An Apology

Posted on September 15, 2019 by woit

I’m afraid I made a serious mistake in this previous posting discussing Sean Carroll’s new book. Since the book was relatively reasonable, while the jacket and promotional material that came with it were nonsense, I assumed that Carroll was just being ill-served by his publisher. It’s now clear I was very wrong. He’s on a book tour, and the nonsense is exactly what he is putting front and center as a revelation to the public about how to understand quantum mechanics. For a couple examples, here’s what was on the PBS News Hour

The “many worlds” theory in quantum mechanics suggests that with every decision you make, a new universe springs into existence containing what amounts to a new version of you. Bestselling author and theoretical physicist Sean Carroll discusses the concept and his new book, “Something Deeply Hidden,” with NewsHour Weekend’s Tom Casciato.

and here’s something from his talk down the street from me.

Using your public platform to tell people that the way to understand quantum mechanics is that the world splits depending on what you decide to do is simply What the Bleep? level stupidity. Those in the physics and science communication communities who care about the public understanding of quantum mechanics should think hard about what they can do to deal with this situation. They may however come to the same conclusion I’ve just reached: best to ignore him, which I’ll try to do from now on.

Posted in Quantum Mechanics | Comments Off on An Apology

Regarding Papers about Fundamental Theories

Posted on September 9, 2019 by woit

Discussion in the comment section of the previous blog entry led me to do a little bit of historical research this morning, and I thought I’d write up the results here. First of all, for some interesting comments from people around back then about how attitudes in the physics community changed during the 1970s, see here, here and here.

What I looked into is one specific story, trying to figure out what was behind Sean Carroll’s claim in the New York Times that

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.

Mark Hillery here notes that this is likely a reference to the Physical Review, and that it very much has not been true for the past 15 years, during which he has been an editor there.

Tracing back where Carroll got this from, I guessed that (since it’s the historical source he recommends in his book) it came from Adam Becker’s book, What is Real?. Looking at that book one finds on page 214:

Physical Review actually had an explicit editorial policy barring papers on quantum foundations unless they could be related to existing experimental data or made new predictions that coulhered be tested in the laboratory.

This matches Carroll’s claim (with the part inconvenient for his case deleted…). Becker’s source notes for this text refer to an editorial in the July 15, 1973 issue of Physical Review D (Particles and Fields) written by Samuel Goudsmit, the editor-in-chief. The editorial is entitled “IMPORTANT ANNOUNCEMENT: Regarding Papers about Fundamental Theories”. Goudsmit does not specifically refer to quantum foundations papers, but writes:

The subject matter of these papers usually concerns a fundamental aspect of theoretical physics. Extreme verbosity and vagueness of expression makes these papers hard to read and understand. A paucity of mathematics as compared to wordage distinguishes them from the more conventional theoretical papers. The author proposes new theories, but their specific assumptions are usually hidden behind very lengthy arguments. Sometimes the paper contains a reinterpretation of existing theories which the author considers more satisfactory than the prevailing views, though no new experimental consequences are expected.

He sets forth the following as features expected of articles publishable by the Physical Review:

All implied assumptions must be stated clearly and concisely and as much as possible expressed in mathematical form.

The author must convincingly show

  • that these assumptions lead to the explanation of hither to unexplained observations, or
  • that these assumptions expose new relations between known data or theories, or
  • that these assumptions are simpler and fewer than in existing theories.

Moreover, the author must show that the new assumptions do not contradict existing experimental facts.

He must also investigate possible new consequences of his assumptions and whether these could be tested by new experiments.

Looking some more into this, I realized that I had first seen this story in David Kaiser’s book How the Hippies Saved Physics (see review here), which clearly is Becker’s source (Becker’s next note refers to this). On page 121 Kaiser has:

The longtime editor of the Physical Review… actually banned articles on the interpretation of quantum mechanic. He went so far as to draw up a special instruction sheet to be mailed to referees of potentially offending submissions: referees were to reject all submissions on interpretive matters out of hand, unless the papers derived quantitative predictions for new experiments.

Kaiser goes on to quote John Clauser as pointing out that according to this policy, Bohr’s response to the 1935 EPR paper would not have been publishable. His source notes refer to the Goudsmit editorial and private emails from Clauser on July 8, 2009. The same note also refers to an article by Clauser, Early History of Bell’s Theorem, which has a lot of detailed information about the story of the reception of Bell’s theorem and early efforts to do experimental tests (but nothing about the Physical Review policy). By the way, back in 1964, Bell decided not to submit his important paper to Physical Review, not because of any policy they might have had, but because they had page charges.

