Archive for April, 2008

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

Wednesday, April 30th, 2008

Here’s a great anecdote from Mikhail Simkin and Vwani Roychowdhury at the University of California, Los Angeles:

During the “Manhattan project” (the making of the nuclear bomb), physicist Enrico Fermi asked General Leslie Groves, the head of the project, what was the definition of a “great” general. Groves replied that any general who had won five battles in a row might safely be called great. Fermi then asked how many generals were great. Groves said about three out of every hundred. Fermi conjectured that if the chance of winning one battle is 1/2 then the chance of winning five battles in a row is (1/2)^5 = 1/32 . “So you are right, General, about three out of every hundred. Mathematical probability, not genius.”

Simkin and Roychowdhury’s interest is not generals but World War 1 fighter pilots. They say an ace fighter pilot is one who has who achieved five or more victories. “Can this be explained by simple probability?” they ask.

At first glance this doesn’t seem likely. The German World War 1 ace Manfred von Richthofen had 80 victories to his name.

If the chance of an aerial victory is 1/2, then the chance of winning 80 on the trot is:

(1/2)^80 = 10^(-24)

That’s not very likely by chance alone and it is tempting to think of von Richtoven as an outstanding pilot .

But Simkin and Roychowdhury say that a more careful analysis proves this conclusion wrong. Their argument is based on the fact that the Germans claimed vastly more victories than losses: 6759 victories versus only 810 losses. That makes the rate of defeat:

810/(6759+810) = 0.107.

So the probability of 80 victories in a row is actually:

(1-0.107)^80 = 10(-4).

And the chance of one of the German’s 2894 fighter pilots achieving this feat is:

1 – (1 – 10^(-4))^2894 = 0.29.

“Richthofen’s score is thus within the reach of chance,” conclude Simkin and Roychowdhury.

The paper goes on to work out that far from being outstanding, von Richtoven was probably merely in the top 27 per cent of pilots ranked by skill.

Basically, he was lucky.

The 90th anniversary of von Richtoven’s first and only loss was last week: the Red Baron was shot down and killed over the Somme on 21 April 1918.

Ref: arxiv.org/abs/physics/0607109: Theory of aces: high score by skill or luck?

Solving the faint young Sun problem

Tuesday, April 29th, 2008

We know by studying ancient rocks that liquid water existed on the surface of Earth at least 3.7 billion years ago. That implies that the surface temperature at that time was at least 273K.

We also know by studying stars similar to ours that the Sun must have been significantly less bright than it is now (Sol is thought to have increased in luminosity by 30 per cent since then). That ought to have resulted in temperatures on Earth that were well below freezing.

This contradiction is known as the “faint young Sun” problem and nobody has adequately explained it.

The most common explanation is that the planet must have been warmed by some kind of greenhouse gas effect mediated perhaps by carbon dioxide, atmospheric haze, ammonia or methane. The trouble is that the evidence indicates that these gases existred in quantities at least an order of magnitude too little to have done the job.

So what gives? According to Philip von Paris of the Institute of Planetary Research  at the German Aerospace Centre in Berlin, Germany, und Freunds, the mix up is largely the result of an error in our understanding of how much radiation was absorbed in those days.  His team has used a new atmospheric model of the early Earth to determine that the required green house effect would have been possible with carbon dioxide with a partial pressure of about 2.9mb, about an order of magnitude less than previously thought.

That’s a pretty good match with the amount of carbon dioxide thought to have been around between 2 and 2.5 billion years ago. Nice result.
But von Paris has been a little over ambitious. He says: “thus, the contradiction between sediment data
and model results disappears,” implying that the faint young Sun problem is solved.

But hang on a minute, he seems to have forgotten the billion years or so between 2.5 and 3.7  billion years ago in which the temperature is still unexplained.

Nice try, von Paris. But we’re not that easily foooled.

Ref: arxiv.org/abs/0804.4134: Warming the early Earth – CO2 reconsidered

First superheavy element found in nature

Monday, April 28th, 2008

The hunt for superheavy elements has focused banging various heavy nuclei together and hoping they’ll stick. In this way, physicists have extended the periodic table by manufacturing elements 111, 112, 114, 116 and 118, albeit for vanishingly small instants. Although none of these elements is particularly long lived, they don’t have progressively shorter lives and this is taken as evidence that islands of nuclear stability exist out there and that someday we’ll find stable superheavy elements.

But if these superheavy nuclei are stable, why don’t we find them already on Earth? Turns out we do; they’ve been here all along. The news today is that a group led by Amnon Marinov at the Hebrew University of Jerusalem has found the first naturally occuring superheavy nuclei by sifting through a large pile of the heavy metal thorium.

What they did was fire one thorium nucleus after another through a mass spectrometer to see how heavy each was. Thorium has an atomic number of 90 and occurs mainly in two isotopes with atomic weights of 230 and 232. All these showed up in the measurements along with a various molecular oxides and hydrides that form for technical reasons.

But something else showed up too. An element with a weight of 292 and an atomic number of around 122. That’s an extraordinary claim and quite rightly the team has been diligent in attempting to exclude alternative explanations such as th epresence of exotic molecules formed from impurities in the thorium sample or from the hydrocarbon in oil used in the vacuum pumping equipment). But these have all been ruled out, say Marinov and his buddies.

What they’re left with is the discovery of the first superheavy element, probably number 122.

What do we know about 122? Marinov and co say it has a half life in excess of 100 million years and occurs with an abundance of between 1 and 10 x10^-12, relative to thorium, which is a fairly common element (about as abundant as lead).

Theorists have mapped out the superheavy periodic table and 122 would be a member of the superheavy actinide group. It even has a name: eka-thorium or unbibium. Welcome to our world!

This may well open the flood gates to other similar discoveries. Uranium is the obvious next place to look for superheavy actinides. I’d bet good money that Marinov and his pals are eyeballing the stuff as I write.

Ref: arxiv.org/abs/0804.3869: Evidence for a Long-lived superheavy Nucleus with Atomic Mass Number A = 292 and Atomic Number Z @ 122 in Natural Th

In case ya missed ’em…

Sunday, April 27th, 2008

Iron ‘n’ stone

Saturday, April 26th, 2008

The iron arsenide superconductivity challenge

Friday, April 25th, 2008

Until a few weeks ago, all so-called high temperature superconductors were layered copper oxides of the type discovered by Karl Muller and Georg Bednorz back in 1986. These are so-called because they become superconducting at temperatures above 30K, the theoretical limit predicted by the BCS theory (after Bardeen, Cooper and Schrieffer) of superconductivity that ruled supreme until then. The discovery won Muller and Bednorz the Nobel Prize in 1987.

But in February, Hideo Hosono at the Tokyo Institute of Technology announced that he had discovered a second family of high temperature superconductors based on iron arsenide. Initially these became superconducting at 26K. But within weeks, other groups had upped the temperature to 40K and then to 54K by make some subtle changes to the material’s composition.

Today two groups publish papers on the arXiv claiming versions of this material that superconduct at 41K and 54K. While another group claims to have made low cost wires out of iron arsenides (a considerable feat given that the annealing temperature of the new superconductors is some 1200C ).

These temperatures are s still nowhere near the record of 138K for copper oxides nor the 200K being claimed for aluminum nanoclusters. But its not bad for just 6 weeks work. Who knows what temperatures they might be possible with this material.

And therein lies the challenge. The question now is whether iron arsenides superconduct in the same way copper oxides do. If not, the theorists are going to have one helluva headache working this one out.

Refs:

arxiv.org/abs/0804.3725: Superconductivity at 41.0 K in the F-doped LaFeAsO1-xFx

arxiv.org/abs/0804.3727: Superconductivity at 53.5 K in GdFeAsO1-δ

arxiv.org/abs/0804.3738: Superconductivity of powder-in-tube LaO0.9F0.1FeAs wires

Modelling how birds mistime egg-laying due to climate change

Thursday, April 24th, 2008

Many birds have to time egg-laying to coincide with a peak in food availability, for example , to match the hatching shcedule of a particular type of caterpillar. This is a tricky business because many caterpillars are available for only a few weeks and the birds must lay their eggs around a month in advance to hit the peak. Needless to say, nobody is quite sure how birds do it.

The situaiton has recently become more complicated because a warmer climate in many habitats is causing caterpillars to advance their hatching date. How birds are coping with this is being increasingly studied.

Daniel Campos, a mathematician at the University of Manchester, and a few buddies have come up with a few predictions based on evolutionary game theory, in which agents compete, learn, evolve and are selected for or against. This team’s particular focus is on evolutionary minority games in which agents are rewarded for behaving in a way that puts them in a minority.

It turns out that evolutionary minority games are pretty good models of many real world situations in which individuals compete for a limited resource. That’s because those in the minority have fewer competitors and so are more likely to survive. Campos and co say that the way birds compete for food aftr egg laying is one of these situations and that their model can be used to make some predictions about the behaviour of birds as food resources change.

Their main result seems to be that the best learners among bird species should adapt more easily to changing hatching schedules. But there is a caveat: if the food resource isn’t scarce, this doesn’t apply.

That’s more or less what the ornithologists had predicted themsleves without the help of sophisticated computer models. And it seems to match the observations that some birds have adapted their laying because of a change in the timing of a food peak. Of course, other species haven’t changed and nobody knows whether this is because the birds are poor learners or because there are alternative food sources available, or for some other reason. And all this is against the backdrop of a very limited knowledge about bird learning abilities in the first place.

All in all, this a slightly unsatisfying contribution to a debate that will reach fever pitch as more observational data comes in. But the authors appear to acknowledge this in the title of their paper: “Limited Resources and Evolutionary Learning may help to Understand the Mistimed Reproduction in Birds caused by Climate Change”. Then again, it may not.

Ref: arxiv.org/abs/0804.3485: Limited Resources and Evolutionary Learning may help to Understand the Mistimed Reproduction in Birds caused by Climate Change

First observation of antibonding in artificial molecules

Wednesday, April 23rd, 2008

When electrons are confined in a flat space, they interact in much the same way as electrons in ordinary atoms by forming into pairs of various energy levels. They can even be made to emit light when they jump from one level to the next, just like electrons in the orbitals in real atoms.

These flat spaces are easily created in a thin semiconductor layer. Because of their similarity with the real thing, these electronic patterns are called “artifical atoms” and they form a kind of periodic table depending on the number of electrons they contain. And get this: put two artificial atoms next two each other and the electrons can tunnel across the gap to form a kind of covalent bond. That makes an artificial molecule.

The chemistry of artificial molecules has received widespread interest in recent years because it is possible to use them to make materials with properties that are otherwise not found in nature. And today, Matt Doty from the Naval Research Labraotry in DC and a few pals announce the first experimental observation of one of these strange properties in artificial diatomic molecules: antibonding.

Doty says that as the distance between two atoms increases, the bond linking them together suddenly switches to an antibond that pushes them apart. Antibonds are never seen in nature. They arise because artificial atoms contain “holes”, the absence of an electron in the structure which can be thought of as positively charged. Doty spotted their effect by looking for the characteristic light signature that an antibond can be made to emit in a magnetic field.

Antibonds could turn out to have useful properties. Doty and pals say molecules containing anitbonds could one day be used to manipulate the spin of passing electrons, a property that could be useful for the emerging field of spintronics.

Let’s wait and see.

Ref: arxiv.org/abs/0804.3097: Antibonding Ground States in Semiconductor Artificial Molecules

Bluetooth surveillance secretly tested in the city of Bath

Tuesday, April 22nd, 2008

In 2001 Jose Emilio Suarez Trashorras was jailed in a Spanish prison for drug related offences. Whilst imprisoned, Trashorras established regular contact with Jamal Ahmidan who was serving time for a petty crime. Both individuals embraced radical Islamic fundamentalist ideas within the prison and were recruited in the Takfir wa al-Hijra group, a Moroccan terrorist groups linked with al-Qaida . Following their release, Ahmidan became the leader of the terrorist cell that conducted the Madrid bombing. In a drugs-for-bombs exchange with a third party, Trashorras provided the explosives for the 13 backpack bombs that killed 191 people and injured hundreds.

So write Vassilis and Panos Kostakos in the department of computer science and the University of Bath in the UK, who have come up with a system that they say could spot and monitor these kinds of interactions in prisons.

Their idea? Fit inmates with RFID tags that allow their positions to be monitored, and then number crunch the resulting data sets to see who spends the most time with whom.

Not exactly rocket science but the Kostakos’s have an even more frightening idea. Why not test the idea by anonymously monitoring the movements of students, residents and workers of the city of Bath by listening out for their bluetooth-enabled devices as they move around the city. And that’s what they’ve done.

What the Kostakos found is that it is straightforward to capture data on people’s encounters using bluetooth. In fact they captured data on 10,000 unique devices during the 6 month study. Yep, that’s 10,000.

Exactly how much you can tell about these encounters isn’t clear. But hey, this is only a demonstration (either that or they’re keeping schtum about the juicy details).

Of course, it’s already possible to make inferences about encounters between individuals using the location information from cellphone networks. But that isn’t easily accessible to ordinary folk and in any case has a blunt resolution of a mile or so. Bluetooth, on the other hand, gives your location to within 10 metres or so.

The moral? Turn off your bluetooth enabled devices when in the city of Bath (and anywhere else). In other hands, this kind of data could be dangerous.

Ref: arxiv.org/abs/0804.3064: Intelligence Gathering by Capturing the Social Processes Within Prisons

Monday, April 21st, 2008

Radar astronomy is a crucial tool in measuring the trajectories of Earth-crossing asteroids. If we’re going to be hit, radar is how we’ll work out when. The technique has also been used to image various bodies such as the asteroid 216 Kleopatra, to measure distances with extreme accuracy and to test relativity by monitoring the orbit of Mercury.

The technique was first used in the 1960s and since then astronomers have broadcast some 1400 radar transmissions towards the heavens, says Alexander Zaitsev at the Institute of Radioengineering and Electronics, Russian Academy of Sciences, in Fryazino near Moscow. These transmissions have lit up  about 2×10^-3 part of the sky.

By contrast, humanity has broadcast only 16 messages intended for extraterretrial consumption. And these have illuminated an area of sky some 2000 times less than the radar astronomy signals.

Since the radar signals were 500 times longer than the ET messages, Zaitsev says that ET is about 1 million times more likely to pick these up first. So first contact is likely to be a little more prosaic than most of us had anticipated.

The purpose of Zaitsev’s calculation is to take on the chicken little’s who say that we should keep our interstellar gobs shut lest some hostile super-civilisation decide to turn us all to astroslavery. He says these people should be more worried about radio astronomy than ET messages and since we can’t stop monitoring Earth-crossing asteroids, the party poopers might as well give up and go home.

It’s all irrelevant anyway. If ET was going to make herself known, she’d have done it by now.

Ref: arxiv.org/abs/0804.2754: Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization