Archive for November, 2007

Nanowire magnets

Tuesday, November 20th, 2007

Co-Ni nanowires

Take a handful of Cobalt-Nickel (Co80-Ni20) alloy nanowires and drop them into a mixture of toluene and the synthetic polymer PMMA. Zap the mixture with a decent magnetic field, sit back and wait.

The field causes the nanowires to align and as the toluene evaporates, the PMMA traps them in place as it solidifies.

The result? A reasonably strong permanent magnet that remains magnetised at temperatures up to 250 degrees C.

Not bad eh?

Frederic “Teap” Ott at the Laboratoire Léon Brillouin in France and colleagues say the new material rivals both SmCo and AlNiCo magnets in various aspects. In particular, they reckon the new magnet is twice as good as existing materials for magnetic media recording.

Ref: arxiv.org/abs/0711.1978: Magnetic Nanowires as Permanent Magnet Materials.

The heterohydrogen question

Monday, November 19th, 2007

Kinetic energy operator

Can hydrogen and and antihydrogen bind to form a stable molecule?

That’s the question that a growing number of particlebods have been scratchin’ their eggs over. And it ain’t merely hypothetical, neither.

In the last few years, engineers at CERN in Switzerland and Fermilab near Chicago have been a-tinkerin’ and a-toyin’ with their particle traps and dramatically improved the numbers of antiprotons they can round up. All of a sudden, physicists have the building (and anti-building) blocks with which to test the question.

Of course, they ain’t got round to it yet which has left the field wide open to theorists. This week, it’s the turn of Mohamed Assad Abdel-Raouf at the United Arab Emirates University in Al Ain City. He’s run the first computations to calculate the bindin’ energies of so-called heterohydrogens (presumably ordinary homohydrogen is illegal in most middle eastern states).

He reckons various molecules of hydrogen and antihydrogen are possible. In particular, them eggs at CERN and Fermilab should keep ‘em peeled for antihydrogen-hydrogen, antihydrogen-deuterium and antihydrogen-tritium, says the man.

Go to it fellas.

Ref: arxiv.org/abs/0711.2283: Possible Coexistence of Antihydrogen with Hydrogen, Deuterium and Tritium Atoms

Shells ‘n’ pebbles

Sunday, November 18th, 2007

The pick of the other stories form the physics arXiv this week:

A Survey of Chaos-based Secure Communications

Neutrons from Piezonuclear Reactions

How to build a safe web crawler

Time Transfer Laser Link between China and France

How Much Can We Know about the Universe Before the Big Bang

Could X-ray afterglows be standard candles?

Saturday, November 17th, 2007

Standard candle

Astrobods love standard candles. Love ‘em. And it ain’t hard to understand why.

When ya look out into the darkness, it’s easy to see where everything fits into the celestial sphere. It ain’t quite so simple to see in which layer it sits in the celestial onion. So astronomers look for standard candles, objects whose distance they can measure by some other means.

One of the most famous and useful standard candles are Type Ia supernovas which have a known luminosity. So the brighter they look to us, the nearer they must be (which ain’t true for ordinary stars cos brighter stars might just be big ones a long way off). In fact, astronomers can work out the distance of Type Ia supernovas pretty accurately by measuring the apparent brightness here on Earth (assuming there ain’t no dust in the way).

The trouble is that we can only see Type Ia supernovas in our own galaxy and a few of the nearest others. That leaves a few hundred million galaxies whose distance is almost impossible to measure accurately.

Now Bruce “Which” Gendre at the Istituto di Astrofisica Spaziale e Fisica Cosmica in Rome and buddies reckon they’ve found another standard candle in the form of the X-ray afterglow that follows gamma ray bursts. The sources seem to have a standard luminosity so measuring the amount of light that reaches Earth gives you a good idea how far off they are.

Which Gendre and his pals have even tested the idea against another way of measuring distance from the redshift of the light (technically that just tells you have fast the source is moving away from you, not its distance but alotta bods assume that more redshift = further away.) Bruce reckon his new method is pretty much in agreement with redshift data.

