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
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
terça-feira, janeiro 26, 2010
quinta-feira, janeiro 21, 2010
terça-feira, janeiro 19, 2010
segunda-feira, janeiro 18, 2010
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
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]|
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
quinta-feira, janeiro 14, 2010
quarta-feira, janeiro 13, 2010
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.
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]|
segunda-feira, janeiro 11, 2010
Dentro de quatro ou cinco anos, estimam especialistas, um planeta capaz de abrigar vida deve ser encontrado
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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.