Quantum Supremacy II

About the only thing that has transcended the bitter partisan divisions between Democrats and Republicans in the US during recent years has been quantum mechanics, with the enactment late last year of the National Quantum Initiative Act (the NQI was first mentioned on the blog here). In March there was a National Quantum Coordination Office established at the White House, and last week there was an executive order establishing a National Quantum Initiative Advisory Committee.

The NQI directs the federal government to spend \$1.2 billion over the next five years, with the NSF told to create two to five “Multidisciplinary Centers for Quantum Research and Education” and the DOE two to five “National Quantum Information Science Research Centers”. Besides the NQI, pretty much everywhere you look the past few years you see new well-funded “quantum” centers popping up, two randomly chosen examples would be the Chicago Quantum Exchange and the Yale Quantum Institute. In the private sector, a huge investment in quantum science is taking place, driven by hopes that quantum computing and other applications will lead to a technological revolution and associated vast riches.

Looking at new books on fundamental physics that I’ve seen over the past year and a half, the conventional enthusiastic treatment of string theory/SUSY/extra dimensions is now dead, with Sabine Hossenfelder’s Lost in Math the only popular book addressing these topics, and doing so in a quite negative way. The new trendy topic is the foundations of quantum mechanics, with the recent publication of Adam Becker’s What is Real?, Philip Ball’s Beyond Weird, Anil Ananthaswamy’s Through Two Doors at Once, Lee Smolin’s Einstein’s Unfinished Revolution, George Greenstein’s Quantum Strangeness, and Sean Carroll’s Something Deeply Hidden. Forthcoming from Oxford University Press are two quantum books by Jim Baggott, Quantum Reality and The Quantum Cookbook.

On the whole this change in hot topic is a positive development, although the fact that it’s driven by a lack of anything new to say about particle physics and unification is rather depressing. On the quantum front, while I think it’s great that public attention is being drawn to quantum mechanics, if you look at my reviews you’ll see that I have mixed feelings about the point of view taken by some of the recent books (the best of the lot I think is Philip Ball’s).

The latest example of the high public profile of quantum mechanics is the publication today in the New York Times of a piece by Sean Carroll arguing that Even Physicists Don’t Understand Quantum Mechanics: worse, they don’t seem to want to understand it. Unfortunately I don’t think that this article accurately describes the issues surrounding what we do and don’t understand about “quantum foundations”, nor the dramatically improving funding prospects for research in this area. In addition I don’t think that it’s accurate, fair (or good for public relations) to portray your colleagues as “not really interested in how nature really works”, somehow not curious or bright enough to realize (see here) that there is a crisis at the heart of their subject and that, thanks to Sean Carroll:

the crisis can now come to an end. We just have to accept that there is more than one of us in the universe. There are many, many Sean Carrolls. Many of every one of us.

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26 Responses to Quantum Supremacy II

  1. Blake Stacey says:

    Not everyone was happy that Bohr’s view prevailed, but these people typically found themselves shunned by or estranged from the field.

    Two examples do not add up to a “typically”, no matter how famous those examples are. Nobody mentions Haag’s debate with Bohr, for example, despite it being rather illuminating. Nobody reads the proceedings of the 1938 Warsaw conference, where Hans Kramers declared that “everyone knew the quantum theory was provisional” and Heisenberg was reported as speculating that quantum mechanics would have to break down at high energies. Nobody — and this surprises me — notes that Feynman took Everett seriously enough to criticize in his early ’60s course on gravitation.

    Nor can Bohr coming off better in his debates with Einstein really be equated with his view prevailing, not when even other physicists who disagreed with Einstein (like Heisenberg and Pauli) also disagreed with Bohr.

  2. Blake Stacey says:

    For that matter, the most famous riposte to the view that quantum mechanics can be “considered complete” is the EPR paper, and neither Podolsky nor Rosen were drummed out of the physics profession!

