Ágora y lo que se perdió en Alejandría
October 16th 2009

Este fin de semana he visto la película Ágora, de Alejandro Amenábar, y me ha gustado mucho. No voy a comentar aquí sobre los interesantes temas que trata la película, como son el fundamentalismo religioso, las luchas de poder, el declive del Imperio Romano, la astronomía y la ciencia opuestas a la fe y la ignorancia... Todo esto seguro que es tratado en otros sitios mejor de lo que yo lo haría.

Lo que yo quiero comentar es una simple reflexión que me sobrevino mientras veía la película. Para entrar en antecedentes, explicaré brevemente el contexto de la película. Esta está ambientada en Alejandría, en el año 391 y posteriores. El hecho histórico concreto que quiero reseñar es la destrucción en 391 del Serapeum de la Biblioteca por Teófilo de Alejandría, el (¿cómo no?) Patriarca de la Iglesia Cristiana en Alejandría, bajo decreto del (¿cómo no?) Emperador cristiano Teodosio I. Esta es una de las cuatro destrucciones parciales o totales que dicha Biblioteca ha sufrido en la historia, todas ellas consideradas enormes pérdidas culturales (a mano del fanatismo y la ignorancia, y a veces de los accidentes, como el incendio de Julio César).

Mi reflexión es púramente pragmática, y se resume en una pregunta: ¿por qué se perdió una gran cantidad de obras culturales de valor incalculable en ese y los otros ataques la Biblioteca? La respuesta podría ser que los culpables son los cristianos, o la ignorancia, el fanatismo, el odio... Hay muchas posibles respuestas, pero no son la que busco. La respuesta que busco es tan sencilla como la pregunta: se perdió una cantidad de cultura de valor incalculable porque no se habían hecho copias. Es indudable que los manuscritos originales de grandes autores tienen un gran valor, y son irreemplazables. Pero al menos el contenido de dichas obras puede perdurar si es multiplicado y repartido. Está claro a donde voy: que es la concentración y racionamiento de la cultura lo que la mata, el intentar controlar y limitar su distribución, no compartirla libremente. Si miro al panorama actual, no son los usuarios de p2p los que identifico con los cristianos que asaltaron el serapeum de la Biblioteca. Más bien estos serían los ministerios de "cultura" y SGAEs del mundo. Aquellos (los usuarios de p2p) los identificaría, en todo caso, con los paganos que intentaron salvar la mayor cantidad de libros del fuego "purificador" de los doctores de lo "correcto" personificado por Teófilo y sus fanáticas huestes.

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Is my theory bullshit?
March 15th 2008

This post tries to sketch a rule of the thumb to quickly check whether an idea/theory/belief is utterly useless or not. I have admittedly adapted it from the russellian definition of Science. Recall that utterly useless ideas are not necessarily wrong. They are just that: utterly useless.

There is a single basic question you have to ask yourself when you invent/encounter a flashy new theory or idea like telekinesis or homeopathy:

Can I imagine any conceivable way of refuting this theory?

If the answer is "no", then the theory is bullshit.

If you accept this, you are bound to abandon the theory if someone comes up with a valid experiment at which your theory fails (if someone challenges your telekinetic powers and you can not shut her mouth, you must accept you don't have telekinetic powers).

On the other hand, if you don't accept the above premise, you must, without excuse, believe in any other theory that can not be proved wrong, such as the Invisible Pink Unicorn or the Flying Spaghetti Monster. Failing to do so will undoubtedly qualify you as an absolute hypocrite.

Now, the long explanation...

Proving something true is theoretically impossible, but proving something wrong is trivial: if I say that all swans are white, no matter how many white swans I see, I will never be sure that the theory is true. On the other hand, after the first black swan I see, I will conclude without doubt that the theory was wrong.

Thus, "proving" some theory is usually equaled to designing an experiment in controlled conditions, where a result is expected from the theory, and we get precisely that result in the experiment. Obviously, we could have obtained a different result, and our theory would have been proved wrong. It is precisely the fact that a different result could potentialy refute our theory what makes the desired result confirm it. It follows that, if there is no conceivable circumstance under which the experiment could have failed, our theory can not be disproved, and therefore can not be "proved" through absence of refutation.

Take for example a seer who claims to be able to see the future. Her theory is not necessarily bullshit: one can devise a test, failing which would mean that she is wrong. For example, one can ask her to "see" something that she can not access by normal means, and that she can not guess by chance, for example the next lottery winning number. If she guesses correctly, the theory is temporarily accepted. If she fails, the theory is dropped.

Not it comes the funny twist: any argument that tries to make the precognition theory above survive after a failure (e.g. "I do not control when I can see the future", "I only see abstract visions that I have to interpret afterwards", and so on... you know the thing), automatically turns it into bullshit. Directly. And that because of the little rule of the thumb I present above.

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When patents threaten science
January 30th 2007

I just read in the Science magazine issue of the first week of December an article by the same name as this blog entry (L. Andrews, J. Paradise, T. Holbrook and D. Bochneak, Science 2006 314 (5804) 1395). The online version (only available under subscription, I believe) can be found here.

The article demystifies the idea that patents are necessary to push the development, and warns about an already evident rise of patent abuse, particularly in scientific areas.

The article gives a very interesting example case:

The Court of Appeals for the Federal Circuit (of the USA. Where else could this happen?) has fined LabCorp for USD2M, because they disclosed to physicians a piece of knowledge allegedly patented by Metabolite Laboratories. The piece of info is that an excessive level of the amino acid homocysteine in human body fluids can be a signal of a deficit of B vitamin in the patient.

The Federal Circuit also held that merely thinking about the relationship between homocysteine and B vitamin levels (e.g., to develop new test types) would result in patent breach.

How stupid is that? No, skip that. Not stupid. How criminal is that? What kind of neo-con mindset is necessary to believe it is fair? Patents where not invented for that. Imagine the following situation: a physician meets a patient with a low level of homocysteine, but his/her hospital can not afford paying for that license. The physician knows that high homocysteine means low B vitamin, but can not prescribe the due treatment, lest a patent be breached. The physician must decide between diagnosing a B vitamin deficiency (and break the law), or let the patient suffer or die. If the result of conceding a patent is that a physician must be in such a situation, then, obviously, that patent was wrong in first place.

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¿Popularización de Software Libre acentúa explotación de bugs?
November 22nd 2006

[This entry is also available in English|English PDF|PDF en castellano]

Los excépticos del movimiento FLOSS suelen decir que el Software Libre tiene, en general, menos bugs explotados que el software privativo, solamente porque es menos popular que este. Argumentan que, como el FLOSS tiene menos usuarios, los crackers estarán menos interesados en malgastar su tiempo intentando explotar los bugs que pudiera tener. La mayor base de usuarios del software privativo daría además, según ellos, una mayor publicidad a sus bugs, y sus modos de explotación se difundirían más rápido. El corolario a esta teoría sería que la popularización de aplicaciones FLOSS (p.e. Firefox), llevería a un incremento en el número de bugs descubiertos y explotados, llegando eventualmente a un estado similar al del software privativo actual (p.e. "Cuando Firefox sea tan popular como Internet Explorer, tendrá tantos bugs como Internet Explorer.").

El objetivo de este artículo es demostrar matemáticamente la total insensatez de tal teoría. Específicamente, argumentaré que un aumento en el tamaño de la comunidad de un proyecto FLOSS lo mejora de al menos 3 formas:

  1. Desarrollo acelerado
  2. Menor vida media de bugs abiertos
  3. Menor vida media de bugs explotados

Una explicación más extensa está disponible en formato PDF. Lamento que las fórmulas matemáticas en HTML sean de calidad francamente pobre, pero no tengo ni tiempo ni habilidad para mejorarlas. Si el lector está interesado en fórmulas bonitas, recomiendo acudir al PDF.

Este blog, y el PDF que enlazo, están liberados bajo la siguiente licencia:


Creative Commons License

Esta obra está bajo una licencia de Creative Commons.

Lo que esto significa, básicamente, es que eres libre para copiar y/o modificar este trabajo como gustes, y redistribuirlo cuanto quieras, con dos únicas limitaciones: que no le des uso comercial, y que cites a su autor (o al menos enlaces a este blog).

1 - Proposiciones y derivación

Tenemos un proyecto FLOSS P, nuevas versiones del cual se liberan cada T tiempo. Cada versión se asume que incorpora G nuevos bugs, y las sucesivas versiones serán liberadas cuando todos los bugs de la anterior sean parcheados. En un momento determinado habrá B bugs abiertos (de los G originales).

Asumo que la velocidad de parcheo es proporcional al tamaño de la comunidad de usuarios (U):

dB/dt=-KpU

1.1 - Desarrollo acelerado

De arriba, la dependencia temporal del número de bugs abiertos:

B = G - Kp U t

El tiempo entre versiones (T), de B = 0:

T = G/KpU

De manera que el tiempo entre versiones se acorta para U creciente.

1.2 - Menor vida media de bugs abiertos

En un período de tiempo dt, se parchean (-dB/dt)dt bugs, siendo su edad t. Si llamamos τ a la vida media de los bugs, tenemos la definición:

τ = (∫t(-dB/dt)dt)/(∫dB)

De ahí se deduce:

τ = T/2

Esto es: la vida media de los bugs es siempre la mitad del tiempo entre versiones, el cual (como se ha mencionado) tiene una proporcionalidad inversa con U.

1.3 - Fracción de bugs explotados antes de ser parcheados

Definimos la siguiente velocidad de explotación de bugs, donde Bx es el total de bugs explotados, Kx es la "eficiencia de explotación" de los crackers (cuya cantidad se asume proporcional a U), y Bou es la cantidad de bugs abiertos y sin explotar:

dBx/dt = Kx U Bou

También definimos α = Box/B, donde Box es la cantidad de bugs abiertos y explotados.

Se puede derivar la evolución temporal de α:

α(t) = 1 - exp(-Kx U t)

Tras ello definimos γ = Kp/KxG, y derivamos la fracción de los bugs G que terminan siendo explotados para un tiempo t:

Bx(t)/G = 1 - γ + (γ - 1 + t/T)exp(-Kx U t)

Resolviendo para t=T, y tomando en cuenta que T=G/KpU, obtenemos la fracción de los bugs totales que son explotados en algún momento durante el período entre versiones (Fx):

Fx = 1 - γ + γ exp(-1/γ)

Nótese que Fx es independiente de U, esto es, aunque aumente el tamaño de la comunidad de usuarios, no aumenta la fracción del total de bugs que acaban siendo explotados (aunque un crecimiento de la comunidad traiga asociado un aumento colateral del número de crackers).

1.4 - Menor vida media de bugs explotados

Deseamos saber cuánto tiempo permanecen sin parchear los bug explotados, y llamamos a este tiempo τx. Tras un desarrollo ligeramente complejo, pero derivando siempre de equaciones previamente definidas (ver la versión PDF), obtenemos una expresión realmente simple para τx:

τx = Fxτ

Esto es, el tiempo medio de explotación de los bugs explotados es proporcional a τ, el cual a su vez es proporcional a T, o inversamente proporcional a U.

2 - Conclusiones

El eslogan "Más popularidad = más bugs" es un non sequitur. De acuerdo con el simple modelo que bosquejo aquí, cuanto más amplia sea la comunidad de usuarios de un programa FLOSS, más rápido serán parcheados los bugs, incluso admitiendo que a más usuarios, más crackers entre ellos, dispuestos a acabar con él. Incluso para crackers que sean más efectivos en su desempeño que los usarios de buena fe en su trabajo de parcheo (Kx >>> Kp), aumentar el tamaño de la comunidad reduce el tiempo que los bugs permanecen abiertos y también cuánto tiempo tardan en parchearse los bugs ya explotados. No importa cuán torpes sean los usuarios, y cuán rapaces los crackers, el modelo libre (por medio del cual se da acceso al código a los usuarios, dándoles así poder para contribuir al programa) asegura que la popularización es positiva, tanto para el programa como para la propia comunidad.

