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.

Saturday, 17 November 2018

How strong are the objections to Plan S?

After a coalition of European science funding agencies announced their Plan S initiative for open access, a number of researchers wrote an open letter criticizing the move, under the title “Reaction of Researchers to Plan S: Too Far, Too Risky”. To summarize, they fear that Plan S would increase costs, lower quality, and restrict academic freedom. In order to evaluate how seriously these fears should be taken, let me start with a 5-point analysis of the issues, before discussing the open letter’s specific concerns.

Point 1: Traditional journals are overpriced by an order of magnitude.

Overall, publishers earn about 3800-5000 euros per article they publish. The true costs of organizing peer review are much lower: a few hundred euros per article at SciPost or PeerJ. The difference is therefore not mainly due to commercial publishers’ profits, but rather to large inefficiencies at all traditional publishers. (Have a look at the salaries of the American Chemical Society’s executives.)

Some publishers also use the money for subsidizing other activities, such as conferences (learned socities) or science journalism (Nature, Science). While these activities are valuable, they are not the main purpose of scientific publishing, and should not determine its future.

Saturday, 27 October 2018

CNRS rejects Couperin's claimed victory in Springer big deal

After long and tortuous negotiations, the French consortium Couperin has claimed victory in its recent agreement with Springer, after having secured price decreases. This claim seems reasonable, as prices of big deals with publishers tend to increase steadily. Of course, critics can still point out that Springer remains very expensive compared to smaller, more efficient publishers. But at least Springer seems amenable to some compromises in negotiations. And one should not forget that the greediest and most obnoxious publisher remains Elsevier, who even refused to join the Initiative for open citations.

I was therefore surprised when CNRS announced its rejection of the Springer deal, although CNRS takes part in Couperin and was actively involved in the negotiations. The email announcement came from Alain Schuhl, a CNRS official who is also a member of Couperins’ governing council. (See the email and its English translation below.) This email was a warning to CNRS researchers that access to Springer journals was now cut off. However, articles from 2017 and earlier are still available, as they are coverd by the previous subscription.

The explanation for the rejection is that the deal’s price was too high according to the Ministry’s open science plan. This explanation makes little sense for two reasons:

Thursday, 27 September 2018

SciPost two years on

There are two reasons why we should care about the journal SciPost Physics: it practices open peer review, by publishing the exchanges between authors and reviewers, and it is free to authors and readers, at a time when academia is struggling to escape the stranglehold of predatory publishers such as Elsevier.

Two years after it was launched, SciPost Physics is alive on well, with 155 published articles at the time of this writing. The journal has managed to attract articles of high quality, some of them by well-known physicists such as Cardy, Verlinde, Seiberg, or Rychkov. The main challenges are now to attract many more authors, and sustainable funding. The journal is funded by institutional subsidies, an economic model that works for arXiv but that is rare for journals. If the finances worked out, the journal could ultimately hope to become a megajournal, and publish thousands of articles a year like PLOS One or PeerJ. For the moment, the stated aim of the “Expansion and Sustainability Drive” is to publish 500 articles a year at a cost of 200.000 euros. So each published article should cost 400 euros, a very reasonable price for a lean electronic journal: an order of magnitude more than arXiv, an order of magnitude less than Elsevier. (Unlike Elsevier, SciPost does not pay executives, lobbyists, shareholders, salespersons, lawyers, etc.)

In this post I will comment on SciPost’s workflow and platform, updating my earlier assessment in light of two years’ worth of experience. The aims are to help authors decide whether this journal suits them, to determine which features of SciPost should be emulated (or avoided) by other journals, and to provide feedback to SciPost itself.

Wednesday, 4 July 2018

Directive sur le droit de copie: quelques critiques

Le parlement européen doit prochainement voter un projet de directive sur le droit de copie, qui a fait l'objet de nombreuses critiques. J'en ai parlé il y a une dizaine de jours à Cédric Villani, le député (certes pas européen) de ma circonscription. D'après lui, le gouvernement français pousse pour que la directive soit adoptée. Je lui ai écrit ensuite l'email suivant, qui résume certaines critiques: