Cambridge University Press eBooks

Cosmology and the many worlds interpretation of quantum mechanics

Viatcheslav Mukhanov

Published Jun 21, 2007• 3 citations

Abstract

Introduction Although the mathematical structure of quantum mechanics was understood within a few years after it was invented, numerous quantum paradoxes still disturb ‘simple-minded’ physicists. Most of them, as ‘naïve realists’, would probably never take Bohr's own over-philosophical and over-complicated treatment of these paradoxes seriously if they realized the philosophical consequences of the Copenhagen interpretation. To make my meaning clearer, let me quote Bohr's answer to Professor Hoffding's question regarding the double-slit experiment [1]. Bohr was asked: ‘What can the electron be said to be in its travel from the point of entry to the point of detection?’ And he replied: ‘ To be ? To be ? What does it mean to be ?’ However, if one questions the existence of microscopic constituents of macroscopic bodies, then the next logical step would be to question the existence of the macroscopic bodies and even ourselves. Needless to say, very few (if any) of us, when making experiments or analyzing their results, address the question of what it means ‘to be’ every time. Even in the context of elementary particles, probably nobody doubts that the particles exist and somehow travel from the point of entry to the point of detection. Moreover, within the accuracy allowed by the uncertainty relation, these particles can be localized and described just as well as macroscopic ‘classical’ objects.

DOI: 10.1017/cbo9781107050990.019

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Routledge eBooksSep 8, 20170 citations

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On the Interpretation and Philosophical Foundation of Quantum Mechanics

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Jan 1, 200826 citations

Could Feynman Have Said This?

