/2 to such devices, tacitly admitting quantum effects.huToward the end of April 1957, Everett received a letter from Petersen, reporting on how the two preprints had been received in Copenhagen. The gist of Petersen’s renewed critique was that Everett’s theory did not conform to Bohr’s principle of complementarity or his insistence on the special status of observation. Everett was, however, not at all interested in working within what he took to be a deeply flawed approach to quantum mechanics.
24. 4. 57hl
Dear Hugh,
Thanks for sending the two papers.hm It was good to see that your ideas are now going to be published. During the past months I have often been wondering if we might see you here as we had hoped, and I trust that you have not quite given up coming and seeing us.
As you can imagine, the papers have given rise to much discussion at the Institute, especially with Bohr, Källénhn and Rosenfeld who was here for a few days. Koman also read the manuscripts. Of course, I am not going to report in detail about these discussions, but I think that most of us here look differently upon the problems and don’t feel those difficulties in quantum mechanics which your paper sets out to remove. Accordingly, we cannot agree with you and Wheeler that the relative state formulation entails a further clarification of the foundations of quantum mechanics.
I don’t think you can find anything in Bohr’s papers which conforms with what you call the external observation interpretation.ho Rather, his analysis follows the line of the correspondence argument which was the basic guide in establishing the formalism as a mathematical generalization of classical theories. As emphasized in the Einstein–article there can on this view be no special observational problem in quantum mechanics— in accordance with the fact that the very idea of observation belongs to the frame of classical concepts.hp The aim of the analysis is only to make explicit what the formalism implies about the application of the elementary physical concepts. The requirement that these concepts are indispensable for an unambiguous account of the observations is met without further assumptions and is directly reflected by the way in which c- and q-numbers appear in the formalism. There is no arbitrary distinction between the use of classical concepts and the formalism since the large mass of the apparatus compared with that of the individual atomic objects permits that neglect of quantum effects which is demanded for the account of the experimental arrangement.hq There may be cases, e.g. in the treatments of the γ-ray microscope, where the placing of the separation is to some extent a matter of taste, but the free choice is limited to a region where quantum description is equivalent with the classical one. In the recording of observations, like a mark on a photographic plate, we are also concerned only with measurements performed on heavy bodies. Such recordings may of course be witnessed by any number of observers, and also as regards approximate measurements I can see no new problems.
I do not understand what you mean by quantized observers. Obviously, one can treat any interaction quantum-mechanically, including the interaction between an electron and a photographic plate, but when utilized as an “observer” the definition of the “state” (position) of the plate excludes considerations of quantum effects.hr It seems to me that as far as your treatment of many-body systems is consistent with the proper use of the formalism it has nothing to do with the measuring problem.
Of course, I am aware that from the point of view of your model-philosophy most of these remarks are beside the point.hs However, to my mind this philosophy is not suited for approaching the measuring problem. I would not like to make it a universal principle that ordinary language is indispensable for definition or communication of physical experience, but for the elucidation of the measuring problems hitherto met with in physics the correspondence approach has been quite successful. How radically this approach will have to be modified in order to cope with the many unsolved difficulties remains to be seen.
Betty and I hope to see you sometime when you have finished your work in the Pentagon. As you may know we have got a son who is now learning some elementary communication.
Please give our greetings to the Wheelers.
Yours
Aage
Hugh Everett, III
Arlington Towers, T-438
Arlington, Virginia
May 31, 1957
Dr. Aage Petersen
Blegdamsvej 17
Copenhagen, Denmark
Dear Aage:
It was very good to hear from you again. Perhaps we will be able to talk together again sometime soon. There is a good chance that I will be sent to Europe in the fall on business, and I could probably take a few weeks off and come to Copenhagen. Please let me know what the best times to come are so that I can arrange things (to the extent that I am able) to be most convenient.
In the meantime, lest the discussion of my paper die completely, let me add some fuel to the fire with a number of random comments and criticisms of the “Copenhagen interpretation.”ht
First of all, the particular difficulties with quantum mechanics that are discussed in my paper have mostly to do with the more common (at least in this country) form of quantum theory, as expressed for example by von Neumann, and not so much with the Bohr (Copenhagen) interpretation. The Bohr interpretation is to me even more unsatisfactory, and on quite different grounds. Primarily my main objections are the complete reliance on classical physics from the outset (which precludes even in principle any deduction at all of classical physics from quantum mechanics, as well as any adequate study of measuring processes), and the strange duality of adhering to a “reality” concept for macroscopic physics and denying the same for the microcosm.
Now I do not think you can dismiss my viewpoint as simply a misunderstanding of Bohr’s position. I am willing to admit that Bohr’s complementarity principle, which expresses limitations on the unrestricted use of classical concepts, is a valid principle. I even am prepared to admit that in the initial stages of formulation of quantum theory this principle was very useful in clarifying the theory and showing that it does not lead to any of the more obvious kinds of contradictions. The trouble goes much deeper than this however. I believe that the basing of quantum mechanics upon classical physics was a necessary provisional step, but that the time has come to proceed to something more fundamental.
