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Irish Mathematicians Solve The Guinness Sinking Bubble Problem

Mon, 28 May 2012 13:23:00 GMT

Bubbles sink in Guinness because of the peculiar geometry of pint glasses, say a dedicated group of researchers at the University of Limerick

One of the more intriguing conundrums in fluid dynamics is the puzzling behaviour of bubbles in Guinness, the famous Irish stout.

As many drinkers will attest, the bubbles in Guinness appear to sink as the drink settles and the head forms. How can this be, given that bubbles are less dense than the surrounding fluid and so should rise?

Over the last ten years or so, physicists have begun to pick this problem apart. Most recently they've shown that it is not the bubbles that sink but the liquid, which circulates in a way that is downwards near the glass walls and upwards in the interior. As long as the downward flow of the liquid is faster than the upward motion of the bubbles, they will appear to sink.

But that still leaves a puzzle: why does the liquid circulate in this way?

Today, a dedicated team of Irish mathematicians reveal the answer. Eugene Benilov, Cathal Cummins and William Lee at the University of Limerick say the final piece in this puzzle is the shape of the glass, which has a crucial influence over the circulatory patterns in the liquid.

To understand how, first remember that the motion of every bubble exerts a drag on the liquid around it. Now imagine what would happen if there were a region of liquid containing fewer bubbles near the wall of a pint glass and consequently a region of higher bubble density near the middle of the glass.

Benilov and co say that the drag will be higher in the region where the bubble density is higher, in other words near the centre of the glass. This creates an imbalance that sets up a circulation pattern in which the liquid flows upwards in the centre of the glass and downwards near the walls.

That's exactly as observed in a pint of Guinness. But what causes the region of low bubble density near the glass walls in the first place?

Benilov and co imagine that to start with, the bubbles are distributed evenly throughout the liquid. In a perfect cylinder, they would simply rise together. The bubbles in each volume of liquid are steadily replenished from below.

But imagine a container that is narrower at the bottom and wider at the top so that the walls rise at an angle, as in a pint glass. In this case, the simple act of bubbles rising creates a region of low bubble density next to the angled wall because the bubbles are not being steadily replenished below.

By contrast, the bubble density is higher in the middle of the glass because the bubbles are replenished from below.

That would set up exactly the circulation pattern that is observed, say Benilov and co.

This effect is well known in sedimentation theory as the Boycott eﬀect. "It was ﬁrst observed in test tubes containing red blood cells when it was discovered that sedimentation times could be signiﬁcantly reduced by inclining the test tubes," say Benilov and co.

These guys have even created a computer model of bubble behaviour in Guinness which confirms their thinking.

The icing on the cake, however, is that there is a simple experiment that can easily confirm the theory.

Experiments are not usually the domain of mathematicians. But Benlivo and co demonstrate valour beyond the call of mathematical duty by actually performing the experiment in which they bravely pour Guinness into a cylinder. "If the container is tilted, bubbles will be observed to move upwards near its upper surface and downwards near its lower surface," they say.

They have even created a video of this experiment which you can download here (avi).

Of course, the essence of experimental science is repeatability. Many readers will not be content with mere visual evidence from a video but insist on repeating this experiment on their own terms, perhaps in a hostelry of their own choosing. Quite right.

But if you choose this route, remember that this work is not entirely whimsical. "Understanding these types of bubbly ﬂows is important for a number of applications, such as manufacturing champagne glasses engraved with nucleation sites, and widget and similar technologies for promoting foaming in stouts," say Benilov and co.

Guinness drinkers (and servers) will also be aware of another problem that plagues them--the time it takes for a pint of Guinness to settle, which is significantly longer than with most ales and lagers.

Could this work allow pint glasses to be redesigned in a way that encourages stouts to settle more quickly?

We'll be following future developments closely.

Ref: arxiv.org/abs/1205.5233: Why Do Bubbles In Guinness Sink?

Gates 'n' Doors

Sat, 26 May 2012 04:10:00 GMT

The best of the rest from the Physics arXiv this week

A Huge Reservoir Of Ionized Gas Around The Milky Way: Accounting For The Missing Mass?

Free Flight Of The Mosquito Aedes Aegypti

Tumor Self-Seeding: How Do Cells Find Their Way Home?

