Friday, 4 September 2020

Does this covariant function belong to some 2d CFT?

In conformal field theory, correlation functions of primary fields are covariant functions of the fields’ positions. For example, in two dimensions, a correlation function of N diagonal primary fields must be such that
$$\begin{aligned} F(z_1,z_2,\cdots , z_N) = \prod_{j=1}^N {|cz_j+d|^{-4\Delta_j}}  F\left(\tfrac{az_1+b}{cz_1+d},\tfrac{az_2+b}{cz_2+d},\cdots , \tfrac{az_N+b}{cz_N+d}\right) \ , \end{aligned}$$
where zj ∈ ℂ are the fields’ positions, Δj ∈ ℂ their conformal dimensions, and $\left(\begin{smallmatrix} a& b \\ c& d \end{smallmatrix}\right)\in SL_2(\mathbb{C})$ is a global conformal transformation. In addition, there are nontrivial relations between different correlation functions, such as crossing symmetry. But given just one covariant function, do we know whether it belongs to a CFT, and what can we say about that CFT?

In particular, in two dimensions, do we know whether the putative CFT has local conformal symmetry, and if so what is the Virasoro algebra’s central charge?

Since covariance completely fixes three-point functions up to an overall constant, we will focus on four-point functions i.e. N = 4. The stimulus for addressing these questions came from the correlation functions in the Brownian loop soup, recently computed by Camia, Foit, Gandolfi and Kleban. (Let me thank the authors for interesting correspondence, and Raoul Santachiara for bringing their article to my attention.)

Doesn’t any covariant function belong to multiple 2d CFTs?

In conformal field theory, any correlation function can be written as a linear combination of s-channel conformal blocks. These conformal blocks are a particular basis of smooth covariant functions, labelled by a conformal dimension and a conformal spin. (I will not try to say preciely what smooth means.) In two dimensions, we actually have a family of bases, parametrized by the central charge c, with the limit c = ∞ corresponding to global conformal symmetry rather than local conformal symmetry.

Thursday, 27 August 2020

(2/2) Open access mystery: why did ERC backstab Plan S?

In my first post about the ERC’s recent withdrawal from supporting Plan S, I tried to explain ERC’s announcement using publicly available information on the ERC, Plan S, and their recent news. The potential dangers of this approach were to miss relevant pieces of information, and to give too much weight to calendar coincidences.

We are still waiting for a detailed and convincing explanation from the ERC, and for a description of their open access strategy if they still have one. Meanwhile, I would like to complete the picture based on informal contacts with a few well-informed colleagues. There emerge two potential lines of explanation.

Wednesday, 22 July 2020

(1/2) Open access mystery: why did the ERC backstab Plan S?

The European Research Council (ERC) just announced that they would withdraw their support for Coalition S, the consortium of research funders behind Plan S. Plan S is the valiant but not universally welcome attempt to impose strong open access requirements to research articles, without paying more money to publishers.

The ERC is Europe’s most prestigious research funder, and a main backer of Plan S. Without Plan S, the ERC has no open access strategy, and without the backing of ERC, Coalition S may not be big enough for succeeding. Why would ERC make this U-turn? I do not know, but let me gather a few potentially relevant pieces of the puzzle. The pieces are of three types:
  • some context on the ERC and more generally on Europe’s research plans,
  • the recently announced rights retention strategy by Coalition S,
  • ERC’s meager and not very credible justification for their withdrawal.

Tuesday, 3 March 2020

With open peer review, open is just the beginning

Abstract

Open peer review does not just mean publishing existing reviewer reports, but should also lead to writing reports primarily for the public. We make a specific proposal for structured reviewer reports, based on the three criteria validity, interest and clarity.
This post is partly based on a joint proposal with Anton Akhmerov for improving the structure of reviewer reports at SciPost. Feedback from Jean-Sébastien Caux on that proposal is gratefully acknowledged.

 

Benefits of open peer review: the obvious and the less obvious


In the traditional model of academic peer review, reviewers’ reports on the submitted article are kept confidential, and this is a big source of inefficiency and waste. If the article is published, the readers can neither assess how rigorous the process was, nor benefit directly from the reviewers’ insights. If it is rejected, the work has to start all over again at another journal.

Open peer review, defined here as making the reports public, could help journals remedy the penury of reviewers: if applied to rejected articles, by avoiding duplicating effort, and if coupled with naming reviewers, by giving them better incentives to do the work. However, the consequences of open peer review may be more far-reaching. Published reports can indeed be used for evaluating the article’s interest and quality. In aggregate, they could be used for evaluating journals and researchers. For these purposes, they would certainly be better than citation counts.

Wednesday, 27 November 2019

Preprints and other tools of open research: contribution to the Open Access roundtable at GFP 2019

In the context of the conference GFP 2019 on polymer chemistry, I am taking part in a roundtable on Open Access. Chemists are coming quite late to the Open Access debate. The preprint archive Chemrxiv is young, not widely used, and not independent from publishers. The traditional subscription-based publishing system, and the standard bibliometric indicators, dominate communication and evaluation. And when chemists are dragged into the debate by discipline-agnostic initiatives such as Plan S, their positions tend to be conservative.

Inevitably, chemists are being affected by Open Access and other evolutions of the research system, whether or not these evolutions seem beneficial to them. It would be useful for chemists to know more about preprints and other tools of scientific communication, beyond the traditional journals: not only to comply with Open Access mandates, but also to make their own choices among the existing innovations and best practices.

Tuesday, 1 October 2019

Learning scientific writing from great writers

For scientists, writing well (or well enough) is a critical skill, as written texts are essential for communicating research. Of course, not every scientist should be able to write well, as some may rely on collaborators. In a lecture on "Writing physics", David Mermin emphasizes the importance of language and writing through a famous example:
It is also said that even Landau's profound technical papers were actually written by Lifshitz. Many physicists look down on Lifshitz: Landau did the physics, Lifshitz wrote it up. I don't believe that for a minute. If Evgenii Lifshitz really wrote the amazing papers of Landau, he was doing physics of the highest order. Landau was not so much a coauthor, as a natural phenomenon — an important component of the remarkable coherence of the physical world that Lifshitz wrote about so powerfully in the papers of Landau.

Friday, 31 May 2019

Uniqueness of the $2d$ critical Ising model

This post is motivated by a request from JHEP to review a recent article by Anton de la Fuente. I am grateful to the author for stimulating correspondence.

 

The conformal bootstrap: analytic vs numerical

 

The critical Ising model is described by a unitary conformal field theory. In two dimensions, that theory is part of a family called minimal models, which can be exactly solved in the analytic bootstrap framework of Belavin, Polyakov and Zamolodchikov. Minimal models are parametrized by two coprime integers 2 ≤ p < q, they are unitary when q = p + 1, and the Ising model is the case (p, q)=(3, 4).

These 2d bootstrap results date back to the 1980s. More recently, the bootstrap method has been successfully used in higher dimensional CFTs, such as the 3d Ising model. While the basic ideas are the same, there are important technical differences between 2d and higher d.

Monday, 27 May 2019

Academics and Wikipedia: the WikiJournal experiment

Since November 2017, I have been an editor of the WikiJournal of Science, a Wikipedia-integrated, broad scope, libre open access journal. For me this is one way of encouraging academics to write in Wikipedia, by making it possible to publish Wikipedia articles in a recognized academic journal. The WikiJournals as they now exist may not yet be ideal for that, but they are already providing valuable insights into the difference between Wikipedia standards and academic standards, academics' attitudes towards Wikipedia, etc.

I am discussing these insights in a Wikipedia essay, for which this blog post is an announcement. This leads me to suggest that WikiJournals be radically reformed - or that any organization with similar aims should follow a different approach. The essay can be discussed at its Talk page.

Thursday, 9 May 2019

One software to rule them all? Open source alternatives to Mathematica

This post is based on a joint talk with Riccardo Guida given at IPhT Saclay on May 7th.

Wolfram’s Mathematica has been the dominant computer algebra system for decades (at least in theoretical physics), and in an advertisement it even compared itself to Sauron’s One Ring.

Mathematica’s dominance however does not come from black magic, but rather from its quality and power compared to other available computer algebra systems. But dominant positions are often abused, and Wolfram’s commercial practices can verge on the abusive, though much less systematically than say Elsevier’s. In this blog post we will denounce the problems with Mathematica, and discuss four open source alternatives: SymPy, SageMath, Maxima, and FriCAS. In the case of SymPy, we will also provide a demonstration notebook.

Wednesday, 6 February 2019

Solving two-dimensional conformal field theories

This is the text of my habilitation defense, which took place on December 21st 2018. The members of the jury were Denis Bernard, Matthias Gaberdiel, Jesper Jacobsen, Vyacheslav Rychkov, Véronique Terras, Gérard Watts and Jean-Bernard Zuber.

In this habilitation defense, I gave a subjective overview of some recent progress in solving two-dimensional conformal field theories. I discussed what solving means and which techniques can be used. I insisted that there is much to discover about Virasoro-based CFTs, i.e. CFTs that have no symmetries beyond conformal symmetry. I claimed that we should start with CFTs that exist for generic central charges, because they are simpler than CFTs at rational central charges, and can nevertheless include them as special cases or limits. Finally, I argued that in addition to writing research articles, we should use various other media, in particular Wikipedia. 

 

Introduction


Two-dimensional CFTs are defined by the presence of a Virasoro symmetry algebra. This symmetry is sometimes enough for solving CFTs, and even classifying the CFTs that obey some extra conditions. For example, we can classify CFTs whose spaces of states decompose into finitely many irreducible representations of the Virasoro algebra: they are called minimal models. In some cases, Virasoro symmetry is not enough, but the CFT can nevertheless be solved thanks to additional symmetries. In particular, we can have symmetry algebras that contain the Virasoro algebra.
Let me discuss a few CFTs that I find particularly interesting. I will classify them according to their symmetry algebras, and characterize these algebras by the spins of the corresponding chiral fields. In this notation, the Virasoro algebra is \((2)\), as its generators are the modes of the energy-momentum tensor, which has spin \(2\). The sum of the spins of the generators gives you a rough idea of the complexity of an algebra.

Wednesday, 30 January 2019

The Im-flip condition in the two-dimensional Potts model

I have been using this blog for publishing the reviewer reports that I write for journals, since the journals typically do not publish the reports. However, the new journal SciPost Physics does publish the reports for accepted articles. I have recently reviewed an article by Gorbenko, Rychkov and Zan for SciPost Physics, and written about the experience: it would seem that I need not blog about that article, since my report is already online.

However, not everything that I have to say on the article made it into the report. I will now write on two calculations that I did: the first one is a test of one of the article’s main predictions in more general cases, the second one is a direct derivation and generalization of a technical result that they obtain in a roundabout way.

Sunday, 13 January 2019

Plan S implementation: in danger of mission creep?

The principles behind plan S have already sparked lots of debate, including an open letter denouncing the plan, based on objections that I found not very convincing. Now that the plan’s promoters have published their draft implementation guidance (and are inviting comments on it), the discussion can become more specific. Given the boldness of the principles, their implementation cannot be painless, and is bound to raise criticisms if not resistance. It is therefore both crucial and difficult to get the implementation right, and to inflict the minimum amount of pain that is necessary for achieving the goals.

This means that the implementation should be tightly focussed on open access and cost reduction, to the exclusion of other possible goals that a reform of the publishing system might have. I will discuss whether the draft implementation guidance is indeed focussed enough. But first, let me be more specific about what a minimal implementation might look like.