Archive for July, 2008

The Casimir conundrum

Thursday, July 31st, 2008

When it comes to the Casimir force, physicists are in an embarrassing position.

“Weak intermolecular forces have a truly pervasive impact, from biology to chemistry, from physics to engineering. It may therefore come as a surprise to know that there still exist, in this well established field, unresolved problems of a fundamental character. This is indeed the case with respect to the problem of determining the Van der Waals-Casimir interaction between two metallic bodies at finite temperature. As of now, people simply don’t know how to compute it, and the numerous recent literature on this subject provides contradictory recipes, which give widely different predictions for its magnitude”

So writes Giuseppe Bimonte at the Istituto Nazionale di Fisica Nucleare in Naples, Italy. That doesn’t sound good but Bimonte has a way out of the conundrum.

His proposal is to measure the change in Casimir pressure between two superconducting plates as their temperature is raised through their critical value so that they no longer superconduct.

Bimonte claims that the results should unambiguously distinguish between the various competing theories.

Get to it.

Ref: arxiv.org/abs/0807.2950: The Casimir Effect in a Superconducting Cavity: a New Tool to Resolve an Old Controversy

The painful search for gravitational waves

Wednesday, July 30th, 2008

G-wave data

Gravitational wave detectors have a sorry history of disappointing results.

Joseph Weber at the University of Maryland first claimed to have spotted these waves in 1969. He did it by listening to the way a giant cylindrical bars vibrate, thinking that passing gravitational waves would cause them to ring like a bell. Nobody has been able to reproduce these results and they remain strongly disputed today.

Various groups still listen out for gravitational waves using Weber-like detectors. But the Ferraris in this field are a new generation of laser interferometers that are much more sensitive to the bending and squeezing of space that these waves cause as they pass by.

The trouble is that none of these detectors has ever spotted a gravitational wave either, despite the investment of hundreds of millions of dollars. One way of increasing the sensitivity is to use two or more interferometers in different parts of the world to look for wave simultaneously.

Now the results of the first  combined search using four detectors (three LIGO detectors in the US and the GEO600 in Germany) have been published and the results are again disappointing.  They took data over a period of month between 22nd February and 23rd March 2005, giving them a decent amount of data to play with. But…

“No candidate gravitational wave signals have been identified”

says the team, ominously.

That’s embarrassing because these combined searches should be sensitive enough to pick up gravitational waves from sources such as supernovae and from black holes as they collide.

So why aren’t they seeing anything? One possibility is bad luck, that there weren’t any events during the the time  the data was being taken. That seems unlikely. Another possibility is that the problem is closer to home, perhaps in the equipment, analysis or even the theory itself.

Whatever the problem, they don’t seem to be able to put their finger on it. This data is three years old which means it’s been given one almighty going over before publication.

So I wonder how these guys are feeling given that hundreds of millions of dollars and several years of work has so far produced zilch.

Ref: arxiv.org/abs/0807.2834: First Joint Search for Gravitational-Wave Bursts in LIGO and GEO600 Data

The day the solar wind disappeared

Tuesday, July 29th, 2008

coronal-hole.jpg

It happened on 11 May 1999…nobody knows  why and the event was not related to well known drivers of solar weather such as coronal mass ejections or large flares.

According to Durgess Tripathi at the University of Cambridge, UK, and pals, the cause seems to be linked to the appearance a few days earlier of a coronal hole, where the Sun’s corona appeared darker than usual. Exactly how this could have reduced the solar wind to nothing isn’t yet known.

Ref: http://arxiv.org/abs/0807.2697: The Solar Wind Disappearance Event of 11 May 1999: Source Region Evolution

Why small black holes cannot grow

Monday, July 28th, 2008

Quantum mechanics places a fundamental limit on the minimum quanta of energy that can be associated with a bit of energy.  It’s about 10^-50 Joules, which ain’t much.

That has important implications for black holes, says Scott Funkhouser, a physicist at The Citadel, the military college of South Carolina, in Charleston. As black holes accretes mass, its total energy increases but the energy per bit decreases,  he says.

And this decrease must always be greater than the minimum quanta of energy allowed by quantum mechanics. That only happens when black holes are bigger than 10^11 kg, which is only about a thousandth of the mass of Halley’s Comet.

That’s small on astrophysical  terms. But it should also come as a relief to anybody worried about the possibility of the production of tiny black hole at particles accelerators such as the LHC. According to Funkhouser, these black holes must be safe because they cannot grow.

Phew!

Ref: arxiv.org/abs/0807.1938: The Minimum Mass of a Black Hole that is Capable of Accretion in a Universe with a Cosmological Constant

In case ya missed ‘em…

Sunday, July 27th, 2008

The baubles from the physics arXivblog this week:

Musical relativity

The puzzling wrinkles in graphene

Terminator 0.0.1 (alpha)

How likely is an avian flu pandemic?

Flu ‘n’ achoo

Saturday, July 26th, 2008

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

A New Look at Bell’s Inequalities and Nelson’s Theorem

Controlling Transistor Threshold Voltages using Molecular Dipoles

Exploiting Bird Locomotion Kinematics Data for Robotics Modeling

Atmospheric Calorimetry above 10^19 eV: Shooting Lasers at the Pierre Auger Cosmic-Ray Observatory

Escherlike Quasiperiodic Heterostructures

Thornhill, de Broglie and the Kinetic Theory of Electromagnetic Radiation

How likely is an avian flu pandemic?

Thursday, July 24th, 2008

Poisson distribution

With winter approaching, many governments in the northern hemisphere are stocking up on Tamiflu and fine tuning their civil defense plans to cope with the disruption a bird flu outbreak might cause.

But how likely is an outbreak? While various groups have written about how a pandemic might happen, Rinaldo Schinazi at the University of Colorado says nobody seems to have bothered to work out the probability of such an event.

That sounds unforgivable, given the investment that’s going in to tackling the problem. But in some ways it’s understandable. Schinazi says:

“The occurrence of a pandemic seems to be hopelessly complex in the sense that it depends on a multitude of factors, some known others unknown. Factors mentioned in the literature go from the stability of the current influenza strains to the number of pigs in China! Hence, a model taking into account all known factors would probably be as complex as the phenomenon itself, would not be that accurate and would therefore be useless.

But Schinazi has taken an entirely different approach:

“We do not attempt to incorporate into the model any of the factors that are believed to provoke a pandemic. Instead we treat the occurence of a pandemic as a purely random phenomenon.

And the result is a fascinating read. It turns out there have been only 10 pandemics in the last 300 years. This is the data Schinazi uses to prime his models.

Assuming a Poisson distribution, the mean time between pandemics is 30 years but the probability that no pandemic occurs in 60 years is 14% .

Assume a random walk model and the probability that the time between two pandemics is at least 50 years becomes 11% but the probability that this interval is at least 100 years is 8%, which is hardly much less.

Schinazi’s conclusion is both surprising and re-assuring: the next pandemic might not be as imminent we’ve been led to believe.

Ref: arxiv.org/abs/0807.3524: Will the Announced Influenza Pandemic Really Happen?

Terminator 0.0.1 (alpha)

Wednesday, July 23rd, 2008

NAO robot

The French start up Aldebaran-Robotics based in Paris has high hopes for its humanoid robot called NAO.  The device is 57 cm high and weighs 4.5 kilograms (about the size of a 6 month old baby) and you may be about to see a lot more of it. The company has sent a simplified version to 16 teams playing in the Robocup humanoid football league this year.

NAO looks an impressive device, judging by the design, which the company has posted on the arXiv today.   And others clearly agree. Earlier this year, the company picked up  Euros 5 million in venture capital funding to help commercialise the device. The target market is university research labs involved in developing the next generation of software and hardware for robotics.

That’s a smart move because  it could make NAO a de facto standard.

NAO doesn’t come cheap, however. A single robot will set you back Euros 10K but that is significantly cheaper than most other humanoids. Fujitsu’s HOAP costs $50K, for instance, and Honda hasn’t been able to put price on Asimo.

The company hopes that economies of scale will bring down the price as production scales up. Eventually it hopes to sell NAO to the public for Euros 4K each.

Better start saving.

Ref: arxiv.org/abs/0807.3223: The NAO Humanoid: A Combination of Performance and Affordability

The puzzling wrinkles in graphene

Tuesday, July 22nd, 2008

Graphene

Last year, Jannick Meyer at the Max Plank Institute for Solid State Research in Stuttgart and pals discovered that single sheets of graphene are gently rippled, like the rolling hills of New England. That’s a puzzle because graphene behaves like a perfect 2D crystal. So how do these ripples form  and what role do they play in the crystal properties of the material?

One possibility is that thermal fluctuations cause the wrinkles, in other words, that graphene buckles as it heats up. But today, Rebecca Thompson-Flagg and buddies at the University of Texas at Austin present another idea.

They say that heat is an unlikely to be the cause because the stiffness of graphene ought to ensure that ripples of the observed size vibrate at a frequency of 10^11 Hz. However, Meyer’s observations only make sense if the ripples are static.

Instead, Thompson-Flagg suggests that the ripples are formed by the adsorption of OH molecules at random sites throughout the crystal. In other words, the graphene wrinkles when it’s damp.

They’ve simulated the shape of a damp graphene and the ripples exactly match those seen by Meyer.

That’s an interesting result but not quite a slam dunk. For that, we’ll need to see what graphene looks like and how it behaves when no OH is present.

Ref: arxiv.org/abs/0807.2938: Rippling of Graphene

Musical relativity

Monday, July 21st, 2008

Musical relativity

Here’s a neat idea for a concert that’s going to blow a few minds if it ever takes to the stage.

A combination of three or more notes played together is called a chord. We know that certain musical chords sound happy while others sound sad (although nobody knows why). The mood of a piece of music then depends on the combination of chords being played. More than a few weighty tomes have been written about the way one chord can be transformed into another and the effect this has on the mood of the music.

But Kaca Bradonjic, a physicist at Boston University, says that musicians appear to have ignored one of the fundamental ways of changing the pitch of a note: the Doppler shift. He points out that it ought to be possible for an observer moving at a specific velocity to hear a sad sounding note as a happy one and vice versa.

Which means that the mood of a piece of music depends on the relative  velocities of the audience and performers.

He calculates for example that to hear a C major chord as a C minor, the listener would need to be travelling at about 43 miles per hour, directly away from the source. That’s a fair speed. And the accelerations necessary to vary this effect from one note to another during a concert would make this one helluva roller coaster ride.

Talking of which, a (very quiet) roller coaster might be the perfect venue for  the first concert of this type.

Ref: arxiv.org/abs/0807.2493: Relativity of musical mood