    Louis de Broglie doesn’t seem to have suffered for returning to pilot-wave theory in the 1950s. I have heard that he quashed the careers of Jean-Louis Destouches and Paulette Destouches-Février for taking too Bohrian a turn in their work on quantum logic, but that’s philosopher lore which may or may not be confirmable in a history book. And speaking of quantum logic, that’s a thing that a small but determined set of researchers bustled away at in order to “dig more deeply”, starting with Birkhoff and von Neumann and continuing with Jauch, Piron, Mackey, etc. I doubt that any of Andrew Gleason’s colleagues criticized him for doing quantum foundations instead of getting back to Hilbert’s fifth problem.

  3. GoletaBeach says:

    Sean Carroll’s op-ed piece left me unimpressed. “What’s surprising is that physicists seem to be O.K. with not understanding the most important theory they have.” he says.

    Huh? We had Bell’s work, boiled down nicely by David Mermin, certainly regularly taught in grad and undergrad quantum. We had a whole lot of delayed choice experiments, photon-by-photon build up of the two slit interference pattern, interference measured around solenoids showing the reality of the vector potential. We had just about every weird quantum correlation experiment that arose in decays of the phi meson and Upsilon(4S) interrogated… in the later, Sin(2beta) of CP violation is a bit harder to measure due the the collapse of the wave function caused by the first decay of the correlated pair. Considerable effort checked all of that experimentally.

    Experimentalists have tried really, really hard to get the first empirical clue that the simple-minded Copenhagen is off. Maybe it is Quantum Bayesian in fact, which kinda works, and kinda feels like: everybody, the details of the measurement are and always have been a big correlated mess between the system and the observation apparatus.

    No empirical clue of anything inconsistent has showed up. Maybe the experimental effort has even exceeded that devoted to supersymmetry. Neither experimental quest has found a shred of deviation from simple expectations. Not a reason to give up, but hardly a reason for new billion-$ initiatives… clever folks laboring away in Rutherfordian and Via Panisperna-like genteel poverty are as likely to find the new revolution.

    Many (not sure about “all”) experimental condensed matter folks doing QIS are pretty blunt that they don’t care much about the marquis item… and Gil Kalai points out that noise might deflate the grand expectations of quantum computing…. https://www.ams.org/journals/notices/201605/rnoti-p508.pdf

    Mainly QIS is a good jugular of funding… and their standard old condensed matter funding mechanisms all got wildly political and unproductive. The card “what if the Chinese get quantum computing first and take over the world” is now getting played hard.

    And a card that particle physicists cannot play. Getting supersymmetry first (or any new high energy or dark matter/energy phenomenon) won’t enable anybody to take over the world. Perhaps speculative experimentation will gradually get ironed of particle physics… as all the money is needed for the world and US accelerator neutrino program.

  4. Lena Birkenfeld says:

    Peter,

    If I remember correctly, in one of your past posts you introduced to us that Arnold Neumaier has a new deterministic thermal interpretation of quantum mechanics grounded upon the mathematical concept of coherent spaces, and links to his five papers. Now on one hand, it seems that his thermal interpretation and coherent spaces may be the concepts needed to resolve the issues in the foundations of quantum mechanics that Sean Carroll and others are bringing up, as well as unify quantum and classical mechanics together. On the other hand, Neumaier’s ideas are still relatively new, and apart from this blog and on PhysicsForums, his ideas haven’t attracted much attention (He hasn’t yet had his five papers peer reviewed yet either.) He has a new book coming out on that topic on 24 October 2019 called “Coherent Quantum Physics: A Reinterpretation of the Tradition”: https://www.mat.univie.ac.at/~neum/physfaq/therm/

  5. casper says:

    The responses so far have interpreted Sean’s “quantum foundations” word choice as referring to topics outside of his apparent emphasis on quantum interpretations, to the point of confusing Sean’s critique of physics culture for a critique of mathematics culture (Gleason), confusing quantum foundational work for questioning the empirical adequacy of the standard von Neumann recipe (“Neither experimental quest has found a shred of deviation from simple expectations”), and even broadening the definition to include QIS. It really shouldn’t be all that controversial that working on quantum interpretational issues would not generally help one’s case for tenure in physics departments in the years since WWII.

