Strings 2018

I’m at Strings this week, in tropical Okinawa. Opening the conference, organizer Hirosi Ooguri joked that they had carefully scheduled things for a sunny time of year, and since the rainy season had just ended “who says that string theorists don’t make predictions?”

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There was then a rainstorm during lunch, falsifying string theory

This is the first time I’ve been to Strings. There are almost 500 people here, which might seem small for folks in other fields, but for me this is the biggest conference I’ve attended. The size is noticeable in the little things: this is the first conference I’ve been to with a diaper changing room, the first managed by a tour company, the first with a dedicated “Cultural Evening” featuring classical music from the region. With this in mind, the conference were impressively well-organized, but there were some substantial gaps (tightly packed tours before the Cultural Evening that didn’t leave time for dinner, and a talk by Morrison cut short by missing slides that offset the schedule of the whole last day).

On the well-organized side, Strings has a particular structure for its talks, with Review Talks and Plenary Talks. The Review Talks each summarize a subject: mostly main focuses of the conference, but with a few (Ashoke Sen on String Field Theory, David Simmons-Duffin on the Conformal Bootstrap) that only covered the content of a few talks.

I’m not going to make another pie chart this year, if you want that kind of breakdown Daniel Harlow gave one during the “Golden Jubilee” at the end. If I did something like that this time, I’d divide it up not by sub-fields, but by goals. Talks here focused on a few big questions: “Can we classify all quantum field theories?” “What are the general principles behind quantum gravity?” “Can we make some of the murky aspects of string theory clearer?” “How can string theory give rise to sensible physics in four dimensions?”

Of those questions, classifying quantum field theories made up the bulk of the conference. I’ve heard people dismiss this work on the ground that much of it only works in supersymmetric theories. With that in mind, it was remarkable just how much of the conference was non-supersymmetric. Supersymmetry still played a role, but the assumption seemed to be that it was more of a sub-topic than something universal (to the extent that one of the Review Talks, Clay Cordova’s “What’s new with Q?”, was “the supersymmetry review talk”). Both supersymmetric and non-supersymmetric theories are increasingly understood as being part of a “landscape”, linked by duality and thinking at different scales. These links are sometimes understood in terms of string theory, but often not. So far it’s not clear if there is a real organizing principle here, especially for the non-supersymmetric cases, and people seem to be kept busy enough just proving the links they observe.

Finding general principles behind quantum gravity motivated a decent range of the talks, from Andrew Strominger to Jorge Santos. The topics that got the most focus, and two of the Review Talks, were by what I’ve referred to as “entanglers”, people investigating the structure of space and time via quantum entanglement and entropy. My main takeaway from these talks was perhaps a bit frivolous: between Maldacena’s talk (about an extremely small wormhole made from Standard Model-compatible building blocks) and Hartman’s discussion of the Average Null Energy Condition, it looks like a “useful sci-fi wormhole” (specifically, one that gets you there faster than going the normal way) has been conclusively ruled out in quantum field theory.

Only a minority of talks discussed using string theory to describe the real world, though I get the impression this was still more focus than in past years. In particular, there were several talks trying to discover properties of Calabi-Yaus, the geometries used to curl up string theory’s extra dimensions. Watching these talks I had a similar worry to Strominger’s question after Irene Valenzuela’s talk: it’s not clear that these investigations aren’t just examining a small range of possibilities, one that might become irrelevant if new dualities or types of compactification are found. Ironically, this objection seems to apply least to Valenzuela’s talk itself: characterizing the “swampland” of theories that don’t make sense as part of a theory of quantum gravity may start with examples from string compactifications, but its practitioners are looking for more general principles about quantum gravity and seem to manage at least reasonable arguments that don’t depend on string theory being true.

There wasn’t much from the amplitudes field at this conference, with just Yu-tin Huang’s talk carrying that particular flag. Despite that, amplitudes methods came up in several talks, with Silviu Pufu praising an amplitudes textbook and David Simmons-Duffin bringing up amplitudes several times (more than he did in his talk last week at Amplitudes).

The end of the conference featured a panel discussion in honor of String Theory’s 50th Anniversary, its “Golden Jubilee”. The panel was evenly split between founders of string theory, heroes of the string duality revolution, and the current crop of young theorists. The panelists started by each giving a short presentation. Michael Green joked that it felt like a “geriatric gong show”, and indeed a few of the presentations were gong show-esque. Still, some of the speeches were inspiring. I was particularly impressed by Juan Maldacena, Eva Silverstein, and Daniel Harlow, who each laid out a compelling direction for string theory’s future. The questions afterwards were collated by David Gross from audience submissions, and were largely what you would expect, with quite a lot of questions about whether string theory can ever connect with experiment. I was more than a little disappointed by the discussion of whether string theory can give rise to de Sitter space, which was rather botched: Maldacena was appointed as the defender of de Sitter, but (contra Gross’s summary) the quantum complexity-based derivation he proposed didn’t sound much like the flux compactifications that have inspired so much controversy, so everyone involved ended up talking past each other.

Edit: See Shamit’s comment below, I apparently misunderstood what Maldacena was referring to.

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8 thoughts on “Strings 2018

  1. Pingback: This Week’s Hype | Not Even Wrong

  2. David Brown

    “The questions afterward …” Is Milgrom the Kepler of contemporary cosmology? I say yes. My guess is that almost all string theorists reject MOND as soon as they read the definition of the MOND acceleration constant. String theorists seem to think that McGaugh, Kroupa, Sanders, and Scarpa agree on a value 1.2±.2 * 10^-10 meter/sec^2 because the pro-MOND astronomers and astrophysicists read each other’s publications and then engage in data dredging or p-hacking. I say that Milgrom is one of the world’s greatest living scientists — but what does MOND mean in terms of string theory and general relativity theory?
    Can the string landscape explain the empirical successes of Milgrom’s MOND? Consider 4 hypotheses:
    (1) After quantum averaging, Einstein’s field equations are 100% correct.
    (2) All dark matter particles are WIMPs with D-brane charges.
    (3) Within the string landscape there is a D-brane shock wave that makes it appear that Einstein’s field equations are slightly wrong (due to D-brane charges acting detectably upon some WIMPs but non-detectably upon other WIMPs.)
    (4) In the standard form of Einstein’s field equations replace the -1/2 by -1/2 + fake-function, where the fake-function seems to be there because the pro-MOND astrophysics are unaware of the D-brane shock wave acting upon some of the WIMPs. The fake-function means that the pro-MOND astrophysics have put the weird anomaly into gravitational acceleration instead of into the energy-momentum tensor. The fake-function = approximately 3.9±.4 * 10^-5 where MOND is approximately valid and = 0 where non-relativistic MOND cannot be applied. Are D-branes somehow involved in maintaining the structure of the string landscape or somehow weirding the WIMPs?

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    1. 4gravitonsandagradstudent Post author

      I let this comment through, but as a warning, I would usually mark this as spam, since it’s only tenuously connected to the topic of the post. Even a little “here is a question I would have liked to ask” would have helped make it at least plausibly on topic.

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      1. Lubos Motl

        Dear Tetragraviton, if you have a blacklist, just add David Brown on it. He is posting exactly the same mostly irrational stuff of MOND basically everywhere – I am getting periodic e-mails with that, he’s posting it to my blog, he posted it to the panel discussion YouTube video from Strings 2018 (which had nothing to do with the content of his MOND text). I don’t think he’s been listening to anything I ever wrote to him so you really should classify him as a spammer – it’s no exaggeration at all.

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  3. Shamit Kachru

    I can add one clarifying remark. The complexity Maldacena was referring to was not quantum complexity. There are two questions you could naturally ask about making de Sitter vacua. One is, “Can you make any solution with positive cosmological constant?” This could be a constant comparable to the scale of supersymmetry breaking in the model. A second question you could ask, motivated by Nature, is “Can you make a solution with a positive cosmological constant * much smaller than * the scale of supersymmetry breaking?” The constructions of dS that have been proposed, taken at face value, would naturally produce solutions with cosmological constant of order the SUSY breaking scale. No miracle. To argue that solutions with small cosmological constant are possible in that framework, one needs to argue that tuning in a complex landscape of fluxes allows one to find cancellations in rare (but existing) vacua. Juan conflated both questions, and was discussing the need to invoke tuning in a complex landscape to get CC much smaller than SUSY breaking scale. But the questions can and should naturally be separated.

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  4. sean samis

    “Talks here focused on a few big questions: ‘Can we classify all quantum field theories?’ ‘What are the general principles behind quantum gravity?’ ‘Can we make some of the murky aspects of string theory clearer?’ ‘How can string theory give rise to sensible physics in four dimensions?'”

    That last topic clearly implies that — currently — even its advocates think string theory does not give rise to sensible physics in four dimensions. That topic appears to fit the definition of a “buried lede”. Unless one can show how sensible physics can arise in four dimensions from string theory(s) I don’t think the rest of the topics matter much.

    sean s.

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    1. 4gravitonsandagradstudent Post author

      The word “how” is doing some work here. There are definitely lasting questions about whether string theory can give rise to sensible physics in four dimensions: that’s what the debate over whether it can give a de Sitter space (whether you can get a universe with a cosmological constant like our own) is about, for example, and there are other questions that similarly might rule out string theory as a description of our world. But even if those questions get answered, there’s still a question of how precisely it happens: how the dimensions can be curled up, whether there are any general traits of compactifications that work, whether this lets you make any testable predictions. One thing I was hoping for was a statement of the “state of the art” describing what the “most realistic” compactifications to date can manage: I’ve heard they can get the MSSM, but are they doing that while fixing moduli? Do they have SUSY breaking, or the Higgs? How complete is the most complete picture to date? Unfortunately the relevant review talk was Morrison’s, which was incomplete. If someone does know the current status please chime in in the comments.

      I also don’t think the other questions rely on this all that much. Classifying quantum field theories is something you can do (and find interesting) independent of string theory, string theory can suggest relations that you can prove in other ways. While a lot of the “general principles of quantum gravity” research starts with string theory, they really are often interested in general principles that can be independently justified. Clearing up murky aspects of string theory may depend much more on whether string theory is true, depending on the “aspect”: I’m not sure when any of these people would need to apply String Field Theory, or to know how to construct amplitudes in M theory.

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