So, as far as I can tell, the historical record shows that the documented Physical Review policy didn’t, as the descriptions by Kaiser, Becker and Carroll suggest, explicitly refer to papers on the interpretation of quantum mechanics or quantum foundations. Possibly it was such papers that were annoying Goudsmit and led to his editorial, but I’d be curious to know if anyone knows more about what was specifically bothering Goudsmit. What sort of papers were being submitted to Physical Review D around 1972-73 that would uncharitably fit the negative description he gives quoted above?

Update: In the comments Blake Stacey suggests that the 1972 experiment of Freedman-Clauser may have been what led to papers being submitted to Physical Review that inspired Goudsmit’s July 1973 editorial. Looking more into this, it’s quite possible that the kind of thing Goudsmit was concerned about were the sorts of papers Jack Sarfatti was writing around this time. According to Kaiser’s book (pages 62-63), it was just a few months before this that Sarfatti decided to change the sorts of papers he was writing:

By the early 1970s, having published a few articles in prestigious journals on quantum theory, elementary particles, and even some idiosyncratic ideas about miniature black holes, Sarfatti could list half a dozen distinguished physicists scattered across the United States, Britain and France as references to vouch for the quality of his work…
… Sarfatti began to lose enthusiasm for his position at San Diego State during the early 1970s, and indeed for the sterile direction in which he saw theoretical physics heading. He announced his new plans in a letter to renowned Princeton physicist John Wheeler in the spring of 1973… Sarfatti declared that he would leave his “uninspiring institution” and seek out “the best possible environment to create a great and historic piece of physics. I feel impelled by history – a certain sense of destiny,” he explained. (“I recognize that I may be suffering under some sort of ‘crackpot’ delusion, but I cannot accept that as likely. In any case, I must try,” he averred).

See the comment here for some of the sort of papers Sarfatti was writing at the time, quite possibly submitting them to Physical Review. The opening sentence of Goudsmit’s description of the problem (and the fact that he was publishing it in Phys. Rev. D, Particles and Fields)

The subject matter of these papers usually concerns a fundamental aspect of theoretical physics.

seems to me more likely to be referring to the sort of thing Sarfatti was writing than to papers on the interpretation of quantum mechanics.

Update: It turns out that Goudsmit’s papers are available online, here. A non-exhaustive search turned up no evidence pro or con for my conjecture about Sarfatti or similar papers. I only found one set of files (Box 50, Folder 45, “Leibowitz refereeing, 1973”) referring to his 1973 editorial. These have to do with this paper, which was published September 1973, after two years of refereeing. This publication led to another author writing a paper criticizing the first, leading to another refereeing problem. Goudsmit weighed in (January 5, 1976) by noting that it was exactly this kind of paper and the problems with refereeing such things that his editorial had been concerned with. Note that the paper in question is NOT an interpretation/foundations paper. Goudsmit writes:

The event shows again clearly the necessity of rapid rejections of questionable papers in vague borderline areas. There is a class of long theoretical papers which deal with problems of interpretation of quantum and relativistic phenomena. Most of them are terribly boring and belong to the category of which Pauli said, “It is not even wrong”. Many of them are wrong. A few of the wrong ones turn out to be valuable and interesting because they throw a brighter light on the correct understanding of the problem. I have earlier expressed my strong opinion that most of these papers don’t belong in the Physical Review but in journals specializing in the philosophy and fundamental concepts of physics.

He then refers to his earlier editorial.

Looking at the exchange in these letters, the referee of the second paper writes “I would suggest that [the author] understands neither relativity nor quantum theory.” This example suggests that the problem Goudsmit had in mind when writing his editorial was not a need for “an explicit policy that papers on the foundations of quantum mechanics were to be rejected out of hand”, but just a need to deal with the common problem that is still with us, for both journals and the arXiv. There are lots and lots of people writing low quality papers claiming to say something new about the foundations of physics, on a continuum from the crank to the not so bad. Refereeing such things is difficult and time-consuming, so a journal needs a policy to deal with them quickly and efficiently, otherwise they end up with the mess described in these letters. Goudsmit’s editorial I think was an attempt to come up with such a policy.

Update: Jorge Pullin wrote to remind me of an earlier Goudsmit story, which I described on the blog here, but had completely forgotten about. In 2008 an undated paper from Bryce DeWitt’s files (he passed away in 2004) was posted on the arXiv. It included a claim much like the ones discussed here that refer to the 1973 editorial, but about a much earlier incident:

Most of you can have no idea how hostile the physics community was, in those days, to persons who studied general relativity. It was worse than the hostility emanating from some quarters today toward the string-theory community. In the mid fifties Sam Goudsmidt, then Editor-in-Chief of the Physical Review, let it be known that an editorial would soon appear saying that the Physical Review and Physical Review Letters would no longer accept “papers on gravitation or other fundamental theory.” That this editorial did not appear was due to the behind-the-scenes efforts of John Wheeler.

