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terça-feira, janeiro 26, 2010

A energia escura e o Congresso

How to Build a Dark Energy Detector

Posted: 25 Jan 2010 09:10 PM PST

All the evidence for dark energy comes from the observation of distant galaxies. Now physicists have worked out how to spot it in the lab


The notion of dark energy is peculiar, even by cosmological standards.

Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10^-10 joules per cubic metre.

What's strange about this idea is that as space expands, so too does the amount of energy. If you've spotted the flaw in this argument, you're not alone. Forgetting the law of conservation of energy is no small oversight.

What we need is another way of studying dark energy, ideally in a lab on Earth. Today, Martin Perl at Stanford University and Holger Mueller down the road at the University of California, Berkeley, suggest just such an experiment

The dark energy density might sound small but Perl and Mueller point out that physicists routinely measure fields with much smaller energy densities. For example an electric field of 1 Volt per metre has an energy density of 10^-12 joules per cubic metre. That's easy to measure on Earth.

Of course there are some important differences between an electric field and the dark energy field that make measurements tricky. Not least of these is that you can't turn off dark energy. Another is that there is no known reference against which to measure it.

That leaves the possibility of a gradient in the dark energy field. If there is such a gradient, then it ought to be possible to measure its effect and the best way to do this is with atom interferometry, say Perl and Mueller.

Atom interferometry measures the phase change caused by the difference in two trajectories of an atom in space. So if a gradient in this field exists it should be possible to spot it by cancelling out the effects of all other forces. Perl and Mueller suggest screening out electromagnetic forces with conventional shields and using two atom interferometers to cancel out the the effect of gravitational forces.

That should allow measurements with unprecedented accuracy. Experiments with single atom interferometers have already measured the Earth's gravitational pull to an accuracy of 10^-9. The double interferometer technique should increase this to at least 10^-17.

That's a very exciting experiment which looks to be within reach with today's technology.

There are two potential flies in Perl and Mueller's ointment. The first is that the nature of dark energy is entirely unknown. If it exists and if there is a gradient, it is by no means certain that dark energy will exert a force on atoms at all. That will leave them the endless task of trying to place tighter and tighter limits on the size of a non-existent force.

The second is that some other unknown force will rear its head in this regime and swamp the measurements. If that happens, it's hard to imagine Perl and Mueller being too upset. That's the kind of discovery that ought to put a smile on any physicists face.

Ref:arxiv.org/abs/1001.4061: Exploring The Possibility Of Detecting Dark Energy In A Terrestrial Experiment Using Atom Interferometry


To Understand Congress, Just Watch the Sandpile

Posted: 24 Jan 2010 09:10 PM PST

The behavior of Congress can be modeled by the same process that causes avalanches in sandpiles.

What does it take for a resolution in Congress to achieve sizeable support? It's easy to imagine that the support of certain influential representatives is crucial because of their skill in the cut and thrust of political bargaining.

Not so, say Mikhail Simkin and Vwani Roychowdhury at the University of California, Los Angeles. It turns out that the way a particular resolution gains support can be accurately simulated by the avalanches that occur when grains of sand are dropped onto each other to form a pile.

Simkin and Roychowdhury begin their analysis with a study of resolution HR1207 and a plot of the number of co-sponsors it received against time early last year. This plot is known in mathematics as a Devil's staircase--it consists of long periods without the addition of any new co-sponsors followed by jumps when many new co-sponsors join during a single day. "One might have suspected that the biggest steps of the staircase are due to joining of a highly influential congressman bringing with himself many new co-sponsors which he had influenced," say Simkin and Roychowdhury.

That's uncannily similar to the way in which avalanches proceed in a a model of sandpiles developed by Per Bak, Chao Tang and Kurt Wiesenfeld in 1988. Perhaps Congress can be modelled in a similar way, reason Simkin and Roychowdhury.

Their model assumes that the roles of sand grains is played units of political pressure. They assume that there is a network of influence in Congress through which representatives exert political pressure on each other (just as sand grains exert forces on each other through the network of contacts between them in the pile). When the pressure on representatives reaches a threshold, they co-sponsor the resolution and this, in turn, puts pressure on other member of congress to sign.

This is like the pressure that builds up in a sandpile as grains are dropped onto it. When a threshold is reached at a certain point on the pile, an avalanche occurs which redistributes the pressure to other places.

In addition, the representatives are pressured by their constituents which is analogous to dropping grains of sand at random.

There is a difference between sandpiles and congress however. Once a representative has signed, he or she cannot do it again and so take no further part in the process. Any further pressure on them is simply dissipated. So representatives cannot topple more than once, unlike sand grains which can keep on toppling as the pile gets bigger.

This is a pretty simple model but when Simkin and Roychowdhury ran it, they found that it generates a Devil's staircase that is uncannily similar to the one generated by representatives for HR1207.

Perhaps the most interesting feature is that the model assumes that all representatives have equal influence. "In our model, big steps are a result of evolution of Congress to a sort of critical state, where any congressman can trigger an avalanche of co-sponsors," say Simkin and Roychowdhury.

The pair suggest some interesting ways to follow up their work. They point out that not all resolutions in Congress get the same level of support. In their model, this is due to the amount of public pressure, ie the number of units of political pressure dropped onto the pile at random. If there is no outside pressure, the resolution will not get sizeable support in a reasonable amount of time.

"An obvious extension to the model is to introduce political pressure against the resolution," they say, pointing out that an interesting case would be when the negative pressure exactly balances the positive. "It could explain the cases when a resolution quickly gains some support, which, however, never becomes overwhelming."

So representatives are not as important as perhaps they might imagine. Perhaps the stage should be replacing them with actual grains of sand. By Simkin and Roychowdhury's reckoning, it wouldn't make much difference.

Ref: arxiv.org/abs/1001.3732: Stochastic modeling of Congress

quinta-feira, janeiro 21, 2010

Explorando o relevo entrópico do multiverso


Deriving the Properties of the Universe

Posted: 17 Jan 2010 09:10 PM PST

The properties of the universe can be derived by thinking about the origin of complexity, says a new theory.


Physicists and cosmologists have long noted that the laws of physics seem remarkably well tuned to allow the existence of life, an idea known as the anthropic principle.

It is sometimes used to explain why the laws of physics are the way they are. Answer: because if they were different, we wouldn't be here to see them.


To many people, that looks like a cop out. One problem is that this way of thinking is clearly biased towards a certain kind of carbon-based life that has evolved on a pale blue dot in an unremarkable corner of the cosmos. Surely there is a more objective way to explain the laws of physics.


Enter Raphael Bousso and Roni Harnik at the University of California, Berkeley and Stanford University respectively. They point out that the increase in entropy in any part of the Universe is a decent measure of the complexity that exists there. Perhaps the anthropic principle can be replaced with an entropic one?


Today, they outline their idea and it makes a fascinating read. By thinking about the way entropy increases, Bousso and Harnik derive the properties of an average Universe in which the complexity has risen to a level where observers would have evolved to witness it.


They make six predictions about such a Universe. They say "typical observers find themselves in a flat universe, at the onset of vacuum domination, surrounded by a recently produced bath of relativistic quanta.These quanta are neither very dilute nor condensed, and thus appear as a roughly thermal background."


Sound familiar? It so happens that we live in a (seemingly) flat universe, not so long after it has become largely a vacuum and we're bathed in photons that form a thermal background. That's the cosmic infrared background that is emitted by galactic dust heated by starlight (this is different from the cosmic microwave background which has a different origin).


That's a remarkably accurate set of predictions from a very general principle. The question, of course, is how far can you run with a theory like this.


It certainly has the feel of a powerful idea. But, just like the anthropic principle, it also has the scent of circular reasoning about it: the universe is the way it is because if it were different, the complexity necessary to observe it wouldn't be here to see it.


That may not be so hard to stomach, given the power of the new idea. Even a hardened physicist would have to accept.that Bousso and Harnik have a remarkably elegant way of capturing the state of the universe.


Ref:arxiv.org/abs/1001.1155: The Entropic Landscape


The Entropic Landscape

We initiate a quantitative exploration of the entire landscape. Predictions thus far have focused on subsets of landscape vacua that share most properties with our own. Using the entropic principle (the assumption that entropy production traces the formation of complex structures such as observers), we derive six predictions that apply to the whole landscape. Typical observers find themselves in a flat universe, at the onset of vacuum domination, surrounded by a recently produced bath of relativistic quanta. These quanta are neither very dilute nor condensed, and thus appear as a roughly thermal background. Their characteristic wavelength is of order the inverse fourth root of the vacuum energy. These predictions hold for completely arbitrary observers, in arbitrary vacua with potentially exotic particle physics and cosmology. They agree with observation: We live in a flat universe at the onset of vacuum domination, whose dominant entropy production process (the glow of galactic dust) has recently produced a radiation bath (the cosmic infrared background). This radiation is marginally dilute, relativistic, and has a wavelength of order 100 microns, as predicted.
Comments:40 pages and 3 figures, references added
Subjects:High Energy Physics - Theory (hep-th); Cosmology and Extragalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)
Cite as:arXiv:1001.1155v2 [hep-th]

terça-feira, janeiro 19, 2010

Estatística de encontros com asteróides


Tomando dados de uma tabela de encontro com NEOS (Near Earth Objects) até 2178 dada aqui, fiz um gráfico do número desses encontros (nesta janela de tempo) que apresentam uma aproximação menor que a distância D em unidades astronômicas. Em outras palavras, qual a probabilidade de se ter um encontro menor que D neste período.

Encontrei que essa probabilidade é proporcional à D^2 (uma parábola), aproximadamente. Na verdade, a curva em vermelho é proporcional a D^1.95.

Depois de pensar um pouco, acho que achei um argumento (estatístico e geométrico) de porque existe essa regularidade. Colocarei aqui em um update, mas deixo como um enigma para meus amigos blogueiros que gostam de física e astronomia proporem soluções.

PS: Alguém conhece um paper sobre isso?

Amor em Londres: uma aplicação prática da Equação de Drake

Quem disse que a pesquisa em SETI não serve para nada?


Amor em Londres é tão raro quanto encontrar aliens, indica cálculo


da Folha Online

Romances podem acontecer diariamente, mas encontrar um verdadeiro amor em Londres é tão raro quanto alienígenas na galáxia, segundo divulgou um economista londrino nesta terça-feira (19).

Peter Backus, professor de economia na Universidade de Warwick, calculou que ele tem 0,00034% de chances de encontrar um amor na capital inglesa, usando a equação de Drake, que é aplicada por cientistas para determinar o número potencial de extraterrestres na nossa galáxia.

Bridget Jones -27.set.99/AP
Casal observa peças de ouro em joalheria de Londres; cálculo prevê 0,00034% de chances de encontrar um amor na capital inglesa
Casal observa peças de ouro em joalheria de Londres; cálculo prevê 0,00034% de chances de encontrar amor na capital inglesa

O nome da equação provém do astrônomo e astrofísico Frank Drake, responsável por desenvolver a equação no começo dos anos 1960.

Backus, 31, vive na região central de Londres, não idealizou nada em especial na parceira ideal, pedindo apenas que ela fosse uma mulher radicada em Londres, com idade entre 24 e 34 anos, e também com formação universitária.

"Não estou tentando ser elitista ou qualquer outra coisa", disse sobre a exigência educacional. "Qualquer um tem preferências. Eu acho só que teríamos mais em comum."

Reduzindo ainda mais as suas chances, ele estimou que se sentiria fisicamente atraído por apenas 5% das mulheres com todos estes critérios.

Isso significa que há por volta de 10,5 mil mulheres no Reino Unido que preenchem todos os critérios de Backus, disse ele --há por volta de 10 mil civilizações com potencial de comunicação que podem existir na Via Láctea, de acordo com a equação Drake.

Então apenas 0,14% dos londrinos e 0,017% da população britânica se enquadra nas exigências de Backus.

Mas aí a coisa se complica.

Se a mulher dos sonhos do economista é tão exigente quanto ele, suas chances de encontrar alguém que corresponda ao nível é apenas de 0,00034%.

"Há 26 mulheres no Reino Unido com as quais eu talvez tivesse um relacionamento maravilhoso. Então, em uma noite londrina, há 0,00034% de chances de encontrar uma dessas pessoas especiais", diz ele.

"Isso é uma em 285 mil chances. Nada formidável."

segunda-feira, janeiro 18, 2010

Astrobiologia Computacional


Numerical Testing of The Rare Earth Hypothesis using Monte Carlo Realisation Techniques

Duncan H. Forgan (1), Ken Rice (1) ((1) SUPA, Institute for Astronomy, University of Edinburgh)
The Search for Extraterrestrial Intelligence (SETI) has thus far failed to provide a convincing detection of intelligent life. In the wake of this null signal, many "contact pessimistic" hypotheses have been formulated, the most famous of which is the Rare Earth Hypothesis. It postulates that although terrestrial planets may be common, the exact environmental conditions that Earth enjoys are rare, perhaps unique. As a result, simple microbial life may be common, but complex metazoans (and hence intelligence) will be rare. This paper uses Monte Carlo Realisation Techniques to investigate the Rare Earth Hypothesis, in particular the environmental criteria considered imperative to the existence of intelligence on Earth.
By comparing with a less restrictive, more optimistic hypothesis, the data indicates that if the Rare Earth hypothesis is correct, intelligent civilisation will indeed be relatively rare. Studying the separations of pairs of civilisations shows that most intelligent civilisation pairs (ICPs) are unconnected: that is, they will not be able to exchange signals at lightspeed in the limited time that both are extant. However, the few ICPs that are connected are strongly connected, being able to participate in numerous exchanges of signals. This may provide encouragement for SETI researchers: although the Rare Earth Hypothesis is in general a contact-pessimistic hypothesis, it may be a "soft" or "exclusive" hypothesis, i.e. it may contain facets that are latently contact-optimistic.
Comments:13 pages, 10 figures, accepted for publication in the International Journal of Astrobiology
Subjects:Earth and Planetary Astrophysics (astro-ph.EP); Galaxy Astrophysics (astro-ph.GA)
Cite as:arXiv:1001.1680v1 [astro-ph.EP]

sexta-feira, janeiro 15, 2010

A principal diferença entre gripe suína e gripe comum

15/01/10 - 16h27 - Atualizado em 15/01/10 - 17h03

EUA estimam que nova gripe adoeceu 39 milhões a 80 milhões de americanos

Números referem-se ao período entre abril e 12 de dezembro de 2009.
Hospitalizações causadas pelo vírus H1N1 foram 173 mil a 362 mil.


O Centro para Controle e Prevenção de Doenças dos Estados Unidos (CDC) divulgou nesta sexta-feira (15) seu boletim de avaliação do quadro epidemiológico da nova gripe. O órgão estimou que o vírus H1N1 adoeceu entre 39 milhões a 80 milhões de americanos e levou a 173 mil a 362 mil hospitalizações. O total de mortes causadas pela influenza foi de no mínimo 7.880 e no máximo 16.460.

O boletim do CDC confirma que pessoas com menos de 65 anos são mais gravemente afetadas pela doença. Cerca de 90% das hospitalizações estimadas e 88% das mortes foram de americanos com menos de 65 anos. No caso da gripe comum, a situação é inversa: 60% das hospitalizações e 90% das mortes são de pessoas com 65 anos de idade ou mais.


Referencia para o paper da Ariadne:


Scaling in the Global Spreading Patterns of Pandemic Influenza A and the Role of Control: Empirical Statistics and Modeling

The pandemic of influenza A (H1N1) is a serious on-going global public crisis. Understanding its spreading dynamics is of fundamental importance for both public health and scientific researches. In this paper, we investigate the spreading patterns of influenza A and find the Zipf's law of the distributions of confirmed cases in different levels. Similar scaling properties are also observed for severe acute respiratory syndrome (SARS) and bird cases of avian influenza (H5N1). To explore the underlying mechanism, a model considering the control effects on both the local growth and transregional transmission is proposed, which shows that the strong control effects are responsible for the scaling properties. Although strict control measures for interregional travelers are helpful to delay the outbreak in the regions without local cases, our analysis suggests that the focus should be turned to local prevention after the outbreak of local cases. This work provides not only a deeper understanding of the generic mechanisms underlying the spread of infectious diseases, but also an indispensable tool to decision makers to adopt suitable control strategies.
Comments:13 pages, 16figures
Subjects:Physics and Society (physics.soc-ph)
Cite as:arXiv:0912.1390v1 [physics.soc-ph]

Navegar, em redes complexas, é preciso...

The Global Shipping Network Revealed

Posted: 14 Jan 2010 09:10 PM PST

Despite carrying 90 per cent of the planet's trade, nobody has mapped the network of links between the world's ports. Until now.


The International Maritime Organization based in London estimates that 90 per cent of the world's trade is moved around the planet by sea. Given the fascination that complexity scientists have with rail, air and road networks, it seems strange that so little attention has been paid to the maritime network.


That's a wrong that's put right today by Pablo Kaluza and pals at Carl von Ossietzky University in Oldenburg, Germany. These guys have used the itineraries of 16,363 cargo ships during 2007 to construct a network of links between the world's top 951 ports. The results, at least in part, are eyebrow raising.


First, the unsurprising news. Kaluza and co show that these links form a small world network in which it is easy to move from one port to another in a small number of jumps. That's just what you'd expect given what we know about other transport networks.


However, the maritime network shows some surprising differences from the network that flight paths make between airports. For example, on average, it takes just 2.5 jumps to move from one port to another on the maritime network compared to an average of 4.4 to move between one airport and another. The maximum shortest path between ports is 8 jumps while between airports it is 15 jumps. It looks to be decidedly easier to travel the world by ship than by plane, at least in some respects.


One oddity, however, is that the maritime network is decidedly asymmetric: more than half of all ports are linked in only one direction, meaning that cargo ships do not routinely make round trip journeys between ports.


Perhaps most interesting of all is that different types of ship move in different patterns. Kaluza and co study three types of ship: container ships, bulk dry carriers and oil tankers. While container ships tend to follow regular schedules, the movement of bulk dry carriers and oil tankers is much less regular. That's because their routes are determined by the price of the commodities they carry and which vary enormously. Bulk dry carriers and oil tankers are also more likely to sail empty.


That's important because it gives a unique insight into the pattern of world trade. But there is another reason too.


One of the most significant methods of cross species invasion is from water sucked into ships' ballast tanks in one part of the world and discharged in another; a particularly important factor when ships sail empty.


So the new network map should give marine biologists an insight into how bioinvasion occurs and what steps they can take to tackle it.


Ref: arxiv.org/abs/1001.2172: The complex network of global cargo ship movements

quarta-feira, janeiro 13, 2010

Novas metáforas físicas em Economia


Parodiando Engels, "todo economista vivo é escravo de um físico morto". A questão não é se novas idéias e conceitos físicos podem ser aplicados às ciências econômicas e sociais, mas por que os velhos conceitos da física newtoniana ("equilíbrio", "forças" econômicas, "tensão" social etc.) permanecem no discurso sócio-econômico.


The (unfortunate) complexity of the economy

This article is a follow-up of a short essay that appeared in Nature 455, 1181 (2008) [arXiv:0810.5306]. It has become increasingly clear that the erratic dynamics of markets is mostly endogenous and not due to the rational processing of exogenous news. I elaborate on the idea that spin-glass type of problems, where the combination of competition and heterogeneities generically leads to long epochs of statis interrupted by crises and hyper-sensitivity to small changes of the environment, could be metaphors for the complexity of economic systems. I argue that the most valuable contribution of physics to economics might end up being of methodological nature, and that simple models from physics and agent based numerical simulations, although highly stylized, are more realistic than the traditional models of economics that assume rational agents with infinite foresight and infinite computing abilities.
Subjects:General Finance (q-fin.GN); Physics and Society (physics.soc-ph)
Journal reference:Physics World, April 2009, p.28-32
Cite as:arXiv:0904.0805v1 [q-fin.GN]

Asteróide passa perto da Terra


13/01/2010 - 10h21

Asteroide "ex-lixo espacial" passa perto da Terra nesta manhã

Um asteroide descoberto neste domingo (10) passa perto da Terra nesta quarta-feira (13), às 10h46 do horário de Brasília.

O objeto foi identificado pelos Laboratórios Lincoln do MIT (Massachussets Institute of Technology), e vai passar a cerca de 122.300 quilômetros do planeta.

Em razão de a sua órbita ser bem parecida com a da Terra no período de um ano, alguns cientistas haviam sugerido antes que o objeto seria um estágio de foguete em órbita ao redor do Sol, ou seja, um lixo espacial.

Chamado 2010 AL30, o asteroide não representaria riscos mesmo que se chocasse com a Terra.

Ele possui entre 10 e 15 metros de diâmetro, e espera-se que objetos abaixo de 25 metros como esse apenas se queimem na atmosfera do planeta.

Interessante que, com essa figura, podemos estimar a velocidade do objeto em relação à Terra. Na minha conta deu 30-40 mil Km/h.

Terremoto no Haiti, hora a hora


A blogosfera é uma rede sensorial que podemos seguir hora a hora no Blogpulse. O número de posts em inglês é de cinco a seis vezes maior que em português ou castelhano (ver abaixo). Mas o número de posts sobre o Haiti é de três vezes a quase uma igualdade. Existem muitos brasileiros na missão de paz no Haiti (e foi noticiado agora que Zilda Arns morreu no terremoto). Ou seja, na blogosfera como na vida, dói mais quanto mais próximo estamos uns dos outros.

segunda-feira, janeiro 11, 2010

Terra a vista


A menos que a Hipótese da Terra Rara esteja correta (ou seja, de que a evolução de seres multicelulares e explosões Cambrianas sejam muito raras), não vejo como escapar o Paradoxo de Fermi Local: se existem planetas habitados num raio de 1000 anos-luz daqui, por que eles ainda não nos colonizaram?

Descoberta de planeta habitável é iminente, dizem astrônomos

Dentro de quatro ou cinco anos, estimam especialistas, um planeta capaz de abrigar vida deve ser encontrado

Associated Press

Tamanho do texto? A A A A

WASHINGTON - Astrônomos afirmam que estão à beira de encontrar planetas semelhantes à Terra em órbita de outras estrelas, um passo essencial para determinar se estamos sozinhos no Universo.

Planeta semelhante à Terra pode ser inferno vulcânico

Nasa descobre planetas gigantes fora do Sistema Solar

Cerca de 15% dos sistemas solares são como o da Terra

Moléculas orgânicas descobertas em mais um planeta distante

Um alto funcionário da Nasa e outros importantes cientistas dizem que, dentro de quatro ou cinco anos, o primeiro planeta semelhante à Terra e capaz de abrigar vida deve ser encontrado, ou talvez até já tenha sido. Um planeta com o tamanho aproximado da Terra pode até mesmo ser anunciado ainda este ano, se certas pistas detectadas por um telescópio espacial se confirmarem.

Na reunião anual da Associação de Astronomia dos Estados Unidos, cada uma das descobertas a respeito de "exoplanetas" - os localizados fora do Sistema Solar - aponta para a mesma conclusão: planetas onde a vida pode surgir provavelmente abundam, a despeito da violência do ambiente espacial, repleto de explosões, buracos negros e colisões.

domingo, janeiro 10, 2010

Novo blog de (uma) matemática?


Faz tempo que não comento sobre novos blogs de ciência aqui. Bom, como o ABC está desligado para reparos (ele parece o LHC! - alguém sabe como bloquear malwares no Joomla?), vou anotando aqui blogs que encontrei recentemente - no caso através de um comentário de sua autora Sandy, foto ao lado. O nome do blog não é muito curto, tenho medo de pensar no nome que ela irá colocar em sua tese de pós-graduação...

A VIDA É UMA CAIXINHA DE SURPRESAS QUE NÃO OBEDECE A GEOMETRIA EUCLIDIANA.


OU SEJA, A CAIXINHA É BEM MAIOR POR DENTRO DO QUE POR FORA, E CONTÉM BEM MAIS SUPRESAS DO QUE APARENTA. O DETALHE É QUE ELAS NEM SEMPRE SÃO BOAS... =P MAS, A SITUAÇÃO SEMPRE PODE PIORAR, POR ISSO NEM TUDO ESTÁ PERDIDO NEM NUNCA ESTARÁ... EM OUTRAS PALAVRAS, NÃO FAÇO A MÍNIMA IDÉA DE QUAL SERIA A DESCRIÇÃO ADEQUADA PARA O BLOG... =P

É um blog novo, seu primeiro post é de julho de 2009. Se tivesse participado do II EWCLiPO, levaria o prêmio do Blog-bebê!

Exemplo de post:


(...)

-Pessoa humana;
essa é a minha favorita. A definição mais geral da palavra "pessoa" é "ser pensante". É total "umbiguicentrismo" dizer que para ser pessoa é preciso ser humano. Para ser humano é preciso ser pessoa (not exactly,but...) mas a recíproca não necessariamente é verdadeira.


Outras pérolas das expressões populares são frases como:

"não foi nada."- se não foi nada então foi algo??

"não há ninguem aqui."- se não há ninguém então há alguém? (considerando que "ninguém" seja o oposto de "alguém").
e por aí vai:

"não há ninguém melhor do que eu";
"eu não quis dizer nada";
"eu não sei de nada";

O problema é que essas frases são negações de negações, mas negações de negações são "afirmações". Em lógica o símbolo ¬representa uma negação. Utilizar essa operação duas vezes seguidas faz com que a negação seja negada, se a negação é negada então não há mais negação.
A palavra "nada" é a negação de "algo", "niguém" é a negação de "alguém". Se colocamos a palavra "não" em frases que usam essas palavras estamos negando a negação. Então:

-não ninguém melhor do que eu ->há alguém melhor do que eu.
-eu não quis dizer nada -> eu quis dizer algo.
-eu não sei de nada -> eu sei de algo.

Mas o que não faz a força do hábito né ¬¬....eu não costumo não usar essas expressões...

ps: não vou nem comentar sobre a nossa gramática.... =/