and should not require a sales pitch. In 1986, shortly after the RMP article appeared, I did write a letter to the Scientific American offering do write a popular article about the transactional interpretation. However, they never even replied to my letter. I was somewhat irritated and dropped the idea of a large popular-language article. However, in my science-fact columns published in Analog Magazine (see http://www.npl.washington.edu/av). I have dealt with various aspects of the TI and quantum interpretation a number of times. Eventually I may do my own book on the subject.

Last summer, on the 10th anniversary of the RMP publication, I did a citation index search on the TI papers, and I have placed the results at the end of the version of the TI paper that is on WorldWideWeb (see http://www.npl.washington.edu/tiqm). There are some references, but far fewer that I would have expected by now when I wrote the paper.

Perhaps I should mention that the Everett-Wheeler interpretation of quantum mechanics had a similar history. It was published in Reviews of Modern Physics in 1957 and largely ignored until 1971, when Bryce DeWitt published an article about it in Physics Today. Since then, the Everett-Wheeler interpretation has been embraced by the much of the theoretical community (in spite of its conspicuous inadequacy in dealing with the problem of nonlocality) and the various versions which include "decoherence" as a substitute for wave function collaps are widely discussed in the literature. Perhaps QM interpretations, like wines, have to be aged for 15 years before the paradigm shifts they bring are palatable.

As for fundamental objections, I'm not aware of any fatal ones. Below I list some of the objections and criticisms that have been raised, along with my responses:

(1) From Carl von Weiszacker at the 1988 New Orleans QM conference: "The TI differs from the Copenhagen Interpretation only in using different terms for the same quantum constructs, so that the difference between the two is basically semantic."

My reply to this, published in the 1988 "Overview" paper, is that while there may (or may not) be a 1:1 correspondence between terms in the TI and CI, the causal relations in some cases are clearly inverted, particularly for issues involving the role of the observer. The CI gives the illusion that the observer has a mysterious central role in the quantum process while the TI demonstrates that the observations of the observer are only a side effect of the central transaction.

(2) A question raised by someone named Levine at the 1988 New Orleans QM conference and recently repeated by one Jack Sarfatti of Berkeley, (who seems to have achieved some status on the Net from the sheer volume of his diatribes): "Some quantum mechanical systems (e.g. an ensemble of fermions with spin 1/2) have no known formalism capable of representing the system in position space and can be formulated only in a multidimensional momentum phase space. Therefore, it is inappropriate to discuss `waves physically present in space' as portrayed in the TI, and so the TI cannot be considered as a general interpretation."

I've always found this objection rather silly. Of course it is always possible to represent a quantum system in momentum space, where the formalism and solutions may be more tractable. Does this mean that there is no possible (if intractable) representation of the system in configuration space and that the same waves cannot be considered to exist in space? Of course not. That's what multidimensional Fourier transformations are for.

(3) An objection raised by several field theorists: "The TI interprets the solutions of wave equations, e.g., the Schroedinger, Dirac, and Klein-Gordon equations. However, modern quantum field theory does not use or need wave equations, relativistic or otherwise, and so the TI is not applicable."

I think the argument, like the one above, starts from mathematical convenience and ends with undue generalization. It has not been demonstrated that any aspect of quantum field theory CANNOT be converted to a representation in terms of relativistic wave equations and their solutions. There are even some indications in the literature (e.g., Konopinski) that quantum field theory, with its embarrassment of infinities and self-energies that must be renormalized away, could profit from a representation in terms of a time-symmetric (advanced + retarded) formalism as suggested by the TI.

(4) Another objection raised by Jack Sarfatti: "Figures 2-5 of the RMP paper show the waves propagating along 45-degree space-time trajectories, i.e., traveling at the speed of light. Further, Gribbin has argued that in the light-cone reference frame there is no space-time interval between the emission and absorption events, so they are superimposed, facilitating the TI handshake. Therefore, the TI is implicitly only about photons and cannot be applied to massive particles like electrons. Also, Dirac has taught us that time-reversed electrons are positrons, so their wave representations can't be combined in an advanced- retarded handshake."

The photon argument is bogus. The figures in the RMP paper use 45 degree trajectories because essentially all of the EPR experiments that have demonstrated the intrinsic nonlocality of QM employ photons. This does NOT mean that the trajectories of the advanced and retarded waves HAVE to be at 45-degrees in a Minkowski diagram. Clearly, massive particles will have steeper timelike trajectories, which presents no problems to the application of the interpretation. Gribbin's zero-interval observation is interesting but was not contained in the TI paper and is not a part of the interpretation.

The positron argument is also bogus. Representing charged particles with advanced and retarded solutions requires four kinds of waves which are characterized by positive or negative charge and positive or negative energy, all of which are solutions, for example, of the Dirac equation. The advanced-retarded handshake for an transaction involving the emission and absorption of an electron will use a negative charge with positive energy wave and a positive charge with negative energy wave, the latter reinterpreted as a positive energy electron according to the usual Dirac rules.

These are all the "fundamental objections" to the TI of which I am aware. None are particularly damaging, and the first two have been addressed in a journal publication with no published refutations. In this context I should also mention that Steve Gull of Cambridge, in a recent FOP publication about the Clifford Algebra version of the Dirac equation, remarked as a sort of aside that the TI could not be applied to the solutions he was looking at. I sent him E-mail (which I know he received) asking for a clarification of that statement, but in ~6 months he has not replied. If you know him, perhaps you could encourage him to do so.


Anyhow, that's about all I can say about the status of the transactional interpretation. I'd be interested in hearing how your presentation goes at Cambridge. Please keep me informed.

Best regards,

John G. Cramer Professor of Physics University of Washington

Source : Foundations of Quantum Mechanics, Mike Towler, Cavendish Laboratory, University of Cambridge

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