Earth Rotation Prevents Exact Solid Body Rotation Of Fluids In The Laboratory

Large Social Networks can be Targeted for Viral Marketing with Small Seed Sets

Universal Properties of Mythological Networks

Quantum Spring

Extraction of Historical Events from Wikipedia

A Photonic C-NOT Gate Breakthrough for Quantum Computing

Thu, 24 May 2012 10:23:00 GMT

Physicists have built a quantum logic gate that combines a quantum dot that fires photons with a photonic circuit that processes them

In the race to build powerful quantum computers, many groups are competing to build logic gates that can process quantum information and still be connected together on a large scale.

One important question remains unanswered, however: what should the devices use to carry quantum information?

Schemes involving charged particles such as Ion traps, electron circuits and superconductors have long looked promising because the qubits they hold can be easily manipulated with electric and magnetic fields. Charged particles also interact easily with each other in a way that can be made to process data.

The problem, of course, is that stray fields also interact with charged particles, causing the quantum information they carry to leak away. Stray fields litter the universe like the plague and this severely reduces the utility of these types of devices.

One alternative is the humble photon, which is unaffected by stray fields and can travel many kilometres through a waveguide without interacting with the environment.

The trouble with photons is twofold. First, they are hard to produce individually, tending instead to come in bursts of several. That makes it hard to create them.

Second, they tend to pass through one another unnoticed, like ships in the night. That makes it hard to process the information they carry.

However, various groups have made significant process in solving these problems. Lots of labs have developed photon guns that can be made to emit single photons, one at a time.

At the same time, other labs have created photon circuits in the form of interconnecting waveguides that force photons to interact and thereby process the information they carry. These circuits are like tiny Scalectrix sets in which the cars collide where the track narrows.

In this circuit, the qubits are path-encoded meaning that the presence of a photon in one track is a 1 and its absence is a 0, for example. When they come together they interfere, thereby processing the information they carry.

But the efficiency of this interaction depends crucially on both photons being identical. Small differences in wavelength, for example, can dramatically reduce the performance. But making identical photons is hard.

Today, Andrew Shields at Toshiba Research Europe Limited in Cambridge, UK, and a few buddies say they've solved this problem by integrating both these developments into a single device that acts like a C-NOT logic gate.

"The Controlled-NOT (CNOT) gate we demonstrate is the basic building block of quantum logic, since in combination with one qubit gates it can be used to perform any quantum operation," say Shields and co.

This logic gate consists of a pillar of indium arsenic that acts as a quantum dot--it emits a single photon when zapped with laser light of a specific frequency. This is coupled to a photon racetrack carved out of silicon.

A C-NOT logic gate requires two input photons. So the circuit works by zapping the quantum dot twice, generating two photons. These are identical because they've come from the same dot.

The photons then travel to a beam splitter that sends them down the appropriate paths (one of which has a built in delay that means determines when the photons enter the circuit relative to each other).

Shields and co have measured the truth table of their logic gate. They say it matches their theoretical predictions and can be made better with a few tweaks.

What's significant about this approach is its scalability. Shields and co say it ought to be possible to build many quantum dots and circuits onto a single integrated chip. And the differences between photons from different quantum dots can be minimised by triggering them all with the same laser pulse.

That's handy but it's not all plain sailing. The device must be cooled to 4.5 Kelvin, the operating temperature of the quantum dot, and the results of a single logic operation take some 30 minutes to collect.

Clearly that will have to change to make these devices viable. But that's an engineering challenge these guys will surely relish.

Ref: arxiv.org/abs/1205.4899: Controlled-NOT Gate Operating With Single Photons

How Men and Women Manage Their Social Networks Differently

Wed, 23 May 2012 11:40:00 GMT

A new study of online behavior reveals that men and women organize their social networks very differently.

One of the interesting insights that social networks offer is the difference between male and female behaviour.

In the past, behavioural differences have been hard to measure. Experiments could only be done on limited numbers of individuals and even then, the process of measurement often distorted people's behaviour.

That's all changed with the advent of massive online participation in gaming, professional and friendship networks. For the first time, it has become possible to quantify exactly how the genders differ in their approach to things like risk and communication.

Gender specific studies are surprisingly rare, however. Nevertheless a growing body if evidence is emerging that social networks reflect many of the social and evolutionary differences that we've long suspected.

Earlier this year, for example, we looked at a remarkable study of a mobile phone network that demonstrated the different reproductive strategies that men and women employ throughout their lives, as revealed by how often they call friends, family and potential mates.

Today, Michael Szell and Stefan Thurner at the Medical University of Vienna in Austria say they've found significance differences in the way men and women manage their social networks in an online game called Pardus with over 300,000 players.

In this game, players explore various solar systems in a virtual universe. On the way, they can mark other players as friends or enemies, exchange messages, gain wealth by trading or doing battle but can also be killed.

The interesting thing about online games is that almost every action of every player is recorded, mostly without the players being consciously aware of this. That means measurement bias is minimal.

The networks of friends and enemies that are set up also differ in an important way from those on social networking sites such as Facebook. That's because players can neither see nor influence other players' networks. This prevents the kind of clustering and herding behaviour that sometimes dominates other social networks.

Szell and Thurner say the data reveals clear and significant differences between men and women in Pardus.

For example, men and women interact with the opposite sex differently. "Males reciprocate friendship requests from females faster than vice versa and hesitate to reciprocate hostile actions of females," say Szell and Thurner.

Women are also significantly more risk averse than men as measured by the amount of fighting they engage in and their likelihood of dying.

They are also more likely to be friends with each other than men.

These results are more or less as expected. More surprising is the finding that women tend to be more wealthy than men, probably because they engage more in economic than destructive behaviour.

"These results conﬁrm quantitatively that females and males manage their social networks drastically different," say Szell and Thurner.

Of course, there are important questions over the extent these findings reflect gender differences in the real world.

One obvious problem is that of gender swapping: men who play as women and vice versa. Szell and Thurner say that other studies have shown that around ten per cent of online gaming populations engage in gender swapping. They say there's no reason to think this would be any different in Pardus and that it shouldn't affect the results.

A more serious problem could be the well known phenomenon that women tend to receive better treatment in male-dominated online gaming communities. Indeed, Szell and Thurner say they can see evidence of this in their data. That's something they'll need to look into in more detail.

There s one group for whom this kind of study will be invaluable: advertisers and marketeers. That makes it potentially valuable form a commercial point of view.

But the more interesting scientific question is whether this kind of work can tease apart gender differences with a resolution that reveals new and surprising patterns.

We'll need to wait a little longer to get a better answer to that.

Ref: arxiv.org/abs/1205.4683: How Women Organize Social Networks Different From Men

Biophoton Communication: Can Cells Talk Using Light?

Tue, 22 May 2012 10:27:00 GMT

A growing body of evidence suggests that the molecular machinery of life emits and absorb photons. Now one biologist has evidence that this light is a new form of cellular communication.

One of the more curious backwaters of biology is the study of biophotons: optical or ultraviolet photons emitted by living cells in a way that is distinct from conventional bioluminescence.

Nobody is quite sure how cells produce biophotons but the latest thinking is that various molecular processes can emit photons and that these are transported to the cell surface by energy carying excitons. A similar process carries the energy from photons across giant protein matrices during photosynthesis.

Whatever the mechanism, a growing number of biologists are convinced that when you switch off the lights, cells are bathed in the pale fireworks of a biophoton display.

This is not a bright phenomena. Biophotons are usually produced at the rate of dozens per second per square centimetre of cell culture.

That's not many. And it's why the notion that biophoton activity is actually a form of cellular communication is somewhat controversial.

Today, Sergey Mayburov at the Lebedev Institute of Physics in Moscow adds some extra evidence to the debate.

Mayburov has spent many hours in the dark watching fish eggs and recording the patterns of biophotons that these cells emit.

The question he aims to answer is whether the stream of photons has any discernible structure that would qualify it as a form of communication.

The answer is that is does, he says. Biophoton streams consist of short quasiperiodic bursts, which he says are remarkably similar to those used to send binary data over a noisy channel. That might help explain how cells can detect such low levels of radiation in a noisy environment.