I don’t know of any other evidence for this (took a quick look in the Goudsmit online archive, didn’t see anything). It seems highly likely that this claim about Goudsmit and the Physical Review is not accurate. One minor problem with a claimed “mid-fifties” planned editorial for Physical Review Letters is that PRL wasn’t even started until mid-1958. More seriously, the idea that the Physical Review in the mid-fifties would consider banning “papers on gravitation or other fundamental theory” is just completely implausible, and if that phrase is accurate, it surely is very much taken out of context. This story is very similar to the Carroll one about the 1973 editorial, and I’m guessing the true story about the mid-fifties incident is again just that Goudsmit was even then struggling with how to deal with bad “not even wrong” theory papers about fundamental physics.

Update: Steven Weinberg has a version of the “mid-fifties” Goudsmit story, in his biographical notice for DeWitt, in the context of a discussion of the January 1957 Chapel Hill conference on gravity:

Samuel Goudsmit had recently threatened to ban all papers on gravitation from Physical Review and Physical Review Letters because he and most American physicists felt that gravity research was a waste of time.

Again, there’s a problem with this that PRL wasn’t even started until a year and a half later, and he has Goudsmit specifying just GR research, not the wider “gravitation or other fundamental theory” which DeWitt gave in quotes.

Posted in Favorite Old Posts, Quantum Mechanics | 26 Comments

Quantum Supremacy II

Posted on September 7, 2019 by woit

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.

Posted in Quantum Mechanics | 26 Comments

Some Math News

Posted on August 27, 2019 by woit

My Columbia colleague Patrick Gallagher passed away a few months ago at the age of 84. He had only recently retired, and for many years was the longest serving member of the department and an important part of its institutional memory. On October 10 there will be a memorial conference here at Columbia.

It’s too bad Pat didn’t live to see the latest from Terry Tao, who describes recent results which are related to old work of Gallagher’s by “Our proof of this theorem proceeds more or less along the same lines as Gallagher’s calculation, but now with k allowed to grow slowly with x.”

Turning to other topics, Peter Scholze continues to come up with new ideas about the foundations of mathematics at a pace far too fast for me to fool myself into thinking I might be able to follow what he’s doing. In recent months he has run a course on “Condensed Mathematics”, which involves new ideas about topology developed with Dustin Clausen. For a video of a recent talk where he explains this, see here.

On the Langlands/representation theory front, some interesting things are:

Finally, the 2019 PCMI was devoted to the topic of Quantum Field Theory and Manifold Invariants, videos are here.

Update: Also on the Langlands/representation theory/quantum front, this week at Northeastern there’s a conference going on (rumored to be celebrating the fact that Etingof and Okounkov share the same birthday, and are now 50). Videos are starting to appear here, including one of Edward Frenkel discussing the paper with Etingof and Kazhdan mentioned above.

Posted in Langlands, Obituaries | 7 Comments

Something Deeply Hidden

Posted on August 20, 2019 by woit

Sean Carroll’s new (available in stores early September) book, Something Deeply Hidden, is a quite good introduction to issues in the understanding of quantum mechanics, unfortunately wrapped in a book cover and promotional campaign of utter nonsense. Most people won’t read much beyond the front flap, where they’ll be told:

Most physicists haven’t even recognized the uncomfortable truth: physics has been in crisis since 1927. Quantum mechanics has always had obvious gaps—which have come to be simply ignored. Science popularizers keep telling us how weird it is, how impossible it is to understand. Academics discourage students from working on the “dead end” of quantum foundations. Putting his professional reputation on the line with this audacious yet entirely reasonable book, Carroll says that 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.

This kind of ridiculous multi-worlds woo is by now rather tired, you can find variants of it in a host of other popular books written over the past 25 years. The great thing about Carroll’s book though is that (at least if you buy the hardback) you can tear off the dust jacket, throw it away, and unlike earlier such books, you’ll be left with something well-written, and if not “entirely reasonable”, at least mostly reasonable.

