Archive for February, 2009

The telescope that Antarctica broke

Tuesday, February 17th, 2009


First light from any instrument is always exciting but particularly so when exotic locations and exciting goals are involved.  The CORONA experiment offers both.

CORONA is a stellar coronagraph designed to spot extrasolar planets orbiting other stars. It is based at Dome C, some 10,000 feet above sea level in in Antarctica, a location that boasts some of the best seeing on the planet.

The goal of the project is to test the feasibility of operating such a device in the harsh conditions that exist in the Antarctic, where temperatures can drop to -80 degrees C in winter.

The result: at -65 degrees C the telescope showed some strong aberrations in test images of Sirius, probably the result of thermal distortions.

Geraldine Guerri from the Université de Nice Sophia-Antipolis in France and buddies say: “The coronagraph could not be operated in these conditions, and thus, no nighttime images are presently available. CORONA has been sent back to France to be modified.”

That must have been disappointing, although their paper puts a brave face on things. Their plan is to eventually build a much more sophisticated coronagraph with adaptive optics.

That won’t be easy and given their experience so far, it looks highly, perhaps overly, ambitious.

Ref: First Light from the Dome C (Antarctica) of a Phase Knife Stellar Coronagraph

What should robots do for us?

Monday, February 16th, 2009


Robots have great potential for assisting the old and disabled (not to mention the rest of  us) .  But if you’re developing one of these devices, where do you start? What kind of assistance you should concentrate on providing?

Young Sang Choi and buddies from the Healthcare Robotics Lab at Georgia Institute of Technology have worked it out for us. They asked a group of people suffering from amyotrophic lateral sclerosis (Stephen Hawking’s condition) what they most wanted a robot to be able to fetch for them.

Here are the top five requests (out of 43)…

  1. TV remote
  2. medicine pills
  3. prescription bottle
  4. glasses
  5. cordless phone

Seems sensible to compile a list like this. Other notable entries:  14 cellphone, 21 keys, 36 wallet, 38 drinking can (drink not specified)

Does such a list exist for able-bodied  people? And if not, what should be on it?

Ref: A List of Household Objects for Robotic Retrieval Prioritized by People with ALS

Swings ‘n’ roundabouts

Saturday, February 14th, 2009

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

Black Hole Hair Removal

Quantum Critical Analog of Cloud Chamber

Tsetse Fly Population Outcomes for a Number of Aerial Spray Scenarios at Different Temperatures

Self Collimation of Ultrasound in a 3D Sonic Crystal

Some Reports of Snowfall from Fog During the UK Winter of 2008/09

How to build a desktop Foucalt’s Pendulum

Friday, February 13th, 2009


Foucalt’s pendulum–a 28 kilogram bob suspended on a 67 metre wire– famously hangs in the Pantheon in Paris where Leon Foucalt himself demonstrated it in 1851.

The pendulum oscillates in a vertical plane which slowly rotates. The rotation is explained by the Earth’s motion which spins beneath the pendulum.

The length of the wire is important because it helps to iron out any irregularities that can always occur in the swing of a pendulum. In particular, it’s hard to set a pendulum in motion without imparting some ellipsoidal motion. The problem is that this ellipse always precesses, a motion that inevitably swamps the rotation caused by the Earth’s spin.

This is much less of a problem for long pendulums because the properties of the ellipsoidal motion are inversely related to the length of the wire.

If you have access to a 67 metre ceiling, you’re fine. But try it with a shorter pendulum and you’ll just never see the effect you want. That’s why Focault’s Pendulums are rather rare devices.

That may change thanks to some interesting work by Reinhard Schumacher and Brandon Tarbet at Carnegie Mellon University in Pittsburgh. What they’ve done is to work out how to drive a much shorter pendulum without allowing its ellipsoidal motion to precess.

They’ve done this with a motor that uses Faraday induction and magnetic repulsion to push a pendulum in a controlled fashion. The system is cleverly designed to cancel out any tendency to precess, whatever the size of the ellipse that the pendulum is describing.

But while the precession is suppressed, the rotation due to the Earth’s motion is not.

They’ve tested their idea on a 3 metre pendulum and say it works well.  They end their paper saying: “We plan to further test this method on even shorter pendula, since there is no lower limit at which the model given here should apply.”

Anybody want a desktop Focault Pendulum?

Ref: A Short Foucault Pendulum Free of Ellipsoidal Precession

The frightening origins of glacial cycles

Thursday, February 12th, 2009


Climatologists have known for some time that the Earth’s motion around the Sun is not as regular as it might first appear. The orbit is subject to a number of periodic effects such as the precession of the Earth’s axis which varies over periods of 19, 22 and 24 thousand years, its axial tilt which varies over a period of 41,000 years and various other effects.

The combined effect of these variations are often cited to explain the 41,000 and 100,000 year glacial cycles the Earth appears to have gone through in the past.

But there is a problem with this idea: the change in the amount of sunlight that these variations cause is not enough to trigger glaciation. So some kind of non-linear effect must amplify the effects to cause widespread cooling.

That’s not so surprising given that we know that our climate appears to be influenced by all kinds of non-linear factors. Even still, nobody has been able to explain what kind of processes can account for the difference.

Now Peter Ditlevsen at the University of Copenhagen in Denmark thinks he knows what might have been going on. He says that the change in the amount of sunlight the Earth receives acts as a kind of forcing mechanism in a climatic resonant effect. The resulting system is not entirely stable but undergoes bifurcations in which the cycle switches from a period of 41,000 years to 100,000 years and back again, just as it seems to have done in Earth’s past.

“This makes the ice ages fundamentally unpredictable,” says Ditlevsen.

Quite, but the real worry is this: if bifurcations like this have happened in the past, then they will probably occur in the future. The trouble is that our current climate models are too primituve to allow for this kind of bifurcation and that means their predictions could be even more wildly innacurate than we know they already are .

Kinda frightening, don’t you think?

Ref:  The Bifurcation Structure and Noise Assisted Transitions in the Pleistocene Glacial Cycles

Watch this space for the chemistry of dust

Wednesday, February 11th, 2009


It’s not often that chemists get new tools with which to investigate the building blocks of the world around us, so  a paper on the arXiv today gives them good reason to pop a few corks.

Vladlen Shvedov at the Australian National University in Canberra and a few mates have today unveiled a way of confining and steering aerosol particles in a beam of light.

That hasn’t been possible until now because aerosols are slippery blighters. They absorb huge amounts of light that increases their temperature making them hard to hang on to.

That makes the conventional  way of handling small particles using optical tweezers more or less useless. Optical tweezers rely on radiation pressure to push their charges around. But this is dwarfed by thermal forces and so only works for particles that are relatively cool.

Thermal heating makes particles misbehave because  nonuniform heating makes one side of a particle hotter than the other, causing gas molecules on either side to bounce off with different velecities. This generates a force known as photophoresis.

What Shvedov and his cobbers have done is exploit the photophoretic force to trap partices using  two light beams in the shape of doughnuts.

The result is a device that can trap aerosol particles up to 10 micrometres across  and steer them along trajectories several millimetres long at a rate of around a centmetre per second.

All of a sudden that makes possible a whole host of experiments that were previosuly impossible: developing ecologically clean and safe nanotechnologies, modeleling the chemical proceses at work in the atmosphere and best of all (IMO) simulating interstellar dust .

If you see any chemists with smiles on their faces, you’ll know why. In the meantime look out for some fascinating insights into the chemistry of dust.

Ref: Optical Guiding of Absorbing Nanoclusters in Air

Simulating Sweden

Tuesday, February 10th, 2009


If you want to model how infectious diseases spread, you need a decent simulator to see how the various coping strategies pan out. Your simulation needs to take into account the population, its age and gender distribution, where people live and how far they travel from home to work and which people share homes.

But making this data realistic would be hard. After all, would anybody willingly agree to have their real data entered into such a simulation?

Actually yes. Swedes. All nine million of them.

Yep, the personal details of the entire Swedish population have been used to create what must be the world’s largest and most realistic computer simulation of the way infectious diseases spread.

Lisa Brouwers at the Swedish Institute for Infectious Disease Control and buddies have built a simulation called Microsim in which every member of the Swedish population is represented with details including their sex, age, family status, school, workplace and their geographic location at these places to within 100 metres.

That makes for potentially fantastic simulations but it also raises extraordinary questions over privacy. The data is only minimally anonymized: each individual is given a random identifier but otherwise their personal data is intact.

Given that the team is combining data from three different sources, this doesn’t sound like nearly enough protection.

But Brouwers must know what she’s doing. Or at least be praying that the rest of Sweden doesn’t find out what she’s done.

Ref: MicroSim: Modeling the Swedish Population

Econophysicists identify world’s top 10 most powerful companies

Monday, February 9th, 2009


The study of complex networks has given us some remarkable insights into the nature of systems as diverse as forest fires, the internet and earthquakes. This kind of work is even beginning to give econophysicists a glimmer of much-needed insight in the nature of our economy. In a major study, econophysicists have today identified the most powerful companies in the world based on their ability to control stock markets around the globe. it makes uncomfortable reading. (more…)

Terror ‘n’ terroir

Saturday, February 7th, 2009

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

Striped Superconductors: How the Cuprates Intertwine Spin, Charge and Superconducting Orders

Measuring Accurately Liquid and Tissue Surface Tension with a Compression Plate Tensiometer

Structure and Evolution of the Foreign Exchange Networks

The Minimal Temperature of Quantum Refrigerators

Comparing Bird and Human Soaring Strategies

The Puzzle of Neutron Lifetime

The power laws behind terrorist attacks

Friday, February 6th, 2009


Plot the number of people killed in terrorists attacks around the world since 1968 against the frequency with which such attacks occur and you’ll get a power law distribution, that’s a fancy way of saying a straight line when both axis have logarithmic scales.

The question, of course, is why? Why not a normal distribution, in which there would be many orders of magnitude fewer extreme events?

Aaron Clauset and Frederik Wiegel have built a model that might explain why. The model makes five simple assumptions about the way terrorist groups grow and fall apart and how often they carry out major attacks. And here’s the strange thing: this model almost exactly reproduces the distribution of terrorists attacks we see in the real world.

These assumptions are things like: terrorist groups grow by accretion (absorbing other groups) and fall apart by disintegrating into individuals. They must also be able to recruit from a more or less unlimited supply of willing terrorists within the population.

Being able to reproduce the observed distribution of attacks with such a simple set of rules is an impressive feat. But it also suggests some strategies that might prevent such attacks or drastically reduce them in number . One obvious strategy is to reduce the number of recruits within a population, perhaps by reducing real and perceived inequalities across societies.

Easier said than done, of course. But analyses like these should help to put the thinking behind such ideas on a logical footing.

Ref: A Generalized Fission-Fusion Model for the Frequency of Severe Terrorist Attacks