June 1996, “Cyclic Never-Endings” Yes, yes, I like Montréal! Does that finally answer your question? The only thing I really miss about the states is having a swarm of good physicists around me. El Jeffy 1: You know, I really miss listen to you babble about quantum mechanics. Nobody around any more to tell me that the moon isn't there when I'm not looking; you even had me believing for a while . Damn, if I had only stayed there a bit longer…perhaps you would have remained a true believer! August 1996, “Laws of Thought” [Referring to the phrase “Quantum Mechanics is a Law of Thought.”:] El Jeffy 2: I was just reading about the mathematician George Boole, the guy who invented Boolean Algebra. In 1854 he published a book called An Investigation of the Laws of Thought, on which are founded the mathematical theories of logic and probabilities. Is that where your lovely little phrase comes from? Yes indeedy, that's exactly where it comes from. Quantum Mechanics is a Law of Thought. God I like the ring of that. August 1996, “Plagiarists” El Jeffy 3: I am disappointed in you. I never realized that you plagiarized that from poor ol' Boole. Can't think up your own catch phrase, eh? I think the phrase bothers me far less when applied to something like Boolean algebra. It still gives me the heebies when you refer to real live physical systems with such terminology .
June 1996, “Cyclic Never-Endings” Yes, yes, I like Montréal! Does that finally answer your question? The only thing I really miss about the states is having a swarm of good physicists around me. El Jeffy 1: You know, I really miss listen to you babble about quantum mechanics. Nobody around any more to tell me that the moon isn't there when I'm not looking; you even had me believing for a while . Damn, if I had only stayed there a bit longer…perhaps you would have remained a true believer! August 1996, “Laws of Thought” [Referring to the phrase “Quantum Mechanics is a Law of Thought.”:] El Jeffy 2: I was just reading about the mathematician George Boole, the guy who invented Boolean Algebra. In 1854 he published a book called An Investigation of the Laws of Thought, on which are founded the mathematical theories of logic and probabilities. Is that where your lovely little phrase comes from? Yes indeedy, that's exactly where it comes from. Quantum Mechanics is a Law of Thought. God I like the ring of that. August 1996, “Plagiarists” El Jeffy 3: I am disappointed in you. I never realized that you plagiarized that from poor ol' Boole. Can't think up your own catch phrase, eh? I think the phrase bothers me far less when applied to something like Boolean algebra. It still gives me the heebies when you refer to real live physical systems with such terminology .
February 1996, “QM is a Law of Thought” By the way, did you ever read the paper Carl and I wrote titled “Quantum Information: How Much Information in a State Vector?” In it we talk at some length about this idea of “Quantum Mechanics as a Law of Thought” and other Bayesian sounding things. (We even quote Gertrude Stein?!”!) Of course, all the ideas are loose now, but one day they'll be tight. September 1996, “Silly Latin” Allow me to continue my tradition of writing you only when I need something translated!! I've decided on an epithet for myself. Either: Quantum Theory is a Law of Thought or Quantum Mechanics is a Law of Thought What do you think? (…and don't tell me I'm plagiarizing Boole!) Anyway, the problem is that I would like to have these translated as closely as possible into Latin. Do you think you're up to the task? I would appreciate it so much. I can't wait to see you in Japan; I really miss the old gang. Rüdiger's First Reply: 19 September 1996 I like “Quantum Mechanics is a Law of Thought” better, but I can't give a good reason for that.
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.
Attitudes toward quantum mechanics differ interestingly from one generation of physicists to the next. The first generation are the founding fathers, who struggled through the welter of confusing and self-contradictory constructions to emerge with the modern theory of the atomic world and supply it with the “Copenhagen interpretation.” On the whole they seem to have taken the view that while the theory is extraordinarily strange (Bohr is said to have remarked that if it didn't make you dizzy then you didn't really understand it), the strangeness arises out of some deeply ingrained but invalid modes of thought. Once these are recognized and abandoned the theory makes sense in a perfectly straightforward way. The word “irrational,” which appears frequently in Bohr's early writings about the quantum theory, is almost entirely absent from his later essays. The second generation, those who were students of the founding fathers in the early postrevolutionary period, seem firmly—at times even ferociously—committed to the position that there is really nothing peculiar about the quantum world at all. Far from making bons mots about dizziness, or the opposite of deep truths being deep truths, they appear to go out of their way to make quantum mechanics sound as boringly ordinary as possible. The third generation—mine—were born a decade or so after the revolution and learned about the quantum as kids from popular books like George Gamow's. We seem to be much more relaxed about it than the other two. Few of us brood about what it all means, any more than we worry about how to define mass or time when we use classical mechanics. In contemplative moments some of us think the theory is wonderfully strange and others think it isn't; but we don't hold these views with great passion. Most of us, in fact, feel irritated, bored, or downright uncomfortable when asked to articulate what we really think about quantum mechanics. I'm one of the uncomfortable ones. If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!” But I won't shut up. I would rather celebrate the strangeness of quantum theory than deny it, because I believe it still has interesting things to teach us about how certain powerful but flawed verbal and mental tools we once took for granted continue to infect our thinking in subtly hidden ways.
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.
November 1997, “The Radical Eye” Let me send you some prep work…while I'm procrastinating from things I really need to be doing. I'll convey the information in my favorite way: quoting/forwarding old emails! The first, an old – kind of poorly written – note “Dreams of a More Ethereal Quantum,” concerns one aspect of the below-the-water stuff: the idea about distilling the MaxEntlooking part out of quantum theory. It doesn't say much more than I said in the proposal…but you can't expect too much, it's still just a vague program for a program. That stuff is at the bottom of the note; the early part of the note says a few – even more sketchy – things about Mermin and Quantum Probabilities…so I thought I would include it for that reason. The only other readable thing (I think) I've written on this aspect of the below-the-water stuff is on pages 22–26 of my paper with Caves, “Quantum Information: How Much Information in a State Vector” (it's on quant-ph). These same pages also contain an extended discussion on probability. The second note, “Rainy Mondays and Consistent Histories,” is a note I wrote Bob Griffiths concerning what I think of his Consistent History stuff. I think it says quite plainly and clearly what I think about probability in quantum mechanics. It's pretty self-contained (i.e., not referring to old conversations, etc.), so you might find it useful reading.
08 April 1996, “Hartle/Coleman Comments” I like the grass fire analogy: I agree we should stomp it out if we can. However, as with most debates, I imagine we'll just have to wait until it falls by the wayside … or wait until someone gives it a name like the Modern Interpretation of Quantum Mechanics with Sugar on Top, so that everyone else will just discount it and move on. Do you know whether Coleman has published any of this? Wojciech first told me about it at least a year ago (and told me I should contact Coleman concerning it to “help your career along”). If you have a copy of the paper, please send it my way. I've been at a little disadvantage in thinking about this because I cannot find my copy of Hartle's paper and the library has been shut down since Wednesday evening; I'll be able to get a new copy Tuesday morning. I know I liked the paper very much after turning semi-Bayesian because of what it said in words (even though I never agreed with the mathematics); however, it certainly did have a frequentist streak running through it … and that's what's bringing us back now. I think what I'll do in this note is just write down various thoughts as they come to me; I've got a few pieces of junk spread out on my desk to prod me along.