Archive for May, 2008

In case ya missed ’em…

Sunday, May 11th, 2008

…the pearls from this week’s physics arXiv blog:

First evidence that water forms in interstellar space

Why tiny helicopters are so hard to fly

The puzzling discovery of a motor made from liquid film

Blind date gives astronomers a new love of the stars

The mathematics of tackling tax evasion

Taxes ‘n’ tasters

Saturday, May 10th, 2008

The other highlights from the physics arXiv this week:

Fermi at Los Alamos and the Early Britain’s Way to Nuclear Energy

The Silencing of Neuronal Activity by Noise and the Phenomenon of Inverse Stochastic Resonance

Molecular Thin Films: a New Type of Magnetic Switch

No Many-Scallop Theorem: Collective Locomotion of Reciprocal Swimmers

Probing a Bose-Einstein Condensate with an Atom Laser

Search for Variation of the Fundamental Constants in Atomic, Molecular and Nuclear Spectra

To What Extent does Genealogical Ancestry imply Genetic Ancestry?

The mathematics of tackling tax evasion

Friday, May 9th, 2008

Tax evasion

In recent years, economists have gained the luxury of actually being able to test their ideas in experiments involving the behaviour of real people. And one particularly new and promising area of experimental economics focuses on tax evasion, which ought to be of keen interest to many governments around the world.

A couple of years ago, Simon Gachter at the University of Nottingham carried out a number of experiments on the way people co-operate which had profound implications for tax evasion. Gachter’s conclusion was that people decide whether or not to pay taxes based on the behaviour of their peers. The implication is that in certain circumstances, tax evasion may be a kind of fashion that spreads through society like bell-bottomed jeans.

Today, Georg Zaklan from the University of Bamberg in Bavaria, Germany, and pals show just how this might work in the real world by constructing a model of tax evasion behaviour in society.

His society is an Ising spin model (most commonly used to show critical behaviour in magnetic materials) in which agents can chose to evade taxes or not based on the behaviour of their neighbours.

Sure enough, the model shows that without any control on tax evasion, the behaviour can spread rapidly, disappear equally quickly and re-appear again later (just like bell-bottoms).

But the beauty of Zaklan’s simulation is that it suggests a way in which governments can very easily prevent the spread of tax evasion. The team has modelled the effect of increasing the probability that a tax evader will be caught and show that a small increase could have profound effects on tax evasion.

So what governments should do is increase the number of tax audits they carry out (as well as making sure there are adequate punishments for offenders). Zaklan says the model implies that if only 1 % of the population is tax audited, tax evaders would be brought to heel for good.

That sounds interesting and might be worth a try in some countries, were it not for some important gaps in the paper.

The biggest of these is this: what evidence is there that tax evasion fluctuates in the real world in the way that the Ising model predicts? Zaklan doesn’t present any, so while this work is interesting, I’ll need some better evidence before I’m convinced that his model really describes what’s going on.

Ref: arxiv.org/abs/0805.0998: Controlling tax evasion fluctuations

Blind date gives astronomers a new love of the stars

Thursday, May 8th, 2008

Star date

When it comes to studying the night sky, astronomers aren’t short of images. There are huge archives of both amateur and professional images taken in the the age before digital imaging. The Harvard College Observatory Astronomical Plate Stacks contain enough images to cover the entire sky 500 times over.

But although the image quality is excellent, the problem is the indexing. When logs go missing or when data has been badly transcribed, it can be almost impossible to work out exactly what appears in an image or when it was taken.

The error rate in many older collections is high enough to make astronomers think twice about using them. And as astronomy moves towards its goal of creating a Virtual Observatory, in which all images are available online in a kind of giant virtual planetarium, a lack of trust in the data is a serious problem.

If, as an astronomer, you’ve been losing sleep over this issue, you can rest easy. David Hogg at New York University and buddies (including the search giant Google), have solved the problem by reducing it to one of image matching.

