“Partners in crime again?” CAF to AP 29 September 1999 Introduction Irving John Good once titled a paper, “46,656 Varieties of Bayesians.” With little stretch of the imagination, one can believe that a similar paper on the interpretations of quantum mechanics exists somewhere out there too. And that may be true even though 46,656 is greater than the whole membership of the American Physical Society! It is no wonder that taking a stand on quantum foundations is treacherous business. Nevertheless, in the March 2000 issue of Physics Today , Asher Peres and I flouted the usual rewards of a calm life and did just that. Our article was titled, “Quantum Theory Needs No ‘Interpretation’.” This chapter is about the evolution of that article and a later “reply to critics.” When I was an undergraduate at the University of Texas, Bryce DeWitt made a great impression on me by saying something I have not forgotten: “We learn mathematics so that we don't have to think when we do physics.” My paper with Asher contained no mathematics. We had to think, and think very hard. The fruits of that labor, for my part, were in a greatly enhanced keenness for seeing through various foundational issues. How I wanted to share that newfound keenness with the reader! But it was difficult, if not impossible, to convey such an aspect of the paper-writing process in the very paper itself.
“Partners in crime again?” CAF to AP 29 September 1999 Introduction Irving John Good once titled a paper, “46,656 Varieties of Bayesians.” With little stretch of the imagination, one can believe that a similar paper on the interpretations of quantum mechanics exists somewhere out there too. And that may be true even though 46,656 is greater than the whole membership of the American Physical Society! It is no wonder that taking a stand on quantum foundations is treacherous business. Nevertheless, in the March 2000 issue of Physics Today , Asher Peres and I flouted the usual rewards of a calm life and did just that. Our article was titled, “Quantum Theory Needs No ‘Interpretation’.” This chapter is about the evolution of that article and a later “reply to critics.” When I was an undergraduate at the University of Texas, Bryce DeWitt made a great impression on me by saying something I have not forgotten: “We learn mathematics so that we don't have to think when we do physics.” My paper with Asher contained no mathematics. We had to think, and think very hard. The fruits of that labor, for my part, were in a greatly enhanced keenness for seeing through various foundational issues. How I wanted to share that newfound keenness with the reader! But it was difficult, if not impossible, to convey such an aspect of the paper-writing process in the very paper itself.
June 1997, “Painting My Stripes” I am just returning from the library, where – completely by accident – I ran into and read an entertaining little article: A Peres, “Einstein, Gödel, Bohr,” Foundations of Physics 15 (2), 1985, 201–205. I was especially entertained by your closing line, “Any attempt to inject realism in physical theory is bound to lead to inconsistencies.” Do you have any other writings (beside the AJP paper with Zurek) where you expand upon the ideas presented in this? In connection to this, I have another question. If I recall correctly, you once told me that you finally came to grips (“interpretationally,” that is) with quantum mechanics in 1986. (Perhaps I have the year wrong, but that's part of the question.) Would you be willing to expand upon that point? In particular, was your arrived-at comfort founded upon the line I quote above? I suppose I ask these questions for several reasons, perhaps the strongest of which is a renewed dilettante interest in the foundations of quantum theory. Among other things, I am trying to plan (or dream up!) a talk for the Hull conference in September to be titled something like: “Quantum Information Theory's Implications for the Foundations of the Quantum as a Whole.” My own pet “idea for an idea” (as Wheeler would say) is that the quantum theoretical description – along with its Hilbert-space structure – is forced upon us when the information we have about a physical situation/system is of a certain variety.
22 August 1999, “Your Article” I am writing to let you know that I enjoyed your article quant-ph/9812050, “Why Quantum Mechanics is Hard to Understand,” very much. I was in particular very pleased with your Section IV “Dynamics vs. History.” It may interest you to know that Markus Fierz expressed a similar point of view in his article: “Does a physical theory comprehend an ‘objective, real, single process’?” in Observation and Interpretation in the Philosophy of Physics , edited by S. Körner (Dover, NY, 1957), pp. 93–96. I will append the full text below for your enjoyment. [See note to Rüdiger Schack, dated 29 August 1999.] Most importantly, I would like to ask you for the final coordinates of your paper. Was it published somewhere? I would like to cite it in an upcoming paper. 28 August 1999, “Not Much” I don't have a good idea about where to send your paper. I think chances are that AJP will be tougher than FP . In any case, it had an impact on me. My old advisor used to say of our joint papers, “If three people read this paper, we'll be doing OK!” If you know of two others with yours, you're probably doing OK. I like this focus on the concept of “object.” I look forward to anything you might have to say about it. I have some slides that I use as a rallying cry to the idea that quantum information theory may have some impact on quantum foundations. The first is a list of axioms for quantum mechanics.
October 1997, and to Asher Peres, “Premonition?” I was awakened this morning just in the middle of a complicated dream. I only remember very fuzzy fragments, but there is one in particular that I thought you two would enjoy hearing about. Someone walked up and handed me a newspaper clipping. The headline read, “Bill Wootters and World Renowned Teleporter Asher Peres Attain New Heights.” I didn't see the article, but the headline was accompanied by a picture of you two both wearing lederhosen and Alpine hats, smiling at the camera!! July 1998, “Quantum Giggles” I'm sitting at a little sidewalk cafe in Benasque, Spain thinking of you and all your efforts to find a compelling structure underneath quantum mechanics. It's strange, but for me this has turned out to be a very foundation-oriented conference. Carl Caves and I have been working to construct a quantum version of de Finetti's theorem in classical probability theory. de Finetti's theorem gives the “subjectivist” a way to interpret an “unknown probability” within his framework: probability distributions over the probability simplex are nothing more than shorthand for a certain kind of probability assignment (called “exchangeable”) over a large multi-trial space. What is an unknown quantum state? If you take quantum states to be states of knowledge, as we do, then an unknown quantum state is a troublesome concept. We think that it can be fixed up in roughly the same way – one just has to identify the right notion of an “exchangeable density operator.”
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.
Fifteen years ago, I mused in a Reference Frame column on how different generations of physicists differed in the degree to which they thought that the interpretation of quantum mechanics remains a serious problem. I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be ‘Shut up and calculate!’” In the intervening years, I've come to hold a milder and more nuanced opinion of the Copenhagen view, but that should be the subject of another column. The subject of this one is the habit of misquotation or misattribution that afflicts our profession, a rather different example of which I pointed out a few months ago. Given my capacity for intellectual development (“inconsistency,” in the terminology deployed in the current American political season), it's fortunate that I've now reached an age at which I tend to forget about things I've written more than a few years ago. Indeed, I find it downright irritating when somebody asks me questions about papers I wrote a mere half dozen years ago, naively identifying me with the author of those ancient texts. Until quite recently, I had no memory of ever having written such a childishly brusque dismissal of such an exquisitely subtle point of view, much less of having published it in so widely read a venue. This amnesia, combined with the evolution in my thinking that had distanced me from my long-forgotten words, may explain why I was initially somewhat puzzled by the slight sensation of discomfort that passed over me when, browsing the e-print archive earlier this year, I read a characterization of Max Born's probability rule as “the favorite ingredient of what has been nicknamed, after Feynman's famous dictum, the shut up and calculate interpretation of quantum mechanics.” I yield to nobody in my admiration for Richard Feynman's aphorisms on the nature of quantum mechanics. Indeed, long ago I published a poem consisting of nothing more than a resetting as verse of a paragraph Feynman had written about his own attitude toward the quantum theory, in his now (but not then) famous article that launched the whole field of quantum computation. I like to think I have devoured everything Feynman ever wrote on the character of quantum mechanics.