Minimal string theories and their limits

This text is an introduction to recent work by Collier, Eberhardt, Mühlmann and Rodriguez:

• arXiv:2309.10846 – The Virasoro Minimal String

• arXiv:2409.18759 – The complex Liouville string: the worldsheet

• arXiv:2410.07345 – The complex Liouville string: the matrix integral

I am grateful to the authors for helpful discussions and correspondence, and to SciPost for invitations to review two of these articles. (As always with SciPost, the reviews are online.) I am also grateful to the string theory group at IPhT Saclay for inviting me to discuss this subject in their journal club.

 

Minimal string theories

In the worldsheet approach, a string theory may be constructed from a two-dimensional conformal field theory with the central charge c = 26, where we consider primary fields of conformal dimensions $\Delta=\bar{\Delta}=1$. These conditions on the central charge and conformal dimensions, called respectively criticality and marginality, are necessary for the string theory to be independent from the parametrization of the worldsheet. Alternatively, if we take the physical spacetime to be the worldsheet itself rather than the target space, we obtain a model of two-dimensional quantum gravity. In this interpretation, criticality allows gravity to remain topological at the quantum level.

Which conformal field theories give rise to string theories that are simple enough to be tractable, but complicated enough to be interesting? A simple recipe is to take a product of two theories: a theory called the matter CFT, which can in principle be arbitrary, and Liouville theory, which allows arbitrary complex values of c and Δ, allowing us to fulfill criticality and to build a marginal field from any diagonal field of the matter CFT. The resulting string theory is tractable provided the matter CFT is, given that Liouville theory is exactly solved.

A portrait of the blogger as a peer-reviewer

In a publish-or-perish system of scientific communication, large numbers of articles are written and submitted to journals. Editors face the challenge of finding peer reviewers for all these articles. Scientists who accept invitations to review, usually for free, play a vital role in the system. This gives them a lot of freedom to choose which invitations to accept, and to perform peer review as they wish. 

This blog post states my own current “code of conduct” as a peer reviewer: my answers to questions that arise whenever a researcher receives an invitation to review, or has an article to referee.

Should I review this article?

1. Only review for journals I would publish in. This means the publisher should not be Elsevier, the journal should be open access, should not have obnoxious formatting constraints, should not cost too much to the community, and should publish reviewer reports.

2. See the article’s full text before deciding. And the article should already be available on arXiv as a preprint.

The state and prospects of academic peer review

(A PDF version of this post is available on HAL.)

Peer review plays a crucial role in the communication and management of research. How well is it working? How could it be improved? The answers depend on whom you ask. Here they are debated by seven characters with different points of view.

1. Osman: Now that everyone is here, let us start this debate on academic peer review: should it be strengthened, overhauled, or abandoned? When I arrived, Clementine was already in the room, reviewing a paper and complaining about it. This leads to my first question: is peer review taking too much time and effort? 

2. Clementine: I was indeed complaining, but like most colleagues I realize that peer review is an essential part of our work: this is how we validate each other’s work, this is how we assess its interest and likely impact, and this is how we collectively improve how papers are written. Now, the problem is that so many papers get submitted nowadays, it becomes hard to do a proper job on all of them. Even after declining many invitations to review, I have to be pragmatic and focus on the main question: is the submitted paper worth publishing in that journal? If the paper is obviously good work by well-known people, it can be waved through with minimal scrutiny. If it is mediocre work, the rejection should be short and final – I do not want to get dragged into a technical debate with the authors. If however I am unsure whether the paper should be accepted after a quick first reading, then I need to scrutinize it, and to find a good reason for either rejecting it, or accepting it after some improvements. 

3. Deirdre: So you spend more time reviewing second-rate works than really interesting papers? I am glad I stopped reviewing years ago. 

An environmental boycott of Elsevier

Since 2012, thousands of academics have been boycotting the academic publisher Elsevier, whom they blame for overpricing its journals, and more generally for resisting open access to the scientific literature. Of course, most major academic publishers are guilty of the same, but Elsevier stands out as the worst offender. For instance, Elsevier was the last major publisher to join the Initiative for Open citations, years after all the others. Elsevier did not join the Initiative for open abstracts, and they play a leading role in the legal persecution of Sci-Hub.

In February 2022, Elsevier was denounced for helping the fossil fuel industry via its publishing and consulting activities. Again, other publishers are doing that too, and Elsevier is only the worst (or more prominent) offender. This has led to a (no longer active) petition by the Union of Concerned Scientists. Elsevier was not impressed, and now there is a campaign called Stop Elsevier.

This campaign includes a boycott. Participants may commit to

• Refuse to Review

• Refuse to Submit

• Write to Editors

• Refuse to Edit

• Take Direct Action

• Share Boycott on Social Media

There are also various options for allowing these commitments to be publicly shared, as in the original Cost of Knowledge boycott. For the moment, they are still kept private, though.

The new boycott won’t sink Elsevier by itself, but it could strengthen its well-deserved reputation for unprincipled greed. This would help discourage academic institutions from doing business with Elsevier.

Hypocrites in the air, and Barnabas Calder

Since travel by plane is one of the main sources of carbon emissions by researchers, climate scientists who take the plane have been called hypocrites in the air. The expression could be applied to many other researchers, who worry about climate change (without necessarily working on the subject), but fly much more than is really needed.

But what is “really needed” plane travel for a researcher? Surely, we could eliminate much plane travel without compromising our work, by renouncing useless or marginally useful meetings, conferences or visits. Videoconferencing can be helpful: it is often profitable to save the time and expense of travelling, by accepting some loss in communication quality. However, if we take the climate seriously, this cannot be enough. Emissions need not just be halved, but be brought close to zero, and quickly.

Taking the climate seriously implies reducing emissions even when this is detrimental to research – or other activities. Some researchers are already doing it. Barnabas Calder, author of Architecture: from prehistory to climate emergency, explains in his introduction that he did not visit many of the buildings he discusses:

Parts of the book might have been better with first-hand experience of the buildings, but in a world that can ill afford the carbon burden of jet-fuelled travel, the move towards sustainable energy consumption will involve bigger and tougher compromises than this.

Unlike most research subjects, Calder’s work is relevant to addressing climate change, and yet he accepts the handicap of travel restrictions. How does one call the opposite of a hypocrite in the air? A honest scientist on the ground?

No more rejections: eLife reinvents the academic journal

Academic journals as gatekeepers

Nowadays, the role of academic journals is not so much disseminating information, as choosing which submitted articles to accept or to reject for publication. Gatekeeping was surely important when journals were actually printed, in order to save paper and shelf space. Nowadays, it serves as a token of quality, which can make or break the careers of the articles’ authors.

The journal’s publishing decisions are based on the peer review process, which involves the work of reviewers and editors. The process can be fair — or biased, it can be rigorous — or cursory, it can be constructive — or confrontational. How would we know, when the result is only one bit of publicly available information? Not even one bit actually, since when an article is rejected, we do not even know that it was submitted.

In good cases, the process can in fact yield more useful information: because it results in the article being improved, and/or because the reviewers’ reports are made public. However, this generally does not occur when the article is rejected, even in best-practices-in-their-field journals such as SciPost.

eLife’s new model

To avoid wasting the reviewers’ work, and to make it impossible to judge articles by the journals that publishes them, the obvious solution is to eliminate accept/reject decisions. This is the basis of eLife’s new model, announced here. As with most radical progress, the challenge is to succeed in spite of the system’s corruption and its perverse incentives. In eLife’s case, this requires that the resulting articles count as publications for whoever manages the authors’ careers. To this end, eLife is assigning DOIs, and the authors can designate an official Version of Record.

Will eLife will thrive or perish? Will its new model be emulated? Will much-needed systemic change follow? Hard to tell. What I can do for the moment is to discuss the model’s logic in more detail, and point its strengths and weaknesses.

The editor is back

A striking feature of the new model is the crucial role of editors. In old times, editors used to run their journals without help from reviewers — Einstein was outraged when presented with a report from an anonymous reviewer in 1936, for the first time in his career. Nowadays, editors delegate much work and responsibility to reviewers, who are often asked to provide explicit recommendations on whether to accept or reject submitted articles.

In eLife’s new model, reviewers still write reports, but they can focus on giving their scientific opinion without worrying about journal’s publication criteria. Editors now make important decisions:

• Performing an initial screening to decide whether the submitted article deserves consideration, or should be desk-rejected. Given the large numbers of articles that are written nowadays, some sort of screening is unavoidable.

• After peer review has been done, rating the article for significance and strength of support.

The purpose of the rating system is to help readers quickly decide whether the article is worthy of trust and attention. Crucially, ratings are not optional and delegated to reviewers as in SciPost: they are done systematically, and involve the editors. This will hopefully allow the ratings to be done consistently and meaningfully, by people who are well-placed to compare many articles. To evaluate the quality and interest of an article, explicit ratings are potentially much better than a crude accept/reject decision.

This system requires a lot of work and commitment from editors: this could well be a weakness. However, not rejecting articles and doing all the review work in public can certainly save a lot of the editors’ and reviewers’ time, if not at eLife in particular, at least at a systemic level.

More details on the rating system

The rating system or “eLife assessment” system is described here:

• 5 ratings for significance of findings, from useful to landmark.

• 6 ratings for strength of support, from inadequate to exceptional.

These ratings are supposed to be embedded in a short text that summarizes the editors and reviewers’ opinion of the article. Having standard ratings promises to allow easy comparison between articles, and should be very valuable to readers. At the same time, it makes it possible to do statistics and build metrics, with all their potential for misuse. These metrics would however be directly based on an explicit evaluation of the article, and could therefore be more meaningful than citation numbers, let alone journal impact factors.

This rating system is not far from what I have been proposing in general and in particular. Possible improvements to the eLife system could include letting authors suggest ratings for their own articles, and adding a rating for clarity i.e. quality of writing and presentation.

Reactions

• Thoughtful blog post by Sophien Kamoun.

• The usual criticism that researchers have to play by the rules of the system, and would commit suicide by participating in eLife’s new model. By this logic, eLife should have been content to perform good quality open peer review: attempts at further improvement are doomed.

• HHMI’s enthusiastic support. After Plan S, another case of funders pushing researchers towards good practices.

Suprisingly good CNRS guidelines on open access

When it comes to good practices, research institutions are often good at declarations of principles, and not so good at implementation. For example, it is easy to declare that research assessment should be qualitative and not rely too much on bibliometrics, but harder to do it in practice.

This is why I am pleasantly surprised by recent CNRS guidelines on open access. These guidelines take the form of a letter to CNRS researchers who have to fill yearly reports on their work. 

 

The salient points about open access are:

• Over the last three years, the proportion of articles by CNRS researchers that are openly accessible rose from 49% to 77%. (OK, these data mean little in the absence of more details.)

• CNRS is adopting a rights retention strategy, as proposed by Coalition S: it recommends that all articles be distributed under a CC-BY license. In particular, this allows the articles to be made openly accessible right at the time of publication.

• CNRS is not asking researchers for their lists of publications: instead, CNRS just takes publication lists from the national preprint archive HAL.

• The weak point of all this seems to be the impractical and clunky nature of HAL. However, as reminded in the letter, HAL is increasingly interoperable with disciplinary archives such as ArXiv and BiorXiv. And indeed, my recent articles were automatically harvested by HAL from ArXiv.

This goes in the direction of having a strong open access mandate while requiring no extra work from researchers. To get there, it remains to make the CC-BY license mandatory, and the upload of articles to HAL fully automatic.

The war in Ukraine: directives to French researchers from CEA and CNRS

My previous post reproduced a letter from Maxim Chernodub, suggesting how French scientists could help Ukrainian colleagues, and also calling French scientists to not boycott Russian collaborators. However, it seems that we will not have much choice in the matter, at least if we follow the official directives, which I will paraphrase below.

Directives from CEA (received by email)

Russian scientists may remotely attend online conferences, but only as individual experts, i.e. provided they do not represent an institution. 

Already submitted publications are OK. But it is forbidden to submit a new publication with a coauthor who is affiliated in Russia. 

Press release from CNRS

CNRS is suspending all new scientific collaboration with Russia. Russian researchers who work in France may continue their activities. 

Interpretation

From CNRS we only have a press release so far. The precise directives, when they come, may well look like CEA's. It is unclear how these directives will be enforced. Most scientific journals still allow submissions from Russia-based authors. 

As far as I can tell, such drastic measures against Russian scientists are unprecedented. No similar measures were taken in other cases of human right abuses, including wars of aggression. 

For researchers who want to keep collaborating with Russians without violating the official directives, technical loopholes might help: the directive from CEA is about submitting publications, but preprints are not publications, right? Co-authors may not be affiliated in Russia, but could they publish as private individuals, without citing an affiliation?

The war in Ukraine: a letter from Maxim Chernodub

Since the beginning of the Russian invasion of Ukraine, Western researchers have been wondering how to help our Ukrainian colleagues, and how to behave with our Russian colleagues. A letter by French-Russian-Ukrainian physicist Maxim Chernodub has been circulating, which offers valuable perspective and advice on these issues. (It was written on 27/02/2022.) Below is the text of the letter, reproduced with the author's permission. 

Reforming research assessment: nice declarations, little action?

There seems to be a consensus among universities and research funders that research assessment should not be based on crude quantitative metrics, such as: numbers of articles, numbers of citations, journal impact factors, the h-index, etc. The 2012 San Francisco Declaration on Research Assessment (DORA) formulates principles which could greatly improve research assessment if they were applied, although I would argue that the DORA is misguided in its recommendations to authors. The DORA has been signed by thousands of organizations: just for France this includes the Academy of Sciences, CNRS and HCERES. More recently, the European Commission has issued a report called Towards a reform of the research assessment system, which deals with the same issues and promotes similar principles.

Since the same principles have to be reiterated 9 years later, you may think that little has changed in all that time. And you would be largely right. Significant reforms of research assessment in individual organizations are so rare that they are newsworthy. And some universities are denounced for taking actions that directly contradict the principles they have officially endorsed.

Open access by decree: a success of Plan S?

“Science family of journals announces change to open-access policy”: the title of this Nature news article may sound boring, but the news are not:

Science is allowing authors to distribute their articles under Creative Commons license, free of charge, without embargo.

Sounds too good to be true? Well, of course, there are caveats. First, this is the author’s accepted version, not the published version. (OK, minor point.) Second, this is a trial policy, which may or may not be made permanent. And third, this only applies to authors who are mandated to do so by their Coalition S funders.

Who, you may ask, are these happy authors? I will defer to Wikipedia for reminders about Plan S and Coalition S, and move to the next question: how did Coalition S achieve this? Last summer, Coalition S announced a Rights Retention Strategy that mandates fundees to distribute their work under CC license, without embargo, no matter what the publisher makes them sign. In their own words,

This public licence and/or agreed prior obligations take legal precedence over any later Licence to Publish or Copyright Transfer Agreement that the publisher may ask the author to sign.

This type of “open access by decree” may sound like little more than an exercise in blame-shifting. To an author who is caught between a funder and a journal’s incompatible demands, it should not matter who blacklists the other: the author cannot publish in that journal. However, Plan S got bad press among some researchers for appearing to blacklist journals. Now it is the journals who will have to blacklist funders, and to reject submitted articles for technicalities.

The success of the maneuver depends on publishers’ reactions. Coalition S is too big to ignore, so publishers have to take a stand. Nature journals have recently announced their adoption of Gold open access options, for a charge of the order of 9.500 euros per article. This was denounced as outrageously expensive. Björn Brembs even took it as a refutation of the widespread idea that the author-pays model would lead to lower costs.

However, Coalition S-funded authors can now choose between publishing in Science for free, or in Nature for 9.500 euros. It does seem that Science is competing on price! Not so fast: Nature’s high price is partly an illusion, as it comes in the context of transformative agreements, which are supposed to transition Springer Nature’s journals to open access. The idea is that academic institutions’ journal subscriptions would also cover open access publishing of their researcher’s articles. For an author who is covered by a transformative agreement, publishing open access in Nature is effectively free, which may be why Science had to offer the same for free, just to stay competitive.

At this stage, it is not clear which authors will face which choices of open access options and pricing. It is therefore a bit early for seeing the effects of the Rights Retention Strategy. At least, we now have the admission by an important publisher that open access is not an extra service that should bring them extra revenue.

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 z_j ∈ ℂ are the fields’ positions, Δ_j ∈ 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.

(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.

(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.
  

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.

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.

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.

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.

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.

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.