Carroll gives an unusually lucid explanation of what the standard quantum formalism says, making clear the ways in which it gives a coherent picture of the world, but one quite a bit different than that of classical mechanics. Instead of the usual long discussions of alternatives to QM such as Bohmian mechanics or dynamical collapse, he deals with these expeditiously in a short chapter that appropriately explains the problems with such alternatives. The usual multiverse mania that has overrun particle theory (the cosmological multiverse) is relegated to a short footnote (page 122) which just explains that that is a different topic. String theory gets about half a page (discussed with loop quantum gravity on pages 274-5). While the outrageously untrue statement is made that string theory “makes finite predictions for all physical quantities”, there’s also the unusually reasonable “While string theory has been somewhat successful in dealing with the technical problems of quantum gravity, it hasn’t shed much light on the conceptual problems.” AdS/CFT gets a page or so (pages 303-4), with half of it devoted to explaining that its features are specific to AdS space, about which “Alas, it’s not the real world.” He has this characterization of the situation:

There’s an old joke about the drunk who is looking under a lamppost for his lost keys. When someone asks if he’s sure he lost them there, he replies, “Oh no, I lost them somewhere else, but the light is much better over here.” In the quantum-gravity game, AdS/CFT is the world’s brightest lamppost.

I found Carroll’s clear explanations especially useful on topics where I disagree with him, since reading him clarified for me several different issues. I wrote recently here about one of them. I’ve always been confused about whether I fall in the “Copenhagen/standard textbook interpretation” camp or “Everett” camp, and reading this book got me to better understanding the difference between the two, which I now think to a large degree comes down to what one thinks about the problem of emergence of classical from quantum. Is this a problem that is hopelessly hard or not? Since it seems very hard to me, but I do see that limited progress has been made, I’m sympathetic to both sides of that question. Carroll does at times too much stray into the unfortunate territory of for instance Adam Becker’s recent book, which tried to make a morality play out of this difference, with Everett and his followers fighting a revolutionary battle against the anti-progress conservatives Bohr and Heisenberg. But in general he’s much less tendentious than Becker, making his discussion much more useful.

The biggest problem I have with the book is the part referenced by the unfortunate material on the front flap. I’ve never understood why those favoring so-called “Multiple Worlds” start with what seems to me like a perfectly reasonable project, saying they’re trying to describe measurement and classical emergence from quantum purely using the bare quantum formalism (states + equation of motion), but then usually start talking about splitting of universes. Deciding that multiple worlds are “real” never seemed to me to be necessary (and I think I’m not the only one who feels this way, evidently Zurek also objects to this). Carroll in various places argues for a multiple world ontology, but never gives a convincing argument. He finally ends up with this explanation (page 234-5):

The truth is, nothing forces us to think of the wave function as describing multiple worlds, even after decoherence has occurred. We could just talk about the entire wave function as a whole. It’s just really helpful to split it up into worlds… characterizing the quantum state in terms of multiple worlds isn’t necessary – it just gives us an enormously useful handle on an incredibly complex situation… it is enormously convenient and helpful to do so, and we’re allowed to take advantage of this convenience because the individual worlds don’t interact with one another.

My problem here is that the whole splitting thing seems to me to lead to all sorts of trouble (how does the splitting occur? what counts as a separate world? what characterizes separate worlds?), so if I’m told I don’t need to invoke multiple worlds, why do so? According to Carroll, they’re “enormously convenient”, but for what (other than for papering over rather than solving a hard problem)?

In general I’d rather avoid discussions of what’s “real” and what isn’t (e.g. see here) but, if one is going to use the term, I am happy to agree with Carroll’s “physicalist” argument that our best description of physical reality is as “real” as it gets, so the quantum state is preeminently “real”. The problem with declaring “multiple worlds” to be “real” is that you’re now using the word to mean something completely different (one of these worlds is the emergent classical “reality” our brains are creating out of our sense experience). And since the problem here (classical emergence being just part of it) is that you don’t understand the relation of these two very different things, any argument about whether another “world” besides ours is “real” or not seems to me hopelessly muddled.

Finally, the last section of the book deals with attempts by Carroll to get “space from Hilbert space”, see here, which the cover flap refers to as “His [Carroll’s] reconciling of quantum mechanics with Einstein’s theory of relativity changes, well, everything.” The material in the book itself is much more reasonable, with the highly speculative nature of such ideas emphasized. Since Carroll is such a clear writer, reading these chapters helped me understand what he’s trying to do and what tools he is using. From everything I know about the deep structure of geometry and quantum theory, his project seems to me highly unlikely to give us the needed insight into the relation of these two subjects, but no reason he shouldn’t try. On the other hand, he should ask his publisher to pulp the dust jackets…

Update: Carroll today on Twitter has the following argument from his book for “Many Worlds”:

Once you admit that an electron can be in a superposition of different locations, it follows that person can be in a superposition of having seen the electron in different locations, and indeed that reality as a whole can be in a superposition, and it becomes natural to treat every term in that superposition as a separate “world”.

“Becomes natural” isn’t much of an argument (faced with a problem, there are “natural” things to do which are just wrong and don’t solve the problem). To me, saying one is going to “treat every term in that superposition as a separate “world”” may be natural to you, but it doesn’t actually solve any problem, instead creating a host of new ones.

Update: Some places to read more about these issues.

The book Many Worlds?: Everett, Quantum Theory and Reality gathers various essays, including
Simon Saunders, Introduction
David Wallace, Decoherence and Ontology
Adrian Kent, One World Versus Many

David Wallace’s book, The Emergent Multiverse.

Blog postings from Jess Riedel here and here.

This from Wojciech Zurek, especially the last section, including parts quoted here.

Posted in Book Reviews, Multiverse Mania, Quantum Mechanics | 76 Comments

What’s the difference between Copenhagen and Everett?

Posted on August 18, 2019 by woit

I’ve just finished reading Sean Carroll’s forthcoming new book, will write something about it in the next few weeks. Reading the book and thinking about it did clarify various issues for me, and I thought it might be a good idea to write about one of them here. Perhaps readers more versed in the controversy and literature surrounding this issue can point me to places where it is cogently discussed.

Carroll (like many others before him, for a recent example see here), sets up two sides of a controversy:

  • The traditional “Copenhagen” or “textbook” point of view on quantum mechanics: quantum systems are determined by a vector in the quantum state space, evolving unitarily according to the Schrödinger equation, until such time as we choose to do a measurement or observation. Measuring a classical observable of this physical system is a physical process which gives results that are eigenvalues of the quantum operator corresponding to the observable, with the probability of occurrence of an eigenvalue given in terms of the state vector by the Born rule.
  • The “Everettian” point of view on quantum mechanics: the description given here is “The formalism of quantum mechanics, in this view, consists of quantum states as described above and nothing more, which evolve according to the usual Schrödinger equation and nothing more.” In other words, the physical process of making a measurement is just a specific example of the usual unitary evolution of the state vector, there is no need for a separate fundamental physical rule for measurements.

I don’t want to discuss here the question of whether the Everettian point of view implies a “Many Worlds” ontology, that’s something separate which I’ll write about when I get around to writing about the new book.

What strikes me when thinking about these two supposedly very different points of view on quantum mechanics is that I’m having trouble seeing why they are actually any different at all. If you ask a follower of Copenhagen (let’s call her “Alice”) “is the behavior of that spectrometer in your lab governed in principle by the laws of quantum mechanics” I assume that she would say “yes”. She might though go on to point out that this is practically irrelevant to its use in measuring a spectrum, where the results it produces are probability distributions in energy, which can be matched to theory using Born’s rule.

The Everettian (let’s call him “Bob”) will insist on the point that the behavior of the spectrometer, coupled to the environment and system it is measuring, is described in principle by a quantum state and evolves according to the Schrödinger equation. Bob will acknowledge though that this point of principle is useless in practice, since we don’t know what the initial state is, couldn’t write it down if we did, and couldn’t solve the relevant Schrödinger equation even if we could write down the initial state. Bob will explain that for this system, he expects “emergent” classical behavior, producing probability distributions in energy, which can be matched to theory using Born’s rule.

So, what’s the difference between the points of view of Alice and Bob here? It only seems to involve the question of how classical behavior emerges from quantum, with Alice saying she doesn’t know how this works, Bob saying he doesn’t know either, but conjectures it can be done in principle without introducing new physics beyond the usual quantum state/Schrödinger equation story. Alice likely will acknowledge that she has never seen or heard of any evidence of such new physics, so has no reason to believe it is there. They both can agree that understanding how classical emerges from quantum is a difficult problem, well worth studying, one that we are in a much better position now to work on than we were way back when Bohr, Everett and others were struggling with this.

Posted in Quantum Mechanics | 27 Comments

Where We Are Now

Posted on August 6, 2019 by woit

For much of the last 25 years, a huge question hanging over the field of fundamental physics has been that of what judgement results from the LHC would provide about supersymmetry, which underpins the most popular speculative ideas in the subject. These results are now in, and conclusively negative. In principle one could still hope for the HL-LHC (operating in 2026-35) to find superpartners, but there is no serious reason to expect this. Going farther out in the future, there are proposals for an extremely expensive 100km larger version of the LHC, but this is at best decades away, and there again is no serious reason to believe that superpartners exist at the masses such a machine could probe.

The reaction of some parts of the field to this falsification of hopes for supersymmetry has been not at all the abandonment of the idea that one would expect. For example, today brings the bizarre news that failure has been rewarded with a $3 million Special Breakthrough Prize in Fundamental Physics for supergravity. For uncritical media coverage, see for instance here, here, and here.

Some media outlets do better. I first heard about this from Ryan Mandelbaum, who writes here. Ian Sample at the Guardian does note that negative LHC results are “leading many physicists to go off the theory” and quotes one of the awardees as saying:

We’re going through a very tough time… I’m not optimistic. I no longer encourage students to go into theoretical particle physics.

At Nature, the sub-headline is “Three physicists honoured for theory that has been hugely influential — but might not be a good description of reality” and Sabine Hossenfelder is quoted. At her blog, she ends with the following excellent commentary:

Awarding a scientific prize, especially one accompanied by so much publicity, for an idea that has no evidence speaking for it, sends the message that in the foundations of physics contact to observation is no longer relevant. If you want to be successful in my research area, it seems, what matters is that a large number of people follow your footsteps, not that your work is useful to explain natural phenomena. This Special Prize doesn’t only signal to the public that the foundations of physics are no longer part of science, it also discourages people in the field from taking on the hard questions. Congratulations.

In related news, yesterday I watched this video of a recent discussion between Brian Greene and others which, together with a lot of promotional material about string theory, included significant discussion of the implications of the negative LHC results. A summary of what they had to say would be:

  • Marcelo Gleiser has for many years been writing about the limits of scientific knowledge, and sees this as one more example.
  • Michael Dine has since 2003 been promoting the string theory landscape/multiverse, with the idea that one could do statistical predictions using it. Back then we were told that “it is likely that this leads to a prediction of low energy supersymmetry breaking” (although Dine soon realized this wasn’t working out, see here.) In 2007 Physics Today published his String theory in the era of the Large Hadron Collider (discussed here), which complained about how “weblogs” had it wrong that string theory had no relation to experiment. That piece claimed that

    A few years ago, there seemed little hope that string theory could make definitive statements about the physics of the LHC. The development of the landscape has radically altered that situation.

    and that

    The Large Hadron Collider will either make a spectacular discovery or rule out supersymmetry entirely.

    Confronted by Brian with the issue of LHC results, Dine looks rather uncomfortable, but claims that there still is hope for string theory and the landscape, that now big data and machine learning can be applied to the problem (for commentary on this, see here). He doesn’t though expect to see success in his lifetime.

  • Andy Strominger doesn’t discuss supersymmetry in particular, but about the larger superstring theory unification idea, tries to make the case that it hasn’t been a failure at all, but a success way beyond what was expected. The argument is basically that the search for a unified string theory was like Columbus’s search for a new sea route to China. He didn’t find it, but found something much more exciting, the New World. In this analogy, instead of finding some tedious reductionist new layer of reality as hoped, string theorists have found some revolutionary new insight about the emergent nature of gravity:

    I think that the idea that people were excited about back in 1985 was really a small thing, you know, to kind of complete that table that you put down in the beginning of the spectrum of particles…

    We didn’t do that, we didn’t predict new things that were going to be measured at the Large Hadron Collider, but what has happened is so much more exciting than our original vision… we’re getting little hints of a radical new view of the nature of space and time, in which it really just is an approximate concept, emergent from something deeper. That is really, really more exciting, I mean it’s as exciting as quantum mechanics or general relativity, probably even more so.

    The lesson Strominger seems to have learned from the failure of the 1985 hopes is that when you’ve lost your bet on one piece of hype, the thing to do is double down, go for twice the hype…

Update: The Breakthrough Prize campaign to explain why supergravity is important despite having no known relation to reality has led to various nonsense making its way to the public, as reporters desperately try to make sense of the misleading information they have been fed. For instance, you can read (maybe after first reading this comment) here that

Witten showed in 1981 that the theory could be used to simplify the proof for general relativity, initiating the integration of the theory into string theory.

You could learn here that

When the theory of supersymmetry was developed in 1973, it solved some key problems in particle physics, such as unifying three forces of nature (electromagnetism, the weak nuclear force, and the strong nuclear force)

Update: On the idea that machine learning will solve the problems of string theory, see this yesterday from the Northeastern press office, which explains that the goal is to “unify string theory with experimental findings”:

Using data science to learn more about the large set of possibilities in string theory could ultimately help scientists better understand how theoretical physics fits into findings from experimental physics. Halverson says one of the ongoing questions in the field is how to unify string theory with experimental findings from particle physics and cosmology…

Update: Physics World has a story about this that emphasizes the sort of criticism I’ve been making here.