They take an astrophotograph of dubious provenance and use a computer program called “Blind Date” to look for asterisms (the shapes that constellations make). When they find a match, this immediately locates the image within a part of the sky. But the really cool part of their technique is based on the fact that stars move over time, albeit by tiny amounts. So any small deviation in the location of stars within an image give an immediate time stamp for when the shot was taken.

The team says this technique works for every science-quality image that it has been tested against and 85 per cent of lower quality images. In some cases, it can date images to within a few months.

The plan is to use Blind Date to produce metadata automatically for every image that is entered into the Virtual Observatory, which should reduce errors substantially and also prevent deliberate spoofing of the project.

That should go a long way to restoring trust.
Ref: arxiv.org/abs/0805.0759: Blind Date: Using Proper Motions to Determine the Ages of Historical Images

The puzzling discovery of a motor made from liquid film

Wednesday, May 7th, 2008

liquid-film-motor

Here’s an interesting effect discovered by a group of Iranian physicists at Sharif University of Technology in Tehran, Iran (it’s not often we hear from these guys).

They placed a thin film of water in a square cell and applied two perpendicular electric fields. One was an external electric field. For the other, they used two copper electrodes to generate a voltage across the cell like an electrolysing cell (although no chemical reaction took place).

So they had a pair of electric fields at right angles acting on this thin film.

The unexpected result is that the film of water begins to rotate. The team has a number of movies of the effect on its website. They call it a liquid film motor and it’s a quite extraordinary effect. At one point they divide their cell into nine smaller ones and the liquid in each cell rotates in exactly the same way.

The question is: what’s causing the rotation? The team can easily control the direction and speed of rotation by varying the relative angle and direction of the electric fields, which rules out the possibility that convection is causing the rotation (something that is seen when a field is applied to some thin films of liquid crystals). Neither does adding salt to water change the effect, ruling out the possibility that ion movement directs the flow.

The rotation occurs in polar liquids but not in non-polar ones so the intrinsic dipole moment of the molecules seems to be crucial. People have been observing the electrohydrodynamics  of various types of thin films for a good few years but nobody has seen anything like this. Just what’s going on remains a mystery.

But the puzzle shouldn’t overshadow what looks like an important discovery that could have widespread industrial application in microfluidic devices for mixing.

Ref:  arxiv.org/abs/0805.0490: A Liquid Film Motor

Why tiny helicopters are so hard to fly

Tuesday, May 6th, 2008

Helicopter

Tiny remote control helicopters have become all the rage in the last few years as lightweight motors and materials have plummeted in price. But if you’ve ever played with one, you’ll know how hard they are to control.

That’s not the result of poor construction. Small helicopters are harder to control than big ones because of the laws of physics: moments of inertia drop in proportion to the fifth power of vehicle size. This gives small helicopters quicker response times, making them more agile. But the real killer is that the main rotor tip speed in a small helicopter is the about the same as it is for a large helicopter. So the ratio of the rotor moments to the moments of inertia can become huge and unmanageable.

That’s when you need to develop a model of helicopter dynamics so you can design remote control systems or an autonomous flight control system that can manage this agility, say Hardian Reza Dharmayanda and pals at Konkuk University in Seoul, South Korea.

And that’s what they’ve done in this paper: built and tested a control system for a Yamaha R-50 helicopter, which uses a two-bladed main rotor with a Bell-Hiller stabilizer bar. The next step, they say, is to make the helicopter fully autonomous using their model.

These guys may be re-inventing the wheel but it’s interesting to see how they’re doing it.

Ref: arxiv.org/abs/0804.4757: Analysis of Stability, Response and LQR Controller Design of a Small-Scale Helicopter Dynamics

First evidence that water forms in interstellar space

Monday, May 5th, 2008

Star juice

Water is the most abundant solid material in space. Astronomers see it on various planets, on moons, in comets and in interstellar clouds. But how did it get there? Nobody really knows how water could possibly form in the freezing darkness of interstellar space.

At least they didn’t until now. Today, Akira Kouchi and buddies at the Institute of Low Temperature Science at Hokkaido University in Japan say that have created water for the first in conditions similar to those found in interstellar space.

