Archive for November, 2008

Counting negative links make network models more realistic

Tuesday, November 18th, 2008


Spotting communities within networks is a big deal. Not least for search engines that rely heavily for their results on the communities that form when websites point to each other. If a lot of websites point to another site then that proves it is of value.

At least that’s what everyone has assumed. But links can be negative as well as positive. If lots of websites point to another site specifically to say how bad it is, then the community is actually saying the site has little value.

Being able to tell the difference, then, is crucial, not only for search results but in understanding the structure of the network and the communities that emerge.

Vincent Traag at the University of Amsterdam in the Netherlands and a buddie say that including negative as well as positive links, profoundly changes the pattern of communities that you find in a network.

They’ve applied the idea to a dataset called the Correlates of War that provides details of agreements and disputes between 138 countries between 1993 and 2001.

In terms of the network, a negative link is the same as a positive link but pointing in the opposite direction (it has the opposite sign).

By putting the links into a model of the world, Traag has worked out what global communities existed at the time. The communities that emerge are the standard power blocs well known to historians: the West; Latin America; Russia & China; West Africa; North Africa & the Middle East; and a collection of independents not truly forming a bloc.

That’s almost exactly as historians would put it except for one or two features. For example, west Africa does not normally figure as a power bloc on its own and the independents include New Zealand which would normally be classified as part of the West.

That provides an interesting and somewhat unconventional insight ino the politics of the time.

Ref: Community Detection in Networks with Positive and Negative Links

The exoplanet photo gallery is bigger than you think

Monday, November 17th, 2008


Astronomers tend to get excited by pinpricks of light. And perhaps today they have more reason than usual to celebrate the pixels that Paul Kalas at the University of California, Berkeley, and pals have found in one of the Hubble Space Telescope’s images.

These pixels, they say, represent the first optical image of a planet orbiting another star. The star in question is Fomalhaut in the southern constellation of Piscis Austrinus and one of the brightest in the sky.

Kalas and co say the planet is about three times the mass of Jupiter orbiting at a rather distant 119 AU. By comparison, Neptune orbits at around 30 AU so this is going to be one cold body.

That’s impressive work that has had significant press coverage but let’s put it in perspective.

Last year, the infrared Spitzer Space Telescope photographed HD 189733b, a Jupiter-sized gaseous planet orbiting a yellow dwarf in the constellation of Vulpecula. It even produced a heat map of the surface showing, unsurprisingly, that the planet is warmer at the equator than at the poles. But the map of HD 189733b got almost no coverage. And images of various “hot Jupiters” have been around for perhaps a decade or so.
I guess Hubble just has a better PR team.

Ref: Optical Images of an Exosolar Planet 25 Light Years from Earth∗

Cloak ‘n’ dagger

Saturday, November 15th, 2008

The best of the rest from the physics arXiv this week:

Acoustic Noise in Deep Ice and Environmental Conditions at the South Pole

Gravitational Strings. Do We See One?

Computer Simulations of Pulsatile Human Blood Flow Through 3D-Models of the Human Aortic Arch, Vessels of Simple Geometry and a Bifurcated Artery: Investigation of Blood Viscosity and Turbulent Effects 

Colossal Dielectric Constant up to GHz at Room Temperature

De Broglie-Bohm Pilot-Wave Theory: Many Worlds in Denial?

Quantum cloaking makes molecules invisible

Friday, November 14th, 2008

Cloaking is surely the zeitgeist topic of the moment and for proof, you need look no further than the work of Jessica Fransson from the University of Upssala in Sweden and colleagues. This is a group who have who have applied the ideas of cloaking to the quantum world and come up trumps. the result is a design for a molecular cloak that could turn out to be extremely useful.

First what does it mean to see or not see a quantum object? Fransson and co say that seeing is equivalent to detecting quantum objects and in the case of molecules that means looking for the terahertz radiation they produced when they vibrate.

“We propose a method for detecting and manipulating quantum invisibility based on THz cloaking of molecular identity in coherent nanostructures,” says Fransson and buddies.

In practice, this means designing quantum corals, elliptical nanostructures, that absorb terahertz waves of specific frequencies. When a molecule that emits this frequency is placed at the focus, it cannot be spotted. It is essentially invisible.

Useful? You bet. Such a quantum coral would be ideally suited to detecting molecules of specific species while ignoring others. For example, if you have a particular molecular species that poisons your measurements, then what you need is a cloak that will make it invisible to your detectors

It’s ideas like this that are going to make cloaking mighty useful one of these days.

Ref: Quantum Detection and Invisibility in Coherent Nanostructures

And here is the sunspot forecast…

Thursday, November 13th, 2008

Astronomers have been monitoring sunspot numbers since 1700 and using them as an indicator of solar cycles since 1913. Today we know that peaks in sunspot numbers have an important influence on the Earth, increasing the amount of drag on satellites and contributing to telecoms and and power outages. Accurate forecasts of sunspot activity could help mitigate against these effects.

Ali Kilcik at Akdeniz University in Turkey and an international group of buddies have used a variety of techniques to analyse the data going back to the 1700s and use it to make predictions about the forthcoming maximum solar cycle 24, which we entered earlier this year.

Is prior performance a good guide to the future? It certainly hasn’t been in the past. Predicting the number of sunspots at the solar max has been notorioulsy unreliable.

That hasn’t stopped Kilcik and co sticking their necks out. They say solar cycle 24 will peak in December 2012 with 89 sunspots, a relatively small number considering that cycle 23 peaked with 170 or so in 2000.

Guess we’ll have to wait and see.

Ref: Nonlinear Prediction of Solar Cycle 24

The cosmic ray revolution

Wednesday, November 12th, 2008

Cosmic rays, the high energy protons and helium nuclei that constantly bombard the Earth, have puzzled astronomers for the best part of one hundred years. Where do they come from and how are they accelerated to energies in excess of 10^20 eV—that’s about the energy that Roger Federer gives a tennis ball during a serve? (By contrast, the  Large Hadron Collider will be able to accelerate protons to a mere  10^12 eV.)

To tackle these questions, astronomers have built a giant cosmic ray telescope about the size of Rhode Island in Argentina. It’s called the Pierre Auger telescope and in the short time it has been operating, it is already challenging astronomers’ views about the origin of cosmic rays. In particular, it’s beginning to look as if the highest energy comsic rays come from active galactic nuclei.

Serguei Vorobiov from University of Nova Gorica in Slovenia summarises the highlights. Worth a read if you want to get up to speed on a new generation of astronomy.

Ref: The Pierre Auger Observatory — a New Stage in the Study of the Ultra-High Energy Cosmic Rays

Triggering a phase change in wealth distribution

Tuesday, November 11th, 2008


Wealth distribution in the western world follows a curious pattern. For 95 per cent of the population, it follows a Boltzmann Gibbs distribution, in other words a straight line on a log-linear scale. For the top 5 per cent, however, wealth allocation follows a Pareto distribution, a straight line on a log-log scale, which is a far less equitable way of apportioning wealth.

Nobody really understands how this arrangement comes about but Javier Gonzalez-Estevez from the Universidad Nacional Experimental del Tachira in Venezuela and colleagues think they can thrown some light on the problem.

They have created an agent-based model in which each agent’s “wealth” evolves according to the way it interacts with its neighbours. Gonzalez-Estevez and co say that a simple model of this kind accurately reproduces the combination of Boltzmann Gibbs and Pareto distributions seen in real economies.

But get this. The teams says: “it is possible to bring the system from a particular wealth distribution to another one with a lower inequality by performing only a small change in the system configuration”. That’s an intriguing possibility.

In their latest work they say that it is possible to switch between Pareto and Boltzmann-Gibbs distributions, simply by increasing the number of neighbors each agent has.

In other words, this triggers a phase change in which wealth suddenly becomes more equally distributed (or vice versa).

That’s going to be a fascinating area for econophysicists to explore. Economists have always thought about that changing the distribution of wealth means tax collection and redistribution.

Now there’s a whole new way in which it might be approached. Gonzalez-Estevez and team make no suggestion as to how it might be done in real life economies but you can be sure that more than a few econophysicists will be thinking about how to trigger these kinds of phase changes for real.

Taxes as a way of redistributing wealth could become a thing of the past. But it will be as well to remember that not everyone wants to makes wealth distribution fairer.

Ref: Transition from Pareto to Boltzmann-Gibbs Behavior in a Deterministic Economic Model

Predicting the popularity of online content

Monday, November 10th, 2008


The page views for entries on this site in the last week range from more than 17,000 thousand for this story to around 100 for this one.

That just goes to show that when you post a blog entry and there’s no way of knowing how popular it will become. Right?

Not according to Gabor Szabo and Bernardo Huberman at HP Labs in Palo Alto who reckon they can accurately forecast a site’s page views a month in advance by analysing its popularity during its first two hours on Digg.

They say a similar prediction can be made for YouTube postings except these need to be measured for 10 days before a similarly accurate forecast can be made. (That’s almost certainly because Digg stories quickly become outdated while YouTube videos are still found long after they have been submitted.)

That’s not so astounding if all (or at least most) content has a similar long tail-type viewing distribution. Measuring part of this distribution automatically tells you how the rest is distributed.

But actually proving this experimentally is more impressive. In principle, it gives hosts a way of allocating resources such as bandwidth well in advance which could be useful, especially if you can charge in advance too.

Ref: Predicting the Popularity of Online Content

Orbits ‘n’ obits

Saturday, November 8th, 2008

The best of the rest from the physcis arXiv:

Non-Invasive Glucose Monitoring Techniques: A review and Current Trends

Very Slow Surface Plasmons: Theory and Practice

Cosmological Electromagnetic Fields and Dark Energy

MMOGs as Social Experiments: the Case of Environmental Laws

De Broglie-Bohm Pilot-Wave Theory: Many Worlds in Denial?

Web Usage Analysis: New Science Indicators and Co-usage

Saturn’s anomalous orbit flummoxes astronomers

Friday, November 7th, 2008


One of the first tests of Einstein’s theory of general relativity was to explain the precession of the perihelion of Mercury, which had long bamboozled astronomers. Newton’s law of gravity simply cannot account for it. But relativity does.

Now it’s Saturn’s turn to flummox astrophysicists. The Russian astronomer Elean Pitjeva, who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied Astronomy in St Petersburg, has analysed a huge data set of planetary observations dating back to 1913, including 3D observations of the Cassini spacecraft now orbiting Saturn.

She says that the precession of Saturn’s perihileon, as predicted by general relativity, needs to be corrected to fit the data. The correction is tiny: -0.006 arcseconds per century.

That’s an astonishing claim but perhaps not surprising given the growing body of evidence that some kind of correction to gravity is needed to explain various puzzling phenomena such as the Pioneer and Flyby anomalies.

Obviously Pitjeva’s work needs to be independently verified but already the astronomy-mill is hard at work guessing what might cause the deviation from Einsteinian physics.

It’s possible that known physics will do the trick: for example, our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction.

Lorenzo Iorio at the National Institute of Nuclear Physics in Pisa Italy, outlines various explanations of known physics:

Our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction but this turns out to generate a prograde precession no the retrograde precession found by Pitjeva

The Lense-Thirring effect generates a force that is four orders of magnitude too small to account for the difference

Mutual cancellations among unmodelled or mismodelled effects may have conspired to cause the effect but Iorio says this looks exceedingly unlikely

Neither do various exotic modifications of gravity or the DGP braneworld model explain the figures, says Iorio

So what’s left? A magnificent conundrum for astronomers to puzzle over until they get better data and/or a new theory of gravity that explains all.

Ref: On the Recently Determined Anomalous Perihelion Precession of Saturn