18 November 1997, “Bolt from the Blue” I saw Adrian Kent's four papers on the server early this morning, and I was reminded of Léon Rosenfeld's description of Bohr's reaction to the EPR paper: This onslaught came down upon us as a bolt from the blue. Its effect on Bohr was remarkable. We were then in the midst of groping attempts at exploring the implications of the fluctuations of charge and current distributions … A new worry could not come at a less propitious time. Yet, as soon as Bohr had heard my report of Einstein's argument, everything else was abandoned … I suspect the air in Montréal will be just as this when you all awake today. How I wish I were there to see the excitement! I hope for the sake of our field, and a generally more exciting world, that Kent is right! As you all know, this sceptical physicist (i.e., me) was always a little wary of the Mayers–Lo–Chau strong claim of having considered all possible quantum protocols. I would like to hear the Montréal verdict once it is in. A good day to all! 31 August 1999, “Subliminal Messages” Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography
18 November 1997, “Bolt from the Blue” I saw Adrian Kent's four papers on the server early this morning, and I was reminded of Léon Rosenfeld's description of Bohr's reaction to the EPR paper: This onslaught came down upon us as a bolt from the blue. Its effect on Bohr was remarkable. We were then in the midst of groping attempts at exploring the implications of the fluctuations of charge and current distributions … A new worry could not come at a less propitious time. Yet, as soon as Bohr had heard my report of Einstein's argument, everything else was abandoned … I suspect the air in Montréal will be just as this when you all awake today. How I wish I were there to see the excitement! I hope for the sake of our field, and a generally more exciting world, that Kent is right! As you all know, this sceptical physicist (i.e., me) was always a little wary of the Mayers–Lo–Chau strong claim of having considered all possible quantum protocols. I would like to hear the Montréal verdict once it is in. A good day to all! 31 August 1999, “Subliminal Messages” Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography Quantum Foundations in the Light of Quantum Cryptography
03 April 2000, “Foundations in the Light of Information” Below is a slightly modified version of the invitation letter I sent everyone. (The only change is an update on the attendance.) Looking forward to seeing you again. (BTW, I hope you'll take the problems list seriously.) Dear friends, Gilles Brassard and I are organizing a little meeting in Montréal from Wednesday May 17 through Friday May 19 on the subject of “Quantum Foundations in the Light of Quantum Information and Cryptography.” We hope you can come. Mainly the purpose of the meeting is to gather a group of our friends who think that some aspects of quantum foundations are a little more mysterious than they ought to be and, importantly, are intrigued by the idea of applying quantum information to the task of cleaning things up. The atmosphere of the meeting should be quite relaxed with plenty of time for discussion and/or private brooding: we only ask that you be prepared to give a 20 to 30 minute talk (though longer talks are welcome too) expressing your point of view and any avenues for progress you foresee. Attendants are also encouraged to compile and share a list of concrete problems whose solution would tell us something novel about the quantum foundations.
30 August 1998, “Desiderata” I just read over your four desiderata for an acceptable “über-quantum theory” for the third time. (When I finish this note, I'll read them over for the fourth time … at the very least for simile's sake.) They're not so uninspired! (… to use your words, that is.) But I do hope you will keep your promise of writing a longer exposition. In the mean time, let me ask you for one point of clarification about the connection between your Desideratum #1 and my latest happy thoughts. Kentism 1: 1. A theory in which some elementary, and precisely defined, components constitute the basic statements from which propositions about an external and objective reality can be built. Maybe trajectories à la Bohm, maybe collapse events à la GRW, maybe events of more general form à la consistent historians, maybe the evolution of the classical gravitational field in some (unknown) theory that links classical metric and quantum matter. Maybe something quite different from any of the above. But, at any rate, something which allows us to talk about what happened in the past, in the absence of observers, and which allows us to resolve the measurement problem by replacing the vague notion of measurement by something precise.