Obviously this is an idea that’ll need a little more testing but it could be a refreshing new way to determine distances over cosmological scales. And that’s gonna get them astrobods excited, bless ‘em.

Ref: arxiv.org/abs/0711.2222: X-ray Afterglow Light Curves: towards a standard candle?

Web traffic and sand piles

Friday, November 16th, 2007

Web traffic

Drop grains of sand onto a flat surface and they form a pile. Keep adding grains and eventually ya’ll witness an avalanche. The curious thing about avalanches is that yer can’t tell how big they is going to be. A single dropped grain could dislodge a handful of other grains or hundreds of grains or thousands or perhaps tens of thousands of ‘em. How odd that the same trigger, the dropped grain of sand, could generate results that vary over several orders of magnitude.

Of course, the size of the avalanche don’t depend on the size of the grain at all. Instead, it depends on the complex network of forces that exist within the pile when the grain hits. It turns out that when these forces are balanced just right, the scale of avalanches cannot be determined in advance. This balance is known as self-organized criticality and it is true of earthquakes, forest fires, stock market crashes, the size of which cannot be predicted in advance.

Now it looks as if web traffic is in this same state of self organized criticality.

At least that’s what Mikhail Simkin and a friend at the University of California, Los Angeles, tell us. They have looked at the traffic at various popular websites and say it looks remarkably like the way avalanches occur, with very little activity for long periods interspersed with huge spikes in traffic that can vary by orders of magnitude.

Simkin argues that the web is like a pile of sand in a state of self organized criticality. Instead of a network of forces between grains, the web depends on a constantly changing network of links between pages. At any moment, these links could generate a flurry of activity as the page becomes dugg or slashdotted, for example. But the traffic from such an ‘avalanche’ can vary over several orders of magnitude in an entirely unpredictable way.

(This seems so obvious that ah can’t quite believe that Simkin is the first to suggest this, but if he is, well done.)

The lesson for all ya bloggers out there is to keep dropping the grains of sand.

Ref: arxiv.org/abs/0711.1235: A Theory of Web Traffic

The terrible truth about extremism

Thursday, November 15th, 2007

Extremism

Why is our way o’ living threatened by extremists? A natural question for anybody a-fretting and a-worrying about the state of world order. But the answer ain’t gonna please ya’ll. It’s looking increasingly as if extremism is an ordinary emergent property of societies like ours that we can’t do nothing about.

Andre Martins at the University of Sao Paulo in Brazil has modeled the way opinions meander ‘n’ flow through society. His laboratory is a virtual world populated by thousands of opinionated “agents” that interact with each other and change their minds according to various rules. Martins’ work consists of setting the model running and then putting his feet up for a snooze until a suitable amount of opinion forming has gone on.

Various people have tried to model opinions in this way but Martins is the first to reproduce the extremism we actually see in society. In other models each agent can take one of two views according to the opinion of those agents nearby. For example, one rule might be that an agent is obliged to change its view if two or more neighboring agents hold the opposite view. This kind of model can show how opinions spread through a society but extremism never evolves because the agent can hold only one view or the other.

Instead, Martins allows his agents to have a continuous spectrum of opinions that range from one extreme to another. This is then influenced by those agents nearby. The result is that:

“The appearance of extremists is naturally observed and it seems to be a characteristic of this model. This can help explain cases where people are led, by social pressure, to believe blindly in whatever opinion is shared by its local group, despite divergent voices in the larger society they live in.”

So what’s the bottom line? According to Martins, extremism is a natural property of social networks like ours. And that means we ain’t ever gonna get rid of it.

Ref: arxiv.org/abs/0711.1199: Continuous Opinions and Discrete Actions in Opinion Dynamics Problems

The electrifying physics of windblown sand

Wednesday, November 14th, 2007

Saltation

Wind blown sand is a damned nuisance. And not just cos it sandblasts cars, fills the atmosphere with aerosols and makes yer eyes water.

Geophysicists ain’t never been able to explain it good ‘n’ proper. Why does the average height of the sand above the ground remain constant as wind speed increases, when their models say the height should increase?

