Archive for November, 2007

The tricky task of river delta modelling

Friday, November 30th, 2007

Mississippi delta formation

Geobods have a problem: they can’t model any geophysical process worth its salt on computer because of the complexity of the processes involved and the timescales over which they take place. It’s just too computationally demanding. One process in particular has defied attempts to model it: river delta formation.

But Hansjorg Seybold from the Swiss Federal Institute of Technology in Zurich and buddies finally seem to have pulled it off and have produced a pretty good simulation of the Mississippi River delta.

Previous simulations have all attempted to model the flow of water and sediment using equations that describe the interaction between the processes of erosion, deposition and shallow water flow. The hydrodynamics this leads to is hugely complex.

But Seybold and co have simplified things by using conservation laws instead of flow laws. So, for example, the total amount of water is constant during each iteration. That seems to have made a big difference. The teams says it has simulated a number of features of river deltas for the first time, such as river mouth switching. Neat huh?

Ref: Modeling River Delta Formation

Solar System heading thataway

Thursday, November 29th, 2007

This way

Take a look at the cosmic microwave background radiation and ya can see a slight bias caused by the movement of the solar system.

Now Christopher “He’s Alive” Gordon at the University of Oxford and a coupla pals have worked out where we’re goin’ by estimating our motion relative to the CMB using Type Ia supernovae standard candles.

So which way are we headed? [Cue: drum roll] We’re a-trundlin and a-tootlin’ along at a sedate 475 kilometers per second in the direction of the constellation of Sextans in the northern hemisphere. (In galactic co-ordinates that’s l=238 and b=45.)

Ah’ve had a good look with the arXivblog binoculars and there don’t seem to be nothing of interest in that direction. Disappointing really.

Ref: Determining the Motion of the Solar System Relative to the Cosmic Microwave Background using Type Ia Supernovae

The smoking gun that may prove our climate models wrong

Wednesday, November 28th, 2007

Hot Europe

If ya live in Europe, fall 2006 musta been one helluva season.

Many climatologist worry that climate models severely underestimate the effects of global warming. The extraordinary temperatures recorded in Europe in Autumn 2006 might just be the smoking gun that proves that these models really do get it wrong, big time.

Here’s what happened. The immediate causes of the heat ‘n’ sweat were southerly winds driving heat northwards and persistence from the very hot July along the North Sea coast. These generated record temperatures all over Europe but some records were spectacular. In some places, the average temperature was 4 degrees C higher than average. The average temperature in the Netherlands was 1.6 degrees C higher than the previous record which was set in 1730 and 1731 (they got long memories over there).

Now Geert van Oldenborgh from the Royal Netherlands Meteorological Institute has analysed how global warming might have contributed to the weather that autumn and he has some worrying news. Current climate models simply cannot reproduce the kind of warming seen in Europe during that period probably because they do not account for nonlinear effects that result in extreme conditions.

Van Oldenborgh’s conclusion?

“Either autumn 2006 was a very rare event, or these climate models do not give the correct change in temperature distribution as the temperature rises.”

Who knows what to expect now?

Stock up on suncream

Ref: How Unusual was Autumn 2006 in Europe?

Breaking the Netflix Prize dataset

Tuesday, November 27th, 2007

Netflix data

Hell, this is good work. In October last year, Netflix released over 100 million movie ratings made by 500,000 subscribers to their online DVD rental service. The company then offered a prize of $1million to anyone who could better the company’s system of DVD recommendation by 10 per cent or more.

Of course, Netflix assured everybody that the data had been anonymized by removing any personal details.

That turns out to have been a tad optimistic. Arvind Narayanan and Vitaly Shmatikov at the the University of Texas at Austin have just de-anonymized it.

Here’s how: turns out that an individual’s set of ratings and the dates on which they were made are pretty unique, particularly if the ratings involve films outside the most popular 100 movies. So it’s straightforward to find a match by comparing the anonymized data against publicly available ratings on the Internet Movie Database (IMDb).

That’s exactly what Narayanan and Shmatikov have done. And get this, once the match is made, it immediately links the user to the any private ratings on the Netflix database.

“Given a user’s public IMDb ratings, which the user posted voluntarily to selectively reveal some of his (or her; but we’ll use the male pronoun without loss of generality) movie likes and dislikes, we discover all the ratings that he entered privately into the Netflix system, presumably expecting that they will remain private.”

So what, I hear ya ask.

Here’s what the dynamic duo have to say about one person whose data they outed:

First, we can immediately find his political orientation based on his strong opinions about “Power and Terror: Noam Chomsky in Our Times” and “Fahrenheit 9/11.” Strong guesses about his religious views can be made based on his ratings on “Jesus of Nazareth” and “The Gospel of John”. He did not like “Super Size Me” at all; perhaps this implies something about his physical size? Both items that we found with predominantly gay themes, “Bent” and “Queer as folk” were rated one star out of five. He is a cultish follower of “Mystery Science Theater 3000”. This is far from all we found about this one person, but having made our point, we will spare the reader further lurid details. “

So Netflix may have inadvertently revealed the political affiliation, sexual orientation, BMI and God-knows-what else of 500,00 of their subscribers. Way to go!

Next up the mobile phone datasets we talked about a coupla weeks back

Ref: : Robust De-anonymization of Large Datasets (How to Break Anonymity of the Netflix Prize Dataset)

Perfect steganography

Monday, November 26th, 2007


Steganography is the practice of hiding a message within another object such as an image, video or audio file. In some sense, it’s akin to camouflage, a-lurkin’ and a-hidin’ behind an image. By contrast, cryptographers make no attempt to hide their work but instead merely encrypt it.

“Steganography differs from cryptography in that the presence of the message needs to remain secret, rather than the value of the message,” says Pierre Moulin “Rouge” at the University of Illinois at Urbana-Champaign with his buddy Ying Wang.

The question is: how good can steganography get? The trick is to ensure that the message matches the statistical properties of the covertext (the object in which it is hidden). The better the match, the more difficult it is to spot using statistical techniques.

But get this: steganography ain’t just good, it can be perfect. That’s the conclusion that Moulin Rouge and Wang come to after analysing how much data can be sent in this way through a noisy channel (in other words, how well it works in real life).

That’s impressive but there’s a catch. Perfect steganography requires the message statistics to exactly match those in the covertext. And for that to work, the message has to be essentially random. That means encrypting it using a one time pad before it is sent. So ya gotta have perfect encryption before ya can get perfect steganography (although it is possible to trade a small amount of security for another type of encyrption such as one with a public key).

That’s not beyond what most governments and military organisations are capable of but Moulin Rouge’s work raises another interesting possibility. Messages can be hidden not just in images and video but in data streams from a computer, for example, in the timing of data packets. So a clever thief might hijack a computer’s own data stream for sending data stolen from it.

How many computer security packages check for that? Not many, ah’d wager. At least, not yet!

Ref: Perfectly Secure Steganography: Capacity, Error Exponents, and Code Constructions

Nuts ‘n’ bolts

Sunday, November 25th, 2007

The best of the rest this week from the physics arXiv

An Exceptionally Simple Theory of Everything

Imaging the Internal Structure of the Earth with Atmospheric Neutrinos

A Novel Design of Dielectric Perfect Invisibility Devices

Odyssey: a Solar System Mission

Neural Synchronization and Cryptography

Indications of a Past Flare of the Galactic-Center Black Hole

How to maximise your PageRanking

Saturday, November 24th, 2007


Google’s PageRank system rules the web, right? This is the algorithm that determines how far up the list yer site appears in a given Google search. A better ranking can mean big bucks for some sites

The PageRank algorithm is closely guarded secret. But a growing number of academics are trying to reverse engineer the algorithm so that they can better understand how it works (and presumably to boost their own ranking).

Everyone knows that ya can boost yer ranking by getting other popular sites to link to you. That ain’t always so easy.

But another question is how best to arrange links between pages within yer own site to boost your ranking.

When you have a group of pages, various theories have been put forward: should you make your links into a ring structure with no center or a star-like structure with hub or some other shape?

Nobody knows, or at least they didn’t until Cristobald de Kerchove at the Universite Catholique de Louvain in Belgium came along. Using some reasonable assumptions about the algorithm and a few impressive mathematical techniques, he’s worked out the optimal linking strategy for boosting your PageRanking.

This is it: arrange your pages in a simple forward chain so that each links to the next but at the same time include every possible backward link (see diagram above).

Simple when ya know how.

Ref: Maximizing PageRank via outlinks

The frightening prospect of flu

Friday, November 23rd, 2007

Flu transmission

Bird flu may get all the headlines but the number of deaths it causes each year is currently measured in hundreds. The real killer, the one that should set yer spine a-shiver, is ordinary fly which kills hundreds of thousands each year.

With winter nearly upon us up here in the northern hemisphere, the spectre that we’ll be a-shakin and a-sweatin our way through a flu pandemic in the coming months is raising its ugly head again.

So what can science tell us about the way pandemics spread? Not enough, according to Gerardo Chowell from Arizona State University in the US and his friend Hiroshi. They’ve posted a comprehensive review of epidemic science dating back to the 1918 Spanish flu epidemic that killed as many as 100 million people by some estimates. We’ve learnt an awful lot since then about the way flu spreads but the frightening thing about Gerardo’s review is how much more we need to understand.

It’s not just small things that are up in the air but sizable pieces of the jigsaw. We still don’t know some basic probabilities associated with infection. For instance, given that an individual is infected with flu, what are the chances that the disease will manifest itself clinically? And given that the disease has manifested itself clinically in an individual, what are the chances of that person dying. And if a virus can be caught from a number of diffferent host species (as it might eventually be with bird flu) what is the probability of transmission?

Without a good understanding of these kindsa factors, it’s gonna be difficult for authorities to plan an effective response to a flu pandemic.

Mah advice? Stock up on food and water.

Ref: Quantifying the Transmission Potential of Pandemic Influenza

Bent on plastic electronics

Thursday, November 22nd, 2007


Ain’t we expecting alot from organic transistors: flexible electronics, inkjet-printed microchips and so on. But one reason that they’ve yet to hit the market big time is that their electronic behavior is hard to pin down and, even worse, seems to change with time. What’s going on?

Bertram Batlogg and buddies from the Swiss Federal Institute of Technology in Zurich are among those who are slowly teasing these characteristics apart, particularly with pentacene, the most promising organic semiconductor.

One thing they know is important is the mobility of charge carriers, how easily they can move around the structure. In pentacene, this seems to change with time and Batlogg and pals think they’ve worked out why.

It looks as if pentacene heals itself at room temperature, that is it corrects the defects within its structure. What the Swiss team has found is that oxygen and nitrogen seem to interfere with the healing process, which also explains why various labs have measured the phenomenon so differently.

Now Batlogg says he can standardize the process of healing (or self annealing, as it is called) by carrying it out in a vacuum at room temperature. This should guarantee a uniform quality of pentacene based-chips in future.

So keep ’em peeled for functional crisp packets and edible microchips. Them plastic chips are a-comin’.

Ref: Defect Healing at Room Temperature in Pentacene Thin Films and Improved Transistor Performance

A habitable planet in Libra?

Wednesday, November 21st, 2007

GL 581

Where are all them habitable planets that we gonna move to when Earth becomes too hot ‘n’ nasty for us? (By “habitable”, astrobiobods mean host to liquid water.)

Turns out that a coupla good candidates are orbiting Gliese 581, a red dwarf star about 20 light years from here in the constellation of Libra (Gliese 581 is our 87th closest neighbor).

Now Franck “Ex” Selsis and pals from the Centre de Recherche Astrophysique de Lyon in France have been a-dreamin and a-wondrin what conditions are like on these planets, called GL581c and Gl 581d, which are just a little larger than Earth.

After mucho guessin’ calculatin’, they say 581c receives 30 per cent more energy than Venus and so is unlikely to have liquid water. 581d could be a better bet with conditons there close to those on early Mars, which is thought to have liquid water. Ex Selsis says that with the right kinda greenhouse gases, liquid water may be a distinct possibility on 581d. But how such a “super-greenhouse” atmosphere could be created and remain stable isn’t yet known (although it looks like we’re about to find out here on Earth).

So don’t getcha hopes up but do tell them astrobods to keep searchin’.

Ref: Habitable Planets Around the Star Gl 581?