Comparemos esto con un modelo cerrado, en el que una base de usuarios mayor puede incrementar el número de crackers atacando a un programa, pero ciertamente añade poco o nada a la velocidad con que el código es parcheado y corregido. Es de hecho el software privativo el que debe temer su popularización. Es fácil de ver que cuando una pieza determinada de software privativo alcanza una cierta "masa crítica" de usuarios, los crackers pueden potencialmente desmoronar su evolución (digamos, haciendo τx = T, Fx = 1), porque (a diferencia del FLOSS), G, P, y por lo tanto T, son constantes (ya que dependen únicamente de los vendedores del software).

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Popularity of Free Software generating bug exploitation?
November 22nd 2006

[Esta entrada está también disponible en castellano|English PDF|PDF en castellano]

It is often said (by FLOSS-skeptics), that Free Software has less exploited bugs than the Proprietary Software because it is less popular. They argue that, since less people uses FLOSS, the crackers are less inclined to waste their time exploiting the bugs it could have. The greater user base of the proprietary software would also, in their words, make bugs more prominent, and their exploits spread faster. The corollary of this theory would be that popularization of FLOSS applications (e.g. Firefox), would lead to an increase in the number of bugs discovered and exploited, eventually reaching a proprietary-like state (e.g. "Firefox will have as many bugs as IE, when Firefox is as popular as IE").

In this blog entry I will try to outline a mathematical model, proposed to demonstrate the utter nonsense of this theory. Specifically, I will argue that an increase in community size benefits a FLOSS project in at least 3 ways:

  1. Faster development
  2. Shorter average life of open bugs
  3. Shorter average life of exploited bugs

A more thorough explanation is available in PDF format. Recall that the math display in HTML is generally poor (much more so when I don't have the time nor skills to tune it). If you like pretty formulas, the PDF is for you.

Both this blog entry and the linked PDF are released under the following license:

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 License.

What this basically means is that you are free to copy and/or modify this work, and redistribute it freely. The only limitations are that you can not make it for profit, and that you have to cite its original author (or at least link to this blog).

1 - Propositions and derivations

We have a FLOSS project P, new versions being released every T time, and each version incorporating G new bugs. Each new version will be released when all bugs from the previous release have been patched. At any point in time, there will be B open bugs (remaining from G).

The patching speed is assumed proportional to the size of the comunity of users (U):

dB/dt=-KpU

1.1 - Faster development

From above, the time dependency of open bugs:

B = G - Kp U t

The inter-release period (T), from B = 0:

T = G/KpU

So the inter-release time (T) is shortened for growing U.

1.2 - Shorter average life of open bugs

In a dt time period, (-dB/dt)dt bugs are patched, their age being t. If we call τ the average lifetime of bugs, we have the definition:

τ = (∫t(-dB/dt)dt)/(∫dB)

From that it follows:

τ = T/2

So, the average life of open bugs equals half the inter-release time, which (as stated above) has an inverse proportionality with U.

1.3 - Fraction of bugs exploited before being patched

We define the following bug exploitation speed, where Bx is the total amount of exploited bugs, Kx is the "exploiting efficiency" of the crackers (whose amount will be proportional to U), and Bou is the amount of open and unexploited bugs:

dBx/dt = Kx U Bou

We also define α = Box/B, where Box is the amount of open and exploited bugs.

It can be derived the evolution of α with time:

α(t) = 1 - exp(-Kx U t)

We then define γ = Kp/KxG, and derive the fraction of G bugs that end up exploited by time t:

Bx(t)/G = 1 - γ + (γ - 1 + t/T)exp(-Kx U t)

Solving for t=T, and taking into account that T=G/KpU, we get the fraction of total bugs that gets exploited ever, during the inter-release period (Fx):

Fx = 1 - γ + γ exp(-1/γ)

Recall that Fx is independent of U, that is, increasing the size of the user community does not increase the fraction of total bugs that get exploited ever, even though the amount of crackers is increased along with the user base.

1.4 - Shorter average life of exploited bugs

We want to find out how long exploited bugs stay unpatched, calling this time τx. After some slightly complex algebra, but always deriving from the previously defined equations (see PDF version), we obtain a fairly simple expresion for τx:

τx = Fxτ

That is, the average exploitation time of exploited bugs is proportional to τ, which is to say it is proportional to T, or inversely proportional to U.

2 - Conclusions

The "Increasing popularity = Increasing bugginess" motto is a non sequitur. According to the simple model outlined here, the broader the user community of a FLOSS program, the faster bugs will be patched, even admitting that an increase in user base brings an equal increase in the number of crackers committed to doom it. Even for crackers that are more effective in their cracking work than the bona fide users in their patching work (Kx >>> Kp), increasing the community size does reduce how long the bugs stay unpatched and also how long the exploited bugs stay unpatched. No matter how clumsy the users, and how rapacious the crackers, the free model (whereby the users are granted access to the code, and thus empowered to contribute to the program), ensures that popularization is positive, for both the program and the community itself.

Compare that with a closed model, in which an increased user base may boost the number or crackers attacking a program, but certainly adds little, if anything, to the code patching and correcting speed. It is actually proprietary software that should fear popularization. It is easy to see that when a particular proprietary software piece grows over a certain "critical mass" of users, the crackers could potentially disrupt its evolution (say, τx = T, Fx = 1), because G, P and thus T, are kept constant (depend only on the sellers of the code).

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An inconvenient truth
November 16th 2006

95% screaming and 5% crying. That's what I felt when watching An incovenient truth (es: Una verdad incómoda) (IMDb|FilmAffinity).

Crying because some problems are huge, and are real. And it is sad. Screaming because there are so many sons of a bitch trying to get away with their dirty business, disguised as "skeptics" of Global Warming.

The movie is a kind of documentary about Global Warming, featuring Al Gore as conductor. Good ol' Al may ring a bell to you: yes he's the one who came on top of George Bush in the 2000 US Presidential Elections, but nonetheless was declared loser due to highly controversial decisions by the US Supreme Court.

The movie is tagged by some people as "boring" or "not really saying anything new". I disagree. I am no GW expert, but by no means illiterate, either. I have a B.Sc. in Chemistry, soon to get the Ph.D. (albeit in Quantum Chemistry, which has little direct link with enviromental matters), and I did find the movie interesting.

Some of the data presented is, of course, redundant to some extent, but nonetheless it seems appropriate to mention it once again, even if "we all know it". However, there is a non-trivial amount of data that, at least for me, was new. To name three such points:

  • Of over 900 scientific articles studied, NOT ONE cast any doubt about the human source of the excess CO2 in the atmosphere, or this excess being the culprit of the global warming. There is no controversy among scientist, but rather a complete agreement. On the other hand, a surprising 53% of the studied mass media (newspapers,...) did mention some "disagreements" or "doubts", making it look like we don't really know what causes the global warming, or if it even exists! Clearly someone is trying to intoxicate the general public with doubts, when there's none. Think of it.
  • The CO2/temperature increase in the last decades is alleged by some to be just "a part of a trend of ups and downs", because "temperatures have always fluctuated". Bullshit. Gore shows studies of deep Antarctic ice that go back a friggin' 650,000 years, and plots of atmospheric CO2 concentration and temperature along these years. There are fluctuations, and even several glaciations could be seen. However, the CO2 concentration and temperature deviation from the average is now more than double what has ever been! Such a trend has never ever happened before.
  • Underdeveloped countries have loose environmental policies, that make them pollute a lot. Developed countries pollute more just because they have more industry, even if it is relatively cleaner. Not completely true. For example, the environmental policies regarding car manufacturing are tighter in China than in the USA. In fact, Chinese cars can be sold in the USA, but most north-american cars can not be sold in China, because they pollute too much. And that China being a book example of reckless industrialization, with little care for environment!