  6. Alessandro Strumia says:

    Writing that physicists are not interested in understanding quantum mechanics and suggesting that physicists pushed out of the field those who tried is worse than inaccurate. It’s fake news. EPR, Bell and others who achieved interesting results were welcomed and discussed. The problem is that there are not so many interesting results, in particular when trying to address the key issue: why a probabilistic theory?

  7. Well, we should just consider the process when Carroll makes a statement, for example,

    “We just have to accept that there is more than one of us in the universe. There are many, many Sean Carrolls. Many of every one of us.”

    from the viewpoint of many worlds interpretation. During this process there appears a lot of Sean Carrolls speaking any possible statements including the negation of the above statement. Hence, why should we pay attention to a statement of just one of these copies?

    P.S. The statement “Everett didn’t even try to stay in academia, turning to defense analysis after he graduated” seems to be incorrect. According to the book The Many Worlds of Hugh Everett III by Peter Byrne, Everett did not want to stay by academia, he wanted to earn good money.

  8. Your formulation “Forthcoming from Oxford University Press are two quantum books …” reminds me of a character from Saki who does not agree with his cook. He says something along the line that a rabbit served with a sauce with exotic ingredients does not become an exotic rabbit.

  9. Peter Woit says:

    Lena Birkenfeld,
    I hope to find time to better understand what Neumaier is doing, likely will write here something about his book when it comes out. It’s a quite different sort of thing though than the Carroll and other books mentioned here, much more technical, aimed at those with some expertise in the subject rather than the general public.

  10. Peter Woit says:

    Blake Stacey/casper,

    Carroll’s history, like that of Adam Becker’s in his book, is just a caricature drawn for ideological reasons and I doubt anyone interested in the history will take it seriously.

    More likely to be taken seriously is his description of the current situation, which also seems to me an unrecognizable caricature. As I pointed out in the posting, at the level of what the public is being told about fundamental physics, “foundations” is the hot topic and getting more attention than almost anything else. Of the huge influx of “quantum” grant money, new institutes and new positions, only a small fraction of it will go to “foundations”, but some of it will, especially to work that has any sort of connection to the real world and the new possibilities being opened up for experiment by new technology. If your idea of “foundations”research is arguing about what is “real”, you may continue to find your colleagues dubious of its value.

    By the way, on this topic I recommend this from Will Kinney:
    https://twitter.com/WKCosmo/status/1170668355847643136

  11. Blake Stacey says:

    Carroll’s history, like that of Adam Becker’s in his book, is just a caricature drawn for ideological reasons and I doubt anyone interested in the history will take it seriously.

    This is a good point. However, I suspect that many people are more interested in having a Rationally Correct(TM) view of physics history, and so they will be content with the caricature because it was taught under the auspices of capital-R Rationality.

    David Mermin coined the phrase “shut up and calculate!” to express the attitudes of his own professors during his graduate-school years, the late 1950s. He himself has been publishing on quantum foundations since 1980, starting with generalizing the Bell inequality in the pages of the Physical Review. Even if the “shut up and calculate” sentiment endures, I find it extremely implausible that the set of questions to which it is applied has remained invariant over the last sixty-odd years.

    My day job is actually a problem that could be classed into quantum foundations or quantum information, depending on which way the light shines. Conversations about it tend to be richer and more fulfilling in the latter context, since the former field prefers to keep having the same old arguments with itself that it’s been having for generations, while the latter is looking for new experiments to do. Even better still, in my experience, have been the interactions with the mathematicians whose attention is caught by quantum information reaching out into algebraic number theory. When I was in physicist school, I never expected I’d be teaching myself about Hilbert’s twelfth problem.

    Thank you for the pointer to Will Kinney’s Twitter thread; I find much to agree with in it.

  12. Peter Shor says:

    Why do all these people who are unhappy with the current state of quantum foundations think the universe owes us a simple, intuitive picture of what is going on?

    We have at least three intuitively very unsatisfactory, but perfectly consistent interpretations of quantum mechanics (“many worlds”, Bohmian pilot wave, and Copenhagen) which all seem to predict exactly the same thing. Mathematically, it’s a coherent and quite beautiful theory (at least until you try to add gravity). Why does there need to be a simple, intuitive foundation to it? There’s nothing really wrong with looking for one, but there’s also nothing wrong with looking for experimental evidence for dark matter candidates. You just shouldn’t be surprised if you don’t find them.

  13. Michael Weiss says:

    Peter, in 1977 Sidney Coleman prominently concluded his undergraduate course in introductory QM with a final lecture on Bell’s Theorem (“Physics 143” at Harvard). Given Sidney’s stature in that decade as both a leading theoretical physicist and a master lecturer (indeed, graduate students and post-docs from around Boston routinely attended his QFT lectures in Physics 253), surely this was a clear sign that Bell’s Theorem had entered the mainstream of physics education by that time.

    Prior to his final illness, Sidney gave a lecture on further Bell-related developments in the early 1990s, which is still available on the Harvard Physics website.

    I also remember chatting with Ed Purcell somewhere in the 1985-1988 period about David Mermin’s exposition of Bell’s argument. Ed shared that Mermin had felt honored and thrilled that Richard Feynman wrote him a personal note congratulating David on putting together such as lucid account. My impression was that this was a very special form of peer recognition to Mermin.

    Finally, Harvard’s Francis Pipkin was a major advocate for Alain Aspect in the 1970s (as I gleaned in 1978 in the course of Physics 191; a lab course with much informal time for wide-ranging discussions); I understand that Pipkin helped ensure Aspect’s promotion and tenure (told to me by someone else).

    These anecdotes argue against Sean Carroll’s suggestion that “mainstream elite” physicists had dismissed Bell’s work as uninteresting and irrelevant.

  14. Mark Hillery says:

    I agree with pretty much everything Blake Stacey, Will Kinney, and Peter Shor have said, but I would like to comment on a few points in the article. First this:

    “For years, the leading journal in physics had an explicit policy that papers on the foundations of quantum mechanics were to be rejected out of hand.”

    Assuming the journal in question is the Physical Review, I can say that as a soon to be retired associate editor at Physical Review A, who has handled parts of quantum information and, on and off, parts of quantum foundations for 15 years, this simply is not correct. We have published a very large number of papers on Bell inequalities and variations on them. There are papers on axiomatizations of quantum mechanics, and attempts to explain why there is a gap between the maximum correlations allowed by mathematics and those allowed by quantum mechanics (for more information on this see Tsirelson’s theorem and Popescu-Rohrlich boxes). We recently published a paper in which experimental data was re-analyzed in order to place bounds on the collapse parameter in the GRW model (mentioned in the Carroll piece). This list is by no means exhaustive. Exactly what the policies were before my time, I cannot say, but there are papers in the Physical Review on Bell inequalities from at least the early 1980’s.

    Second there is this:

    “The current generation of philosophers of physics takes quantum mechanics very seriously, and they have done crucially important work in bringing conceptual clarity to the field.”

    This certainly is not the way I see things. The few philosophers of physics I am aware of have a strong preference for Bohmian mechanics, which is a subject almost completely ignored by working physicists. This very much limits their influence. Also, from what I have seen, the philosophers further diminish whatever effect they may have by their aggressive style.

    Finally:

    “After almost a century of pretending that understanding quantum mechanics isn’t a crucial task for physicists, we need to take this challenge seriously.”

    Again, this ignores a lot. The decoherence program pioneered by Zeh and currently being pushed by Zurek, is taken quite seriously and has been going on for quite a while.

    In conclusion, as has been noted by other commenters, Carroll’s piece paints a very misleading picture of the past and present of research in quantum physics.

  15. Peter says:

    I can confirm that Bell, EPR etc. were part of the curriculum in the early 1980s when I studied physics (somewhere in Europe …). It wasn’t very thorough and nobody left the classroom with the idea this was a hot research topic(*), but it’s not correct that foundational questions were neglected.

