The cherries from this week’s physics arXiv blog:
Archive for April, 2008
The best of the rest from the physics arXiv this week:
Just how birds use the earth’s magnetic field to navigate has puzzled researchers for decades.
But in recent years, a growing body of evidence points to the possibility that a weak magnetic field can influence the outcome of a certain type of chemical reaction in bird retinas involving radical ion pairs.
The idea is that the chemical outcome of the recombination of the ion pairs depends on whether the radical electrons are in a singlet or triplet state. A magnetic field creates a bias towards the triplet state which in turn leads to a one chemical output being preferred over another.
To test the idea, various experimenters have successfully confused the navigational abilities of birds such as robins by zapping them with magnetic fields specifically designed to disrupt this reaction. Case closed.
Not quite. The problem is that the ion recombination is known to happen too quickly for the Earth’s magnetic field to have any effect. So how can this mechanism work?
The claim made today by Iannis Kominis at the University of Crete is that the quantum zeno effect explains all. This is the watched-pot-never-boils effect on the qauntum scale. It states that the act of observing a quantum system can alter its evolution in a way that maintains the state for longer than expected.
One well known example is that it is possible to slow down the rate at which molecules convert from ortho to para isomers when they are constantly involved in collisions.
Kominis says a similar thing happens in birds: the presence of a geomagnetic field extends the lifetime of the singlet-triplet mixture from which the ions recombine. This gives the magnetic field time to bias the outcome of the recombination.
Interesting idea. But what’s most impressive is that it accounts for a number of unexplained observations about avian magnetoreception, such as the heading error of about 30 degrees that often afflicts birds (Kominis says a change in heading angle causes a change in the coherence time) and that avian compasses appear sensitive to only a certain window of magnetic field strength (Kominis says the window depends on the hyperfine couplings of the atoms involved which have been selected for by evolution).
If Kominis is correct, this is extraordinary news: it means a quantum sensor determines the macroscopic behaviour of magnetic sensitive birds.
Kominis says we may well see similar effects elsewhere, and mentions that a similar mechanism might be at work in photosynthesis.
But there’s another system closer to home that is bound to come up. Kominis is careful not to mention it but the quantum consciousness people are going to be all over this like freshmen at a sorority party.
Ref: arxiv.org/abs/0804.2646: Quantum Zeno Effect Underpinning the Radical-Ion-Pair Mechanism of Avian Magnetoreception
The Scottish author Robert Louis Stevenson once said: “politics is perhaps the only profession for which no preparation is thought necessary.”
Given that these people run the world’s biggest (and smallest) economies, how many are needed to do a decent job?
It is well known in management circles that decision making becomes difficult in groups of more than 20 or so. The British historian Northcote Parkinson studied this idea in relation to British politics and conjectured that a cabinet loses political grip as soon as its membership passes a critical size of 19-22 due to its inability to make efficient decisions.
Now Peter Klimek and pals from the Complex Systems Research Group at the Medical University of Vienna in Austria have found a similar relationship between the efficacy of political systems around the world and the size of the cabinets they employ to make decisions.
Using data supplied by the CIA (which must obviously be 100 per cent correct), they compared the cabinet sizes in 197 self-governing countries with various indicators related to those countries’ economic, social and democratic performance. For example, the UN’s Human Delevopment Indicator which assesses a country’s achievement in areas such as GDP, life expectancy at birth and literacy.
The size of cabinets varied from just 5 in Liechtenstein and Monaco to 54 in Sri Lanka.
Klimek and co say that the various indicators of success are negatively correlated with cabinet size. Their message is, rather predictably, that too many cooks spoil the broth.
More interesting is their claim that there is a critical value of around 19-20 members, beyond which consensus is more difficult to achieve. They build a (somewhat unconvincing) mathematical model to show that at this critical value “dissensus” becomes more likely because it is easier to form multiple opposing factions in groups of more than 20 members. However, the transition from consensophile to dissensophile groups doesn’t look very critical to me.
All this is of more than passing relevance in Europe where the recent expansion of the European Union has resulted in a club of 27 nations. How will effective decision making be made? By reducing the size of the cabinet, called the European Commission, to 18 members, with various countries coming in and out on a rotation basis.
That means a third of the members will not be represented at the exective level. Which is praisworthy for its practicality but dubious from a democratic point of view. But that’s politics.
Ref: arxiv.org/abs/0804.2202: To How Many Politicians should Government be Left?
The pantheon of impossible photon tricks grows ever larger. Today, a new addition from Andrew Shields and pals at Toshiba Research Europe in Cambridge, UK:
“We report an experiment in which two-photon interference occurs between degenerate single photons that never meet. The two photons travel in opposite directions through our fibre-optic interferometer and interference occurs when the photons reach two different, spatially separated, 2-by-2 couplers at the same time.”
