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Friday, May 4, 2012
Network Science Reveals The Cities That Lead The World's Music Listening Habits
If you live in North America, you'll soon be listening to the music now playing in Atlanta whereas in Europe, Oslo leads the scene, according to a new analysis of global listening habits 
The evidence that ideas and fashions spread through society like viruses or like wildfire is compelling. Numerous studies have examined the networks in which this spread takes place and with increasingly large data sets to work with, researchers have become increasingly confident in their network-centric view of the world. These tools are teasing apart the large scale behaviour of humanity in ever increasing resolution.
In the fashion world, London, New York and Paris are generally considered the leaders that everyone else follows. So an interesting question is whether network science can tell us which cities play a similar role for music.
That's exactly the question that Conrad Lee and Pádraig Cunningham at the Clique Research Cluster in Ireland set out to answer by analysing data from Last.fm, an social website for music.
Last.fm is interesting because it publishes lists of the most listened to artists divided geographically. So if you live in Seattle, for example, you can see what people in your area are listing to.
So Lee and Cunningham have studied the way these charts vary in time and looked to see whether some cities consistently lead others in terms of listening habits.
These guys studied the Last.fm data for 200 cities around the world dating back to 2003. This is compiled from some 60 billion pieces of data that Last.fm collects from its users.
This is a noisy data set. Some cities have so few listeners that their data is hard to distinguish from noise. So Lee and Cunningham have to apply some fairly robust cleaning techniques to remove this noise.
They then use recently developed statistical techniques to decide which cities lead others. They then construct a network in which a link pointing from one city to another indicates that one follows the other.
The results are interesting. They show that certain cities appear to lead others for various genres of music. For example, Montreal seems to lead North American in indie music listening habits and the leader for hip hop is Atlanta. In Europe, Paris leads for indie music whereas Oslo leads for music as a whole.
There are other interesting patterns too. For example, cities that have similar listening habits are not linked in this network. For example, Portland and San Francisco; Cracow and Warsaw; and Birmingham and Manchester.
Lee and Cunningham suggest that when two cities' listening habits are synchronised there is little to be gained from following the listening habits in the other city so residents look elsewhere.
There's another interesting pattern. It's easy to imagine that the biggest cities ought to be those furthest ahead of the curve because they have biggest populations from which new and interesting bands can emerge. That doesn't seem to be the case in this data--big cities such as New York, LA and London do not lead. "We find only weak support for this hypothesis," say Lee and Cunningham.
That may cause some alarm bells to ring. An interesting body of work has recently suggested that big cities benefit disproportionally for their size since qualities such as efficiency, productivity and innovation all scale super linearly with population.
An important question for Lee and Cunningham is why that doesn't hold for music too.
There is also a question over whether the trends that Cunningham and Lee have found really reflect their hypothesis that some cities' listening habits lead others. Humans are notoriously good at finding patterns in random data.
The ultimate test, of course, is whether their discovery has any predictive value. For example, could they predict how listening habits will change in the near future? "We have not yet demonstrated that our models have this predictive power, although we plan to attempt this validation in future work," they say.
So we must wait and see. If they manage any kind prediction based on this work, it'll be an impressive feat.
In the meantime, if you want to know what you're likely to be listening to in the near future, cough, tune in to the music now playing in Atlanta and Oslo.
Ref: arxiv.org/abs/1204.2677: The Geographic Flow of Music
Molecular "Wankel Engine" Driven By Photons
Chemists say exotic clusters of boron atoms should behave like rotary Wankel engines when bathed in circularly polarised light 
One of the great discoveries of biology is that the engines of life are molecular motors--tiny machines that create, transport and assemble all living things.
That's triggered more than a little green-eyed jealousy from physicists and engineers who would like to have molecular machines at their own beck and call. So there's no small interest in developing molecular devices that can be easily harnessed to do the job.
Today, Jin Zhang at the University of California Los Angeles and a few pals say they've identified a machine that fits the bill.
A couple of year ago, chemists discovered that groups of 13 or 19 boron molecules form into concentric rings that can rotate independently, rather like the piston in a rotary Wankel engine. Because of this, they quickly picked up the moniker "molecular Wankel engines". The only question was how to power them.
Now Zhang and buddies have calculated that this should be remarkably easy--just zap them with circularly polarised infrared light. That sets the inner ring counter-rotating relative the outer one, like a Wankel engine.
Of course, nanotechnologists have identified many molecular motors and even a few rotary versions (ATP springs to mind).
What makes this one special is that the polarised light doesn't excite the molecule's electronic ground state, leaving it free to be chemically active.
By contrast, other forms of molecular power such as chemical or electric current can generate heat that has a critical effect on the system.
For the moment, the photon-powered molecular Wankel engine is merely an idea, the result of some detailed chemical modelling.
Zhang and co leave it to others, who are happy to get their hands dirty, to actually get one of these molecules turning.
If they've got their sums right, that should be sooner rather than later.
Ref: arxiv.org/abs/1204.2505: Photo-driven Molecular Wankel Engine, B13+
Thursday, May 3, 2012
Mathematics of Eternity Prove The Universe Must Have Had A Beginning -- Part II
Heavyweight cosmologists are battling it out over whether the universe had a beginning. And despite appearances, they may actually agree
Earlier this week, Audrey Mithani and Alexander Vilenkin at Tufts University in Massachusetts argued that the mathematical properties of eternity prove that the universe must have had a beginning.
Today, another heavyweight from the world of cosmology weighs in with an additional argument. Leonard Susskind at Stanford University in California, says that even if the universe had a beginning, it can be thought of as eternal for all practical purposes.
Susskind is good enough to give a semi-popular version of his argument:
"To make the point simply, imagine Hilbertville, a one-dimensional semi-infinite city, whose border is at x = 0: The population is infinite and uniformly fills the positive axis x > 0: Each citizen has an identical telescope with a finite power. Each wants to know if there is a boundary to the city. It is obvious that only a finite number of citizens can see the boundary at x = 0. For the infinite majority the city might just as well extend to the infinite negative axis.
Thus, assuming he is typical, a citizen who has not yet studied the situation should bet with great confidence that he cannot detect a boundary. This conclusion is independent of the power of the telescopes as long as it is finite."
He goes on to discuss various thermodynamic arguments that suggest the universe cannot have existed for ever. The bottom line is that the inevitable increase of entropy over time ensures that a past eternal universe ought to have long since lost any semblance of order. Since we can see order all around us, the universe cannot be eternal in the past.
He finishes with this: "We may conclude that there is a beginning, but in any kind of inflating cosmology the odds strongly (infinitely) favor the beginning to be so far in the past that it is eff ectively at minus infinity."
Susskind is a big hitter: a founder of string theory and one of the most influential thinkers in this area. However, it's hard to agree with his statement that this argument represents the opposing view to Mithani and Vilenkin's.
His argument is equivalent to saying that the cosmos must have had a beginning even if it looks eternal in the past, which is rather similar to Mithani and Vilenkin's view. The distinction that Susskind does make is that his focus is purely on the practical implications of this--although what he means by 'practical' isn't clear.
That the universe did or did not have a beginning is profoundly important from a philosophical point of view, so much so that a definitive answer may well have practical implications for humanity.
But perhaps the real significance of this debate lies elsewhere. The need to disagree in the face of imminent agreement probably tells us more about the nature of cosmologists than about the cosmos itself.
Ref: arxiv.org/abs/1204.5385: Was There a Beginning?
Mathematics of Eternity Prove The Universe Must Have Had A Beginning
Cosmologists use the mathematical properties of eternity to show that although universe may last forever, it must have had a beginning 
The Big Bang has become part of popular culture since the phrase was coined by the maverick physicist Fred Hoyle in the 1940s. That's hardly surprising for an event that represents the ultimate birth of everything.
However, Hoyle much preferred a different model of the cosmos: a steady state universe with no beginning or end, that stretches infinitely into the past and the future. That idea never really took off.
In recent years, however, cosmologists have begun to study a number of new ideas that have similar properties. Curiously, these ideas are not necessarily at odds with the notion of a Big Bang.
For instance, one idea is that the universe is cyclical with big bangs followed by big crunches followed by big bangs in an infinite cycle.
Another is the notion of eternal inflation in which different parts of the universe expand and contract at different rates. These regions can be thought of as different universes in a giant multiverse.
So although we seem to live in an inflating cosmos, other universes may be very different. And while our universe may look as if it has a beginning, the multiverse need not have a beginning.
Then there is the idea of an emergent universe which exists as a kind of seed for eternity and then suddenly expands.
So these modern cosmologies suggest that the observational evidence of an expanding universe is consistent with a cosmos with no beginning or end. That may be set to change.
Today, Audrey Mithani and Alexander Vilenkin at Tufts University in Massachusetts say that these models are mathematically incompatible with an eternal past. Indeed, their analysis suggests that these three models of the universe must have had a beginning too.
