## 19 December 2015

### What is an order parameter?

For those of us working with liquid crystals, the answer tends to be fairly automatic: "the average of the second Legendre polynomial over the orientation distribution". Only in a second step do we think to qualify the definition: it concerns the quadrupolar order parameter (call it S) in a three-dimensional system. S = 0 for isotropic orientation, S = 1 when the molecules are perfectly oriented along an axis (the director) and S = –1/2 when they are all perpendicular to the director.

Whether it is the appropriate one depends on the problem at hand: if the particles constituting the system do not have inversion symmetry we should probably use the dipolar order parameter. It is less obvious that, although S describes well the tendency of molecules to align along an axis or perpendicular to it, it is not appropriate for any preferred angle: if all molecules make the "magic angle" θm = arccos(1/√3) ≅ 54.7° with the director, S is again zero so it cannot help distinguish between this situation and a purely isotropic distribution. One should then resort to the octupolar order parameter (or possibly a combination of quadrupolar and octupolar terms, for other angles?)

It may be useful to look at order parameters pragmatically, as Landau did for his theory of phase transitions: they are constructed such as to be zero in one phase and finite in another one. It is up to us to identify the phases and to define the most convenient parameter with regard for the particular system but also for the information we want to extract.

## 12 December 2015

### Note to writers

I do not want to read about your personal history. Unless you have led a very interesting life (literary or otherwise) or you have such an uncommon talent that everything you write is captivating, no matter the topic, do not bother. To wit, if you are not Goethe (or Feynman, or Leigh Fermor) or Proust, your recounted experience had better stand for something larger than itself.

## 29 November 2015

### Solution scattering from colloidal curved plates: barrel tiles, scrolls and spherical patches

My paper just got published in the Journal of Applied Crystallography.

## 28 November 2015

### Tattoo printer

The tattoo printer sounds like the punishment device from Kafka's In the Penal Colony.

## 25 November 2015

### The influence of intellectuals: hubris and humility

Intellectuals (and I take the term in its widest acceptance) have often believed that their ideas could change the world. Some of them, from Plato to Heidegger, tried to influence directly those in power; others hoped for a posthumous effect (or at least for cultural immortality). Criticism of this belief is as old as the belief itself and has two aspects: a pragmatic one (attempts to influence the course of history through ideas usually fail miserably) and a moral one (knowledge engenders obligations and must be used responsibly).

Elias Canetti, in his short essay The Poet’s Profession1, looks at this attitude from a very different angle: the responsibility of a writer not to humanity, but to words themselves, as tools of his trade. The true poet both understands the tremendous power of words and accepts his inability to change the world using this power.

1. In Das Gewissen der Worte [The Conscience of Words], Hanser (1975).

## 22 November 2015

### Do nano-objects have color?

I've been reading Jim Pivarski's blog Coffeeshop Physics for some time, and I always find the topics interesting and the perspective refreshing. However, I think that his latest post "Viruses have no color" contains a number of fundamental errors, beyond the imprecisions inherent in a simplified account.

Pivarski's stated point is that objects smaller than the wavelength of light have no color, and he explains this by the uncertainty principle. Instead, he illustrates that small objects scatter less light than large ones, using a "geometrical" point of view that ignores the composition of the objects and sees them simply as opaque to the incoming light. Of course, in this approximation even large objects are colorless, since their scattering properties will not change much over the visible spectrum1.

The relevant parameter when discussing the color of an object is not the wavelength but the frequency of the incoming light. For instance, gold nanoparticles a few tens of nanometers in diameter both absorb and scatter green light more effectively than at other visible frequencies because in this range the electromagnetic field couples very effectively with the oscillation modes (plasmons) of the conduction electrons in the particle. Dispersions of such particles are therefore green when seen in reflection and red in transmission, as illustrated by the Lycurgus cup. Even atoms can be said to "have color" if we think of their characteristic transition lines (for instance, sodium lamps glow yellow).

The uncertainty principle2 only tells us that the image of the nanoparticles cannot be sharper than the wavelength used to look at them, not that this image is colorless (see such colored images here and here).

