18 December 2016

Business as usual at the White House

After the generalized commotion surrounding the US elections died down, the feeling of surprise lingered in the press, combined with predictions of imminent disaster. Against this alarmist tendency, I'm putting forward three obvious points:
  1. Although Trump's victory was unexpected (i.e. went against poll predictions), it follows a long-term pattern of Republican and Democratic presidents alternating every eight years. This has been the case since Eisenhower, with the exception of Reagan's first mandate.
  2. Speaking of Reagan, there are some striking similarities with Trump: both are (were) charismatic showmen, but not very intellectual and with a penchant for made-up stories. Time will tell how deep this resemblance goes.
  3. In contrast with his populist campaign speeches, Trump's post-election declarations and his cabinet choices signal that he will probably follow very closely the Republican platform: pro-big business, anti-abortion, pro-Israel, anti-environment control, for increased military spending, tax cuts for the rich, free trade and reductions in welfare programs. How many of these points were also on Clinton's agenda is left as an exercise for the reader.
Finally, what alarms me is not how far Trump is from mainstream Republicans, but rather how close the Republican party is to Trump.

1 December 2016

CNRS positions - the 2017 campaign

The detail of the 2017 campaign for permanent research positions at the CNRS (Centre national de la recherche scientifique) has been published in the Journal Officiel (see links below) and the submission site is open. The submission deadline is January 6th 2017. There are 211 open positions at the CR2 level (4 less than in 2016), 75 CR1 (2 less), 256 DR2 (+3) and 2 DR1 (+2). The total number has been stable over the last five years, as shown in the graph below:

The official texts: CR2, CR1, DR2, DR1.

20 November 2016

The Ewald sphere

The Ewald sphere is a widely used concept, but one that is quite difficult to grasp in the beginning (at least it was for me, as well as for some of my colleagues.) It can be seen as a way of converting vectors between the "real" space, in which the experiment is performed, and "reciprocal" space.

18 November 2016

Solution Self-Assembly of Plasmonic Janus Nanoparticles

Our paper appeared in Soft Matter.

Congratulations to Nicolò Castro for his first paper as first author!

21 October 2016

10 October 2016

Curvature of a planar curve

I have done this calculation several times over the years, so I might as well write it down in detail, in case it may be of use to someone else.

We are interested in the curvature \(C = 1/R\) of a planar curve \(y=f(x)\) at a given point A, where \(R\) is the curvature radius at that particular point, defined with respect to the curvature center \(O\) (intersection of the normals raised to the curve in A and its infinitesimal neighbor B.)

The angle subtending AB is: \(\displaystyle \mathrm{d}\alpha = \mathrm{d}s/R \Rightarrow C = \frac{\mathrm{d}\alpha}{\mathrm{d}s}\)
The length of the curve element AB is: \(\displaystyle \mathrm{d}s = \sqrt{\mathrm{d}x^2 + \mathrm{d}y^2} \Rightarrow \frac{\mathrm{d}s}{\mathrm{d}x } = \sqrt{1+ f'(x)^2}\)

The derivative of \(f\) is directly related to the angle \(\alpha\): \(\displaystyle f'(x) = \frac{\mathrm{d}y}{\mathrm{d}x} = \tan \alpha \Rightarrow \alpha = \arctan \frac{\mathrm{d}y}{\mathrm{d}x} = \arctan [f'(x)] \Rightarrow \frac{\mathrm{d}\alpha}{\mathrm{d}x} = \frac{1}{1+f'(x)^2} f''(x)\)

Putting together the three relations above yields:
\[C = \frac{\mathrm{d}\alpha}{\mathrm{d}s} = \frac{f''(x)}{\left [ 1 + f'(x)^2\right ]^{3/2}}\]

12 August 2016

Identification of a major intermediate along the self-assembly pathway of an icosahedral viral capsid

Our paper appeared in Soft Matter!
The modelling and fitting of the SAXS data required lengthy analysis and intensive calculations. In particular, we used the analytical model for scattering from spherical patches that I had published last year.

2 August 2016


I learned today that the title of the fourth book in Proust's Search of Lost Time (Sodome et Gomorrhe) was translated in English as Cities of the Plain, same as Cormac McCarthy's novel, and that the expression comes from King James Version of the Bible. Looks like most novels in the English language borrow their title from the KJV (Yeats's poems are a close second, though).

I also found out that the title to Wittgenstein's Tractatus Logico-Philosophicus was inspired by Spinoza's Tractatus Theologico-Politicus. Also, 60 years before Spinoza Robert Fludd wrote his Tractatus Theologo-Philosophicus, which is closer in title (although probably not in content).

2 July 2016


Higgs ! En voilà, un nom à particule !

28 June 2016

Climate and Geography

Is aggression correlated to the climate? Researchers from the Vrije Universiteit Amsterdam and Ohio State University seem to think so (read the press release and the preprint of the paper).

They put forward a supposedly new model titled CLimate, Aggression, and Self control in Humans (CLASH). I can't comment on the merits and novelty of this approach, but it reminds me of Montesquieu's remarks in The Spirit of the Laws (book XIV, chapter II):

In northern countries we meet with a people who have few vices, many virtues, a great share of frankness and sincerity. If we draw near the south we fancy ourselves removed from all morality; the strongest passions multiply all manner of crimes, every one endeavouring to take what advantage he can over his neighbour, in order to encourage those passions.

27 June 2016

Alternative positions in the two-party system

I've always been intrigued by the American two-party system and its difference with respect to Continental politics. I think the comparison is very instructive and can shed light on recent political developments. Case in point: in the current issue of The Atlantic, Jonathan Rauch deplores the state of Americal politics, in particular the lack of discipline in both the Democratic and the Republican parties, whose "politicians, activists, and voters" are less and less likely to line up behind the party leaders. Terms and phrases in quotes are Rauch's.

Rauch explains this by the reform of the political system over the last few decades, which rendered the "middlemen" less powerful, and thus unable to defend against such "symptoms" as "insurgence" and "radicalization". The author really likes the allegory of the immune system fighing the "pathogens".

These "symptoms" are also common in Europe, where they gave rise to powerful political movements and parties. This has not happened in the United States, probably for the simple reason that, in a two-party system, it can be easier for new political orientations to work from within the existing parties, rather than create new ones, as in Europe. The religious conservatives and the libertarians have to go through the Republican party and the socialists through the Democratic one. As long as they were in the minority, this was not a problem for the establishment, but when the "outsiders and insurgents" start getting the upper hand, the "body politic" (which for Rauch seems to mean the political status quo) may not recover.

The comparison with Europe also casts doubt on Rauch's diagnostic: the radicals gained prominence at about the same time in the US and all over Europe, although the political systems are quite diverse. A much simpler explanation is that the sluggish economy and the resulting social problems caused the discontent and the anti-establishment feeling on both sides of the Atlantic.

To summarize: as in Europe, in the United States alternative positions became politically relevant in the last 10-15 years, probably for reasons unrelated to political reform; unlike (Continental) Europe, however, they had to go through one of the two major parties due to their stability, itself due to the first-past-the-post electoral system.

26 June 2016

Hierarchy and inequality

In Left and Right: The Significance of a Political Distinction, Norberto Bobbio mentions an interesting variation in defining what is called generically in politics "the right". For Bobbio, this side of the spectrum is defined by a defense of inequality, and he dismisses the use of the term "hierarchy" by Elisabetta Galeotti as unrelated to the left/right polarity. This may well be so (at least in Bobbio's system of definitions) but I think that the hierarchy/inequality distinction is quite useful in separating different right-wing positions.

14 May 2016

Impresii despre Soldaţii de Adrian Schiop

Soldaţii. Poveste din Ferentari de Adrian Schiop. Editura Polirom, 2013


În 250 de pagini, Schiop ne prezintã o relaţie sentimentalã, de la începutul timid pânã la previzibilul sfârşit lamentabil. Mai important însã, el reuşeşte performanţa de a schimba treptat perspectiva, astfel cã la sfârşit nu mai este atât de clar cine pe cine exploateazã, iar implozia relaţiei nu decurge aşa cum am fi bãnuit la început.

Soldaţii este o poveste de dragoste, înainte de a fi o poveste din Ferentari sau una despre homosexuali. Aceste douã elemente, ca şi tensiunea dintre ele, sunt indispensabile acţiunii, dar — dacã e sã judec dupã multe recenzii şi reacţii — cred cã mare parte din succesul cãrţii se va datora lecturii în cheie socialã (ca o meditaţie pe tema homosexualitãţii în România sau pe cea a Ferentarilor ca vestigiu al unei perioade trecute).

Stilul este destul de brut, fãrã fraze şlefuite, cam prea în genul "Flori de mucigai", pe care nu îl apreciez foarte tare, dar care se potriveşte într-adevãr cu subiectul. Mai ales în capitolul No more parties, analiza socialã e desfãşuratã în fraze tãioase şi categorice, combinînd termeni tehnici sau erudiţi şi expresii de argou (à la Houellebecq). Partea aceasta mi s-a pãrut cel mai puţin convingãtoare.

Pe scurt, o carte care meritã din plin cititã, în ciuda unei realizãri destul de inegale (narativ şi stilistic).

16 March 2016

Chartres cathedral (again)

This was my second time in Chartres (I have already taken the same photo three years ago), but now I was more interested in the juxtaposition of the various textures (hence the B&W rendition).

10 March 2016

Social reform as arbitrage

Changes in laws and regulations (including for instance tax rates) are presented by their proponents as improvements to society in general, but it is generally obvious (and accepted) that they favor one group to the expense of another: for instance, changes in labor law can be to the advantage of the employer or the employee.

Sometimes, however, these measures are advertised as benefitting all involved, with no negative effects. Instead of altering the balance between the various players, the new laws are expected to eliminate inefficiencies in the social and economical system, as the practice of arbitrage levels price differences between markets.

This is counterintuitive, and so are the arguments used by the advocates of change, who forecast an outcome exactly opposite to the one expected in normal settings, because the situation is supposedly very unbalanced, even untenable (hence the urgency of the changes.) The problem is that the system to be changed has often existed for decades: the claims that some extreme imbalance has persisted (or has gradually evolved) and that something needs to be done right now (before the next election cycle, preferably) are difficult to take seriously.

A recent example are the proposed ammendments to French labor law: they are supposed to reduce unemployment by relaxing conditions on layoffs and overtime. I guess this could only work if the labor market were completely unbalanced in France.

This is similar to Reagan's proposal of increasing tax revenue by reducing tax rates, which would only have been possible in very specific circumstances (on the decreasing side of the Laffer curve.)

13 February 2016

Water is HO2 (for at least one philosopher)

This evening I've been listening to some podcasts of talks given at the Nietzsche on Mind and Nature conference (Oxford, 2009).

There are some interesting points to be made, so I'll probably write a couple more posts on this, but what I found striking is Günter Abel's affirmation (about 08:55 into his talk; see the video) "[...] saying for example (famous example) 'water is HO2'..." Now, everyone can misspeak, but he did not correct himself and there were no reactions from the audience (then again, this was a philosophy conference held in Oxford, so maybe all attendants were being exceptionnally polite). Is this the general level of scientific education in the population of philosophy professors?!

The topic of Abel's talk is not directly related to science, although he does address the tension between consciousness and neurobiology (and how one should not identify conscious states and physical processes etc.)

11 February 2016

24 January 2016

Sample median for a Lorentz (Cauchy) distribution - part 2

In a previous post I derived the distribution function for the median of a sample of size \(n = 2k +1\) (with \(k \geq 0\) integer) drawn from a Lorentz (or Cauchy) distribution:
g(x) = \frac{n!}{(k!)^2 \, \pi ^{n} \, \gamma} \left [ \frac{\pi ^2}{4} - \arctan ^2 \left (\frac{x - x_0}{\gamma} \right )\right ]^k \frac{1}{1+ \left (\frac{x - x_0}{\gamma} \right )^2 }
I will now consider some of its properties.

23 January 2016

Sample median for a Lorentz (Cauchy) distribution

The Lorentz (or Cauchy) distribution
f(x) = \frac{1}{\pi \gamma}\frac{1}{1+ \left (\frac{x - x_0}{\gamma} \right )^2 }
\end{equation}is pathological, in that it has no finite moments apart from the zero-order one (which ensures proper normalization of the density function.) Many results we usually take for granted (e.g. the central limit theorem) do not apply and, when sampled, the sample mean and sample variance are not good predictors for the position parameters \(x_0\) and \(\gamma\). This should not come as a surprise, since the distribution mean and variance do not even exist. How can we then estimate the position parameters?

19 January 2016

Scattering from a bunch of parallel wires

I got interested in this problem by trying to understand a result in [1], and also because it may be useful for some stuff I'm currently working on. Consider a collection of \(N\) very (infinitely) long objects, parallel to the \(z\)-axis and whose centers have positions \(\mathbf{R}_{j}\) in the \((x,y)\) plane. We are interested in the orientationally averaged scattering signal.