Image (taken from the Wikimedia), showing date much like the one Gore presents in the movie. Notice the point to the extreme right: the CO2 concentration goes over the roof (380 ppm), and is not shown.

Go watch the movie, it's quite informing (and no, not too boring).

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Article in Science
November 15th 2006

I have just read a rather interesting article in Science about the economics of information security (R. Anderson and T. Moore, Science, 2006, 314, 610), and I would like to comment some quotes of it:

There has been a vigorous debate between software vendors and security researchers over whether actively seeking and disclosing vulnerabilities is socially desirable. Rescorla has argued that for software with many latent vulnerabilities (e.g. Windows), removing one bug makes little difference to the likelihood of an attacker finding another one later[1].

Quite interesting! First, even a paper on Science not only regards Windows as a piece of software with a virtually endless reservoir of internal errors, but it even uses it as a paradigmatic example of such a case. Second, it deems such software as not worth patching, and bugs not worth being disclosed (security through obscurity), because they are so many.

[...] [Rescorla] argued against disclosure and frequent patching unless the same vulnerabilities are likely to be rediscovered later. Ozment found that for FreeBSD[2] [...] vulnerabilities are indeed likely to be rediscovered[3]. Ozment and Schecher also found that the rate at which unique vulnerabilities were disclosed for the core and unchanged FreeBSD operating system has decreased over a 6-year period[4]. These findings suggest that vulnerability disclosure can improve system security over the long term.

I have read [1] and [3] very briefly, and Ozment seems very critical of Rescorla's results. However, the comparison between Windows and FreeBSD (I think they mean OpenBSD), which is FLOSS, is quite nice. Windows is so buggy that patching it is hopeless. FreeBSD has seen a decline in the number of disclosed bugs (remember that, being FLOSS, all the bugs found by developers, mantainers and users are disclosed), related to the fact that each bug fixed actually means a reduced probability of finding new bugs (because the total is not endless).

The bottom line is that, for a good piece of software (one that is not so bug-ridden that crackers never "rediscover" an old bug, because there are sooo many new ones to discover), disclosing the bugs is better. It is so because it speeds the patching rate, which in turn reduces the amount of exploitable bugs, which in turn improves the security. The connection between patching bugs and reducing significantly the amount of exploitable bugs can be made when the amount of bugs is small enough that new crackers are likely to rediscover old bugs, and then it would have paid to patch those bugs. Notice also that this is an auto-catalytic (self-accelerated) process: the more bugs disclosed, and more bugs patched, the less bugs remain, so the more it pays to further disclose and patch the remaining bugs, because the less bugs, the relatively more it pays to patch.

Vulnerability disclosure also helps to give vendors an incentive to fix bugs in subsequent product releases[5]. Arora et al. have shown through quantitative analysis that public disclosure made vendors respond with fixes more quickly; the number of attacks increased, but the number of reported vulnerabilities declined over time[6]

Good point! Not only disclosing the bugs is good for the consumers because it directly increases its quality, but also because it helps enforce a better behavior of the vendors. This is a key idea in the article, which delves in the fact that security policies are best when the one enforcing them is the one suffering from their errors. However, nowadays there is little pressure on the vendors to produce more secure software, because the buyer has little knowledge to judge this aspect of the quality, and ends up favoring a product for its looks or the alleged features, regardless of stability or security. Disclosing the bugs helps the buyer to assess the security of a program, thus making a better-balanced choice when buying. This, in return, leads to a more secure software in general, because vendors will have a big incentive to make their products more secure (which they don't really have now).

[1] E. Rescorla, paper presented in the Third Workshop on the Economics of Information Security, Minneapolis, 13 to 14 May 2004 (PDF)
[2] I suspect the authors are mistaking OpenBSD for FreeBSD
[3] A. Ozment, paper presented at the Fourth Workshop on the Economics of Information Security, Cambridge, MA, 2 to 3 June 2005 (PDF)
[4] A. Ozment, S.E. Schechter, paper presented at the 15th USENIX Security Symposium, Vancouver, 31 July to 4 August 2006 (HTML).
[5] A. Arora, R. Telang, H. Xu, paper presented at the Third Workshop on the Economics of Information Security, Minneapolis, 13 to 14 May 2004 (PDF)
[6] A. Arora, R. Krishnan, A. Nandkumar, R. Telang, Y. Yang, paper presented at the Third Workshop on the Economics of Information Security, Minneapolis, 13 to 14 May 2004 (PDF)

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Aclarado ancestral misterio
November 7th 2006

Acabo de dar respuesta a la antigua pregunta de:

¿Qué pesa más, un kilo de hierro o un kilo de paja?

La respuesta es clara: un kilo de hierro. No, no estoy de coña. Es así.

Si la pregunta fuera "¿Qué tiene más masa[...]", obviamente la respuesta es que tienen la misma masa (mil gramos). Ahora bien, nos preguntan por el peso. El peso de un objeto es la fuerza con la que un objeto es atraido hacia el centro de la Tierra ("hacia abajo"). En general, se entiende que esta fuerza corresponde al producto de la masa por la aceleración gravitatoria, y por lo tanto es directamente proporcional a la masa (y por tanto hierro y paja pesarían lo mismo).

Pero no es completamente cierto. Tanto el kilo de paja como el de hierro son objetos inmersos en un fluido: la atmósfera. Dado que el un fluido, el gas atmosférico (el aire) ejerce una fuerza de flotación (por el que un globo de helio asciende, por ejemplo), dado que esta fuerza se contrapone a la de la gravedad, y dado que es proporcional al volumen (el cual es mucho mayor en el caso de la paja), hemos de concluir que el kilo de paja es atraido hacia el centro de la Tierra con una fuerza menor, y por lo tanto pesa menos.

Esto es tan físico y real como pueda serlo cualquier otra cosa: si pones en los dos platos de una balanza simple un kilo de paja y un kilo de hierro, caerá del lado del hierro (si tiene suficiente sensibilidad), una balanza romana suficiente precisa dará el hierro como más pesado, y si cargamos con el kilo de paja por un lodazal, nuestras huellas se hundirán menos en el barro que si lo hacemos con el kilo de hierro. Sea como sea que lo medimos, el kilo de hierro pesa (ligeramente) más.

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Txema's SCF recipe for Gaussian
October 11th 2006

According to an e-mail by Txema, if you have a Gaussian job whose SCF does not converge, you can try the following:

scf(noincfock,conver=6,maxcyc=2025) iop(5/22=20)

Explanation:

  • NoIncFock: Prevents use of Incremental Fock formation
  • Conver=6: Request a mere 10-6 convergence criterion (whereas the default is 10-9, I believe).
  • iop(5/22=20): Use DIIS extrapolation, with energy-based mixing. OK, OK, I have no idea what this means, and I use it out of superstition... yeah, sue me.

Other keywords with (in)direct impact on convergence: QC, VShift, FullLinear.

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Gaussian shared memory
October 9th 2006

If you are running Gaussian in shared memory mode (in parallel in a multi-CPU computer, for example), you might get the following error (last line of output file):

shmget failed

It means that it was not possible to get the amount of shared memory required by the input. This can mean that the computer does not have so much physical RAM, but usually it is just a somewhat silly system setting.

Check the file /proc/sys/kernel/shmmax. Inside it there should be a single number, namely the amount of permited shared memory use (in bytes). If you need more (in my computer it was like 32MB... puaff), just echo xxx > /proc/sys/kernel/shmmax, where xxx is the desired amount of bytes (e.g. 500000000).

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