N. David Mermin

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 (Physics Today, Physics Today 0031-9228 42 4 1989 9 https://doi.org/10.1063/1.2810963. April 1989, page 9 ). 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 (February 2004, page 10).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 (Physics Today, April 1985, page 47) 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. But while "shut up and calculate" sounded dimly familiar to me as a characterization of a certain interpretive stance, I couldn't recall where Feynman had written it. Mulling this over, a terrible thought began to dawn on me. Could it be that I myself had once used the phrase? If so, then it would appear that I had picked it up from something by Feynman, forgotten the source, and presented it as my own. Devastating!It was devastating because I have a horror of writing or uttering any witticism that is not original with me, unless I make it absolutely clear where and (if known) from whom I got it. I don't even like to tell jokes unless I've made them up myself. (I digress to offer you my favorite:Question: What is the difference between theoretical physics and mathematical physics? Answer: Theoretical physics is done by physicists who lack the necessary skills to do real experiments; mathematical physics is done by mathematicians who lack the necessary skills to do real mathematics. Mathematical physicists tend not to like this joke, but other physicists seem to. Nonphysicists, of course, are entirely immune to its charms.)So with growing trepidation, I searched through my past writings on quantum mechanics. I was dismayed when I came upon my 1989 column, which confirmed my worst fears. Not only had I appropriated without proper attribution a Feynman quote, but it appeared to be a famous one. How humiliating! I was afflicted with visions of knowledgeable Physics Today readers shaking their heads 15 years ago at what must have struck them as my shameless attempt to ride to literary glory on the unacknowledged shoulders of Feynman.So I went to the Web to find the source, hoping I could then salvage my reputation by persuading Physics Today to print an addendum or erratum. Google gives more than 130 hits containing both "shut up and calculate" and "feynman." Most of these do not directly link the two, but about a dozen do. Here are a few: "Shut up and calculate" was a motto of Richard Feynman.For example, there's Feynman's "shut up and calculate."My personal philosophy is that of the famous physicist Richard Feynman, who said: "Shut up and calculate."When asked which interpretation of QM he favored, Feynman replied "Shut up and calculate."Richard Feynman foreslog ligefrem en "shut up and calculate" fortolkning af kvantemekanikken.Shut up and calculate—Richard Feynman.Just to make sure, I also searched for "shut up and calculate" and "mermin." I found only 10 hits, all of them mentioning me in ways that had nothing to do with their use, elsewhere, of "shut up and calculate." So it would have been clear to the world that I had indeed passed off Feynman's words as my own.Or had I …?I noticed that not a single one of the Web sites attributing the phrase to Feynman cited a source or hinted at the circumstances under which he had said it. A ray of hope flickered on: Could I once again have become a victim of the Matthew effect?The Matthew effect was enunciated by the great sociologist of science, Robert Merton. 1 1. R. K. Merton, Science 159, 56 (1968). https://doi.org/10.1126/science.159.3810.56 Merton worked in those innocent days when sociologists were interested only in the behavior of scientists and not in the content of their science. (To be fair to contemporary sociologists of science, I should modify that last phrase to "and not in the manifestations of that behavior in the content of their science.") I first learned of the Matthew effect more than 20 years ago, on the occasion of my first and, perhaps until now, only, victimization at the hands of the New York Times.I learned the name for what the Times had done to me when I received a very nice note from P. W. Anderson in which he expressed his regret that the newspaper had given him exclusive credit for a nomenclatural advance that was entirely due to me. "A depressingly typical example of the Matthew effect" was how he characterized the misattribution. (I reported the entire history of this contretemps in these pages back in those dark ages [April 1981, page 46] before there were Reference Frame columns.) When I wrote back asking him what the Matthew effect was, he referred me to Merton.It was Merton who identified and named the tendency always to assign exclusive scientific credit to the most eminent among all the plausible candidates. At least I hope it was he, though I'm sure Merton, who invented many wonderful jokes himself, would have been delighted if the credit for it turned out to be misattributed to him. Merton named the effect after the Gospel According to Matthew, because there it is written, For unto every one that hath shall be given, and he shall have abundance: but from him that hath not shall be taken away even that which he hath.—Matthew 25:29.Could the widespread attribution of my wretched witticism to Feynman be another instance of this same deplorable practice? Had I once again been matthewed?Although I didn't say so in my old Reference-Frame article, what inspired this not so terribly bon mot were vivid memories of the responses my conceptual inquiries elicited from my professors—whom I viewed as agents of Copenhagen—when I was first learning quantum mechanics as a graduate student at Harvard, a mere 30 years after the birth of the subject. "You'll never get a PhD if you allow yourself to be distracted by such frivolities," they kept advising me, "so get back to serious business and produce some results." "Shut up," in other words, "and calculate." And so I did, and probably turned out much the better for it. At Harvard, they knew how to administer tough love in those olden days.This bit of history is relevant to the question of whether Feynman's abundance might have been augmented by a portion of the little that I had. Can you imagine the young Feynman ever having had a similar experience that seared "shut up and calculate" into his tender consciousness? No, of course you can't! Nobody could ever have had the slightest reason to direct the best human calculator that ever was to shut up and calculate.But perhaps Feynman was offering such advice to others who were searching for a better understanding of the quantum mechanical formalism. I can't believe that. He said that he "always had a great deal of difficulty understanding the world view that quantum mechanics represents," and added, "I still get nervous with it." 2 2. R. P. Feynman, Int. J. Theor. Phys. 21, 471 (1982). https://doi.org/10.1007/BF02650179 Nobody who felt that way would ever respond with "shut up and calculate" to conceptual inquiries from the perplexed.Well maybe Feynman, like me, was merely dismissing an interpretive position of others by lampooning it as a "shut up and calculate interpretation." I find this unlikely. For one thing, his strong preference for working things out for himself and, of course, his well-known disdain for philosophy make me doubt that he ever paid much attention to the interpretive positions of others. For another, would one for whom calculation was so effortless and understanding so important be likely to translate anybody's admonition against fruitless speculation into such terms?In short, I suspect that it is only Feynman's habitual irreverence that has linked him in the minds of many to the phrase "shut up and calculate." Who else among the high and mighty—and Merton has taught us that it is only among the high and mighty that people tend to look—could have said it? Albert Einstein? Don't be silly. Erwin Schrödinger? Of course not. Niels Bohr? Don't make me laugh. None of them besides Feynman could have said it. Does that mean that Feynman said it? No!Broaden the search to embrace the low and powerless. Among them am I, who hereby put forth the hypothesis that I was the first to use "shut up and calculate" in the context of quantum foundations. I'm not proud of having said it. It's not a beautiful phrase. It's not very clever. It's snide and mindlessly dismissive. But, damn it, if I'm the one who said it first, then that means I did not, even unconsciously, appropriate the words of Richard Feynman and pass them off as my own. So I have nothing to be ashamed of other than having characterized the Copenhagen interpretation in such foolish terms—a lesser offense than unconscious plagiarism, in my moral bookkeeping.So, dear reader, if you have evidence that Feynman really did say "shut up and calculate," please send it to me. I will not be happy to receive it. I'd rather be a Matthew victim than a plagiarist. But I'd like to know the truth.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. R. K. Merton, Science 159, 56 (1968). https://doi.org/10.1126/science.159.3810.56 , Google ScholarCrossref, ISI2. R. P. Feynman, Int. J. Theor. Phys. 21, 471 (1982). https://doi.org/10.1007/BF02650179 , Google ScholarCrossref, ISI© 2004 American Institute of Physics.

