Tuesday, 20 March 2018

The open secrets of life with arXiv

If you only think of arXiv as a tool for making articles openly accessible, consider this: in 2007, a study showed that papers appearing near the top of the daily listing of new papers on arXiv, will eventually be more cited than papers further down the list – about two times more cited. And there is a daily scramble for submitting papers as soon as possible after the 14:00 EDT deadline, in order to appear as high as possible on the listing. The effect is not as perverse as it seems, as there is no strong causal relation between appearing near the top and getting more citations. (More likely, better papers are higher in the listing because their authors want to advertise them.)

The consequences of arXiv’s systematic use in some communities are actually so deep that a speciation event has occurred among researchers, and a new species of arXivers has appeared. Here I will try to explain how arXivers live, in order to help non-arXivers understand arXivers, and have an idea of what could happen to them if the currently proliferating clones of arXiv gained widespread use.

Thursday, 1 March 2018

Uniqueness of Liouville theory

The original definition of Liouville theory by Polyakov in the 1980s was written in terms of a Lagrangian, motivated by applications to two-dimensional quantum gravity. In the 1990s however, Liouville theory was reformulated and solved in the conformal bootstrap approach. In this approach, the theory is characterized by a number of assumptions, starting with conformal symmetry. In order to actually define the theory, the assumptions have to be restrictive enough for singling out a unique consistent theory.

After assuming conformal symmetry, it is natural to make assumptions on the theory’s spectrum, i.e. its space of states. For any complex value of the central charge \(c\), the spectrum of Liouville theory is
\[\mathcal{S} = \int_{\frac{c-1}{24}}^{\frac{c-1}{24}+\infty} d\Delta\ \mathcal{V}_\Delta \otimes \bar{\mathcal{V}}_\Delta\ ,\]

Sunday, 25 February 2018

Couperin vs Springer and Elsevier: towards less extortionate deals?

Historically, the French consortium Couperin has obtained poor results in negotiating with predatory publishers, mostly consenting to their high and increasing prices. This is not necessarily Couperin’s fault, although it does not help that Couperin’s leadership appears weak and ill-informed. Rather, this is a consequence of the basic economics of scientific publishing, with publishers systematically abusing their strong position. Even the Finnish consortium FinELib, which was determined to seek a good deal and enjoyed a fair amount of support from researchers, recently consented to one more extortionate deal with Elsevier.

However, recent developments suggest that Couperin could fare better in current and upcoming negotiations:

Saturday, 27 January 2018

Germany won't pay for Nature's "scientific porn", and other messages from Couperin's open science days

Earlier this week, there was a mini-workshop in Paris called Couperin’s open science days 2018. (Original title in French: Journées sciences ouverte 2018.) I followed most of it via webcast, and I will now summarize some of the salient points. The videos are available online, but most of them are in French.

The German way: Horst Hippler and Ralf Schimmer


The most important messages came from Germany: the country whose academic institutions have thought seriously about scientific publishing, and have organized themselves so as to drive the needed reforms. The most salient manifestation so far has been the standoff with Elsevier, and it was nice to have further details on the strategy.

Sunday, 21 January 2018

Will no one rid me of these tiresome Latin plurals?

The English language has inherited many scientifc words from Latin: a spectrum, an index, a torus, a formula. Then which plural forms should we use: the Latin plurals two spectra, two indices, two tori, two formulae? Or the English plurals two spectrums, two indexes, two toruses, two formulas? The Latin and the English plurals of these words are both considered correct, but the Latin plurals are more widespread. I will nevertheless argue that using Latin plurals is impractical and illogical, and should often be avoided.

Thursday, 11 January 2018

On single-valued solutions of differential equations

This post is about the issue of solving a nonlinear matrix equation that I raised on MathOverflow. This matrix equation determines the existence of single-valued solutions of certain meromorphic differential equations. The motivating examples are the BPZ differential equations that appear in two-dimensional CFT. For more details on these examples, see my recent article with Santiago Migliaccio on the analytic bootstrap equations of non-diagonal two-dimensional CFT.

Thursday, 7 December 2017

After Elsevier, should we boycott Springer?

While the ongoing “Cost of knowledge” boycott of Elsevier may not be very effective, the likely “no deal” hard exit of Germany from Elsevier subscriptions renews the boycott’s relevance, and maybe its urgency. It is indeed likely that most German universities and research institutions will lose access to Elsevier articles in 2018.

As a researcher, why would I continue publishing in journals that are in principle inaccessible to most of my German colleagues? Universal access to the literature via Sci-Hub is under increasing legal assault and should not be taken for granted. In these circumstances, boycotting Elsevier is no longer only a matter of fighting an obnoxious publisher, but also a basic necessity of ensuring that articles are accessible to their intended audience. (Unless one thinks that the intended audience is not the scientific community, but the paying Elsevier subscribers.)

Now it turns out that if I boycott Elsevier because of Germany, I may have to boycott Springer because of France.

Monday, 23 October 2017

With weight-shifting operators, \(d\neq 2\) looks increasingly like \(d=2\) in CFT

When working on conformal field theory, your life is very different depending on whether the dimension is two or not. In \(d=2\) you have that infinite-dimensional symmetry algebra called the Virasoro algebra, and in some important cases such as minimal models you can classify your CFTs, and solve them analytically. In \(d\neq 2\), your symmetry algebra is finite-dimensional, and you mostly have to do with numerical results. This not only makes you code a lot, but also incites you to make technical assumptions that are physically restrictve, such as unitarity.

Degenerate fields in \(d=2\) CFT

What makes \(d=2\) CFT solvable in many cases is the existence of degenerate primary fields.

Saturday, 9 September 2017

Self-publishing a book with Glasstree

Three years and three major revisions after it first appeared on Arxiv and GitHub (why GitHub? see this blog post), my review article on two-dimensional conformal field theory may be mature enough for appearing in book form. But with which publisher?
To answer this question, I should first say why I would want to have a book in the first place, since the text is already on Arxiv.

Friday, 8 September 2017

Differential equations from fusion rules in 2d CFT

In two-dimensional conformal field theory, correlation functions are partly (and sometimes completely) determined by the properties of the fields under symmetry transformations. In particular, correlation functions of primary fields are relatively simple, because by definition primary fields are killed by the annihilation modes of the symmetry algebra. On top of that, there exist degenerate primary fields that are killed not only by the annihilation modes, but also by some combinations of creation modes. As a result, correlation functions that involve degenerate primary fields sometimes obey nontrivial differential equations, for example BPZ equations. Usually, these equations are deduced from the relevant combinations of creation modes, called null vectors.
Determining null vectors in representations of a symmetry algebra is often complicated, as the algebraic structures of the relevant algebras and of their representations can themselves be complicated. Even in the case of the Virasoro algebra, it is not easy to explicitly determine null vectors. It is however much easier to determine which representations do have null vectors, using the fusion product. For example, if we know degenerate representations \(R_{(1,1)}\) and \(R_{(2,1)}\) with null vectors at levels \(1\) and \(2\) respectively, we can deduce that the fusion product \(R_{(2,1)}\times R_{(2,1)}\) is degenerate and contains \(R_{(1,1)}\). The remainder of \(R_{(2,1)}\times R_{(2,1)}\) must therefore be a degenerate representation, which can be identified as \(R_{(3,1)}\), and has a null vector at level \(3\). (See Section 2.3.1 of my review article for more details.)
An important idea is therefore that it is not the structures of the algebras and representations that matter, but rather the structure of the category of representations, in other words their fusion products. This idea has in particular been developed in the works of Fuchs, Runkel and Schweigert. But how does this help us compute correlation functions, and determine the differential equations that they obey? In other words, can we determine differential equations from fusion products, without computing null vectors?

Tuesday, 21 February 2017

Germany vs Elsevier: a puzzling maneuver

In the tense negotiations between the German consortium DEAL and Elsevier, there is a new twist: on February 13th, Elsevier announced that it was restoring the access of the affected German institutions to its journals.

Elsevier’s two explanations for this maneuver fall short of being convincing. The first explanation, given to Nature, is that “it is customary [...] to retain access to content after a contracted period is concluded and as long as renewal discussions are ongoing”. Why then cut off access in January, and restore it in February?

Wednesday, 1 February 2017

Germany vs Elsevier, and the race for legal open access

The debate about green versus gold open access leaves aside a more fundamental difference: that between legal open access and pirate open access. This difference is essential because, as Bjorn Brembs put it,
In terms of making the knowledge of the world available to the people who are the rightful owners, [pirate] Alexandra Elbakyan has single-handedly been more successful than all [legal] open access advocates and activists over the last 20 years combined.
With Sci-Hub, pirate open access is so successful that one might wonder whether legal open access is still needed. The obvious argument that pirate open access is parasitic and therefore unsustainable, because someone has to pay for scientific journals, is easily disposed of: with up-to-date tools, journals could cost orders of magnitude less than they currently do, and be financed by modest institutional subsidies. A better reason why pirate open access is not enough is that it is subject to technical and legal challenges. This makes it potentially precarious, and unsuited to uses such as content mining.