Various and Sundry

A few things that may be of interest:

  • Survey articles prepared for the 2018 ICM proceedings are starting to appear on the arXiv, and Peter Scholze (who will be getting a Fields Medal in Rio) has put his on his web-site. His title is p-adic Geometry, and it gives an overview of the ground-breaking work he has been doing over the last few years. The last section tells us that

    Currently, the author is trying to understand to what extent it might be true that the “universal” cohomology theory is given by a shtuka relative to Spec Z. It seems that this is a very fruitful philosophy.

    For some background about that section, I’d recommend his talk at the 2015 Clay Math conference.

  • The New Yorker has a very detailed and interesting profile of Jim Simons and what he is up to with the Flatiron Institute he is now funding here in New York. This new Institute is costing him \$80 million a year, characterized as “a lark” for someone with his assets. David Spergel is running the Center for Computational Astrophysics there, and doing a lot of hiring. When I wrote here about his characterization of multiverse research, his final comment about being able to speak freely because he had tenure left me wondering “wait, what about grants, jobs, etc.?”. From the New Yorker article, I realized that while having tenure may give you some ability to speak freely, having a guy with \$18.5 billion willing to write large checks for you gives you a lot more…
  • I’ve just finished teaching a course this semester which concentrated on the formalism for describing geometry in terms of connections and curvature. From the point of view of physicists, this formalism should be of interest because it applies equally well to gauge theory and general relativity. I’d been starting to think again about what light this formalism might shed on how to think about these two subjects together, when last night I noticed a wonderful new article on the arXiv, Gravity and Unification: A review by Krasnov and Percacci.

    This article is an extremely lucid and comprehensive survey of the sort of thing I was thinking about, which can be re-expressed as the question of trying to find, at the classical level, a formalism uniting the vector potentials/field strengths of the SM and the different possible fields used to describe geometry in GR. Some of this has a very long history, going back to the things Einstein was trying in his later years. There have been many different ideas that people have tried since then, and the survey article does a great job of both explaining these ideas, as well as indicating why they haven’t worked out.

    A couple of the general ideas that have always fascinated me make an appearance in the article. One of these is that of what mathematicians call a “Cartan connection”, the idea that you should think of a geometry as locally looking like a quotient space G/H of two Lie groups. A version of this is known to physicists as the MacDowell-Mansouri formulation, which gets a detailed treatment in the article. Another is the idea of using the fact that the complexified orthogonal group in 4 dimensions breaks up into two pieces, sometimes thought of at the Lie algebra level as self-dual vs. anti-self-dual pieces under the Hodge star operation. A version of this idea is known as the Plebanski formulation, and this decomposition is behind the story of Ashtekar variables. These variable have played a crucial role in modern treatments of GR by Hamiltonian methods, as well as the quantization program of loop quantum gravity.

    The focus of the article is on Lagrangian and classical field theory methods for studying these ideas. There’s relatively little about the Hamiltonian story, and also relatively little about the geometry of spinors, two topics that I suspect might provide additional needed insights. For anyone interested in thinking about non-string theory-based ideas about unification of the SM and gravity, there’s a wealth of ideas, references and history here to think about. Perhaps future progress on unification will come from some new breakthrough in this field that shows how to get around the problems identified clearly in this article.

  • For surveys of recent work on quantization of gravity and discussion by experts, a good place to look is videos of talks at a recent conference held at the IHES. Videos available here include my fellow Princeton student Costas Bachas surveying the approach growing out of string theory in Holographic Dualities and Quantum Gravity, Carlo Rovelli the opposition in Current Quantum Gravity Theories, Experimental Evidence, Philosophical Implications, and an even-handed overview from Steven Carlip with Why We Need Quantum Gravity and Why We Don’t Have It.

Finally, for fans of Lenny Susskind’s introductory level books on theoretical physics, Andre Cabannes writes to tell me that the most recent volume (which I wrote about here) is being translated by him into French, to appear next year. He also has notes from the lectures on General Relativity, Cosmology, and Statistical Mechanics, for which no book form has yet appeared.

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This Week’s Hype/Fake Physics

On the Fake Physics front, Jerry Coyne at Why Evolution is True has a post claiming New evidence for the multiverse-and its implications. You would think that recent history should have made clear the danger of using Youtube videos as a reliable source of information, but this posting is based mainly upon a Youtube video, one that claims Evidence for a Multiverse in NASA and ESA Satellite Data? (Coyne seems to have missed the question-mark).

As usual, a large part of the problem here is people looking for material helpful to their arguments, without worrying much about whether the material is accurate or not. In Coyne’s case, he wants to counter the theological fine-tuning argument with the multiverse counter-argument, which requires a multiverse with a wide variety of different physical laws. The Youtube video he found makes the standard tenuous argument that the CMB provides evidence for inflation, inflation should be eternal, thus there should be a multiverse. As I explained in detail here, the models of inflation one supposedly has “evidence” for are not models that lead to the kind of multiverse of different physical laws that Coyne needs for his argument with religion.

I should make it clear that I’m on Coyne’s side in the argument of evolution vs. religion, but scientists arguing on the basis of science should take care that they’re using good science if they don’t want to discredit themselves. And, as a general rule for anyone who cares about what’s true and what isn’t, looking for things on Facebook or Youtube that help your side of an argument is now an extremely bad idea.

The question of how to stand up for truth in a post-truth era was the main topic of this year’s Nobel Week Dialogue (video here). David Gross gave a rousing and inspirational talk on Truth and the Scientific Method (starting about 36:30), which ended with the assurance that “Science will survive Donald J. Trump and his ilk”, because of its rigorous honesty and grounding in experimental testability. Gross is someone well-aware of the multiverse Fake Physics danger, although he didn’t mention it. I’d feel a lot better about his Stockholm talk though if I hadn’t just recently attended this disturbing one.

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The Last Refuge of Cowards

The talks and panel discussions from the 2018 Breakthrough Prize symposium are available via Facebook video. They ended with the following, from prize winner David Spergel:

Well, alright, I’m going to say something that I probably shouldn’t say in Palo Alto. I don’t think the multiverse is a testable and interesting scientific hypothesis. I think it doesn’t explain anything.

The way the multiverse tends to be used is together with the anthropic principle. The idea is that the universe is the way it is because that’s the way we get to live in it. I find the multiverse solutions to these problems, it’s a lot like if you ask me “why am I wearing a black shirt today”. My answer would be: “you wouldn’t have asked the question if I wasn’t wearing a black shirt”. That’s not a satisfactory answer.

The way we have advanced in science is by falsifiability. By developing hypotheses, testing them (that’s why we do experiments) and ruling things out.

Ideas that are not testable, it’s interesting metaphysics, perhaps interesting for philosophers. What has driven four hundred years of scientific progress is the fact that ideas can be wrong. And, the multiverse, I think is kind of the last refuge of cowards… [nervous laughter from the audience] That’s why it’s great to have tenure.

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String Theory Fails Another Test

Back in 2004, the KITP put out a press release (which I wrote about in an early blog post here) announcing that “Newly Devised Test May Confirm Strings as Fundamental Constituent of Matter, Energy”. The press release announced that Polchinski and collaborators had found “the most viable test to date for determining whether string theory is on the right track”, that this test would be performed by LIGO, which “could provide support for string theory within two years.”

This got a lot of attention and was often quoted as evidence that string theory was testable science. In a 2007 article in Physics World, David Gross answers string theory critics with:

String theory is full of qualitative predictions, such as the production of black holes at the LHC or cosmic strings in the sky, and this level of prediction is perfectly acceptable in almost every other field of science,” he says. “It’s only in particle physics that a theory can be thrown out
if the 10th decimal place of a prediction doesn’t agree with experiment.”

LIGO never found any evidence of cosmic strings within two years after 2004, and now the vastly more sensitive Advanced LIGO experiment has just released results of a search. As expected, the results are negative.

Any guess on the probability of a KITP press release announcing that string theory has failed an experimental test? Or of an acknowledgement by Gross that all the “qualitative predictions” of string theory he was using to justify it ten years ago have now all failed, so, by the standard of “every other field of science”, it should be abandoned?

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Scientific Controversy No. 13

I made the mistake yesterday evening of spending it out in Red Hook, at an event billed as addressing the scientific controversy over string theory. The venue was an arts space called Pioneer Works, the brain-child of artist Dustin Yellin (whose formative early experience with physics is described here). The event was sold out (tickets were free, courtesy of the Simons Foundation), and drew a huge crowd of several hundred, mostly twenty-something Brooklyn hipsters.

The guests brought in to discuss the controversy were David Gross and Clifford Johnson, and the moderator was Janna Levin. Levin began the discussion by asking the two of them where they stood on string theory: pro, con or agnostic? This flustered Gross a bit (he’s one of the world’s most well-known and vigorous proponents of string theory) and Levin somehow took this as meaning that he was agnostic. Finally Gross clarified things by saying something like “I’ve been married to string theory for 50 years, not going to leave her now”.

Things then moved on to the usual well-worn hype about GUTs, string theory and unification. The LHC made a quick appearance, with no mention of falsified string theory “predictions” of supersymmetry. Instead Johnson characterized the discovery of the Higgs as somehow a vindication for this unification program. Gross went on to explain that unfortunately testing string theory requires going to the Planck scale where strings would be obvious, but that this was out of the question with any conceivable technology.

Besides being immune to experimental test, Gross also described string/M-theory as not a theory at all, since we don’t know its equations or principles (according to him, it’s a “framework”, see here). The conversation then degenerated into a long and meandering discussion of the black hole information paradox (to her credit, Levin countered Gross’s claim that string theory successfully explained it by reminding him of Polchinski and the firewall business).

The Q and A session consisted of a series of mostly crackpot questions from the audience. Johnson responded to a woman saying she thought that we were oscillating between two universes by telling her that she could see she was wrong by testing her theory. The sudden appearance of testability as a criterion to shoot down vague ideas surely confused her.

On a positive note, neither Johnson nor Gross were interested in promoting the multiverse, and the audience was spared that.

Johnson has a new book out called The Dialogues, written in graphic novel form. My previous experience with him was a rather unpleasant one more than ten years ago, after the publication of my book. He wrote a long sequence of blog posts about what he called the “Storm in a Teacup”, attacking Smolin and me and our books. Attempts to discuss the issues involved with him in the comment section there were confusing at first, until things finally became clear when he explained that he was refusing to read my book or Smolin’s. Dialogue about science was not something he seemed interested in if it involved uncomfortable criticism of string theory.

His book addresses this controversy with a panel in which the physicist figure explains:

Frankly, that’s mostly driven by the press, and a few attention-seeking individuals. Most people have a more nuanced view… It just does not sell newspapers or books.

On the question of “attention-seeking”, one might want to consult Johnson’s forty-plus long series of blog postings about his participation and appearance in TV and movie programs. As far as books go, in an end-note for this panel Brian Greene’s The Elegant Universe is recommended. After the Q and A, a long line formed for people to hand their credit cards over to an assistant, then get a copy of Johnson’s book and have it signed.

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Secret Link Uncovered Between Pure Math and Physics

There’s a very intriguing new article out today by Kevin Hartnett at Quanta magazine, entitled Secret Link Uncovered Between Pure Math and Physics (also a video here). It’s about ideas relating number theory and physics from arithmetic geometer Minhyong Kim. He’s evidently on tour talking about them, with two talks on Gauge theory in arithmetic and a colloquium talk on “Gauge theory in geometry and number theory” in Heidelberg, and a talk on Gauge theory in arithmetic geometry in Paris.

In recent years Kim has been working on what he calls “arithmetic Chern-Simons” theory. For details about this, there are papers here, here, here and here, a workshop here, talks here and here. These ideas grew out of a beautiful and well-known analogy between topology and number theory that goes under the name “Arithmetic Topology”. For more about this, see the book Knots and Primes by Morishita, or the course notes by Chao Li and Charmaine Sia.

While these ideas look quite interesting and I have some idea what they’re about, the Quanta story seems to indicate that Kim has something new, an idea about “Diophantine gauge theory” going beyond the arithmetic Chern-Simons business, and with potential applications to deep problems in arithmetic geometry. Unfortunately the mathematical background here is beyond me (you can try to look at Jordan Ellenberg here, and this earlier paper of Kim’s), and as far as I can tell, the only source for details on the conjectured relations to gauge theory is Kim’s recent talks, which aren’t documented anywhere I can see.

I’m sure we’ll be hearing more about this as time goes on. It joins a host of other ideas relating gauge theory and number theory (in the context for instance of the Langlands program), and promises deeper links to come between fundamental ideas about physics and about mathematics.

: Some personal background on this story from John Baez.

Update: More about this story here, including a discussion of the use of Kim’s methods to deal with the “Cursed Curve”.

Update: Reddit has a report of the Heidelberg talk here, unfortunately giving just enough information about the talk to make it sound very interesting, too little to figure out what the ideas discussed actually were. For yet more frustration on this front, Kim gave a general talk on the subject last year here with video supposedly available, but no browser I’ve tried can access it. I do hope we’ll soon see slides, notes, a paper, something, anything, so we can figure out what the Quanta article actually was about.

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A Physicist’s Physicist Ponders the Nature of Reality

Quanta magazine has an interesting new piece up, an interview of Witten by Natalie Wolchover.

One topic covered in the interview is the question discussed in a recent posting, that of whether a different formulation of QFT exists, one not based on a choice of Lagrangian. Here Witten is non-committal, leaning to the idea such a thing might exist only in special cases:

Now, Nati Seiberg [a theoretical physicist who works down the hall] would possibly tell you that he has faith that there’s a better formulation of quantum field theory that we don’t know about that would make everything clearer. I’m not sure how much you should expect that to exist. That would be a dream, but it might be too much to hope for; I really don’t know…

I find it hard to believe there’s a new formulation that’s universal. I think it’s too much to hope for. I could point to theories where the standard approach really seems inadequate, so at least for those classes of quantum field theories, you could hope for a new formulation. But I really can’t imagine what it would be.

The standard example of where such a formulation might be needed is the 6d superconformal (2,0) theory, about which Witten says:

From the (2,0) theory’s existence and main properties, you can deduce an incredible amount about what happens in lower dimensions. An awful lot of important dualities in four and fewer dimensions follow from this six-dimensional theory and its properties. However, whereas what we know about quantum field theory is normally from quantizing a classical field theory, there’s no reasonable classical starting point of the (2,0) theory.

About the current state of M-theory, there’s this exchange:

You proposed M-theory 22 years ago. What are its prospects today?

Personally, I thought it was extremely clear it existed 22 years ago, but the level of confidence has got to be much higher today because AdS/CFT has given us precise definitions, at least in AdS space-time geometries. I think our understanding of what it is, though, is still very hazy. AdS/CFT and whatever’s come from it is the main new perspective compared to 22 years ago, but I think it’s perfectly possible that AdS/CFT is only one side of a multifaceted story. There might be other equally important facets.

What’s an example of something else we might need?

Maybe a bulk description of the quantum properties of space-time itself, rather than a holographic boundary description. There hasn’t been much progress in a long time in getting a better bulk description. And I think that might be because the answer is of a different kind than anything we’re used to. That would be my guess.

Are you willing to speculate about how it would be different?

I really doubt I can say anything useful. I guess I suspect that there’s an extra layer of abstractness compared to what we’re used to. I tend to think that there isn’t a precise quantum description of space-time — except in the types of situations where we know that there is, such as in AdS space. I tend to think, otherwise, things are a little bit murkier than an exact quantum description. But I can’t say anything useful.

The hope of 22 years ago was that it was non-perturbative string theory which would provide the desired “description of the quantum properties of space-time itself”. Over the years though studies of gauge-gravity duality have moved away from the use of string theory to provide this bulk description. Witten’s take on the current situation: “There hasn’t been much progress in a long time in getting a better bulk description. And I think that might be because the answer is of a different kind than anything we’re used to.” seems reasonable.

It’s interesting to hear that Witten was going back to Wheeler to see if he had any inspiration to offer the current “It from Qubit” program. This requires a patience for the “vague but inspirational” that Witten has more of these days than he used to:

Why do you have more patience for such things now?

I think when I was younger I always thought the next thing I did might be the best thing in my life. But at this point in life I’m less persuaded of that. If I waste a little time reading somebody’s essay, it doesn’t seem that bad.

This patience is not infinite though: among Witten’s many admirable qualities are the way he responds to:

Do you have any ideas about the meaning of existence?

No. [Laughs.]

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Theorists Without a Theory

I had been intending to write something here on the blog about this essay by George Ellis, so when I was contacted by someone at Inference about writing a letter in response, I did so for publication there. It has now appeared in their latest issue, with the title Theorists Without a Theory.

The topic is one I’ve addressed here all too often, but the main point I was trying to make is perhaps a new one. When I was writing here about the controversy over inflation one thing that struck me was that the pro-inflation side was responding to arguments that their theory didn’t solve the problem it was supposed to by in effect saying “the real theory is much more complicated” (see the paragraph beginning “Besides our disagreement…” on page 3 here). One way of seeing part of what is going on here is that most of what gets advertised as “theories” of inflation are actually more appropriately described as toy models. They involve a single inflaton field with a simple potential and unknown couplings to matter, intended as a toy model for the real theory (which will have lots of fields, complicated potentials and specified couplings to matter). An aspect of the controversy is one side pointing out that this theory doesn’t solve problems it is supposed to solve, with the other side arguing that it’s just a toy model.

People sometimes note that there’s a terminological problem with “string theory”, in that the public is often told that “theories” are solidly tested parts of science, which is not true in this case. The actual usage among physicists is different though, with “theory” often used to mean a specific mathematical model or set of models, with no implicit claim of a successful experimental test. A lot of the problem with the usage “string theory” is that no one knows what the actual theory is: it’s a conjecture that a theory with certain specific limits exists. The main point I was trying to make in this piece is that to a large degree the arguments over the scientific status of string theory (and of its supposed landscape and multiverse) revolving around its lack of testability are moot, since the underlying problem is something different: that there is no real theory to argue about. String theorists often try and evade this problem by a terminological shift: string theory is not a “theory”, it’s a “framework”. “Framework” is a much more ill-defined term than the already ill-defined “theory”. A theorist who says “I have a framework, not a theory” is actually saying nothing more than that they are a theorist without a theory.

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Various Physics News

First, two local events, involving well-known physics bloggers:

  • Last Thursday I had the pleasure of attending an event at NYU featuring Sabine Hossenfelder and Natalie Wolchover in conversation. You can watch this for yourself here. If you’re not following Hossenfelder on her blog and at Twitter (and planning to read her forthcoming book), as well as reading Wolchover’s reporting at Quanta magazine, you should be.
  • Next week there will be an event out in Brooklyn advertised as covering the Scientific Controversy over string theory. The idea seems to be to address this controversy by bringing to the public two well-known and very vocal proponents of one side of it.

For a Q and A with another well-known physics blogger, there’s Tommaso Dorigo at Physics Today.

For a couple of encouraging indications that the theoretical physics community may finally be taking seriously the need to give up on failed thinking and try something new, there’s

  • A conference next month in Italy on Weird Theoretical Ideas (Thinking outside the box).
  • An interesting talk at a recent IPMU conference by Yuji Tachikawa. I like his conclusion:

    Basically, all the textbooks on quantum field theories out there use an old framework that is simply too narrow, in that it assumes the existence of a Lagrangian.

    This is a serious issue, because when you try to come up e.g. with a theory beyond the Standard Model, people habitually start by writing a Lagrangian … but that might be putting too strong an assumption.

    We need to do something

In General Relativity related news, there’s a new edition out of Misner, Thorne and Wheeler, the book from which many of us learned both geometry and GR. It comes with new prefaces from David Kaiser as well as Misner and Thorne (which an appropriate search on the Amazon preview might show you…). In other Wheeler-related news, Paul Halpern has a new book out, The Quantum Labyrinth, which tells the entangled stories of Feynman and Wheeler.

Finally, also GR related, the Perimeter Institute has announced the formation of a new cosmology-focused “Centre for the Universe”, funded by an anonymous 10-year $25 million donation. It will be led by cosmologist Neil Turok, who is soon to step down as director of Perimeter.

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Breakthrough Prize 2018

The Breakthrough Prizes for 2018 will be awarded at a ceremony on December 3, I believe at the usual NASA Hangar 1 in Mountain View. The next day Stanford will host the 2018 Breakthrough Prize symposium, which one will be able to watch live from the Breakthrough Prize Facebook page.

The symposium schedule is available here, and while it does not list the Prize awardees, it does appear to list the titles of the talks. From this it looks like the math \$3 million will go to a geometer, who will talk about “Geometry at Higher Dimensions”. There may be several \$100,000 New Horizons Prizes for younger mathematicians, but at least one will be to an analytic number theorist, who will talk about “Analytic Number Theory in Everyday Life”.

For the \$3 million physics prize, it looks like it is going to be split five ways and go to cosmologists/astrophysicists. The talks by laureates are “The Next Decade in Cosmology”, “Gravitational Waves and Cosmology”, “Search for Extraterrestrial Intelligence”, “A New Instrument for Listening to the Universe” and “The Beginning and End of the Universe”.

Update: Some details about the prize ceremony here. Perhaps there really is a problem with the public understanding of mathematics:

This year, a total of seven \$3 million prizes will be awarded – five in life sciences, one in fundamental physics, two in mathematics.

Update: The \$3 million for physics went to the WMAP team. For mathematics, it was Hacon and McKernan. The posted titles for the mathematics prize winners were a red herring, they have been changed to “A Tour of Algebraic Geometry” (McKernan) and “Sphere Packing in High Dimensions” (Viazovska).

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