Physics TodayMay 1, 2004133 citations

A Philosopher Looks at Quantum Mechanics (Again)

Hilary Putnam

‘A Philosopher Looks at Quantum Mechanics’ (Putnam [1965]) explained why the interpretation of quantum mechanics is a philosophical problem in detail, but with only the necessary minimum of technicalities, in the hope of making the difficulties intelligible to as wide an audience as possible. When I wrote it, I had not seen Bell ([1964]), nor (of course) had I seen Ghirardi et al. ([1986]). And I did not discuss the ‘Many Worlds’ interpretation. For all these reasons, I have decided to make a similar attempt forty years later, taking account of additional interpretations and of our knowledge concerning non-locality. (The Quantum Logical interpretation proposed in Putnam [1968] is not considered in the present paper, however, because Putnam [1994b] concluded that it was unworkable.) Rather than advocate a particular interpretation, this paper classifies the possible kinds of interpretation, subject only to the constraints of a very broadly construed scientific realism. Section 7 does, however, argue that two sorts of interpretation—ones according to which a ‘collapse’ is brought about by the measurement (e.g. the traditional ‘Copenhagen’ interpretation), and the Many Worlds interpretation or interpretations—should be ruled out. The concluding section suggests some possible morals of a cosmological character. 1. Background 2. Scientific realism is the premise of my discussion 3. What ‘quantum mechanics’ says—and some problems 4. Other interpretations of quantum mechanics 5. The problem of Einstein's bed 6. Classification of the possible kinds of interpretation 7. Which interpretations I think we can rule out 8. The ‘moral’ of this discussion

The British Journal for the Philosophy of ScienceSep 10, 200595 citations

Mermin habitually answers opinions, real and abstract

N. David Mermin

Mermin replies: Rather than attempting a general definition of “real” and “abstract,” I illustrated with examples what it means to reify an abstraction. Michael Nauenberg and Derek Walton insist on a definition, though Walton agrees with me that it’s not easy to provide one. In their own definitions, Nauenberg assigns reality to phenomena or events that can be recorded by a device, and Walton says reality is that which can be observed. These can be compared with what I came up with toward the end of my essay, though I offered it as a sufficient condition for reality rather than a definition: “What impinges directly upon us is real.” My warning against extending reality to the abstractions that help us impose coherence on our perceptions works almost as well if you replace “our perceptions” by “what the device records” or “what we observe.”In quoting my phrase that “spacetime is an abstract four-dimensional mathematical continuum of points that approximately represent phenomena,” Nauenberg drops the rest of my sentence: “whose spatial and temporal extension we find it useful or necessary to ignore.” Representing phenomena (or, if you prefer, the location of phenomena) by abstract geometric points is invariably an idealization of a state of affairs that in reality is not sharply defined. The fact that crude (on some scale) spatial or temporal distances can be recorded irreversibly by macroscopic instruments does not confer reality on the continuum of ideal points we use to represent such data.Nauenberg remarks that interpretational problems in quantum physics usually come from attempts to impose views of reality learned from classical physics on the microscopic world. I would have said “sometimes.” As Mark Alford critically notes, I also believe that even in classical contexts we should look more skeptically at some of our classical ideas of what is real.I mentioned Werner Heisenberg’s views on the acquisition of knowledge only in the context of whether wave-function collapse is a real physical process, produced, for example, by Philip Pearle’s randomly fluctuating field. Heisenberg believed, on the contrary, that “collapse” was merely our updating of information. Although Nauenberg dismisses Heisenberg’s views as irrelevant to the question of what is real, he seems to agree with Heisenberg in declaring that wavefunction collapse is no more mysterious than the change in a probability distribution after an outcome is recorded.Nauenberg’s colleagues, Fred Kuttner and Bruce Rosenblum, come at me from quite a different direction, reading me as deploring books that honestly and interestingly present the strangeness of quantum physics. But I’ve even tried to write such a book myself. What I do deplore is making quantum mechanics sound more peculiar than it already is. Physicists can be as guilty of this as mystics.In particular, separating the strangeness of the uninterpreted data from the strangeness of the formalism that accounts for those data is a subtle business. One of the things the pilot-wave interpretation of quantum mechanics does perfectly well is to provide a straightforwardly unweird explanation of two-slit particle diffraction: A wave goes through both slits and directs a real particle to the screen on the other side, guiding it through one slit or the other. So I do not agree with Kuttner and Rosenblum that the two-slit data, in and of themselves, boggle the mind, independent of one’s perspective on the quantum theory. And I would say that what is usually called “the measurement problem”—the issue that Pearle addresses—is impossible to formulate without invoking the orthodox quantum formalism.It’s a pleasure to become reacquainted with my old friend Rodney Brooks after half a century. I’m glad he agrees that the field operators are mathematical tools, and I apologize for misconstruing his 50-year-old views. But I do think appealing to quantum field theory to solve the paradoxes of special relativity is using a sledgehammer to crack a walnut. Abandoning the reification of time does the job all by itself.Leonardo Colletti wonders what I think about Galileo’s condemnation by the church. I’m with Galileo. It was the church that was (and still is) guilty of reifying abstractions. I do not “agree with those who refuse to recognize any objective truth in physical theories,” but I also think that Colletti’s “mere calculational device” undervalues the beauty and power of the best abstractions physicists have come up with. A coordinate system fixed in the rotating Earth, with real—to use a dangerous term—centrifugal and Coriolis forces, is just fine for most terrestrial purposes. It’s pretty poor for describing the solar system, and a disaster for cosmology. What’s important is not to succumb to the belief that the correct coordinate system is built into the nature of things, as the church did for one coordinate system, and as Colletti seems to favor for another. We should choose the one that best suits our purpose. In an only slightly different context, Galileo understood this very well. That’s why we talk to this day of Galilean transformations.Experimentalists have been much more sympathetic than theorists to my views on reification. They seem to be less enchanted by their abstractions. I’m glad Amin Dharamsi understood what I was trying to say, and would only add to his examples the simplest of all geometric abstractions—the single point—which plays a central role in my remarks on the