Water forms quite easily when oxygen and atomic hydrogen meet. The problem is that there is not enough of it floating around as gas in interstellar dust clouds. So instead, the thinking is that water must form when atomic hydrogen interacts with frozen solid oxygen on the surface of dust grains in these clouds.

Kouchi and co recreated this process by creating a layer of solid oxygen on an aluminum substrate at 10K and then bombarding it with hydrogen. Sure enough, infrared spectroscopy confirmed the presence of water and hydrogen peroxide, and in the right quantities to explain the abundance of water seen in interstellar clouds.

That’s cool and in more ways than one. All the water in the solar system–in comets, on Mars and in the oceans on Earth–must have formed in exactly this way in the interstellar dustcloud which pre-dated Sol and the planets.

So that’s not just any old water you’re sipping, that’s interstellar star juice.

Ref: arxiv.org/abs/0805.0055: Formation of Hydrogen Peroxide and Water from the Reaction of Cold Hydrogen Atoms with Solid Oxygen at 10 K

In case ya missed ’em…

Sunday, May 4th, 2008

…the gold and silver from this week’s arxivblog:

First superheavy element found in nature

Solving the faint young Sun problem

Stats prove Red Baron’s WW1 victories were down to luck

The next high temperature superconductor?

How antineutrino monitoring could prevent nuclear proliferation

Particles ‘n’ waves

Saturday, May 3rd, 2008

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

Signatures of a Hidden Cosmic Microwave Background

White-Light Imaging in a Two Gratings Diffraction Process

The “Quantum Mousetrap”: Entangled States and Gravitational Waves

Predictions for the LHC: an Overview

Ideal and nonideal electromagnetic cloaks

The Peculiar Volatile Composition of Comet 8P/Tuttle: A Contact Binary of Chemically Distinct Cometesimals?

Evolution of the Genetic Code. Why are there Strong and Weak Letter Doublets?

How antineutrino monitoring could prevent nuclear proliferation

Friday, May 2nd, 2008

Antineutrino monitoring

The Nuclear Non-Proliferation Treaty has been ratified by more countries than any other arms limitation or disarmament treaty (187 at the last count). Its goal is to prevent the spread of nuclear weapons and weapons technology.

The task of monitoring compliance of the treaty is the job of the International Atomic Energy Authority and one area of particular concern is the spread of fissile material, particularly of weapons grade. But how to monitor this?

Ideally, the IAEA would like a non-invasive device that can be placed in the vicinity of a nuclear reactor that monitors its power output and its fissile isotopic content.

Now Adam Bernstein from the Lawrence Livermore National Laboratory in California and colleagues have developed and tested just such a device which works by measuring the reactor’s rate of antineutrino production, which in turn depends on the reactor’s fissile isotopic content and its power output.

The detector is about a cubic metre in volume and consists essentially of a liquid core doped with 0.1% gadolinium. This core is surrounded by various shields to screen out unwanted signals and various detectors to pick up the interesting ones. The detectors are looking for two pulses of energy (from a positron-electron annihilation followed by a neutron capture by a gadolinium nucleus) which are the characteristic signature of an antineutrino reaction.

Bernstein says he and his team have been testing the device for the last two years at unit 2 of San Onofre Nuclear Generating Station in Southern California and all the evidence is that it works well. They say the prototype can determine whether a reactor is on or off with a time resolution of 5 hours with greater than 99% confidence, which is handy because down times are when fuel is removed. It also directly measures power levels over month long periods. The isotopic content is a little more tricky to measure because it changes as fuel is used up within the reactor but should be within reach of the design.

Obviously such a device would also need to be able to operate remotely and record information securely but that’s for development further down the line

The detector’s biggest advantage, however, is its simplicity, says the group: “Our experience is that the simplicity of the detector design will play a key, even decisive role in determining whether this technology is adopted by the IAEA or other safeguards regimes.”

Interesting stuff.

Ref: arxiv.org/abs/0804.4723: Monitoring the Thermal Power of Nuclear Reactors with a Prototype Cubic Meter Antineutrino Detector