Ref: arxiv.org/abs/0709.0847 : Experimental Position-time Entanglement with Degenerate Single Photons
Astronomers have recently wondered whether carbon might form a supercooled liquid under the huge pressures that exist in side carbon-rich white dwarf stars and even inside medium-sized gaseous planets such as neptune and uranus. If that’s the case, then small disturbances in the liquid could trigger the formation of diamonds the size of automobiles.
The trouble is that nobody has been able to create these conditions on Earth so the way in which nucleation might occur is more or less unknown.
Now Daan Frenkel and pals from the University of Amsterdam in The Netherlands say that computer simulations of the behaviour of several thousand carbon atoms under these circumstances have given us the first inkling of how nucleation occur. And the odds are that it’s a tricky process to set in motion.
Applying these results to astrophysical bodies gives two insights, both of which are bad news for diamond hunters.
First, nucleation may be so rare that “not a single diamond could have nucleated in a Uranus-sized body during the life of the universe.”
And second, the appropriate conditions may be common in certain white dwarf stars.
Either way, these diamonds are outta bounds for the foreseeable future.
Ref: arxiv.org/abs/0804.1671: State-of-the-art models for the Phase Diagram of Carbon and Diamond Nucleation
The world’s longest suspension bridge could easily be built if its cables were made from bundles of carbon nanotubes, say Alberto Carpinteri and Nicola Pugno from the Polytechnic University of Turin.
The thinking for this rather unsurprising news is that carbon nanotube cables would allow the main span of a suspension bridge to increase in length by a factor of three, to more than 6 km.
The motivation for this analysis appears to be the problem of building a bridge across the Strait of Messina, the water that separates Sicily from mainland Italy and which would require a main span of some 3 km.
But a little more interesting is a corollary in which the authors have analysed the length of the record setting suspension bridges built since the Brooklyn Bridge in 1883.
Carpinteri and Pugno say they’ve found a Moore-like exponential relationship. That’s kinda interesting (even if the exponent is only 1.0138) because we usually associate exponential increases with hi-tech industries.
Carpinteri and Pugno says the bridge could be built now with carbon nanotube bundle technology. The bad news, however, is that by extrapolating along this curve, it’s clear the Messina Bridge will not be built until about 2050. Probably with carbon nanotube bundle cables.
Their’s is clearly the triumph of hope over extrapolation.
Ref: arxiv.org/abs/0804.1446: Super-bridges Suspended over Carbon Nanotube Cables
…a round up of this week’s plums from the physics arxivblog:
The best of the rest from the physcis arxiv this week:
Wow! Every now and again a paper on the arxiv leaps out at you and today there’s work from Indiana University in Bloomington that has got my eyeballs on stalks.
Get this: a team led by Martin Jarrold is claiming to have found evidence of superconductivity in aluminium nanoclusters at 200 K .
Yep, 200 K. The current world record for high temperature superconductivity is 138 K for a cuprate perovskite so that’s a massive jump.
The background to this is that two years ago Yuri Ovchinnikov at the Landau Institute for Theoretical Physics in Moscow and Vladimir Kresin at the Lawrence Berkeley Laboratory in California predicted that metal nanoclusters with exactly the right number of delocalised electrons (a few hundred or so) could become strong superconductors.
Now Jarrold and his buddies (Kresin and Ovchinnikov among them) have found the first evidence that this prediction is correct in individual aluminium nanoclusters containing 45 or 47 atoms . And they found it at 200 K.
A few caveats. Before a claim of superconductivity can be made, physicists require three unambiguous and repeatable lines of evidence. The first is obviously zero electrical resistance. The second is the Meisner effect in which the superconductor reflects an external magnetic field. And finally there must be evidence of a superconducting phase transition, such as a jump in the material’s heat capacity when superconductivity occurs.
What Jarrold’s team have measured is the last effect–a massive change in an individual nanocluster’s heat capacity at 200 K. That’s an important pillar of evidence which is consistent with superconductivity but it is not yet a slam dunk.
Jarrold and his team are simply time-stamping their efforts by publishing on the arxiv and you can bet your bottom dollar that they’re looking for other evidence right now.
Even with that proviso, this looks to be an important breakthrough which should be straightforward for other groups to replicate. The group’s work is not yet peer-reviewed. That’ll be an important step too.
Jarrold will be only too mindful that the field of high temperature superconductivity is littered with the corpses of physicists who have made premature claims.
But for the moment, sit back and admire. 200K…wow! That’s room temperature in Siberia at certain times of the year.
Ref: arxiv.org/abs/0804.0824: Evidence for High Tc Superconducting Transitions in Isolated Al45 and Al47 Nanoclusters
Earlier ref: arxiv.org/abs/cond-mat/0603733: Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters