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The Future of Gaming: The Hot Potato Experience

Mon, 08 Feb 2010 05:00:00 GMT

The next generation of pervasive games are beginning to appear.

It's no secret that the Nintendo Wii has changed the landscape for gaming. The question is what comes next.

Sony has been developing camera-based games for some time and Microsoft has a similar system in the works. But there are plenty of people who think that neither of these approaches will be game-changers (ahem).

Instead, the most exciting developments are coming from the world of mobile phones or other sensor networks where engineers are testing a new generation of games that can be played anywhere there is a mobile phone or wireless network. These games are location aware, involve multiple players, rapid physical activity and Wii-like gesturing.

So-called pervasive games generate an entirely new set of challenges--and not just for the people who play them. They must work with multiple types of input-an iPhone must be able to play against a Nexus One. They involve many players communicating rapidly, so these devices need to synchronise with each other.

To test how such games might work and the problems they generate, Ioannis Chatzigiannakis and a few mates at the University of Patras in Greece created one called Hot Potato. The game works like this:

A hot potato is a kind of virtual timer that is passed between mobile devices that players hold. During the game, the hot potato counts down to zero when it explodes. When this happens, the person holding it is out. The game repeats until only one player is left. Players can "throw" the hot potato to another player by moving close and making a throwing action with their arm (while holding their device).

Moving too far from the other players increases the chances that a new hot potato will be generated on your device. That keeps the players together. Until one of them receives a hot potato, in which case it pays to move away from them so they can't throw it to you.

Chatzigiannakis and company created the game using Sun's Spot sensor network device, which has an 180MHz ARM 9 processor with 512KB of RAM and 4MB Flash. It is IEEE 802.15.4 compliant and relies on a CC2420 Chipcon transceiver for communication. The device has two buttons, eight LEDs and a number of sensors such as an accelerometer, a thermistor, and a light sensor.

Today, the team reveal the results of tests with the game including user surveys. And it looks as if it works well. They say that players reacted very positively to the game.

As for the limits of the gaming experience, they say: "Up to 14 players can participate in a game session simultaneously in a completely distributed environment; above this limit, inherent technology factors come into play and prevent a seamless gaming experience."

Hot Potato isn't the first pervasive game, by any means. But others such as CatchBob!, SupaFly, and Human Pacman have had limited impact. This is an area that is waiting for its breakthrough game.

Hot Potato sounds like fun and a potential money spinner too. But now comes the difficult part: the final beta testing to take any rough edges off the product and then the sales and marketing.

iPhone app, anybody?

Ref: arxiv.org/abs/1002.1099: The "Hot Potato" Case: Challenges in Multiplayer Pervasive Games Based on Ad Hoc Mobile Sensor Networks and the Experimental Evaluation of a Prototype Game

The Mobile Phone Conundrum: If I Call You, Will You Call Back?

Sun, 07 Feb 2010 05:00:00 GMT

The study of reciprocity between mobile phone users reveals surprising insights about the flow of information in society.

What do your mobile phone habits say about you? Probably more than you might imagine.

At least, that's the suggestion from Lauri Kovanen and pals at the Aalto University School of Science and Technology, Finland. These guys have studied the 350 million calls made by 5.3 million customers over an unnamed mobile phone network during a period of 18 weeks. The primary question they ask is whether mobile phone calls are mutually reciprocated: in other words, does somebody who calls another individual receive in return as many calls as he or she makes, a phenomenon known as reciprocity.

Mobile phone calls are a particularly good way to study reciprocity because they are directed in a way that sms messages and email are not. In a mobile phone call, the caller initiates the conversation and then both parties invest a certain amount of time in the event. But afterwards there is usually no immediate reason for the recipient to call back. So it's clear who initiated the event.

But SMS messages or e-mails are entirely different: here a conversation usually means sending a sequence of reciprocated messages and this makes it much more difficult to study reciprocity by simply counting the number of messages.

This has allowed Kovanen and company to unearth a number of interesting phenomena. For a start, the calling patterns of prepaid users is very different from those with a contract who pay later. Postpaid users tend to be more prolific, having on average 5.41 people they call.

Prepaid users, by contrast, have only 3.41 contacts on average (although the notion of "average" is a little strange here since there is a very long tail on these distributions).

Not only that but postpaid users make 10 times as many calls as prepaid users. "We can also see that prepaid users receive more calls than they make, while the most active postpaid users make more calls than they receive," says Kovanen and company.

Prepaid users are also have more skewed relationships. Among prepaid users, the relationships where one participant makes more than 80 percent of all calls make up over 25 percent of the total.

The figures for postpaid users are far less skewed but they are greater than you'd expect from an ordinary probabilistic distribution in which each party in a relationship was just as likely to call the other.

So what's the difference between prepaid and postpaid callers? One of the most important is probably that prepaid users are much more likely to be young people. And sociologists already know that relationships between young people tend not to be equally reciprocated.

A few years ago, the National Longitudinal Study of Adolescent Health asked US students to name up to five of their best friends. Between them, the students named 7,000 individuals but only 35 percent of the nominations were reciprocated. So perhaps it's not suprising that a similar picture emerges from the study of mobile phone calls.

More puzzling is the skew in reciprocity in postpaid users which may not be as significant as for prepaid users but is still worthy of note.

What Kovanen and co are uncovering may be some fundamental property of human relationships; only more study will reveal that.

But the work is important for another reason: the skewed reciprocity between mobile phone users may influence other things such as the spread of ideas and information in society or, just as likely, the spread of viruses.

And that could have important implications for the way antivirus efforts are organised and directed.

Ref: arxiv.org/abs/1002.0763: Reciprocity of Mobile Phone Calls

V 'n' III

Sat, 06 Feb 2010 05:10:00 GMT

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

Complex Networks: New Trends For The Analysis of Brain Connectivity

Soccer: Is Scoring Goals a Predictable Poissonian Process?

Optical Chirality Without Optical Activity: How Surface Plasmons Give A Twist To Light

Long-Lived Quantum Coherence in Photosynthetic Complexes at Physiological Temperature

Locomotion of Electrocatalytic Nanomotors due to Reaction Induced Charge Auto-Electrophoresis

Doped Graphane Should Superconduct at 90K

Fri, 05 Feb 2010 05:10:00 GMT

New calculations reveal that p-doped graphane should superconduct at 90K, making possible an entirely new generation of devices cooled by liquid nitrogen.

There's a problem with high temperature superconductors. It's now more than two decades since the discovery that certain copper oxides can superconduct at temperatures above 30 K.

Those years have been filled with promise, hyperbole and feverish research. Physicists know that copper oxides superconduct in an entirely different way to conventional BCS superconductors (after Bardeen, Cooper and Schrieffer, who worked out the theory behind them). And yet, nobody agrees on precisely what the new mechanism is. Neither has anybody created a superconductor that works at a usable temperature, that is above the temperature of liquid nitrogen.

Even the resurgence of excitement last year over the discovery that magnesium diboride superconducts at high temperatures, probably in the old fashioned BCS way, soon gave way to malaise as physicists found they were unable to build on the breakthrough to make better superconductors. It's tempting to think that superconductors will never pass the liquid nitrogen barrier.

But today hope is restored thanks to a fascinating set of calculations carried out by Gianluca Savini at the University of Cambridge in the UK and a couple of buddies. They calculate the properties of p-doped graphane from first principles and say that it ought to superconduct at a balmy 90K or more, well within the range of liquid nitrogen cooling.

What's more p-doped graphane should superconduct in the same way as the old fashioned BCS superconductors. That's curious because everybody believes that BCS superconductivity cannot work at high temperatures.

The reason is the energy of the interaction between the superconducting electrons and the surrounding material. In ordinary BCS superconductors this is thought to be just a few tens of meVs. In the copper oxides, however, these interactions have an energy of a few hundred meVs. It's this difference, that makes physicists think that BCS superconductors will never work at the temperature of copper oxides.

And yet the discovery that magnesium diboride superconducts challenges that thinking--energy of these interactions in MgB2 is much higher. Three factors seem to come together to make it possible, say Savini and co. First is the characteristic energy of the phonons in MgB2 which is due to bond stretching and plays an important part in helping superconductors through the structure. Second is the electron density of states in the material and finally they point to the balance between the attractive electron-phonon coupling and the repulsive electron-electron interaction in MgB2.

Might it be possible to find materials in which these quantities can be manipulated further? You betcha. Savini and co noticed that p-doped diamond has two of these characteristics but superconducts only at 4K.

However, they calculate that p-doped graphane fits the bill exactly and should superconduct in the old-fashioned BCS way at 90K. What's more they say there are hints that p-doped diamond nanowires might have similar properties.

Various groups are already playing around with doped diamond nanowires.

The implications of all this are astounding. First up is the possibility of useful superconducting devices cooled only by liquid nitrogen. At last!

But there's another, more exotic implication: by creating transistor-like gates out of graphane doped in different ways, it should be possible to create devices in which the superconductivity can be switched on and off. That'll make possible an entirely new class of switch.

Before all of that, however, somebody has to make p-doped graphane. That will be hard. Graphane itself was made for the first time only last year at the University of Manchester. It should be entertaining to follow the race to make and test a p-doped version.

Ref: arxiv.org/abs/1002.0653: Doped Graphane: a Prototype High-Tc Electron-Phonon Superconductor

The Curious Case of the Evolving Apostrophe

Thu, 04 Feb 2010 05:10:00 GMT

A new technique for analyzing early English texts is gradually revealing the history of the apostrophe.

Last year, grammatical tragedy struck in the heart of England when Birmingham City Council decreed that apostrophes were to be forever banished from public addresses. To the horror of purists and pedants alike, place names such as St Paul's Square were banned and unceremoniously replaced with an apostrophe-free version: St Pauls Square.

The council's reasoning was that nobody understands apostrophes and their misuse was so common in public signs that they were a hindrance to effective navigation. Anecdotes abounded of ambulance drivers puzzling over how to enter St James's Street into a GPS navigation system while victims of heart attacks, strokes and hit 'n' run drivers passed from this world into the (presumably apostrophe-free) next.

Why the confusion? Part of the reason is that apostrophes are not particularly common in the English language: In French they occur at a rate of more than once per sentence on average. In English, they occur about once in every 20 sentences. So English speakers get less practice.

But the rules governing apostrophes are also more complex in English. In both French and English, apostrophes indicate a missing letter, such as the missing i in that's or the v in e'er. But in English, apostrophes also indicate the possessive (or genitive) case. They are used to show that one noun owns another: St James's Street is the street belonging to St James.

The complexity is compounded because in English, the plural is often formed by adding an s. So the word boys means more than one boy. How then do you form the possessive to indicate, for example, a ball belonging to the boys? Is it the boy's ball or the boys's ball or the boys' ball?

And then there are the exceptions. Pronouns, for example, do not take a possessive apostrophe: you can't say I's ball or me's bat. The truth is that knowing when to use an apostrophe is not always easy.

That may be partly because the rules for using apostrophes are evolving. Today, Odile Piton and Hélène Pignot at the University of Panthéon-Sorbonne in Paris present an analysis of the use of apostrophes in English texts from the 17th century and show that the usage was much simpler in those days.

Their main challenge was how to recognize an apostrophe. Apostrophes are often the same as single quotation marks and are entered on a computer keyboard using the same key. So it's easy to get false positives.

Spotting the absence of an apostrophe where there ought to be one can be tricky, too. They give the example of this sentence: "First, that no other mans errors could draw either hatred, or engagement upon me." The automated analysis missed the absent apostrophe in mans, thinking instead that it was the transitive verb "to man."

What Piton and Pignot have yet to study how the use of the apostrophe changes in time. But they now have the automated analysis tools that should make this possible. That could reveal the forces at work that change our language.

For the moment, Piton and Pignot's conclusion is merely that the world was simpler in the 17th century as far as apostrophes go. They say: "The possessive genitive in 's was not very common yet. The apostrophe mostly marks the omission of letters in a wide range of words and the plural of certain words."

Rather like the road signs in Birmingham.

Ref:arxiv.org/abs/1002.0479: "Mind your p's and q's?": or the Peregrinations of an Apostrophe in 17th Century English

Physicist Discovers How to Teleport Energy

Wed, 03 Feb 2010 05:10:00 GMT

First, they teleported photons, then atoms and ions. Now one physicist has worked out how to do it with energy, a technique that has profound implications for the future of physics.

In 1993, Charlie Bennett at IBM's Watson Research Center in New York State and a few pals showed how to transmit quantum information from one point in space to another without traversing the intervening space.

The technique relies on the strange quantum phenomenon called entanglement, in which two particles share the same existence. This deep connection means that a measurement on one particle immediately influences the other, even though they are light-years apart. Bennett and company worked out how to exploit this to send information. (The influence between the particles may be immediate, but the process does not violate relativity because some informatiom has to be sent classically at the speed of light.) They called the technique teleportation.

That's not really an overstatement of its potential. Since quantum particles are indistinguishable but for the information they carry, there is no need to transmit them themselves. A much simpler idea is to send the information they contain instead and ensure that there is a ready supply of particles at the other end to take on their identity. Since then, physicists have used these ideas to actually teleport photons, atoms, and ions. And it's not too hard to imagine that molecules and perhaps even viruses could be teleported in the not-too-distant future.

But Masahiro Hotta at Tohoku University in Japan has come up with a much more exotic idea. Why not use the same quantum principles to teleport energy?

Today, building on a number of papers published in the last year, Hotta outlines his idea and its implications. The process of teleportation involves making a measurement on each one an entangled pair of particles. He points out that the measurement on the first particle injects quantum energy into the system. He then shows that by carefully choosing the measurement to do on the second particle, it is possible to extract the original energy.

All this is possible because there are always quantum fluctuations in the energy of any particle. The teleportation process allows you to inject quantum energy at one point in the universe and then exploit quantum energy fluctuations to extract it from another point. Of course, the energy of the system as whole is unchanged.

He gives the example of a string of entangled ions oscillating back and forth in an electric field trap, a bit like Newton's balls. Measuring the state of the first ion injects energy into the system in the form of a phonon, a quantum of oscillation. Hotta says that performing the right kind of measurement on the last ion extracts this energy. Since this can be done at the speed of light (in principle), the phonon doesn't travel across the intermediate ions so there is no heating of these ions. The energy has been transmitted without traveling across the intervening space. That's teleportation.

Just how we might exploit the ability to teleport energy isn't clear yet. Post your suggestions in the comments section if you have any.

But the really exciting stuff is the implications this has for the foundations of physics. Hotta says that his approach gives physicists a way of exploring the relationship between quantum information and quantum energy for the first time.

There is a growing sense that the properties of the universe are best described not by the laws that govern matter but by the laws that govern information. This appears to be true for the quantum world, is certainly true for special relativity, and is currently being explored for general relativity. Having a way to handle energy on the same footing may help to draw these diverse strands together.

Interesting stuff. There's no telling where this kind of thinking might lead.

Ref: arxiv.org/abs/1002.0200: Energy-Entanglement Relation for Quantum Energy Teleportation

Best Connected Individuals Are Not the Most Influential Spreaders in Social Networks

Tue, 02 Feb 2010 05:10:00 GMT

Who are the best spreaders of information in a social network? The answer may surprise you.

The study of social networks has thrown up more than a few surprises over the years. It's easy to imagine that because the links that form between various individuals in a society are not governed by any overarching rules, they must have a random structure. So the discovery in the 1980s that social networks are very different came as something of a surprise. In a social network, most nodes are not linked to each other but can easily be reached by a small number of steps. This is the so-called small worlds network.

Today, there's another surprise in store for network connoisseurs courtesy of Maksim Kitsak at Boston University and various buddies. One of the important observations from these networks is that certain individuals are much better connected than others. These so-called hubs ought to play a correspondingly greater role in the way information and viruses spread through society.

In fact, no small effort has gone into identifying these individuals and exploiting them to either spread information more effectively or prevent them from spreading disease.

The importance of hubs may have been overstated, say Kitsak and pals. "In contrast to common belief, the most influential spreaders in a social network do not correspond to the best connected people or to the most central people," they say.

At first glance this seems somewhat counterintuitive but on reflection it makes perfect sense. Kitsak and co point out that there are various sceanrios in which well connected hubs have little influence over the spread of infromation. "For example, if a hub exists at the end of a branch at the periphery of a network, it will have a minimal impact in the spreading process through the core of the network."

By contrast, "a less connected person who is strategically placed in the core of the network will have a significant effect that leads to dissemination through a large fraction of the population."

The question then is how to find these influential individuals. Kitsak and co say that the way to do this is to study a quantity called the network's "k-shell decomposition". That sounds complicated but it isn't: a k-shell is simply a network pruned down to the nodes with more than k neighbours. Individuals in the highest k-shells are the most influential spreaders.

The team has tested the idea on a number of networks including the network formed by 5.5 million members of LiveJournal.com, the network of email contacts in the computer science department at University College London and the network of actors who have co-starred in adult films as defined by the internet movie database.

Perhaps the most interesting outcome is that the new approach emphasises the location of the individual within the network relative to the information or virus that is being spread. In hindsight, that seems like an obvious point but one that has not been well accounted for in the past.

The team also use their new ideas to study the spread of infections which do not confer immunity on recovered individuals. They conclude that "high k-shell layers form a reservoir where infection can survive even if its contagiousness is well below the epidemic threshold." Not an altogether reassuring result.

Ref: arxiv.org/abs/1001.5285: Identifying Influential Spreaders in Complex Networks

Physicists Discover How to Grow Graphene

Sun, 31 Jan 2010 05:10:00 GMT

The discovery of a way to grow graphene should make possible the widespread manufacture of graphene-based electronics.

The world of materials science is aflutter with stories about graphene, a supermaterial that is capable of almost anything (if you believe the hype). This form of carbon chickenwire, they tell us, is stronger, faster and better than almost any other material you care to name.

But not cheaper. At least not yet. The big problem with graphene is making it. The only way to get it is to chip away at a bigger block of graphite and then hunt through the flakes looking for single layers of the stuff. That's not a technique that's going to revolutionise the electronics industry, regardless of how much cheap labour is available in China.

That's why an announcement from Hirokazu Fukidome at Tohoku University in Japan and a few buddies is hugely important. These guys say they have found a way to grow graphene on a silicon substrate. To show off their technique they've combined it with conventional lithography to create a graphene-on-silicon field effect transistor--just the kind of device the electronics industry wants to build by the billion.

That's a big deal for two reasons. First, being able to grow graphene from scratch is going to be a huge boost to the study of this stuff and its myriad amazing properties. Second, being able to grow it on silicon makes it compatible (in principle at least) with the vast silicon-based fabrication industry as it stands.

One fear was that graphene's many advantages would be passed over because the electronics industry has so much invested in silicon. That need no longer be a worry.

Of course, Fukidome's graphene transistor is not the first: we've seen several interesting graphene devices in recent years (for example here and here).

But the breakthrough is significant because it is likely to lead to the rapid widespread adoption of graphene in electronics.

If we're about to enter an age of graphene, you could make a good argument that this is where it starts.

Ref: arxiv.org/abs/1001.4955: Epitaxial Graphene on Silicon toward Graphene-Silicon Fusion Electronics

Cities 'n' Towns

Sat, 30 Jan 2010 05:10:00 GMT

The best of the rest from the Physics arXiv this week

Epitaxial Graphene on Silicon toward Graphene-Silicon Fusion Electronics

Trajectory Clustering and an Application to Airspace Monitoring

Henry Eyring: Statistical Mechanics, Significant Structure Theory, and the Inductive-Deductive Method

Modes of Random Lasers

An Example of Complex Pedestrian Route Choice

Alternatives To Dark Matter: Modified Gravity As An Alternative To Dark Matter

RFID Data Reveals London's Polycentric Pattern of Commuting

Fri, 29 Jan 2010 05:10:00 GMT

Data from London's commuters reveals travel patterns in unprecedented detail. It also raises privacy fears.

Since 2003, some 10 million RFID cards have been issued to commuters using the London transport network. The cards are precharged with credit which is then used to pay for journeys on buses and trains.

For scientists studying the behaviour of commuters, the system has generated a firehose of data. Camille Roth at the Institut des Systemes Complexes in Paris and a few buddies have got their hands on the data for the 11.2 million trips that took place during the week of 31 March to 6 April 2008. This data included the start and finishing point of each journey as well as the journey time.

The data gives one of the most detailed insights into commuting patterns ever assembled. It has long been known that London has three main centres for commuting: the West End, the City and the Midtown area between them. However, nobody has been able to tease apart the structure of commuting in any higher resolution.

Until now. The new data shows for the first time that that there are numerous smaller centres as well. These can be ranked according to their total inflow each day. Just behind the big three are centres such as the Docklands area in East London, the area around Parliament and the area to the south west of this near Victoria and Green Park, which the authors call the Government area.

At even higher resolution, it is possible to see centres of inflow around the museum area of South Kensington, the Northern business area around Camden Town and so on.

That's not entirely unexpected but it is a step beyond the standard theoretical description of city commuting.

The reason it is important, of course, is that it gives city planners a starting point from which they can model the effects on th transport infrastructure of new urban projects.

The big fear, of course, centres on privacy issues. It isn't clear to what extent the data is anonymised before it is released. Could it be possible that some suitably clever individual with an electoral register and a few business directories could begin to link individual journeys with specific individuals?

After the Netflix and AOL scandals, only the brave (or perhaps stupid) would rule it out.

Ref: arxiv.org/abs/1001.4915: Commuting In A Polycentric City

Virtual Reality for Fruit Flies: Plans Unveiled

Thu, 28 Jan 2010 05:10:00 GMT

The ability to track fruit flies in real time is the foundation of a virtual reality system that could revolutionize the study of animal flying behavior.

In 1939, John Kennedy at Imperial College, London, discovered a curious fact about mosquitoes. In flight, they tend to turn toward vertical landmarks such as fixed posts. A similar behavior is also observed in fruit flies.

This is a straightforward repeatable observation suggesting that it is hard-wired into a fly's neurobiology. Changing the neurobiology, either genetically or chemically, should generate other flying behaviors. And studying these flying activities should give neurobiologists a way of understanding exactly how neurons and genes determine behavior. All you have to do is watch the flies.

That's easier said than done. High-resolution cameras generally have a small field of view and the data they produce is so great that it cannot be processed in real time. That severely limits the kind of experiments scientists can do.

But all that looks set to change with the development of a system called Flydra by Andrew Straw and buddies from the California Institute of Technology in Pasadena. Flydra is formed from two words: fly, the object of the team's study, and hydra, the many-headed creature of Greek mythology.

The system consists of many relatively low-resolution cameras each with a wide field of view, recording the same, relatively large volume of space. The images are processed in real time by a number of computers that track the 3-D motion of many flying animals at once. This tracking is entirely automated.

That's an impressive system because it opens up the possibility of the systematic study of the flying behavior of many creatures over long periods of time. That's never been done before. Potentially, it could revolutionize our understanding of flying behavior.

The team has already studied the flying behaviour of 20 female fruit flies in a box over a period of 12 hours. During this time, the team varied the contrast of a pattern beamed onto the walls of the box. (To give you an idea of how difficult these experiments must be without automatic tracking, the teams says the flies spent most of their time walking on the walls, floor and ceiling and the entire 12-hour run yielded a total of only 1,760 seconds of flight.)

Animal behaviorists have always assumed that flying animals use these patterns to determine their flying speed.

Sure enough, as the contrast of this pattern is reduced, the flies' flying speed becomes more erratic, suggesting they're having more difficulty controlling it.

That's already a useful result (and confirms the results of other experiments)..

But the real power of Flydra is its ability to track movement in real time (or at least with a lag of only 40ms). This allows the pattern to be changed in real time in response to a fly's motion--a kind of virtual reality for drosophila.

That opens up an entirely new way to study flying behavior. Imagine, for example, a fly veering towards a vertical post which then continually moves. Fruit fly aerobatics, anyone?

And of course, it's not just fruit flies that can be studied. Straw and co are already looking at hummingbird behavior. "We are also studying maneuvering in solitary and competing hummingbirds and the role of maneuvering in establishing dominance," they say.

It looks as if a golden age is nigh for the study of flying animal behavior. John Kennedy, who died in 1993, would have been amazed.

Ref: arxiv.org/abs/1001.4297: Multi-camera Realtime 3D Tracking of Multiple Flying Animals .

Moon May Have Formed in Natural Nuclear Explosion

Wed, 27 Jan 2010 05:10:00 GMT

The moon formed after the explosion of a runaway nuclear georeactor in the Earth's mantle, according to a new theory of lunar formation.

The standard theory of the origin of the Moon is called the giant impact hypothesis. It supposes that early in the Solar System's history, a massive object smashed into the Earth, cleaving it into two unequal parts. The smaller of these condensed into the Moon.

The best simulations of this process suggest that about 80 percent of Moon ought to have come from the impactor and 20 percent from the Earth.

That's hard to reconcile with the measured make up of Moon rock, which is almost identical to Earth rock in terms of isotopic content. Some planetary geologists say this could be explained if, soon after the impact, the debris mixed well before forming into solid bodies. But others counter that this might explain the similarity in the isotopic ratios of lighter elements such as oxygen but can't easily account for the identical ratio of heavier elements such as chromium, neodymium and tungsten.

But there's another theory called the fission hypothesis that could account for the similar isotopic content. This idea is that the Earth and Moon both formed from a rapidly spinning blob of molten rock. This blob was spinning so rapidly that the force of gravity only just overcame the centrifugal forces at work.

In this system, any slight kick would have ejected a small blob of molten rock into orbit. This blob eventually formed the Moon.

The fission hypothesis has been studied for 150 years but ultimately rejected because nobody has been able to work out where the energy could have come from to kick a lunar-sized blob into orbit.

Now Rob de Meijer at University of the Western Cape and Wim van Westrenen at VU University in Amsterdam say they know where that kick might have come from.

Their idea is that centrifugal forces would have concentrated heavier elements such as uranium and thorium near the Earth's surface on the equatorial plane. High concentrations of these radioactive elements can lead to nuclear chain reactions which can become supercritical if the concentrations are high enough.

The question is how concentrated could these elements have become. De Miejer and van Westrenen calculate that it is quite possible for the concentration to be high enough for a runaway nuclear reaction.

Their theory is that the explosion of a natural nuclear georeactor after it became supercritical ejected the material that eventually formed the Moon.

They also say that there ought to be telltale evidence that such an explosion took place, particularly in the lunar abundance of helium-3 and xenon -136, which would both have been produced in great quantities in a natural georeactor.

Future measurements from the surface could provide the evidence needed to confirm their theory but the analysis will not be easy. It is well known that the solar wind deposits vast amounts of these substances onto the lunar surface so that will have to be taken into account.

Of course, georeactors are by no means hypothetical. The most famous is in Oklo in Gabon, not so far from the equator, where a natural nuclear reactor was clearly in operation until about 1.5 billion years ago, leaving telltale signs in the uranium deposits now being mined.

One interesting corollary of this discussion centres on the origin of this theory which is credited to George Darwin, son of the more famous member of this family. Not content with settling the debate over the origin of the species, could it be that the Darwin family might eventually account for the origin of the Moon, too?

Ref: arxiv.org/abs/1001.4243: An Alternative Hypothesis For The Origin Of The Moon

How to Build a Dark Energy Detector

Tue, 26 Jan 2010 05:10:00 GMT

All the evidence for dark energy comes from the observation of distant galaxies. Now physicists have worked out how to spot it in the lab.

The notion of dark energy is peculiar, even by cosmological standards.

Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10^-10 joules per cubic metre.

What's strange about this idea is that as space expands, so too does the amount of energy. If you've spotted the flaw in this argument, you're not alone. Forgetting the law of conservation of energy is no small oversight.

What we need is another way of studying dark energy, ideally in a lab on Earth. Today, Martin Perl at Stanford University and Holger Mueller down the road at the University of California, Berkeley, suggest just such an experiment.

The dark energy density might sound small but Perl and Mueller point out that physicists routinely measure fields with much smaller energy densities. For example an electric field of 1 Volt per metre has an energy density of 10^-12 joules per cubic metre. That's easy to measure on Earth.

Of course there are some important differences between an electric field and the dark energy field that make measurements tricky. Not least of these is that you can't turn off dark energy. Another is that there is no known reference against which to measure it.

That leaves the possibility of a gradient in the dark energy field. If there is such a gradient, then it ought to be possible to measure its effect and the best way to do this is with atom interferometry, say Perl and Mueller.

Atom interferometry measures the phase change caused by the difference in two trajectories of an atom in space. So if a gradient in this field exists it should be possible to spot it by cancelling out the effects of all other forces. Perl and Mueller suggest screening out electromagnetic forces with conventional shields and using two atom interferometers to cancel out the the effect of gravitational forces.

That should allow measurements with unprecedented accuracy. Experiments with single atom interferometers have already measured the Earth's gravitational pull to an accuracy of 10^-9. The double interferometer technique should increase this to at least 10^-17.

That's a very exciting experiment which looks to be within reach with today's technology.

There are two potential flies in Perl and Mueller's ointment. The first is that the nature of dark energy is entirely unknown. If it exists and if there is a gradient, it is by no means certain that dark energy will exert a force on atoms at all. That will leave them the endless task of trying to place tighter and tighter limits on the size of a non-existent force.

The second is that some other unknown force will rear its head in this regime and swamp the measurements. If that happens, it's hard to imagine Perl and Mueller being too upset. That's the kind of discovery that ought to put a smile on any physicists face.

Ref:arxiv.org/abs/1001.4061: Exploring The Possibility Of Detecting Dark Energy In A Terrestrial Experiment Using Atom Interferometry

To Understand Congress, Just Watch the Sandpile

Mon, 25 Jan 2010 05:10:00 GMT

The behavior of Congress can be modeled by the same process that causes avalanches in sandpiles.

What does it take for a resolution in Congress to achieve sizeable support? It's easy to imagine that the support of certain influential representatives is crucial because of their skill in the cut and thrust of political bargaining.

Not so, say Mikhail Simkin and Vwani Roychowdhury at the University of California, Los Angeles. It turns out that the way a particular resolution gains support can be accurately simulated by the avalanches that occur when grains of sand are dropped onto each other to form a pile.

Simkin and Roychowdhury begin their analysis with a study of resolution HR1207 and a plot of the number of co-sponsors it received against time early last year. This plot is known in mathematics as a Devil's staircase--it consists of long periods without the addition of any new co-sponsors followed by jumps when many new co-sponsors join during a single day. "One might have suspected that the biggest steps of the staircase are due to joining of a highly influential congressman bringing with himself many new co-sponsors which he had influenced," say Simkin and Roychowdhury.

That's uncannily similar to the way in which avalanches proceed in a a model of sandpiles developed by Per Bak, Chao Tang and Kurt Wiesenfeld in 1988. Perhaps Congress can be modelled in a similar way, reason Simkin and Roychowdhury.

Their model assumes that the roles of sand grains is played units of political pressure. They assume that there is a network of influence in Congress through which representatives exert political pressure on each other (just as sand grains exert forces on each other through the network of contacts between them in the pile). When the pressure on representatives reaches a threshold, they co-sponsor the resolution and this, in turn, puts pressure on other member of congress to sign.

This is like the pressure that builds up in a sandpile as grains are dropped onto it. When a threshold is reached at a certain point on the pile, an avalanche occurs which redistributes the pressure to other places.

In addition, the representatives are pressured by their constituents which is analogous to dropping grains of sand at random.

There is a difference between sandpiles and congress however. Once a representative has signed, he or she cannot do it again and so take no further part in the process. Any further pressure on them is simply dissipated. So representatives cannot topple more than once, unlike sand grains which can keep on toppling as the pile gets bigger.

This is a pretty simple model but when Simkin and Roychowdhury ran it, they found that it generates a Devil's staircase that is uncannily similar to the one generated by representatives for HR1207.

Perhaps the most interesting feature is that the model assumes that all representatives have equal influence. "In our model, big steps are a result of evolution of Congress to a sort of critical state, where any congressman can trigger an avalanche of co-sponsors," say Simkin and Roychowdhury.

The pair suggest some interesting ways to follow up their work. They point out that not all resolutions in Congress get the same level of support. In their model, this is due to the amount of public pressure, ie the number of units of political pressure dropped onto the pile at random. If there is no outside pressure, the resolution will not get sizeable support in a reasonable amount of time.

"An obvious extension to the model is to introduce political pressure against the resolution," they say, pointing out that an interesting case would be when the negative pressure exactly balances the positive. "It could explain the cases when a resolution quickly gains some support, which, however, never becomes overwhelming."

So representatives are not as important as perhaps they might imagine. Perhaps the stage should be replacing them with actual grains of sand. By Simkin and Roychowdhury's reckoning, it wouldn't make much difference.

Ref: arxiv.org/abs/1001.3732: Stochastic modeling of Congress

Motors 'n' Engines

Sat, 23 Jan 2010 05:10:00 GMT

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

On the Helical Behavior of Turbulence in the Ship Wake

The Influence of Cosmic Rays on the Size of The Antarctic Ozone Hole

The Evolution of Virulence in RNA Viruses Under a Competition-Colonization Trade-Off

Emergent Matter From 3d Generalised Group Field Theories

On-Chip Single Plasmon Detection

Physicists Shrink Heat Engines by Seven Orders of Magnitude

Fri, 22 Jan 2010 05:10:00 GMT

Heat engines are the mainstay of our transportation system but they are huge. Now physicists have built a microscopic version.

The vast majority of motors that power our planes, trains, and automobiles are heat engines. They rely on the rapid expansion of gas as it heats up to generate movement.

These engines play a crucial role in our infrastructure. But attempts to shrink them by any significant amount have mostly ended in failure. Today, the smallest heat engines have a volume of some 10 ^ 7.

That looks set to change with the announcement by Peter Steeneken and pals at NXP Semiconductors in the Netherlands that they have shrunk the heat engine by seven orders of magnitude. Yep, seven!

Their heat engine has a volume of just 0.5 cubic micrometers.

Here's how it works. The new heat engine is essentially a bar of piezoelectric material whihc expands and contracts when an alternating current is applied. So far so good.

The expansion and contraction also changes the resistance of the bar and so also the amount of resistive heat that is generated. So passing a DC current through the bar at the same time as the AC current causes the bar to heat up and cool down. This heating and cooling also causes an expansion and contraction of the bar.

Crucially, there is a phase lag between the piezoelectric and thermal expansion and contractions.

So by choosing just the right driving frequency, it is possible to create a resonant effect in which the mechanical displacement of the bar is amplified.

This is the regime in which the bar acts like a heat engine, pushing and pulling to do work.

Steeneken and co have tested the idea in a preliminary design and shown it to work well. And they've also shown that reversing the thermodynamic cycle turns the the engine into a heat pump or refrigerator.

Of course, there are other, similarly sized refrigerators that work using the Peltier effect but this one has an important advantage. "In contrast to the Peltier effect, the direction of the thermal current does not depend on the direction of the electrical current," say Steeneken and company.

Refrigeration may turn out to be the most important near term application. The ability to pump heat away from microchips efficiently is nothing to be sniffed at.

It's future as a motor is less clear. It's relatively straightforward to make electrostatic motors that work on this scale and we've looked at plans to build electric motors on the quantum scale.

Then there is the tricky question of efficiency. Today's macroscopic heat engines are clearly more efficient than biological ones. But Steneeken readily admits that "it remains to be seen whether they can ever compete with biological or artificial molecular motors on the microscale."

Nevertheless, worth keeping an eye on.

Ref:arxiv.org/abs/1001.3170: Piezoresistive Heat Engine and Refrigerator

A New Way to Deal with the Cargo Container Security Problem

Thu, 21 Jan 2010 05:10:00 GMT

Can a single machine solve the complex problem of scanning cargo containers for conventional and nuclear weapons?

In 2007, the U.S. government set itself the goal of screening all aviation cargo loaded onto passenger planes and all maritime cargo entering the country for both explosives and nuclear materials. And to set up a system to do this within five years.

That's a big ask given that the maritime traffic alone amounts to more than six million cargo containers per year.

And that's before you get to the sheer technical difficulty of reliably spotting every kind of explosive along with all nuclear materials with a minimal percentage of false positives.

Today, Mark Goldberg at the Soreq Nuclear Research Center in Israel and a number of pals, mainly in Germany, outline plans for a single machine that they say could do the job.

The team propose using a particle accelerator to alternately smash ionised hydrogen molecules and deuterium ions into targets of carbon and boron respectively. The collisions produce beams of gamma rays of various energies as well as neutrons. These beams are then passed through the cargo.

By measuring the way the beams are absorbed, Goldberg and company say they can work out whether the cargo contains explosives or nuclear materials. And they say they can do it at the rate of 20 containers per hour.

That's an ambitious goal that presents numerous challenges.

For example, the beam currents will provide relatively sparse data so the team will have to employ a technique called few-view tomography to fill in the gaps. It will also mean that each container will have to be zapped several times. That may not be entirely desirable for certain types of goods such as food and equipment with delicate electronics.

Just how beams of gamma rays and neutrons affect these kinds of goods is something that will have to be determined.

Then there is the question of false positives. One advantage of a machine like this is that it has several scanning modes is that if one reveals something suspicious, it can switch to another to look in more detail. That should build up a decent picture of the cargo's contents and reduce false positives.

But some compromises will have to be accepted. For example the machine will be able to reliably distinguish uranium and plutonium from certain heavy metals such as lead and mercury but not from others such as tungsten or gold. Goldberg and co say this is a reasonable trade off since it would be in customs' interests to detect these metals if they had been undeclared.

Finally, there is the issue of reliability and maintenance. This is not a simple device. Until now, particle accelerators have only been operated reliably in a few laboratories with highly trained staff. Making such a device reliable enough to operate in a real world environment may be the most difficult task of all.

Provided that the U.S. government is willing to pay, of course. The U.S. has 300 ports and 5,119 paved airports. Suppose it orders 1,000 of these machines at $5m each ( a highly conservative estimate), that comes to a total of $5 billion. And that's before operating costs come into play.

One thing is for sure: even if the government decides that this is the machine for the task, Goldberg and co have yet to build one. Today's paper just outlines the plan. That makes the 2012 deadline almost impossible to meet.

Ref:arxiv.org/abs/1001.3255: A Dual-Purpose Ion-Accelerator for Nuclear-Reaction-Based Explosives-and SNM-Detection in Massive Cargo

Making Light of Ion Traps

Wed, 20 Jan 2010 05:10:00 GMT

Physicists have captured an ion using an optical trap for the first time.

The manipulation of single atoms and ions using light and electric fields is one of the defining features of modern science. These tools have given researchers a way to play with the building blocks of our world in a way that previous generations could only dream about. And the number of major breakthroughs that have come from this technology is legion: everything from Bose Einstein Condensates to quantum computing.

Curiously, though, the trapping of ions and atoms relies on different technologies. Since 1980, single ions have been trapped in radio frequency electric fields. These fields exert forces on an ion that are powerful enough to overcome the inevitable stray electric fields that might otherwise influence an ion.

On the other hand, researchers rely on optical traps to grab atoms. The first single atom was trapped in this way only in 1999.

But despite these differences, the trapping mechanism is quite similar in both these techniques. In an RF trap, the electromagnetic field acts on the ion's dipole moment, generating a force that keeps it confined. In an optical trap, the confinement arises from the optical dipole force. This is somewhat weaker than the forces in an RF trap. Physicists have never bothered with optical trapping on ions for fear that any stray electric field would whip the ion away.

But they can now put that fear to rest, say Tobias Schaetz and buddies at the Max Planck Institute for Quantum Optics in Germany. The reason is that these guys have done it: trapped a single magnesium ion in an optical trap for a period of milliseconds. What's more, their experiment is merely a proof of principle: significant improvements to the technique and the trapping should be relatively straightforward to achieve.

The ability to trap ions with optical fields opens up a number of new horizons. Optical traps can be more complex and more easily and finely manipulated than RF traps. They should give physicists a better bag of tricks to play on ions. These traps could be used to build better ion-based quantum computers and quantum simulators. They might also allow physicists to create and study the properties of 3D ionic crystals in more detail.

And after years of manipulating atoms and ions separately, physicists will find they can now play with both at the same time in an optical trap. That should allow researchers to study the interaction between atoms and ions in more detail than ever before.

All in all, cool stuff!

Ref: arxiv.org/abs/1001.2953: Optical Trapping of an Ion

Starlight Reveals a Clue to Biomolecular Handedness

Tue, 19 Jan 2010 05:10:00 GMT

Nobody knows why life on Earth is built from left-handed amino acids. But an important clue lies in the light coming from the Orion Nebula, say astrobiologists.

One of the great mysteries of life on Earth is that it is built almost exclusively from one enantiomer: left-handed amino acids and right-handed sugars. The puzzle is that left and right-handed versions of molecules are produced in equal amounts in all but the most exceptional circumstances. So what conditions led to terrestrial life's preference for one over the other?

Today, Tsubasa Fukue at the National Astronomical Observatory of Japan in Tokyo and a few mates say they think they know how so-called biomolecular homochirality may have come about. The evidence comes from their study of light from the Orion Nebula (above), a well known star nursery and one of the most spectacular objects in the sky.

The Orion Nebula is important because it contains regions that must be similar to the one in which the Sun formed some 5 billion years ago. "Studies of the Orion star-forming region enable us to investigate processes that may have occurred during the birth of our own solar system," say Fukue and co.

Here's the interesting discovery: Fukue and buddies say their study of the nebula reveals a huge area some 400 times the size of our Solar System that is bathed in circularly polarised visible light, probably generated by synchrotron radiation from particles accelerated in powerful magnetic fields.

That's an important observation because on Earth, circularly polarised light can bias chemical reactions in a way that ensures that one enantiomer forms preferentially over another.

Could the same thing happen in space? Possibly. One problem is that amino acids require UV light for photolysis rather than visible light. But it's simply not possibly to see UV light (polarised or not) from the Orion Nebula because it is absorbed by dust in the line of site between here and there.

Fukue and co say that various models of star formation predict that circularly polarised UV light ought to be produced in the region. We also know that amino acids are common in interstellar space.

So it's quite possible that conditions are ripe in the Orion Nebula for homochirality. And if so, it is also possible that the Sun formed in a similar region of space and that the Earth was seeded with these molecules during a period of heavy bombardment soon after it formed.

That's a neat idea but not an entirely new one. The debate over biomolecular homochirality and the role of circularly polarised light goes back many years and while Fukue and co add some interesting new evidence, it is by no means a slamdunk.

The best they can say is that their work supports the idea but it's going to take something a little more substantial to settle the issue.

Ref: arxiv.org/abs/1001.2608: Extended High Circular Polarization in the Orion Massive Star Forming Region: Implications for the Origin of Homochirality in the Solar System

Deriving the Properties of the Universe

Mon, 18 Jan 2010 05:10:00 GMT

The properties of the universe can be derived by thinking about the origin of complexity, says a new theory.

Physicists and cosmologists have long noted that the laws of physics seem remarkably well tuned to allow the existence of life, an idea known as the anthropic principle.

It is sometimes used to explain why the laws of physics are the way they are. Answer: because if they were different, we wouldn't be here to see them.

To many people, that looks like a cop out. One problem is that this way of thinking is clearly biased towards a certain kind of carbon-based life that has evolved on a pale blue dot in an unremarkable corner of the cosmos. Surely there is a more objective way to explain the laws of physics.

Enter Raphael Bousso and Roni Harnik at the University of California, Berkeley and Stanford University respectively. They point out that the increase in entropy in any part of the Universe is a decent measure of the complexity that exists there. Perhaps the anthropic principle can be replaced with an entropic one?

Today, they outline their idea and it makes a fascinating read. By thinking about the way entropy increases, Bousso and Harnik derive the properties of an average Universe in which the complexity has risen to a level where observers would have evolved to witness it.

They make six predictions about such a Universe. They say "typical observers find themselves in a flat universe, at the onset of vacuum domination, surrounded by a recently produced bath of relativistic quanta.These quanta are neither very dilute nor condensed, and thus appear as a roughly thermal background."

Sound familiar? It so happens that we live in a (seemingly) flat universe, not so long after it has become largely a vacuum and we're bathed in photons that form a thermal background. That's the cosmic infrared background that is emitted by galactic dust heated by starlight (this is different from the cosmic microwave background which has a different origin).

That's a remarkably accurate set of predictions from a very general principle. The question, of course, is how far can you run with a theory like this.

It certainly has the feel of a powerful idea. But, just like the anthropic principle, it also has the scent of circular reasoning about it: the universe is the way it is because if it were different, the complexity necessary to observe it wouldn't be here to see it.

That may not be so hard to stomach, given the power of the new idea. Even a hardened physicist would have to accept.that Bousso and Harnik have a remarkably elegant way of capturing the state of the universe.

Ref:arxiv.org/abs/1001.1155: The Entropic Landscape