As mentioned in the comments, I took a closer look at the citation for the prize. The section on supersymmetry is really outrageous, using “supersymmetry stabilizes the weak scale” as an argument for SUSY, despite the fact that this has been falsified by LHC results.

Update: Jim Baggott writes about this story and post-empirical science here.

Noah Smith here gets the most remarkable aspect of this right. String theory has always had the feature that the strings were not supposed to be visible at accessible energies, so not directly testable. Supersymmetry is quite different: it has always been advertised as a directly testable idea, with superpartners supposed to appear at the electroweak scale and be seen at the latest at the LHC. Giving a huge prize to a theoretical idea that has just been conclusively shown to not work is something both new and outrageous.

Update: Tommaso Dorigo’s take is here, which I’d characterize as basically “any publicity is good publicity, but it’s pretty annoying the cash is going to theorists for failed theories instead of experimentalists”(he does say he wanted to entitle the piece “Billionaire Awards Prizes To Failed Theories”):

[Rant mode on] An exception to the above is, of course, the effect that this not insignificant influx of cash and 23rd-hour recognition has on theoretical physicists. For they seem to be the preferred recipients of the breakthrough prize as of late, not unsurprisingly. Apparently, building detectors and developing new methods to study subnuclear reactions, which are our only way to directly fathom the unknown properties of elementary particles, is not considered enough of a breakthrough by Milner’s jury as it is to concoct elegant, albeit wrong, theories of nature. [Rant mode off]

Going back to the effect on laypersons: this is of course positive. Already the sheer idea that you may earn enough cash to buy a Ferrari and a villa in Malibu beach in one shot by writing smart formulas on a sheet of paper is suggestive, in a world dominated by the equation “is paid very well, so it is important”. But even more important is the echo that he prize – somewhere by now dubbed “the Oscar of Physics” – is having on the media. Whatever works to bring science to the fore is welcome in my book.

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Quick Links

Posted on July 24, 2019 by woit

A few quick links:

  • Philip Ball at Quanta has a nice article on “Quantum Darwinism” and experiments designed to exhibit actual toy examples of the idea in action (I don’t think “testing” the idea is quite the right language in this context). What’s at issue is the difficult problem of how to understand the way in which classical behavior emerges from an underlying quantum system. For a recent survey article discussing the ideas surrounding Quantum Darwinism, see this from Wojciech Zurek.

    Jess Riedel at his blog has a new FAQ About Experimental Quantum Darwinism which gives more detail about what is actually going on here.

  • This year’s TASI summer school made the excellent choice of concentrating on issues in quantum field theory. Videos, mostly well worth watching, are available here.
  • This month’s Notices of the AMS has a fascinating article about Grothendieck, by Paulo Ribenboim. It comes with a mysterious “Excerpt from” title and editor’s note:

    Ribenboim’s original piece contains some additional facts that are not included in this excerpt. Readers interested in the full text should contact the author.

  • I’ve finally located a valuable Twitter account, this one.
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Prospects for contact of string theory with experiments

Posted on July 13, 2019 by woit

Nima Arkani-Hamed today gave a “vision talk” at Strings 2019, entitled Prospects for contact of string theory with experiments which essentially admitted there are no such prospects. He started by joking that he had been assigned this talk topic by someone who wanted to see him give a short talk for a change, or perhaps someone who wanted to “throw him to the wolves”.

The way he dealt with the challenge was by dropping “string theory”, entitling his talk “Connecting Fundamental Theory to the Real World” and only discussing the question of SUSY (he’s still for Split SUSY, negative LHC results are irrelevant since if SUSY were natural it would have been seen at LEP, and maybe a 100km pp machine will see something, or ACME will see an electron edm).

He did discuss the string theory landscape, and explained it was one reason that about 15 years ago he mostly stopped working on phenomenological HEP theory and started doing the more mathematical physics amplitudes stuff. David Gross used to argue that the danger of the multiverse was that it would convince people to give up on trying to understand fundamental issues about HEP theory (where does the Standard Model comes from?). It’s now clear that this is no longer a danger for the future but a reality of the present.

In order to go over time, Arkani-Hamed dropped the topic of his title and turned to discussing his hopes for his amplitudes work. The “long shot fantasy” is that a formulation of QFT will be found in which amplitudes are given by integrating some abstract geometrical quantities.

The conference ended with a “vision” panel discussion. Others may see things differently, but what most struck me about this was the absence of any sort of plausible vision.

Update: Taking a look at the slides from the ongoing EPS-HEP 2019 conference, Ooguri seems to strongly disagree with Arkani-Hamed, claiming in his last slide here that a CMB polarization experiment (LiteBIRD) to fly in 8 years, “provides an unprecedented
opportunity for String Theory to be falsified.” I find this extremely hard to believe. Does anyone else other than Ooguri believe that detection/non-detection of CMB B-modes can falsify string theory?

Posted in Strings 2XXX | 20 Comments

Against Symmetry

Posted on July 10, 2019 by woit

One of the great lessons of twentieth century science is that our most fundamental physical laws are built on symmetry principles. Poincaré space-time symmetry, gauge symmetries, and the symmetries of canonical quantization largely determine the structure of the Standard Model, and local Poincaré symmetry that of general relativity. For the details of what I mean by the first part of this, see this book. Recently though there has been a bit of an “Against Symmetry” publicity campaign, with two recent examples to be discussed here.

Quanta Magazine last month published K.C. Cole’s The Simple Idea Behind Einstein’s Greatest Discoveries, with summary

Lurking behind Einstein’s theory of gravity and our modern understanding of particle physics is the deceptively simple idea of symmetry. But physicists are beginning to question whether focusing on symmetry is still as productive as it once was.

It includes the following:

“There has been, in particle physics, this prejudice that symmetry is at the root of our description of nature,” said the physicist Justin Khoury of the University of Pennsylvania. “That idea has been extremely powerful. But who knows? Maybe we really have to give up on these beautiful and cherished principles that have worked so well. So it’s a very interesting time right now.”

After spending some time trying to figure out how to write something sensible here about Cole’s confused account of the role of symmetry in physics and encountering mystifying claims such as

the Higgs boson that was detected was far too light to fit into any known symmetrical scheme…
symmetry told physicists where to look for both the Higgs boson and gravitational waves

I finally hit the following

“naturalness” — the idea that the universe has to be exactly the way it is for a reason, the furniture arranged so impeccably that you couldn’t imagine it any other way.

At that point I remembered that Cole is the most incompetent science writer I’ve run across (for more about this, see here), and realized best to stop trying to make sense of this. Quanta really should do better (and usually does).

For a second example, the Kavli IPMU recently put out a press release claiming Researchers find quantum gravity has no symmetry. This was based on the paper Constraints on symmetry from holography, by Harlow and Ooguri. The usually reliable Ethan Siegel was taken in, writing a long piece about the significance of this work, Ask Ethan: What Does It Mean That Quantum Gravity Has No Symmetry?

To his credit, one of the authors (Daniel Harlow) wrote to Siegel to explain to him some things he had wrong:

I wanted to point out that there is one technical problem in your description… our theorem does not apply to any of the symmetries you mention here! …

It isn’t widely appreciated, but in the standard model of particle physics coupled to gravity there is actually only one global symmetry: the one described by the conservation of B-L (baryon number minus lepton number). So this is the only known symmetry we are actually saying must be violated!

What Harlow doesn’t mention is that this is a result about AdS gravity, and we live in dS, not AdS space, so it doesn’t apply to our world at all. Even if it did apply, and thus would have the single application of telling us B-L is violated, it says nothing about how B-L is violated or what the scale of B-L violation is, so would be pretty much meaningless.

By the way, I’m thoroughly confused by the Kavli IPMU press release, which claims:

Their result has several important consequences. In particular, it predicts that the protons are stable against decaying into other elementary particles, and that magnetic monopoles exist.

Why does Harlow-Ooguri imply (if it applied to the real world, which it doesn’t…) that protons are stable?

What is driving a lot of this “Against Symmetry” fashion is “it from qubit” hopes that gravity can be understood as some sort of emergent phenomenon, with its symmetries not fundamental. I’ve yet though to see anything like a real (i.e., consistent with what we know about the real world, not AdS space in some other dimension) theory that embodies these hopes. Maybe this will change, but for now, symmetry principles remain our most powerful tools for understanding fundamental physical reality, and “Against Symmetry” has yet to get off the ground.

Update: Quanta seems to be trying to make up for the KC Cole article by today publishing a good piece about space-time symmetries, Natalie Wolchover’s How (Relatively) Simple Symmetries Underlie Our Expanding Universe. It makes the argument that, just as the Poincaré group can be thought of as a “better” space-time symmetry group than the Galilean group, the deSitter group is “better” than Poincaré.

In terms of quantization, the question becomes that of understanding the irreducible unitary representations of these groups. I do think the story of the representations of Poincaré group (see for instance my book about QM and representation theory) is in some sense “simpler” than the Galilean group story (no central extensions needed). The deSitter group is a simple Lie group, and comparing its representation theory to that of Poincaré raises various interesting issues. A couple minutes of Googling turned up this nice Master’s thesis that has a lot of background.

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