Atendendo a algumas respostas apreciativas e inúmeras ignorativas, aqui está......

31 março 2011

Why is the Cauchy problem for elliptic equations not well posed?

I spent a long morning at the library trying to understand why it so happens that the Cauchy problem for elliptic partial differential equations is not well posed. I still don't - I understand that it is, but what is the geometrical property of elliptic equations that makes it not well-posed, in contrast to parabolic or hyperbolic equations?

In the process I was not impressed by the creativity shown in textbooks. There are some good books out there - but the majority of them are isomorphic to each other. Most are well-written, a few stand out. But mostly they present the same topics in a different order, with more or less space devoted to each of them. Every preface should answer: "Why is this book different from all the other books?" Unfortunately, it seems that the question is not on most publishers' minds.

The good side is that there are plenty of open problems, even in areas that have already been studied a million times. Ask good questions, don't try to read all the literature, and have fun!

29 março 2011

Iracema e Lucíola

Antes que comecem a elogiar o cara porque ficou doente e morreu, na minha opinião deixar um país sem um vice-presidente em condições de exercer o cargo é falta de respeito com os concidadãos, descaso com a atividade do governo, e um tremendo egocentrismo. #prontofalei

The battle is won by strategy; the war by reliable supply lines

I was reminded of the following #fact:

"The fight is won by bravery; the battle by strategy; and the war by reliable supply lines."

by Krugman's blog post. Who proved the theorem above? I don't know, and I don't have time to research - I need to assign reviewers to 15 conference papers before the other editors fill up the reviewers' inboxes with requests.

21 março 2011

Terremoto no Japão: a Usp pode ajudar?

Terremoto no Japão. Deve haver muito estudante deslocado, universidade sem aula. Gente com bom currículo, futuro promissor, só que com habilidades pouco relevantes para a tarefa imediata de reconstrução. Professores também nessa situação. O problema no Japão não é dinheiro nem recursos, mas por algum tempo os recursos e as necessidades não vão estar casadas.

Podemos abrir vagas de emergência em universidades brasileiras, na Usp, especialmente na Poli? Os estudantes mudam de ares, continuam seus estudos, e contribuem para a Usp estudando conosco. Também facilita a reconstrução lá, alivia um pouco a necessidade imediata. Para a Usp, não atrapalha nada. Em alguns cursos, alguns alunos a mais. O professor fala em português, explica um pouco em inglês, quem sabe alguém ajuda em japonês, em 6 meses os alunos estão em casa. Não atrapalha nem o professor nem os outros alunos - pelo contrário, é um desafio a mais que ajuda a sair da rotina e valorizar as aulas.

Custo baixo, benefício para os 2 lados. Adaptação em S Paulo não vai ser problema para quem quiser vir. Não serão muitos - mas cada estudante ou professor que vier, é um ganho para todos. Dá certo? Se há dificuldades, são apenas burocráticas. Mais uma razão para enfrentar a burocracia.

15 março 2011

Why nuclear power is always unsafe

I was asked to explain why nuclear power is inherently and unavoidably unsafe, and will try to give a common sense engineering answer. You know that I am not an expert. This is actually an advantage, because in this case the more people know, the more they say nonsense. Why? Because a little knowledge is a dangerous thing. The nuclear engineering community has answers to all questions they have asked, but the problem is too complex for them to have asked enough questions. Decisions about nuclear power cannot be left to the nuclear power community, the same way that decisions about nuclear weapons cannot be left to the military, and banking regulation cannot be left to bankers. Their excessive familiarity with the subject limits their thinking, distorts their knowledge, and corrupts their judgment.

The amount of energy that can be released by nuclear reactions is enormous. Worse, it can be released very fast, and in a very small space. Even worse, fissile material can continue to release energy for very long period of time.

Nuclear reactors cannot be designed in a safe manner. What can be done is lower the probability of a given undesired event. However, the damage caused by a nuclear disaster is almost unbounded. The total risk will remain very large even if a sophisticated design makes the probability of an individual event small. Contrast that to the worst-case scenario of a airplane crash, or a fire at a thermoelectric plant. Such accidents are comparatively common, however the total damage is limited to people on the plane and at the crash site, or in a neighborhood of the power plant. The area and the number of people threatened by a nuclear plant are incomparably larger. The fact that nuclear accidents are so rare only gives plant operators a misleading sense of security. (Another comparison is to the risk that large, highly-leveraged banks bring to the economy. A failure can cause paralysis to the whole financial system, but bankers reason with basis on the small-scale events that they observe daily. The safer the system, the longer the interval between crashes, the bigger the risk.)

Nuclear reactors cannot be built in a safe manner. The high energy content of the fissile material amplifies the consequences of any disaster. No place is safe enough at the time scales of nuclear fuel decay. Even at the small time scales of reactor operation, disasters happen. Japan is more prepared for earthquakes than anyone else, but still not prepared enough. And earthquakes happen everywhere, not only in Japan. Moreover, nuclear material is an inviting target for acts of war. France, the only one of the 4 major operators of nuclear plants which hasn't had a serious nuclear accident at home yet, built a nuclear power plant in Saddam Hussein's Iraq. Had in not been destroyed by Israel before it could enter operation, it would likely have been the target of Iranian bombs later, with consequences that we can only imagine.

Nuclear reactors cannot be operated in a safe manner. This is a straightforward engineering consideration. In case of malfunction, a nuclear reactor becomes inaccessible and cannot be maintained properly. A disaster becomes inevitable. No design can avoid this consideration, and therefore no design is acceptable from an operational point of view. An appropriate engineering comparison is an iPhone. It is difficult to fix an iPhone if some part goes bad. This is a problematic engineering decision with many detractors. However, it's just an iPhone. If it breaks, it broke. Nuclear reactors cannot be maintained if something goes wrong, and in that is not acceptable.

A common engineering requirement is that a critical system be fail-safe (baka-yoke in Japanese): that in case of failure it goes to a safe mode. The safest fire doors are kept open by electromagnets, and close without any intervention if power is cut. Nuclear power plants are exactly the opposite: they cannot be turned off safely. Even a reactor that has already been turned off is not safe without power.

14 março 2011

Academic boycott of university rankings

In the 2010 Times Higher Education World university rankings, Israeli universities were excluded from the top 200 list. In the list of 400 universities (not available on the web, only as an iPhone app), the Technion appears in position 221 and Bar Ilan University in 354. The Hebrew University and the University of Tel Aviv, which in 2008 were ranked 93 and 114, were not included in the most recent ranking.

The Academic Ranking of World Universities published in Shanghai includes 6 institutions from Israel among the world's top 400: the 4 above plus the Weizmann Institute and Ben Gurion University.

Does anyone have an explanation of why Times Higher Education excluded the top Israeli institutions from its ranking? Preferably an explanation without the words "Qaddafi oil money", please.

3 things wrong with nuclear reactors: design, construction, and operation

There are only 3 things wrong with nuclear reactors: design, construction, and operation. It appears that the atomic energy community considers fission reactors as versions of thermal power plants, only more powerful. They are wrong. They are wrong in the same way that the people in the military and political establishments who thought of nuclear bombs as stronger dynamite were wrong. Top scientists such as Feynman and Oppenheimer knew better, and were terrified of the bomb.

Nuclear energy is completely different from the energy that comes from chemical reactions which combine carbon with oxygen. The underlying nuclear forces are more powerful, longer-lasting, and more destructive to chemical-based lifeforms by orders of magnitude. A quantitative change means a qualitative change.

Because fissile nuclear material can release so much energy and can keep releasing it over long periods of time, it is not possible to design, build, and operate fissions reactors safely - certainly not with the technological and managerial level we currently employ, which is appropriate for burning gas, oil, and coal. This fact was demonstrated by experience: if the Japanese cannot use their reactors safely, then who can? Neither the richest and freest country in the world could; nor the most secretive, militarized, and centrally planned; now we see that the best organized and most disciplined people in the world aren't able to design, construct, and operate nuclear reactors in a safe manner.

Supporters of nuclear power will say that Japan suffered an earthquake. Yes, they have earthquakes and tsunamis more often then most other countries, and therefore are better prepared for them. In the time scales of atomic power, every place will suffer natural disasters for which even the Japanese haven't prepared. Anyone who thinks they can operate a nuclear power plant safely is like a drunkard who thinks only bad drivers get into accidents.

11 março 2011


Geeks of a certain age will remember the great RISC x CISC wars. Back in 1990 I, like everyone else, was convinced that reduced instruction set computers would win and complex instruction set chips such as 486 and Pentium were toast. For example, NeXTStep ran on Intel and Motorola oldstyle CISC chips, and on HP and Sparc wave-of-the-future RISC ones. I was wrong.

All chipmakers except for Intel changed their microprocessor architectures to RISC, but Intel still won, thanks to economies of scale and good engineering incorporating RISC ideas. HPPA, PowerPC, and Sparc are gone or irrelevant. Even Apple was brought back to CISC by the low power and multicore designs coming out of Intel's Haifa shop. The only surviving CISC architecture of any importance is x86, but that is precisely what my computer has inside, and yours.

Unless of course you are reading this in an iPad or iPhone. Then your mobile device runs iOS, a direct descendant of NeXTStep, and has a RISC processor with the ARM architecture (chances that someone would read a blog on other mobile devices are so slim that it's not worth checking what kind of processor they use. ARM RISC also, probably, or lookalike). And where are the technological advances and the profits, in desktop computers or mobiles?

It seems that RISC won after all. As did NeXT. Just took longer than expected.

07 março 2011

Rashid Khalidi no Nation

Sobre o artigo de Rashid Khalidi no Nation (link). Vale a pena ler, como o Chacra indicou, mas está cheio de erros. Os maiores na minha opinião são:

Primeiro, existe a fração da mídia ocidental que mostra os árabes como terroristas, mas não é difícil encontrar uma visão sobre o Oriente Médio rica e cheia de nuances. O exemplo melhor no Brasil é o blog do Guga Chacra! Há também colunistas no NYTimes que conhecem bem a região, para dar outro exemplo. Khalidi parece identificar "Western Media" com Fox News. Uma simplificação quase comparável a identificar muçulmanos com terroristas. (Nessa última frase pode ser lida minha opinião sobre a a imprensa de extrema direita.)

Segundo, justamente agora que os cidadãos de países árabes estão tomando a iniciativa de decidir sobre seus destinos, o artigo busca trazer o debate de volta para o colonialismo, o império Otomano, as potências europeias, e os padrões de interferência americana. Me parece, ou pelo menos tenho a esperança, que é para os termos desse debate que os jovens no Norte da África estão dizendo "Basta!"

Terceiro, Khalidi não resiste a no final invocar o conflito entre Israel e Palestina como a chave de todos os problemas do mundo árabe. Por décadas os tiranos do Oriente Médio tem usado esse conflito entre 2 dos menores países da região para adiar a solução dos problemas de países muito maiores. Os fatos parecem estar se construindo exatamente ao contrário: só após os países árabes estabelecerem regimes razoavelmente livres e democráticos é que israelenses e palestinos vão ter a possibilidade de resolverem seus conflitos com menos interferência externa negativa.

O ponto de vista expresso por Khalidi não é parte da solução. É parte do problema.