1. I neglect here the λ4 dependence in Thompson scattering, leading to the "blue-sky effect".
2. I preserve here the author's terminology, although "the uncertainty principle" is generally associated with quantum mechanics. Here the reasoning is completely classical, so we might as well call the result "the Abbe resolution limit".

## 20 November 2015

### Why the aspect ratio? Shape equivalence for the extinction spectra of gold nanoparticles

My paper just got published in The European Physical Journal E !

In it, I argue that when describing elongated gold nanoparticles as ellipsoids (to the purpose of modelling their light extinction spectra) the natural comparison criterion is the equivalence of the various moments of mass distribution, rather than the length-to-diameter (aspect) ratio generally used in the literature. I also show that it leads to better spectral correspondence between the various shapes.

## 18 November 2015

### CNRS positions - the 2016 campaign

The detail of the 2016 campaign for permanent research positions at the CNRS (Centre national de la recherche scientifique) has been published in the Journal Officiel, but the submission site is not yet open. The submission deadline is January 6th 2016. There are 215 open positions at the CR2 level (3 more than in 2015), 77 CR1 (no change), 253 DR2 (no change) and no DR1 (-2).

See last year's post for more details.

## 3 November 2015

### The myth of innovation

What is it about the Wall Street Journal and science? It looks like any argument against state action (concerning e.g. climate change) is good, no logic required. Last week, it was the turn of state support for basic science. Matt Ridley's essay goes like this:
1. Technology progresses inexorably.
2. Basic science contributes nothing to new inventions.
3. Discoveries and inventions are made independently by many people.
4. Technology is a living organism that may soon "build and maintain itself". Innovation occurs inevitably, and people are only "pawns in the process".
5. Prohibiting technological development does not work, and what cannot be stopped cannot be directed.
6. Patents and prizes are inherently unfair.
7. Technology causes advances in science, and not viceversa.
8. As per the OECD and other sources, "publicly funded research had no economic impact whatsoever."
9. Although publicly funded science does lead to some discoveries, we do not know what would have happened if the state would have refrained from collecting this money via taxes. Private initiative may have led to an even better outcome.
10. Conclusion: instead of doing stuff, the government should just get out of the way!
This is a linear synopsis of the article, but the various points are interrelated (e.g. 3 and 6, 2 and 8 etc.) and fall under a few general arguments:

#### Technology evolves all by itself (points 1, 4 and partly 7)

This is akin to saying that our cities clean themselves: a position that contradicts some fundamental principle, but does seem to explain reality quite well (until the next garbage strike, at least).

#### Credit for discovery (points 3 and 6)

Discoveries are often made simultaneously by several parties (point 3), and this may be relevant to the patent system and the incentives it creates, but what does it have with the usefulness of basic science?

#### The causality relation between basic science and technology (points 2 and 7)

Almost by definition, basic research does not immediately yield practical results. One must follow the causal chain a couple of steps further. Speaking of causality, Ridley confuses the efficient and the final causes (in Aristotle's terms): it may well be that innovation is the (final) cause for advances in basic science by providing the motivation and financial incentive, but basic science can still be the (efficient) cause of innovation.

#### Return on investment (points 8-10)

If basic science contributes nothing to economic growth (a big if, considering that electricity was once studied with no hope for applications), maybe the state should refrain from financing it and let private actors do their work. It so happens that the French state goes even further, by subsidizing private research, with almost no strings attached, to the tune of several billion euros per year (much more than any other OECD country.) However, this does not lead to faster technical advances in France, and the mechanism is largely seen as ineffective.

## 2 November 2015

### Mom and Dad

An interesting point on the universality of language (in particular, that of the words for mother and father). I have written about this topic before (1,2,3).

## 25 October 2015

### The average of five people

It has been said that “You are the average of the five people you spend the most time with.” By a relative of the mean value theorem (used e.g. in electrostatics) it then follows that there are no local extrema in society.

### Varying the counter ion changes the kinetics, but not the final structure of colloidal gels

Our paper just got published in the Journal of Colloid and Interface Science !

## 12 October 2015

### Defining nematic viscosities: Mięsowicz and Leslie-Ericksen

Isotropic fluids only have two viscosities, intervening in shear and extensional deformations. For anisotropic media, such as nematic liquid crystals, more coefficients are needed, as shown by Mięsowicz in the late '30s: three shear viscosities, labeled $$\eta_1$$ to $$\eta_3$$, a fourth one $$\eta_{12}$$ introduced later by Helfrich and a rotational viscosity, $$\gamma_1$$. We do not worry here about extensional deformations.

The whole topic was put on a solid theoretical basis in the '60s by Leslie and Ericksen [brief and clear presentation here] who introduced six coefficients ($$\alpha_1$$ to $$\alpha_6$$), only five of which are independent. As one can expect from dimensional analysis, the two sets of viscosities, $$\left \lbrace \eta_i, \gamma _1 \right \rbrace$$ and $$\left \lbrace \alpha_j \right \rbrace$$ are linearly related.

I only recently realized, while discussing with my former PhD advisor, that the difference between the two definitions is deeper than an arbitrary linear transformation. Mięsowicz had in mind clear experimental configurations, defined by the relative orientation of director, velocity and velocity gradient, while Leslie and Ericksen adopt a more formal approach, based on generalized hydrodynamics, as in the paper of Martin, Parodi and Pershan.

The twist (so to speak) is that the theoretical approach gives a clearer view of the various modes and the constraints on the coefficients, while the Mięsowicz configurations are very difficult to achieve in practice, precisely due to the coupling between flow and director orientation.

## 30 September 2015

### Hierarchy of topics in physics

It seems to me that the prestige of the various subfields of physics depends on the nature of the objects under study as follows (in descending order):
1. Bosons
2. Fermions
3. Atoms
4. Everything else
Cold atoms (when they exhibit bosonic behaviour) go in the first category, with photons and the Higgs boson.

## 27 September 2015

### Ontological gaps

The easiest solution to an ontological separation is via a middle term, e.g. the spirit as intermediary between body and soul.

Sometimes this can take exotic forms, like the gnostic hierarchy of emanations, which is much more complicated than other ways of bridging the distance between God and creation (such as the uncreated energies in Orthodox doctrine) but also psychologically more acceptable: naively, it is easier to understand a higher essence that becomes progressively diluted into creation (which does preserve a divine spark) than two completely separate orders where the first one conveniently has an aspect that can interact with the second.

## 22 September 2015

### The bluebell curve

One of the reasons put forward for men's domination of both science and art is the spread of their IQ distribution: supposedly, there are more men than women at both extremes of the scales.
Novalis held the same point of view, but in reverse: "Doesn't it speak for the superiority of women that the extremes in their education are much more striking than in ours?" (II, 616). I cannot find an English translation of the fragment, but here is the relevant phrase in the original German, with discussion.

## 20 September 2015

### Popper

I think I finally "get" Popper. He has a mathematician's outlook: every statement has (at most) one meaning, which can be either true or false. The truth value is constant and uncontroversial: the same for all people, at any point in history. He is a good Cartesian in this respect.
Unfortunately, the consequence is that anything other than pure mathematics is either meaningless or false. Scientific theories may well be useful, but they are (or will be) falsified; that is all we can say about them. This is not to demean Popper's contribution, which is foundational, but the foundation by itself is not very useful. In particular, there's no place in his system for a new theory that improves upon an old one without merely displacing it.
The same tendency is apparent when he discusses philosophy (and philosophers) in The Open Society. Popper feels no empathy for Plato and does not try to "inhabit" his thought, nor does he try to place the old Greek in the historical context, unless to better condemn him, as in the final section of the first volume. I suppose that if a contemporary author were to hold now Plato's position we would criticize her for her social views (and decide she is a quite poor logician). Should we do the same with Plato himself?
Marx receives the same treatment. Contrast Popper's description with Isaiah Berlin's life of Marx, which shows much more historical and even personal insight, although Berlin was far from being a Marxist.

## 11 September 2015

### Presentation slides - what to avoid

About two years ago, I wrote a post on how to style a presentation. I was at a conference this week and I would like to highlight some easily avoidable mistakes that people made.
• Do not put important information at the edge of the slide. In particular, the reference to the paper you are advertising should not be at the very bottom, where it can be cut off if, for instance, the beamer is not perfectly aligned with the screen. This happened to at least three speakers out of the approximately fifty I listened to.
• Enhance the contrast of the images you intend to show. If you insist on showing the raw images, have an enhanced set on backup slides you can use if the audience cannot see the first ones. Alternatively, use light text on dark background and ask the organizers to dim the lights in the room (if possible.) It works for biologists.
• Do not rely on color contrast between text and background: there should always be an intensity contrast between them. One speaker used blue and red text on green background. It was jarring and, for some colorblind people, probably illegible. I'm a big fan of solid black text on white backgroud, with color only used for highlighting.
Note that the first two points fall under the general principle: "do not expect things to look on the projection screen as they do on your computer."

## 6 September 2015

### Reproducible experiments (again)

Last year I discussed a psychologist's essay on "the emptiness of failed replications". I'm returning to the status of experiment replication because a few days ago Science published a paper of the Open Science Collaboration, which was only able to confirm less than half of the 100 psychological studies it replicated. The study is very interesting in itself, but I'll only comment here on the reaction of psychology professor Lisa Feldman Barrett in a New York Times op-ed. She makes the point that attempting to replicate an experiment in different conditions can lead to different results and, by that fact, point to the importance of previously neglected parameters: context is important.

She illustrates her position by three examples, one of them taken from physics :

[...] when physicists discovered that subatomic particles didn’t obey Newton’s laws of motion, they didn’t cry out that Newton’s laws had “failed to replicate.”

Prof. Feldman Barrett confuses here the epistemological function of experiment and theory. A physical theory cannot be replicated, but it can be tested using (hopefully, reproducible) experiments. Obviously, performing an experiment with elementary particles cannot be seen as replicating one that uses macroscopic objects.

There is, however, a more alarming problem with her position, summarized by the phrase :

Much of science still assumes that phenomena can be explained with universal laws and therefore context should not matter.

There are very few completely general natural laws. Scientific results are "universal" precisely in the sense that the necessary context should be fully specified : A occurs every time conditions B, C, and D are fulfilled, irrespective of parameters E, F and G that are not mentioned.

Of course, the more general the applicability, the more interesting the result, as the psychologists very well know : they say "X correlates with Y and Z in adult humans", and not "X correlates with Y and Z in this set of 58 adults" that they used to infer that particular correlation. The latter finding will not be published in high-profile journals.

Fully specifying the context is certainly more difficult in the social sciences than in physics, but the solution is adding more rigour, not claiming for psychologists the right to generalize their results arbitrarily (what holds for a few dozen psychology undergraduates in an American research university may not apply to all mankind). If the generalisation is shown to be incorrect, this is not proof of a new and exciting result, but simply a sign that the original authors oversold their findings.

## 15 August 2015

### How symbolic are word sounds?

[via Slashdot] A recent study in Royal Society Open Science (featured in Science) discusses sound symbolism in the vocal representations for antonyms (similar to the kiki/bouba effect).

The news here is that the participants created their own "words" during a game of vocal charade and that they were able to communicate their meaning increasingly well in subsequent rounds. The made-up words were also identified by "naive listeners" (not involved in creating them) better than predicted by mere chance. Furthermore, the meaning of the vocalizations was correlated to some acoustic characteristics (e.g. the signals for "up" were reliably shorter, more intense, with higher pitch and pitch change than those for "down".)

On a completely unrelated note, the authors felt the need to warn the reader that:

"Unavoidably, as in similar semiotics experiments, our participants already spoke a language and thus came well acquainted with symbolic communication."

I have already written a few times about language symbolism, in the series on Genette's Mimologics (first part here) and also here and here.

## 12 August 2015

### Law enforcement priorities

The victim is found shot dead, and his encrypted smartphones are found next to him. Four prosecutors from four different countries take to the pages of the New York Times asking for restrictions on encryption. No word about restrictions on firearms.

This is great!

## 6 August 2015

### Materials science...

...is when chemists do physics and physicists do chemistry. It is not always a pretty sight.

## 5 August 2015

### French humour

I finally realized that a distinctive trait of French humour is combining hyperbola and understatement in the same phrase. I'll just post this and look for examples later...

### The Dirac delta "function" - part II

#### The relation between δ(x) and dx

After introducing the Dirac delta "function" $$\delta(x)$$ in the previous post, I'll try now to explain the relation between it and the differential element $$\mathrm{d}x$$. In the process, I'll through all mathematical rigour out the window and aim for an intuitive understanding.

## 4 August 2015

### The Dirac delta "function" - part I

When describing a physical system, one would often like to describe some of its components as pointlike particles, i.e. with no spatial extent. Obviously, all objects have a finite size, but this size can be much smaller than the length scales relevant to the problem at hand. All the attributes of the particle (mass, charge, etc.) can then be assigned to a space point.

## 1 August 2015

### When is science settled?

"By definition, science is never settled. It is always subject to change in the light of new evidence."

The quotation above is from a completely uninteresting, but symptomatic article in the Wall Street Journal. We learn that nothing is certain when it comes to global warming because, among other things, "climatology is only about 170 years old". Also, flat earth and relativity. Caution is an honorable position, but I cannot resist linking to Isaac Asimov's The Relativity of Wrong.

I am however a bit surprised to notice that the same author asserts as a self-evident truth that, should the corporate income tax be abolished, "revenue from other sources, especially the personal income tax, would quickly make up for it and then some". No bet-hedging on this one? Is it a proven result of the venerable science of economics?

## 27 July 2015

### Untranslatable words

I have already written about (im)possible translation on this blog. The series on Genette's Mimologiques (first part here) is also relevant, because Cratylism has strong implications for the theoretical possibility of translation. At some point, I should write something about Steiner's After Babel.

Lately, I've been looking through the monumental Dictionary of Untranslatables: A Philosophical Lexicon. On a lighter note, here are some illustrated examples of hard to translate concepts from several languages. Both sources discuss the term saudade.

P.S. As I'm writing this post, I'm also reading a comment on Slashdot about the different meaning of "government" in British vs. American English...

### Postdoc position: numerical simulations for biophysics

A post-doctoral fellowship is available at the MSC laboratory of the Paris Diderot University (Paris, France), in the framework of our ANR project. Apply before October 1st. The contract should begin on January 1st 2016 at the latest.

Title: Modelling the many-body interactions between protein inclusions in cell membranes
Gross salary: ~ 2500 €/month (varies with seniority).
Duration: ~ 12 months.
Summary of the research topic: The MSC laboratory is a research unit at the very heart of Paris working on three main axes: non-linear physics, soft-matter and interface between physics, biology and medicine. The subject proposed here is part of a wider research effort, pursued in collaboration with three other laboratories from the Paris area, in the framework of a project financed by the ANR (French grant agency). In particular, the successful candidate will work in close collaboration with an experimental group in the Laboratory of Solid State Physics (LPS, Orsay).

## 25 July 2015

### Why good research is like fiction literature

It's not that it requires making things up. I mean this quote by David Foster Wallace:

"[A] certain amount of vanity is necessary to be able to do it all, but any vanity above that certain amount is lethal."

After all, we must be convinced that we can discover something new and important, that nobody else has been able to find, but still remain lucid enough not to fool ourselves.

## 12 July 2015

### Metrics for science

A couple of recent posts at Occam's Typewriter, on journal impact factors and on metrics in general discuss the evaluation of journals, researchers and institutions. In particular, Athene Donald's post links to The Metric Tide, an in-depth evaluation of quantitative indicators.