reification of spacetime. Alford finds spark chamber trajectories analogous to magnetic fields. I would have said they were analogous to Faraday’s iron filings. He goes on to warn against letting quantum mechanics undermine our normal classical sense of reality. The doubts I raised about the reality of classical electromagnetic fields did come entirely from quantum electrodynamics. But my qualms about the reification of the spacetime continuum (which, to my surprise, nobody but Nauenberg objected to) are based entirely on classical physics, untainted by quantum weirdness.It’s a funny coincidence that Alford and Fletcher Goldin ask whether I believe atoms are real. I polished up my column during a three-month visit to Copenhagen, where I argued at some length with Aage Bohr, Ben Mottelson, and Ole Ulfbeck about their view 1 1. A. Bohr, B. R. Mottelson, O. Ulfbeck, Proc. Natl. Acad. Sci. USA 105, 17301 (2008). https://doi.org/10.1073/pnas.0807786105 that all problems in the interpretation of quantum mechanics can be resolved by abandoning “the notion that matter is built of elementary constituents called atoms.” I’m still not persuaded that their program makes sense, but I admire their willingness to reexamine even as apparently unassailable a reification as atoms.Joseph Isler brings up the reification of the ether. The most important culprit caught by special relativity was the reification of time, but the ether is a spectacular example of a reification that most of us today agree was unwarranted, even without an explicit general definition of what it means to be real. The ether was surely making life harder than it had to be.But as Isler notes, and as Alford remarks more generally, reifications can also have considerable heuristic power. If we denied ourselves all our reifications, creative thinking would become difficult, if not impossible. Evolution has hardwired us to reify. It’s important to be aware of the habit, and be ready to consider questioning even our most successful reifications when they start getting us into serious trouble. “At last it came to me,” said Einstein, 2 2. R. S. Shankland, Am. J. Phys. 31, 47 (1963). https://doi.org/10.1119/1.1969236 reminiscing about 1905, “that time was suspect.” Wow!I agree with Alexey Burov that paradoxes and contradictions are a rich source of inspiration. But nature is neither paradoxical nor contradictory. Paradoxes and contradictions arise from our defective understanding of nature. What I advocate is that in trying to improve that understanding we keep in mind, among the possible resolutions of an apparent paradox or contradiction, an unacknowledged and inappropriate reification of an abstraction.I expressed a hope that readers would agree to a certain proposition. Sabine Hossenfelder takes my rhetorical flourish as an attempt to argue, fallaciously, for the truth of that proposition. That was not my intent any more than I intended, by calling attention to the agreement among most of us that the ether is not real, to establish thereby its unreality. Although Hossenfelder takes my column as a shallow, polemical dismissal of both philosophy of science and quantum foundations, I had viewed it as an amateurish attempt to contribute to both disciplines.Pearle recalls that John Bell put me in my place in 1989 in Erice for trying to make a point similar to the one in my column. This surprises me. What I remember is trying unsuccessfully to defend my absent friend and colleague Kurt Gottfried, whose ideas on quantum foundations Bell had tried, in the nicest possible way, to demolish. I developed the views in my recent column a decade later. Conversations with friends in the quantum information community had resonated unexpectedly with some apparently unrelated efforts of mine to make Minkowski spacetime diagrams accessible to non-scientists.On my part, I remember Bell at that same meeting strenuously encouraging Pearle to persist in his efforts to construct a dynamical theory of wavefunction collapse. “Don’t be a sissy!” is what I heard him shout at Pearle, during a very noisy reception. I respect the skill and courage with which Pearle has persisted in these efforts for many decades. But my instincts tell me that he (and Ghirardi, Rimini, Weber, Roger Penrose, and even Tony Leggett) is barking up the wrong tree in seeking an explanation of the measurement problem in a breakdown of quantum mechanics. I wouldn’t (and couldn’t) discourage them from trying, and I’ll be among the first to praise them should they find a squirrel.For me, identifying the bad intellectual habits that induce us to think there is a quantum measurement problem is just as big a challenge (pace Nauenberg) and a lot more fun. Be that as it may, I agree with Pearle that Bell, who is one of my all-time scientific heroes as a thinker, writer, and public performer, would not have approved of my column. I wish he were here to denounce me.Ironically in the present context, Bell and I did agree in Erice that the Fitzgerald contraction, contrary to prevailing opinion, was a real physical phenomenon. Although I’m still nervous about defining “real,” I haven’t changed my mind about that. No reasonable definition of reality could be expected to omit it.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. A. Bohr, B. R. Mottelson, O. Ulfbeck, Proc. Natl. Acad. Sci. USA 105, 17301 (2008). https://doi.org/10.1073/pnas.0807786105 , Google ScholarCrossref, ISI2. R. S. Shankland, Am. J. Phys. 31, 47 (1963). https://doi.org/10.1119/1.1969236 , Google ScholarCrossref, ISI© 2009 American Institute of Physics.

Physics TodaySep 1, 20091 citation