    (*) I left the classroom with the idea that a) something was missing in the “interpretation” of QM, but b) every experiment confirmed the predictions of QM and one could do good physics without worrying about interpretations. Personally, I looked at the achievements of QM from the hydrogen atom to electroweak theory and thought taking position b) had its merits.

  16. Peter Shor says:

    And my previous comment, transmuted to poetry (or at least to words with rhyme and meter):

    If the eternal dance of molecules
    Is too entangled for us mortal fools
    To follow, on what grounds should we complain?
    Who promised us that Nature’s arcane rules
    Would make sense to a merely human brain?

  17. Dave Miller says:

    Peter,

    I’m older than most of the posters here, and perhaps the only one who had a strong interest in QM foundations as early as 1970.

    No one seems to be noticing that the very testimonials here actually document a phase transition in the profession’s attitude towards foundational issues in QM between the early ’70s and mid ’80s.

    Michael Weiss says, “Peter, in 1977 Sidney Coleman prominently concluded his undergraduate course in introductory QM with a final lecture on Bell’s Theorem.” That is the earliest sign I know of the transition. Michael is of course correct that Sidney was highly respected; on the other hand, Sidney was a bit of a maverick, and it is not surprising that he was an early harbinger of the transition.

    When I took QM from Feynman in the 1974-’75 academic year, there was not one word about Bell’s theorem (much less Bohmian mechanics or MWI!). I myself tried to engage Feynman in discussions about QM foundations: I’d known him personally since early in my freshman year, and he was generally open to discussions with undergrads. However, he just would not address QM foundational issues: indeed, the only thing he said about QM foundations in the QM class was a rather stern warning to all of us to avoid research in the area, since it was a career killer.

    And yet, Michael cites evidence that “Mermin had felt honored and thrilled that Richard Feynman wrote him a personal note congratulating David on putting together such as lucid account” in Mermin’s famous Physics Today article in 1985.

    Yes, that is my point: Feynman’s (and the profession’s) attitude changed radically between 1974 and 1985 (with thanks to Sidney Coleman, Dave Mermin, et al.).

    The pre-1975 attitude, by the way, was not that you were an evil person who deserved to be driven out of the field if you were interested in QM foundational issues; the attitude was simply that this was a fruitless line of endeavor, and that work in that area would generally not earn you credit among your colleagues.

    Again, John Bell is an example: he was respected as a legitimate physicist who did real work at CERN but who happened to dabble in foundational stuff on the side.

    I myself did a term paper on Bell’s theorem in the 1974-75 year, not for Feynman’s class but for a “Modern Physics” class. The prof liked it, and I earned an “A.” I knew I would not be punished for my interest, but I also believed Feynman that making it my main research topic was not a good way to advance my career. Indeed, as late as, say, 2000, how many people considered giants in physics had made their name by working on foundations? Witten? Weinberg? Polchinski? Coleman himself?

    I can think of none.

    Again, it is true that a physicist prior to the transition who showed an interest in QM foundations was not vilified and expelled from the field. But, I know of no one prior to 1980 who made QM foundations the core of his research efforts and who was respected as a result by most other physicists.

    It is of course completely different now: our friend Peter Shor is (quite rightly) famous.

    My strong impression from Feynman, by the way, was that his generation had just been exhausted when they were young physicists hearing about the Bohr-Einstein debates, etc. and had understandably decided that all that was a waste of time.

    And they had a point: for decades, the “real action” was in QFT, gauge theories, the quark model, etc. Bell’s 1964 paper seemed not to be fruitful for further research: the initial experimental tests were not until the ’70s and ’80s, and it is really only in more recent decades that Bell’s relevance to quantum computing has made Bell’s work seem fruitful.