There is a good analogy in mathematics. The complex numbers were first defined only in terms of the real numbers. However, with sufficient experience and familiarity with their properties, it became possible and indeed more natural to define them first in their own right without reference to the real numbers, and to derive from them the special case of the reals. I would suggest that the time has come to do the same for quantum mechanics—to treat it in its own right as a fundamental theory without any dependence on classical physics, and to derive classical physics from it. While it is true that initially the classical concepts were required for its formulation, we now have sufficient familiarity to formulate it without classical physics, as in the case of the complex numbers. I am sure that you will recognize this as Bohr’s own example turned against him.
The analogy goes further yet. Just as we no longer regard complex numbers as mere appendages tacked on to the reals to cover annoying inabilities to solve certain equations, we should no longer regard quantum mechanics as a mere appendage to classical physics tacked on to cover annoying discrepancies in the behavior of microscopic systems.
Let me now mention a few more irritating features of the Copenhagen interpretation. You talk of the massiveness of macrosystems allowing one to neglect further quantum effects (in discussions of breaking the measuring chain), but never give any justification for this flatly asserted dogma. Is this an independent postulate? It most certainly does not follow from wave mechanics which leads to quite strange superposition states even for macrosystems when applied to any measuring processes! In fact, by the formulation of your viewpoint you are totally incapable of any justification and must make it an independent postulate—that macrosystems are relatively immune to quantum effects.
Another inconsistency: you vigorously state that when an apparatus can be used as measuring apparatus then one cannot simultaneously give consideration to quantum effects—but proceed blithely to apply the formula ΔxΔP ≥ /2 to such devices, tacitly admitting quantum effects.hu
You say you see no further difficulties with approximate measurements. I have yet to see any adequate account of the phenomena and would appreciate any references you can supply.
Just one final point. I am getting weary of hearing on the one hand that it is the fundamental irreversibility of the measuring process which allows the destruction of phase relations and makes possible the probability interpretation of quantum mechanics, and on the other hand that the fundamentally probabilistic processes of quantum mechanics allow truly reversible processes and for the first time make a satisfactory thermodynamics possible. As a matter of fact, there is nowhere to be found any consistent explanation of this “irreversibility” of the measuring process. It is again certainly not implied by wave mechanics, nor-classical mechanics either. Another independent postulate?hv
I am sure that these points (by no means exhaustive) are poorly and inadequately expressed here, but hope you will think about them until we can have a full discussion. I look forward very much to renewing our always enjoyable arguments. Please give my regards to Betty.
Sincerely,
Hugh Everett, III
Note: Address after August 1:
607 Pelham Street
Alexandria, Virginia
HE:ngehw
hl Wheeler writes at the top of the page: “MBP—Please make a photocopy for me and send this original to Hugh Everett.”
hm Given what Petersen says below, he had just read Everett’s short thesis and Wheeler’s companion paper.
hn Gunnar Källén was a Swedish physicist working at CERN.
ho While Everett sometimes was not perfectly clear in distinguishing between the external observation interpretation of quantum mechanics and the Copenhagen interpretation, there is good reason to suppose that he understood the distinction while working on his thesis since he discusses the Copenhagen interpretation separately in the long thesis, and in his subsequent letter to Petersen, Everett explicitly distinguishes between the Copenhagen and von Neumann–Dirac formulations (pg. 239). See pg. 153 for Everett’s description and criticisms of the Copenhagen interpretation in the long thesis and pgs. 32–34 in the conceptual introduction for a discussion of Everett’s understanding of the distinction.
hp The argument is that since measurement must necessarily be understood classically, there is a sense in which there can be no quantum measurement problem. Everett, of course, disagreed with the assumption that measurement must necessarily be understood classically insofar as he held that he had provided a thoroughgoing quantum mechanical account of measurement.
hq Petersen’s appeal to quantum mechanical considerations to justify treating the experimental apparatus classically is inconsistent with the view that observation is only conceivable as involving a classical apparatus. Everett notes this inconsistency in his reply. Insofar as one simply stipulates that a measuring apparatus cannot be thought of as quantum mechanical, there can be no justification for the stipulation from the quantum mechanical properties one would expect of the apparatus (pg. 240).
hr This is the orthodox Copenhagen line that an observer as an observer can only be conceived of classically.
hs The Copenhagen interpretation did not seek to provide a single, consistent model of all physical systems including measuring devices. Rather, it sought to find a place for classical description in quantum mechanics, and hence what the Copenhagen colleagues took as a precondition for genuine understanding. See pgs. 152–53.
ht This does not mesh well with Wheeler’s report to the Copenhagen colleagues that Everett did not mean to criticize the Copenhagen interpretation. See for example pg. 219. See pg. 236
for Petersen’s discussion of the Copenhagen interpretation and the conceptual introduction (pgs. 32–34) for a discussion of Everett’s dissatisfaction with the Copenhagen interpretation. Everett believed that the Copenhagen interpretation, properly understood, would be subsumed within his relative state formulation.
hu Here Everett notes that Petersen is violating that Copenhagen dogma by treating measuring devices quantum mechanically at all. See (pg. 237).
hv Everett held that pure wave mechanics was fully reversible. Any appearance of irreversibility was just that—an appearance. See for examples pgs. 143–44, 224, and fn. kz on pg. 287.
hw Written at the bottom of the page: “Copy sent to Wheeler”.