Abstract World War II drove several of the founders to Britain or the US. Foundational questions returned with renewed energy in the early postwar years when significant anniversaries came due. The usual suspects rejoined the debate. In the volume celebrating his 70th birthday, Einstein returned to the EPR paper. To its main point, which had been ‘buried alive, so to speak, by erudition’, he now brought exceptional clarity. If the two entangled particles are considered separable and ‘locally real’, then they must communicate via some peculiar mechanism of action at a distance. If they cannot be considered as separable a gaping explanatory hole opens. Either way, quantum theory is not up to the task. But the world of physics, now dominated by America, had moved on. Physicists ‘had no interest in understanding Bohr, and thought that Einstein’s distaste for [quantum mechanics] was just silly . . . It was a very unphilosophical time’.
On 25 March 1935, the Physical Review received a paper from Albert Einstein, Boris Podolsky, and Nathan Rosen, with the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” A few days later, on 30 March 1935, I was born. My life with Einstein was off to a promising start. Some would call it an inauspicious start. Abraham Pais, for example, says in his otherwise admirable biography of Einstein [1] that “the only part of this article that will ultimately survive, I believe, is this last phrase [ No reasonable definition of reality could be expected to permit this ], which so poignantly summarizes Einstein's views on quantum mechanics in his later years.” But today, in this centenary of the Einstein annus mirabilis, as the EPR paper and I both turn 70, it is, in fact, the most cited of all Einstein's papers [2]. The debate over its conceptual implications rages hotter than ever, and for the first time, practical (well, for the moment still gedanken practical) applications of the EPR effect have emerged in cryptography and in other areas of quantum information processing. Being only five weeks old, I was unprepared to pay attention to the article that appeared in the New York Times on 4 May 1935 under an elaborate set of headlines and subheads: And I was completely oblivious to the stern rebuke from Einstein himself, published three days later in the Times , which declared that “any information upon which the article … is based was given to you without my authority. It is my invariable practice to discuss scientific matters only in the appropriate forum and I deprecate advance publication of any announcement in regard to such matters in the secular press.” Apparently Podolsky had tipped off the Times to the article, which did not appear in the sacred press until the 15 May issue of Physical Review. It is not clear that Einstein ever forgave him, and I wish I had been old enough to send Podolsky a cross letter myself. Anyone growing up in America in the 1940s knew that the preeminent genius of our age, and perhaps of any other, lived in Princeton, New Jersey, had a predilection for baggy sweaters, and was always badly in need of a haircut—as far ahead of his time in dress and grooming as he was ahead in science during his 1905 annus mirabilis.
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.
Wonderful news from Copenhagen Heisenberg and Schrödinger invented mathematical formalisms that provided correct answers to all the various problems of (non-relativistic) quantum theory. It was Bohr who uncovered the underlying significance of the theory; in particular he showed how the profound conceptual difficulties encountered as the theory developed – the so-called wave–particle duality, the totally unclassical nature of the uncertainty principle – could be neutralised by a revision, or more accurately a generalisation, of our use of physical concepts. Such was the generally accepted view of things from soon after 1927, when Bohr first put forward his ideas on complementarity at an international meeting of physicists at Como, through the 1930s, when it was felt that Bohr destroyed Einstein's contrary opinions, and certainly up to the time of Bohr's death in 1962. Bohr's analysis is usually spoken of as the Copenhagen interpretation of quantum theory, though it is instructive to realise that such a term was frowned upon by those closest to Bohr, since it appears to suggest that the interpretation is just one among (conceivably) many. Let us examine, for example, the words of Léon Rosenfeld, Bohr's disciple and long-term collaborator, as expounded at a conference in 1957 [52]. Rosenfeld maintained that any idea of ‘interpreting a formalism’ was a ‘false problem’, that in a good theory, the ‘ordinary language (spiced with technical jargon for the sake of conciseness)’ in which it is described, is ‘ inseparably united … with whatever mathematical apparatus is necessary’, that ‘we are here not faced with a matter of choice between two possible languages or two possible interpretations, but with a rational language intimately connected with the formalism and adapted to it, on the one hand, and with rather wild, metaphysical speculations … on the other’.