Their argument focuses on the mathematical properties of eternity--a universe with no beginning and no end. Such a universe must contain trajectories that stretch infinitely into the past.
However, Mithani and Vilenkin point to a proof dating from 2003 that these kind of past trajectories cannot be infinite if they are part of a universe that expands in a specific way.
They go on to show that cyclical universes and universes of eternal inflation both expand in this way. So they cannot be eternal in the past and must therefore have had a beginning. "Although inflation may be eternal in the future, it cannot be extended indefinitely to the past," they say.
They treat the emergent model of the universe differently, showing that although it may seem stable from a classical point of view, it is unstable from a quantum mechanical point of view. "A simple emergent universe model...cannot escape quantum collapse," they say.
The conclusion is inescapable. "None of these scenarios can actually be past-eternal," say Mithani and Vilenkin.
Since the observational evidence is that our universe is expanding, then it must also have been born in the past. A profound conclusion (albeit the same one that lead to the idea of the big bang in the first place).
Ref: arxiv.org/abs/1204.4658: Did The Universe Have A Beginning?
Computer Scientists Build Computer Using Swarms of Crabs
One of the hot topics in computer science is the study of unconventional forms of computation.
This is motivated by two lines of thought. The first is theoretical--ordinary computers are hugely energy inefficient--some eight orders of magnitude worse than is theoretically possible. The second is practical--Nature has evolved many much more efficient forms of computation for specific tasks such as pattern recognition.
Clearly, we ought to be able to do much better--hence the interest in different ways of doing things.
Various groups have tried computing with exotic substances such as chemicals like hot ice and even with a single celled organism called a slime mould.
Today, we look at one of the more curious variations on this theme--a computer that exploits the swarming behaviour of soldier crabs.
First, a little background on the theory behind this idea. Back in the early 80s, a couple of computer scientists--Ed Fredkin and Tommaso Toffoli--studied how it might be possible to build a computer out of billiard balls.
The idea is that a channel would carry information encoded in the form of the presence or absence of billiard balls . This information is processed through gates in which the billiard balls either collide and emerge in a direction that is the result of the ballistics of the collision, or don;t collide and emerge with the same velocities.
Now Yukio-Pegio Gunji from Kobe University in Japan and a couple of pals have built what is essentially billiard ball computer using soldier crabs. "We demonstrate that swarms of soldier crabs can implement logical gates when placed in a geometrically constrained environment," they say.
These creatures seem to be uniquely suited for this form of information processing . They live under the sand in tidal lagoons and emerge at low tide in swarms of hundreds of thousands.
What's interesting about the crabs is that they appear to demonstrate two distinct forms of behaviour. When in the middle of a swarm, they simply follow whoever is nearby. But when they find themselves on on the edge of a swarm, they change.
Suddenly, they become aggressive leaders and charge off into the watery distance with their swarm in tow, until by some accident of turbulence they find themselves inside the swarm again.
This turns out to be hugely robust behaviour that can be easily controlled. When placed next to a wall, a leader will always follow the wall in a direction that can be controlled by shadowing the swarm from above to mimic to the presence of the predatory birds that eat the crabs.
Under these conditions, a swarm of crabs will follow a wall like a rolling billiard ball.
So what happens when two "crab balls" collide? According to Gunji and co's experiments, the balls merge and continue in a direction that is the sum of their velocities.
What's more, the behaviour is remarkably robust to noise, largely because the crab's individuals behaviours generates noise that is indistinguishable from external noise. These creatures have evolved to cope with noise.
That immediately suggested a potential application in computing, say Gunji and co. If the balls of crabs behave like billiard balls, it should be straightforward to build a pattern of channels that act like a logic gate.
And that's exactly what Gunji and co have done. These guys first simulated the behaviour of a soldier crab computer in special patterns of channels. Then they built one in their lab to test the idea with real crabs.
To be fair, the results were mixed. While Gunji and co found they could build a decent OR gate using soldier crabs, their AND-gate was much less reliable.
However, it's early days and they say it may be possible to produce better results by making conditions inside the computer more crab-friendly. (No crabs were harmed in the making of their computer, say Gunji and co.)
So there you have it--a computer in which the information carriers are swarming balls of soldier crabs.
Not a sentence you expect to read every day. But it surely cannot be long before we all have one of these on our desktops.
Ref: http://arxiv.org/abs/1204.1749: Robust Soldier Crab Ball Gate
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