    As Max Planck famously said (often misquoted as “one funeral at a time”), “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”

    Dave

  18. WTW says:

    For a very recent paper on experimental evidence of Quantum Darwinism, by Thomas Unden, with other authors including Zurek, see
    Revealing the emergence of classicality in nitrogen-vacancy centers
    https://arxiv.org/abs/1809.10456
    (submitted to PRL)

    See also: To catch and reverse a quantum jump mid-flight, by Z. K. Minev et al,
    https://arxiv.org/abs/1803.00545
    (in Nature, June 2019, https://www.nature.com/articles/s41586-019-1287-z )

    They seem fairly “foundational” explorations of quantum mechanics, and as far as I can tell done outside of any “Quantum Initiative”. But very interesting explorations of the details underlying quantum processes and measurement, without having to invoke any other copies of Dr. Carroll.

    One comment: I have read multiple accounts of how the personality and position of Bohm stifled dissent re. problems with his ongoing interpretations of QM, including among some other very well known physicists. Even when apparent disagreements were stated, they tended to be swept under the rug or somehow merged together as “quantum weirdness”. See, for example, https://plato.stanford.edu/entries/qm-copenhagen/#DivVie . More recently, physicists working primarily on aspects of string theory or particle physics topics with whom I have spoken also tend to sweep away any such issues as being either uninteresting or irrelevant to the core of the problems on which they are working. (And in fact the only people willing to spend much time talking about such things have been in the Philosophy of Science department : -) I have no idea how general or widespread such things actually are, but from a limited sample, Carroll isn’t too far wrong in terms of how most theoretical physicists have (not) approached these issues.

  19. Lee Smolin says:

    Dear Peter,

    Can I comment as someone whose first class in QM was Freshman QM in 1972, ie just before the phase transition Dave Miller mentions?

    First of all, its true that American physics in general has had a pragmatic, anti-foundational orientation since it became dominant after WWII. Putting aside that there were felt to be good reasons for that (see remarks of Dyson, Feynman etc) , that made it unsurprising when we students were sincerely advised to work on building the present paradigm rather than attempt to over-through Copenhagen (ie text-book QM).

    In the face of this, the very small number of us determined to work on quantum foundations took several strategies, and many succeeded. One was to split one’s time between more mainstream (ie funded) and foundational issues (Bell, Adler, ’Sorkin, ’t Hooft, Mermin, etc), one was to go to teach at a first class small college (Bernstein, Pearle, Wooters), one was to go to or stay in Europe (Penrose, Isham, Deutsch, Valentini, etc,) which was more diverse, finally one could work on QF from a math (Goldstein,Nelson) or philosophy (Shimony, Albert, Maudlin, Sanders, etc) department. Finally, one could work in industry (Bennet, Shor) or defence (Zurek)

    Indeed enough of us succeeded, that in spite of the discouragement, as someone interested in foundations from the early 70’s, there were always new ideas to ponder and work on and people to discuss them with. And what was most important, was that the crucial experiments got done-even if, as with tests of Bell, it took several attempts till the experimentalists had clean results. (And they were done at high status places: Harvard, Berkeley, Paris, MIT, Vienna…)

    So if we are honest, I don’t think we can argue that the progress of QF between the
    1960’s and the QIT revolution in the 2000’s was personnel limited. These are very hard problems that go to the core of our understanding of nature. They require a different temperament than makes a successful mainstream physicist. Not many people have the interest, patience, focus and courage to work on them.

    In addition, from 1972 on was a very tough job market for all of theoretical physics. So I think the overall story is that the problems in QF are so compelling, for those with a taste for foundational kinds of issues, that enough of us succeeded, against advice and expectations that we now have in front of us a much more interesting and diverse set of approaches and hypotheses, than faced the field in the 1950’s.

    Thanks,

    Lee

  20. Yatima says:

    Sean Carroll writes at Quanta Magazine:

    Where Quantum Probability Comes From

    and as expected defends Many-Worlds, and derives the Born rule from first principles:

    Can we resolve this uncertainty in a sensible way? Yes, we can, as Charles Sebens and I have argued, and doing so leads precisely to the Born rule: The credence you should attach to being on any particular branch of the wave function is just the amplitude squared for that branch, just as in ordinary quantum mechanics.

    Still not convinced. “Knowing” or “attaching credence” ultimately means storing classical bits in massively copied physical state (including a fat chunk of previously recorded history and the algorithms needed to process the whole shebang into a prediction or a floating-point value). It’s not some metaphysical essence that a system in quantum superposition could even have.