What I find interesting is that The Metric Tide compares (in Supplementary Report II) various metrics and the REF quality profile, which ranks individual UK researchers in five categories (one to four stars, in increasing order of "originality, significance and rigour" and the bottom drawer, discreetly labeled as "unclassified".)

The authors computed the precision and sensitivity of REF 4* predictions based on each indicator and the Spearman correlation of the indicator with the REF quality profile. In the areas of physics and chemistry, the best predictor seems to be the citation count, with a precision (percentage of correct predictions) of about 50%, a sensitivity (the proportion of REF 4* outputs identified by the metric prediction) of 85% and a correlation of 0.6.

This is fairly imprecise, but the analysis is done over entire thematic fiels (or units of assessment, as they are called in the report). The accuracy would probably improve if the comparison were restricted to sub-fields, which are more homogeneous in terms of audience sizes and citation practices.

What is it clearly missing from the picture (and would be very hard to measure) is the influence of the various metrics themselves on the REF evaluation...

## 11 July 2015

### Weightlifting and the 2/3 power law

At least since Haldane's 1926 paper [1], we know that the various characteristics of an organism scale according to different power laws. For instance, its mass increases as the volume, i.e. as the cube of the length: $$M \sim L^3$$. The strength $$S$$, however, should be proportional to the muscle section, and thus increase as $$S \sim L^2$$. We therefore get $$S \sim M^{2/3}$$.

## 26 June 2015

### The structure factor of a liquid - part V

#### Finite-size effects

In previous posts, we have always considered that the liquid system was infinite and homogeneous. This may no longer be the case if:
• the particles are confined in relatively small spaces or
• their attractive interaction leads to the formation of dense aggregates, separated by more dilute regions.
Although physically very different, those two situations have similar effects on the structure and we will treat them together.

## 29 May 2015

### Global cooling: is the paper really claiming it?

A recent paper in Nature finds a strong correlation between ocean circulation and oscillations in Atlantic surface temperatures which could be moving to a negative phase. According to the authors, "[t]his may offer a brief respite from the persistent rise of global temperatures."

This claim has been taken up by various sites, along a much stronger prediction: decades of global cooling by up to 0.5°C. However, I cannot find this second item anywhere in the original paper. Where does it come from?!

## 25 May 2015

### The structure factor of a liquid - part IV

#### Sum rule for impenetrable systems

The hard sphere liquid is an idealized model, but some of its properties hold for a very large class of systems, those that have an impenetrable core of size $$R_c$$ ($$g(r< 2 R_c = 0$$). Let us write the Fourier relation between $$g(r) -1$$ and $$S(q) -1$$ (the inverse of Eq. (4) in post II):

### The structure factor of a liquid - part III

This is the third part in a series. In part I and part II we defined the basic concepts used in the theory of liquids, in particular the radial distribution function $$g(r)$$ and the structure factor $$S(q)$$.

The simplest system one can imagine is the ideal gas. There is no interaction between particles: $$u(r) = 0$$, leading to $$g(r) = 1$$ (the particle at the origin does not affect the position of its neighbors) and $$S(q) = 1$$. The ideal gas is a trivial case, but it can be seen as the reference state for other systems. In particular, one could say that the functions $$g(r) - 1$$ and $$S(q) - 1$$ that appear in Equation (4) of part II quantify the difference with respect to the ideal gas (due to the interaction potential $$u(r) \neq 0$$.)

## 23 May 2015

### Nîmes

 The Maison Carrée reflected by the façade of the Carré d'Art.

## 16 May 2015

### The structure factor of a liquid - part II

[Continuing the preliminary discussion started in part I.]
We are now interested in an explicit form for $$g(\mathbf{r})$$ (we return here to the general case —where $$g$$ depends on the full vector $$\mathbf{r}$$, and not only on its modulus— simply to avoid the radial integrals). Taking particle 0 as fixed in $$\mathbf{r}_0$$, $$\rho g(\mathbf{r}) {\text{d}}^D \mathbf{r} = \text{d} n (\mathbf{r} - \mathbf{r}_0)$$ is the number of particles (among the remaining $$N-1$$) found in the volume $${\text{d}}^D \mathbf{r}$$ positioned at $$\mathbf{r}$$ with respect to the reference particle. One can formally count these particles by writing:

### The structure factor of a liquid - part I

This post only summarizes some basic concepts and results that will help understand the discussion in the following posts. For a detailed introduction to liquid theory, see one of the many books and review papers [1].

## 23 April 2015

### Projection onto the subspace of spherical harmonics with the same degree

Recently, I've been interested in expanding an angular function over the spherical harmonics, and particularly in retrieving the amplitude of the part corresponding to a given degree $$\ell$$. More precisely, let $$F(\Omega) = F(\theta,\phi) =\sum_{\ell} \sum_{m} Y_{\ell m} (\Omega)$$. The projection of $$F$$ onto the subspace spanned by the harmonics with a given degree $$\ell$$ (I believe this space is generally denoted by $$\mathcal{H}_{\ell}$$) is:

\label{eq:proj1}
\operatorname{Proj}_{\ell} \left [F \right ] (\Omega) = \sum_{m= - \ell}^{\ell} c_{\ell m} Y_{\ell m} (\Omega)

## 13 April 2015

### Temporary positions in science

A very interesting article in Nature on the future of postdoc positions. Well worth reading, although the solutions proposed (increasing the proportion of permanent research staff, in one way or another) are completely impractical. The comments are even more interesting than the article itself.

## 7 April 2015

### Posting on arXiv

Since I'm on vacation these days, I decided to finally submit my published research papers to arXiv. A previous attempt was about as pleasant as a visit to the dentist, but somewhat longer (and unsuccessful). This time around I tried to submit other papers, and things went more or less smoothly, as soon as I learned to follow some guidelines (for documents produced using LaTeX):
• Make sure that all .eps figures are of reasonable size: no files above 6MB and no more that 10MB for the whole submission. I downsized some very large files by first converting them to .pdf and cropping to remove white margins (using Acrobat) then opened the .pdf files in Photoshop and saved a copy in .eps format (without preview and using jpeg compression).
• Check that the figures are correctly invoked in the .tex file (the name should be case-sensitive, something that is not required on Windows systems). File names should not contain special characters (more details).
• Run LaTeX locally until the compilation is error-free.
• Create a .zip archive with the .tex file, the .eps figures and the .bbl file (the compiling process on arXiv does not include Bibtex) and, if necessary, with supplementary material in .ps format.
• Hope that the compilation works without errors, otherwise wade through about three screens of output, fix things and reload files. The supplementary files are simply appended to the final .pdf; this is an easy way to include additional material or tricky parts that fail to compile on the arXiv server. This is how I managed to add to one paper a page-wide table in landscape orientation, which had resisted all other methods.
The final result is here.

For Word documents the process should be much easier, since one can directly submit the .pdf version. I only tried this for one paper, currently on hold because it has line numbers in the margin. This restriction is not mentioned anywhere on the arXiv site (neither is the size limit, by the way.)

Overall, the entire procedure was easier than I thought. Still, the system is far from user-friendly (metadata retrieval using the DOI would be nice), the interface is firmly stuck in the 90s, some limitations seem a bit arbitrary and the help could be more detailed.

## 4 April 2015

### How to read an equation

The mere formal expression of an equation is not very useful, unless complemented by a more or less intuitive understanding. Different people may have different intuitions of a given formula or different mental images of one physical systems (more on that later).

The interesting part is that putting together two such different intuitions of a relation can yield non-trivial results with almost no algebraic manipulation, as I'll show below. What is the meaning of the following formula ?
$\frac{1}{\sqrt{2\pi} \sigma} \int_{-\infty}^{\infty} \text{d}x \exp (i q x) \exp \left (- \frac{x^2}{2 \sigma ^2} \right ) \tag{1}$

### Fundamental physics

The other day at the gym I realized I am a force of nature. The weak force, to be specific. It could have been worse, though: gravity was on a treadmill not far away.

## 27 March 2015

### Real-Time in Situ Probing of High-Temperature Quantum Dots Solution Synthesis

Our paper just got published in Nano Letters!

## 1 March 2015

### Top nine trending physicists

A colleague asked a few days ago who where the most popular physicists on the Internet. I tried to find an answer using Google Trends, and the answer is below:

As expected, only astrophysicists and particle physicists are represented.

I hope the ranking is correct (it is difficult to extract the raw data for more than five topics at a time). Who do you think should complete the top ten?

Here is the time trend (for only four among the nine):

## 21 February 2015

### Pitfalls of logic: modus tollens

From Slate:

Does it mean that if you are alone you do understand the warnings?

## 11 February 2015

### Strain-controlled fluorescence polarization in a CdSe nanoplatelet–block copolymer composite

Our paper has just been published in Chemical Communications as an Advance Article !

## 29 January 2015

### Zero-lens microscopy

Visible light is good for seeing details down to a fraction of a micron. Below this limit, one uses X-rays, with a wavelength of the order of one Ångström. The problem is that there are no high-quality lenses for X-rays, and so, instead of direct images, one has to settle for scattering or diffraction patterns, much more difficult to interpret (but sometimes more useful).

If the magnifying glass is a one-lens optical instrument and the microscope is (schematically) a two-lens device, then an X-ray scattering setup does zero-lens microscopy!

## 23 January 2015

### Is modern science a by-product of Reformation?

There is a strong connection between Reformation and the birth of modern science, not only in time but also with respect to the number of notable Protestant scientists1. Several explanations were proposed:

## 20 January 2015

### Dielectric sphere in a static field

I'll try to give the simplest solution I can think of to the classical problem of a dielectric sphere in a constant external field, see for instance  Landau & Lifshitz, vol. 8, chapter II, §8.

The sphere radius is $$R$$ and its dielectric constant is $$\epsilon_i$$, while that of the surrounding medium is $$\epsilon_e$$. The electric field at infinity is along the $$z$$ axis: $$\mathbf{E}_0 = E_0\, \hat{z}$$. Considering the symmetry of the object we will work in spherical coordinates $$(r,\theta,\phi)$$. Before writing down any equations, let us note the following points:
1. The system has rotational symmetry about the $$z$$ axis. Thus, the field has no component along $$\hat{\phi}$$.
2. The problem is antisymmetric with respect to $$z$$ (changing $$E_0$$ to $$-E_0$$ reverses the sign of the field everywhere).
3. The field scales by $$E_0$$ (doubling the applied field doubles the field at any point in the system).
4. The only length parameter is the sphere radius $$R$$.

## 18 January 2015

### Missing the point on Charlie Hebdo

A few days ago in the New York Times, David Brooks announced his position very clearly: I am not Charlie Hebdo. I'm responding to his Op-Ed because:
• similar positions are quite common in the wake of the Paris attack, but at least here
• the argument is reasoned, carefully presented and respectful of the victims and
• it is published in a very respected, mainstream journal.
Still, I believe it manages to get the "I am Charlie" response completely wrong.

## 7 January 2015

### Mr K

Yet another story about Khodorkovsky (a little more critical of the man and his past than the usual articles). We learn that he thinks about returning to Russia and that even the even the perspective of replacing Putin is not completely out of the question.

Khodorkovsky's fall does resemble something out of Kafka. All the elements are there: the oppressive state, the lack of a real crime, down to the hero's initial.
However, it also reminds me of an older story: Faust makes a pact with the devil, reaps the benefits, and then complains about the stench of brimstone.
The place does stink, and the publicity for a high-profile victim may be very effective in raising the international awareness, but I cannot bring myself to empathize with Mr K. Not because he is still alive and rich, but because for a long time he has been a part of the problem he now decries. In fact, the only reason he became a cause célèbre is his former belonging to an international elite, and the only reason of this elevated status was the favor of the regime.