Bolhas de nada

Bolhas de nada, literalmente bubbles of nothing, é algo que frequentemente aparece em teoria de cordas mas sua origem é mais antiga. Está associado ao possível decaimento não perturbativo de espaço-tempos instáveis. Uma boa descrição desse processo está descrita em Bubbles of "nothing". Para os que nunca ouviram falar sobre vácuos não perturbativos talvez seja melhor ler primeiro algo sobre instantons.

Ghostly photons, axions or noncommutativity?

In March the PVLAS collaboration in Italy found a small polarization rotation when a laser beam went through a region with a large magnetic field. See here. The result can be interpreted as photons turning into axions, a still unobserved particle associated with the conservation of CP symmetry in strong interactions. Axions have no electric charge, a very small mass, and interact very weakly through the strong and weak forces. A very difficult guy to be detected!

Now DESY is planning an experiment using the FLASH, a machine which produces an extremely bright laser beam. The laser will go through a region with intense magnetic field and will be stopped by a wall. In the magnetic field some photons will turn into axions, like in the PVLAS experiment, pass through the wall, and another magnetic field on the other side of the wall will turn these axions back to photons. So photons will apparently come through the wall like ghosts (not the Faddeev-Popov ones!). Very clever! The proposal is here. They are planning to start the experiment by the end of 2006.

If the effect is real and if no axions are found an alternative explanation could be noncommutativity. A noncommutative photon acquires a phase in an electromagnetic background. This causes a rotation in the polarization as observed in PVLAS collaboration. In the DESY experiment no photons would be observed through the wall. That would be great!

Strings 2006: the movie

If you could'nt go to Beijing for the String conference you can now see the movie. You will understand why string is bound by faith as many bloggers say. The link is here. The homepage for the movie is here. Have fun...

Teoria de supercordas e constante cosmológica

Uma das coisas mais surpreendentes das teorias supersimétricas é que não existe uma extensão supersimétrica do grupo de de Sitter mas apenas do grupo de anti-de Sitter. O grupo de de Sitter descreve um espaço-tempo com constante cosmológica positiva enquanto anti-de Sitter tem uma constante cosmológica negativa. Portanto, a supersimetria definitivamente prefere anti-de Sitter (uma das razões da existência da correspondência AdS/CFT).

A descoberta de que o universo está em expansão acelerada, e que essa expansão acelerada pode ser atribuída a uma constante cosmológica positiva, parece fornecer um argumento muito forte de que a supersimetria realmente não existe na natureza. Se a supersimetria não for encontrada no LHC nos próximos anos, a supersimetria estará em maus lençois.

Entretanto, esse não é o único problema relacionado à constante cosmológica. É necessário uma constante cosmológica positiva incrivelmente grande para dar origem à inflação cosmológica. Lembre-se que a inflação é hoje confirmada pelos dados do WMAP. Como a constante cosmológica hoje é positiva e extremamente pequena, é necessário compreender porque ela diminiu tanto nestes 13.7 bilhões de anos. Além disso, a densidade de matéria hoje e a densidade de energia escura hoje são praticamente iguais e não há razão para que isso seja assim. Não existe nenhum teoria que explique todos esses fatos sobre a constante cosmológica.

O que a teoria de supercordas afirma sobre isso? Como ela é supersimétrica não admite soluções de de Sitter e parece estar bastante distante do mundo em que vivemos. Entretanto, há várias possibilidades de compactificar a teoria de cordas e gerar uma constante cosmológica positiva. Um bom trabalho de revisão é The Quest for a Realistic Cosmology in the Landscape of String Theory, de Andrew Chamblin. Apesar de ser de 2004 é bastante atual. Explica, inclusive, a tentativa mais recente de KKLT que usa compactificação com fluxos.

São Paulo in Seed Magazine

There is a small article in Seed Magazine about São Paulo, Science City: São Paulo. It starts with the usual misconceptions: a sprawling metropolis surrounded by ocean and rainforest. It is really a huge city with more than 15 million people but the nearest beach is about 80 Km from São Paulo and no rainforest is nearby. Anyway, the article mentions São Paulo University, Estação Ciência (a multidisciplinary museum), Arte Ciência no Palco (art and science on stage) and Butantan Institute (where you can see snakes and spiders alive). The article is very superficial and misses many important points. For instance, Instituto Butantan makes many vaccines and has to fight against the European and American pharmaceutical industries to export its products and even to sell them in Brazil. It will start to produce the bird flu vaccine soon. Also there are many high tech industries in São Paulo state, in cities like Campinas (telecommunications), São Carlos (optics), São José dos Campos (space and flight technology) and Ribeirão Preto (health and biotechnology). It would be fairer to have the title Science state: São Paulo.

Os senhores do anel e outros filmes

Um pequeno filme sobre o LHC, Lords of the ring, está disponível no Seed Magazine. O anel referido no título é o anel de colisões do LHC. Veja os túneis onde ocorrerão as colisões e os enormes detectores. Quem tiver a oportunidade ir ao CERN não deve deixar de visitar o LHC. Fiz isso e garanto que é impressionante. É necessário atualizar o Qucktime para ver o filme. (Via Reference Frame)

Veja entrevistas com pessoas famosas colecionadas em Peoples Archive. Existem 31 entrevistas atualmente, entre elas: Freeman Dyson, Sir Michael Atiyah, Hans Bethe, Francis Crick, Murray Gell-Mann, Benoit Mandelbrot, Edward Teller e John Wheeler. Voce tem que se registrar primeiro.(Via Not even wrong).

Another hoax in physics?

Ask to a physicist working with gravitational waves to write some questions about his area and then ask another physicist and a social scientist to answer those questions. Deliver the answers to a group of physicists and researchers in human sciences and ask them to find out which is the real physicist. Surprisingly, the physicists group could not recognize his real partner while the human scientists did much better. This was a front page matter of Folha de São Paulo (in Portuguese and subscription required) today. It reminds the Sokal affair, when the physicist Alan Sokal published a pseudo-scientific paper in a humanities journal, and the Bogdanov affair, a controversy about some papers on the Big Bang published by the Bogdanov brothers in respectable physics journals. They all raise serious questions about the strength of the peer-review system that selected the research for publication.

Fortunately the gravitational wave affair is not a hoax but the result of a study made by Harry Collins and collaborators at the Cardiff School of Social Sciences. Collins is a sociologist which works on the nature of scientific knowledge and has some interest in physics. He wrote the book Gravity’s Shadow , where he presents a sociological view of the search for gravitational waves, discusses the nature of knowledge, the processes of scientific discovery and the nature of sociological research (as stated in his website; I did not read the book). In his paper , which he says is to be published in Studies in History and Philosophy of Science 37A, 4 (2006), he reports on his new work described above. The questions made by the gravitational wave physicist are not trivial. Some of them are:

Is a spherical resonant mass detector equally sensitive to radiation from all over the sky?

State if after a burst of gravitational waves pass by, a bar antenna continues to ring and mirrors of an interferometer continue to oscillate from their mean positions? (only motion in the relevant frequency range is important).

Imagine that the end mirrors of an interferometer are equally but oppositely (electrically) charged. Could the result of a radio-wave incident on the interferometer be the same as that of a gravitational wave?

Of course, if you know some physics you can justify any answer, be it right or wrong. The questions were answered by a physicist and by Collins himself. Then 9 physicists were asked to see the answers and to point out who was the real physicist. Just one chose the real physicist, 7 chose Collins and one could not reach any conclusion. In the other group, 8 researchers, mainly from humanities, analyzed the answers. Five of them chose the real physicist, 2 chose Collins and one could not decide among the two.

So what can we learn from all this? This is similar to a Turing test where you have to decide if a computer has intelligence. You ask questions to a computer and to a person, without knowing who is who, and analyze the answers. If you can not decide who is the person then the computer has developed intelligence. What Collins is showing is that even if you have a general knowledge of some area but are not completely immersed in it, you can make comments and even give suggestions as if you were a specialist. In fact, sometimes we do that when refereeing papers or proposals in areas close to ours. This is what happened in the Sokal and Bogdanov affairs. Folha de São Paulo reports that Sokal said that he is strike by Collins work. I can understand why.

I have now found out that this was also published in Nature. A subscription is needed and I can not read it from home. Anyway, here is the link.

UPDATE: Folha de São Paulo article (in Portuguese) can be accessed here. There is also some comments by Marcelo Leite (in Portuguese) published yestarday which can be accessed here. Nature is not accessible unless you have a premium subscription (my instituion doesn't have one).