Now Jasper “Ophelia” Kok and a pal at the University of Michigan in Ann Arbor say they worked it out. Recent measurements show that windblown sand generates significant electric fields because the particles become negatively charged as they bounce along the ground (a similar effect is seen in powder handling facilities). This field gets stronger as the blowin ‘n’ puffin’ gets harder but none of the classical models take this into account.

Now Ophelia has built the first model that incorporates the effects of electric field generation and says it works mighty fine in reproducing the observed characteristics of wind blown sand. As the windspeed increases, so does the electric field. This attracts the particles towards the ground and so prevents their average height from increasing. Voila!

The model even makes a prediction: as wind speeds get even higher, the electric field should become so strong that it begins to lower the average height of the windblown sand. The pair are now on the hunt for evidence that this actually happens.

Ref: arxiv.org/abs/0711.1341: Electrostatics in Wind-blown Sand

The hunt for a repulsive Casimir force

Tuesday, November 13th, 2007

Repulsive Casimir force

Place a couple of infinite metal plates just a few micrometers apart and ya can measure the so-called Casimir force pushing them together.

The thinkin is that virtual particles are constantly a-popping and a-peaking in and outta existence. When the particles bang into the plate, they exert a force which is normally balanced out by the pressure of virtual particles on the other side.

But put another plate close enough and the distance between them is too small to allow for the existence of certain virtual particles. So those on the outside of the plates tend to push ‘em together. Simple really although physicists had to wait until 1996 to measure it for real.

Now they can’t get away from it. One reason why microelectromechanical devices have been slow to permeate our lives is because the Casimir force tends to jam ‘n’ glue the mechanism, and that’s been a-spoilin and a-wreckin’ the party.

What these microengineers need, of course, is a way of getting rid of the Casimir force, or at least a way of balancing it out with another force in the opposite direction.

And now it looks as if they got one. Francesco Intravaia and a pal at Potsdam University in Germany say that making the plates outta a particular combination of metamaterials does the trick, turning the Casimir force from an attractive one into a repulsive force. In particular, one of the plates must be a dielectric while the other is permeable, which could be achieved by engineering them on the nanometre scale, says Intravaia.

So that could make possible a whole new generation of micromachines, with stick free parts and goodness knows what kinda hoverin’ and floatin’.

But the Potsdam work is strictly theoretical. So who’s gonna pull off this trick for real? My money’s on Vlad Shalaev at Purdue although ya’d be a fool to rule out David Smith at Duke. But there are plenty o’ dark horses in this field. Anybody got any inside knowledge they’d care to share?

Ref: arxiv.org/abs/0711.0655: Casimir Intereaction between Absorbing and Metamaterials

Near-to-far field image magnification

Monday, November 12th, 2007

Near field lens

There was a time when magnifying glasses were good for nothing but fryin’ ants and helping the over-60s with newsprint. Now everyone’s a-peekin’ and a-peerin’ at things that are even smaller than the wavelength of visible light.

The conventional thinkin is that you can’t see nothing smaller than about a quarter of the wavelength of light; light’s just gonna go round anything as small as that, right?

Not quite. Turns out that in the region within a wavelength of a light emitter, the so-called near field, light interacts with things in all kindsa interesting ways. And if ya could only see the near field, you might get a handle on what’s going on.

There are various attempts to do this, such as near-field microscopy, which all involve circumventing the near field in some way such as sticking a probe into this forbidden region.

Now Vlad Shalaev at Purdue University in Indiana has designed the gadget we’ve all been a-waitin’ and a-hopin’ for: a lens that can magnify a near-field image into a far field one. Shalaev’s lens is made outta metamaterial that channels electric and magnetic fields away from a point source and redistributes them over a broader area, in other words it magnifies them.

There’s no hope for them ants now.

Ref: arxiv.org/abs/0711.0183: Engineering Space for Light via Transformation Optics

Shells ‘n’ pebbles

Sunday, November 11th, 2007

Other highlights from the arXiv this week

If the LHC is a Mini-Time-Machine Factory, Would We Notice?

Is the evidence for dark energy secure?

Following red blood cells in a pulmonary capillary

Urban and Scientific Segregation: a Review of the Schelling-Ising Model

An Atom Laser is not monochromatic