    (For an earlier reconstruction of the Born rule from first principles, there is this article by Philip Ball, also at Quanta: Mysterious Quantum Rule Reconstructed From Scratch.)

  21. Pingback: Regarding Papers about Fundamental Theories | Not Even Wrong

  22. Peter Woit says:

    Yatima,
    Thanks for the pointer to that new Carroll article, but I’m afraid I want to discourage discussion of it here, since it’s a topic the moderator has no interest in…

  23. Peter Shor says:

    Lee Smolin:

    I don’t believe Feynman was truly anti-foundational. When I was at Caltech as an undergrad, around 1980, he gave a talk on negative probabilities, and said that his motivation was that he had looked at Bell’s theorem, and realized that there was a hidden hypothesis that all probabilities were non-negative. So maybe if you allowed negative probabilities, you could understand the foundations of quantum mechanics.

    He later published a paper on negative probabilities where, with typical Feynman showmanship, he completely omitted to mention his motivation, presumably because he couldn’t get it to work.

    There are lots more mathematicians working on the Riemann hypothesis than there are mathematicians willing to admit it (just like there were lots more mathematicians working on Fermat’s Last Theorem than there were ones willing to admit it). I expect there are currently lots more physicists working on foundations than are willing to admit it.

    If I’m correct about how many physicists have worked on quantum foundational problems without telling anybody, they’re even a lot harder than we think they are.

  24. Blake Stacey says:

    If anyone wants to read Feynman’s paper on “negative probabilities”, it’s here. He defines what would nowadays be called a discrete Wigner function for a qubit (a topic that you can indeed get into PRL for).

  25. Jim Baggott says:

    I think it’s important to realise that the general negativity around quantum foundations in the 20/30-year period to the 1960s or early 1970s wasn’t so much about a commitment to the Copenhagen (shut up and calculate) orthodoxy as a genuine belief that these were questions that couldn’t be answered by experiment. If, as it seemed, these were ‘just’ philosophical questions based on different views about how nature should behave then, arguably, they didn’t belong in physics. This wasn’t just about Bohr’s pragmatism or Heisenberg’s positivism. Remember that in the 1930s von Neumann had published a proof that all hidden variable extensions of quantum theory are impossible.

    Many argue that all this changed with Bell, who dismissed von Neumann’s ‘impossibility proof’ and devised his famous theorem and inequality, but I think this process started with Bohm, under the influence of a conversation he had with Einstein in the early 1950s.

    This change was critical. As soon as it became apparent that there were indeed some foundational questions that could be answered by experiment, interest grew and foundations became a ‘respectable’ part of physics, which is why it started to appear even in a small number of introductory QM courses. I fear that Carroll’s particular spin on history and his arguments in support of a MWI which does nothing more than wrap the standard quantum formalism in a thick blanket of metaphysics does more harm than good to the credibility of quantum foundations in a pragmatic and empirically-grounded physics community. In a piece to be published online early next month by Aeon, I argue that this kind of stuff is dangerous and threatens to undermine the authority of science just when it is under unprecedented attack from anti-scientific and pseudo-scientific propaganda.

  26. Blake Stacey says:

    Remember that in the 1930s von Neumann had published a proof that all hidden variable extensions of quantum theory are impossible.

    I’m glad someone mentioned this. Adam Becker made a fairly big deal out of it in his book, in a way that I found incomplete enough to be misleading. But there’s an angle that’s more significant to the theme of how quantum foundations research is done today. Von Neumann’s flawed theorem is fixable if one generalizes from his notion of measurement to that of POVMs, as is commonly done in quantum information theory. The additional structure of the set of POVMs furnishes a replacement for von Neumann’s additivity assumption, the point where his argument was criticized by Grete Hermann and probably by Einstein. Of course, it is not so shocking to say that changing the assumptions will change what can be proved, but it’s part of the story. And, topically, this part of the story was told in Physical Review Letters.

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