If he's right, then this could help to explain a number of interesting phenomenon that some biologists attribute to biophoton communication.

In several experiments, biophotons from a growing plant seem to increase the rate of cell division in other plants by 30 per cent. That's a growth rate that is significantly higher than is possible with ordinary light that is several orders of magnitude more intense.

Other experiments have shown that the biophotons from growing eggs can encourage the growth of other eggs of a similar age. However, the biophotons from mature eggs can hinder and disrupt the growth of younger eggs at a different stage of development. In some cases, biophotons from older eggs seem to stop the growth of immature eggs entirely.

Mayburov's work won't end the controversy; not by any means. There are still many outstanding questions. One important problem is to better understand the cellular mechanisms at work--how the molecular machinery inside cells produces photons and how it might be influenced by them. Another is to understand the kind of evolutionary pressures that are at work here--how has this ability come about?

Clearly, there's more work to be done here.

Ref: arxiv.org/abs/1205.4134: Photonic Communications and Information Encoding in Biological Systems

European Physicists Smash Chinese Teleportation Record

Mon, 21 May 2012 11:15:00 GMT

The battle over distance records sets up a fascinating race to be the first to teleport to an orbiting satellite

Just a couple of weeks ago, we discussed a Chinese experiment in which physicists teleported photons over a distance of almost 100 kilometres. That's almost an order of magnitude more than previous records.

Today, European physicists say they've broken the record again, this time by teleporting photons between the two Canary Islands of La Palma and Tenerife off the Atlantic coast of north Africa, a distance of almost 150 kilometres.

That's sets the scene for a fascinating prize. Both teams say the next step is to teleport to an orbiting satellite and that the technology is ripe to make this happen.

The Canary islands experiment was no easy ride. In ordinary circumstances, the quantum information that photons carry cannot survive the battering it gets in passing through the atmosphere. It simply leaks away.

Indeed, the European team say that unusually bad weather including wind, rain, rapid temperature changes and even sand storms all badly affected the experiment. "These severe conditions delayed our experimental realizations of quantum teleportation for nearly one year," say Anton Zeilinger at the Institute for Quantum Optics and Quantum Information in Vienna and a few pals.

(However, they are quick to point out that satellite-based quantum communication shouldn't be as susceptible since there is less weather to pass through if you fire photons straight up.)

To perform this experiment, Zeilinger and co had to perfect a number of new techniques to dramatically reduce noise, which would otherwise overwhelm the quantum signal.

Perhaps the most significant of these is a way of using entangled photons to synchronise clocks on both islands. That's important because it allows the team to send photons and then look for them at the receiver at the exact instant they are due to arrive.

This significantly reduces the number of extraneous photons that could swamp the signal. The GPS system allows clocks to be synchronised in a way that allows a 10 nanosecond coincidence window. But entanglement-enhanced synchronisation allowed Zeilinger and co to use coincidence windows just 3 nanoseconds long.

The results sets up an interesting race between east and west. These experiments are proof-of-principle runs for a much more ambitious idea--quantum teleportation to orbiting satellites.

Since teleportation is the basis of more-or-less perfectly secure communication, the prize here is a global communications network that cannot be hacked, even in principle.

"The technology implemented in our experiment thus certainly reached the required maturity both for satellite and for long-distance ground communication," say Zeilinger and co.

The questions, of course, is who will be first to orbit. The Europeans have a space agency that could be persuaded to test this idea but they won't be in a hurry. China is currently showing great ambition in space and will want to show off its technological prowess. Both have the wherewithall to pull off this next step.

The contrast with the US couldn't be clearer.

Ref: arxiv.org/abs/1205.3909: Quantum Teleportation Using Active Feed-Forward Between Two Canary Islands

Fly 'n' Flow

Sat, 19 May 2012 04:10:00 GMT

The best of the rest from the Physics arXiv this week

Decoherence and the Quantum Detection of Classically Undetectable Phenomena

SportSense: Real-Time Detection of NFL Game Events from Twitter

Evolution Of Robust Network Topologies: Emergence Of Central Backbones

Ordinal Boltzmann Machines for Collaborative Filtering

Variation Of The Speed Of Light With Temperature Of The Expanding Universe

Why Shutting Airports Is Not the Best Way to Halt a Global Flu Pandemic

Thu, 17 May 2012 09:58:00 GMT

In a deadly flu outbreak, shutting airports should reduce the spread of the disease. But networks scientists have discovered a better approach that's just as effective.

One of the nightmare scenarios for modern society is the possibility of a global flu pandemic like the 1918 Spanish influenza which infected about a quarter of the global population and killed as many as 130 million of them.

An important question for policy makers is how best to limit the spread of such a disease if a new outbreak were to occur. (The Spanish flu was caused by the H1N1 flu virus that was also responsible for the 2009 swine flu outbreak.)

One obvious idea is to close international airports to prevent, or at least dramatically reduce, the movement of potentially infected individuals between countries. But is this the best approach?

Today, Jose Marcelino and Marcus Kaiser at Newcastle University in the UK, provide an answer. They say a better approach is to cut specific flights between airports because it can achieve the same reduction in the spread of the disease with far less drastic action.

These guys used a standard disease-spreading model to simulate the spread of an H1N1-type infection across a network consisting of the world's top 500 airports and the flights between them. The disease started in Mexico City.

They then reran the simulation to see how different strategies could reduce the spread. They found that shutting entire airports can obviously reduce infection.

But they also studied less obvious strategies such as looking for cities that play an important role in the network and reducing the flights between them by 25 per cent. This turned out to be a much more effective strategy.

They found that shutting entire airports only had a significant effect on spreading if it reduced travel by 95 per cent. By contrast, they could achieve the same effect by removing just 18 per cent of flights between cities ranked by a network measure called edge betweenness.

At best shutting entire airports could only cut infections by 18 per cent whereas removing specific flights reduced infections by up to 37 per cent.

"Selecting highly ranked single connections between cities for cancellation was more effective, resulting in fewer individuals infected with inﬂuenza, compared to shutting down whole airports," say Marcelino and Kaiser. This approach has the added benefit that it disrupts far fewer individuals

Because these guys used a model of the actual global network of airports and flights they were able to identify the specific connections that would need to be targeted. For an infection that starts in Mexico City, the highest ranked routes that would need to be targeted are Sao Paulo to Beijing, Sapporo to New York and Montevideo to Paris.

That seems an eminently sensible suggestion. However, policy makers might want to study this approach in more detail to check that the conclusions still hold if outbreaks occur in other places too.

Another idea worth checking is to see whether smaller airports could also play an important role in disease spreading. Marcelino and Kaiser study a network consisting of the top 500 airports but the world is blessed with some 4000 airports in total.

It's not inconceivable that some of these could play a crucial role in linking different parts of the world in a way that could facilitate disease spreading.

Ref: http://arxiv.org/abs/1205.3245: Critical Paths In A Metapopulation Model Of H1N1: Efﬁciently Delaying Inﬂuenza Spreading Through Fight Cancellation

Humanoid Robot Swarm Synchronized Using Quorum Sensing

Wed, 16 May 2012 10:14:00 GMT

Proof-of-principle experiment shows how humanoid robots can co-operate on a large scale by copying the behavior of social insects and bacterial colonies.

In recent years, various companies and labs have developed impressive humanoid robots that walk, shuffle and even run. Some even dance in groups of up to 20, performing sophisticated choreographed routines.

This kind of synchronisation is no easy task. One way to do it is have one robot as the leader, broadcasting details of its movement and position over a network that the other robots all follow.

The trouble is that network dynamics are not as predictable as choreographers would like. Small delays of half a second or so are common while some messages can be delayed by several seconds. That's clearly not good enough for a dance routine or any other type of synchronised behaviour.

So the approach preferred by roboticists is to program each robot with the dance routine, synchronise their internal clocks at the start of the performance and then leave them to it.

The advantage is that If the performance is reasonably short, the chances of the clocks becoming desynchronised can be made small. The disadvantage is that if the robots become desynchronised--if one falls over, for example--there is no way to regain synchronisation.

So roboticists have been searching for a better form of synchronisation that is more robust to the various trials and tribulations that befall robotic dancers. Today, Patrick Bechon and Jean-Jacques Slotine at the Massachusetts Institute of Technology in Cambridge, reveal a new approach based on the biological phenomenon of quorum sensing.

Biologists have long puzzled over the ability of bacteria and social insects to sense not only the presence of compatriots but their number and to synchronise their behaviour.

It turns out that these creatures perform this synchronisation using a process called quorum sensing. This works by constantly releasing signalling molecules into the environment while at the same time measuring the local concentration of these molecules.

This concentration rises as more creatures join the local population and so is an effective measure of population density. When the concentration rises over some threshold level, it triggers a different behaviour such cell division, pathogen production and nest building.

Now Bechon and Slotine say a similar approach provides a robust way to synchronise humanoid robots. The ideal approach to synchronisation is for each robot to have access to every other robot's position. Instead, the quorum sensing approach gives, each robot access to a global variable such as the average position or average clock time. Each robot can also change this variable because it contributes to the average.

The idea is that if each robot attempts to synchronise with this global average, the swarm as whole should keep good time.

These guys test out their approach with a group of eight NAO robots built by the French robotics company Aldebaran. Each has an internal clock which attempts to synchronise with a global average time maintained by a central server.

It's important to point out that the server is not acting as a master with the robots as slaves that simply follow its signal. If the connection to the central is lost, the robots simply continue with routine but without centralised synchrony.

Instead, the central server is more like a a kind of environment that the robots can sense and interact with.

This arrangement has the significant advantage that if one robot falls over it can simply get back up and join in again when it has resynchronised its movements with the group (see video).

This work is part of a broader development in robotics. The advent of relatively cheap humanoid robots from Aldebaran and other companies means that the large-scale sychronisation of humanoid swarms is now possible.

That's interesting because while synchrony allows large numbers of robots to do the same thing at the same time--such as dancing or marching--it also allows large number so robots to do different but related tasks at the same time.

In other words, synchrony is an enabling technology for large scale co-operation. And that opens the way to an entirely new set of tasks that robots could do--think manufacturing and construction. Perhaps even nest building.

Ref: arxiv.org/abs/1205.2952: Synchronization And Quorum Sensing In A Swarm Of Humanoid Robots

First Simulation of Quantum Tunneling on a Quantum Computer

Tue, 15 May 2012 10:00:00 GMT

Quantum tunneling had always been thought too complex to simulate on today's simple quantum computers. Now a new approach to quantum computing has changed that and opens the door to more complex simulations.

The exploitation of quantum weirdness for computing is one of the great goals of modern physics. It's promise is dramatic for a wide range of number-crunching tasks.

But quantum computers have another trick up their sleeves which is sometimes forgotten--the ability to simulate other quantum systems. Physicists have already shown how quantum computers of various types can simulate phenomenon such as quantum phase transitions and the dynamics of entanglement--things that classical computers simply cannot handle.

There is one quantum phenomenon, however, that has never been simulated--tunnelling. This is the ability of quantum particles to cross a barrier without seeming to have passed through it.

There's no reason in principle why quantum computers can't simulate tunnelling. The problem is the complexity of the task.

The simulations performed so far have all involved so-called analogue processes which are relatively straightforward. The idea here is that the mathematical description of one system, its Hamiltonian, is exactly reproduced in another system.

So watching one system tells you exactly how the other would behave. This is known as analogue quantum particle simulation and it works well provided you can find systems that match in required way. Watching quantum phase transitions is good example because many systems share the same mathematical description.

For more complex problems, physicists have recently been thinking about another approach. The idea here is to break the mathematical system into different parts and simulate them separately. This is known as digital quantum particle simulation and it has huge potential for events that involve more than one object, such as quantum chemistry and tunneling.

The problem is the sheer complexity of these calculations, which require numerous quantum logic gates processing dozens of qubits. That's always been beyond the state-of-the-art for quantum computing.

Earlier this year, however, Andrew Sornborger at the University of Georgia in Athens showed how the case of a single particle tunnelling through a barrier could be made simple enough to simulate on today's quantum computers. Such a demonstration would be the first example of a digital quantum simulation.

And today Guan Ru Feng and pals at Tsinghua University in Beijing say they've done it. To simulate tunnelling, these guys used a quantum computer that relies on nuclear magnetic resonance to manipulate qubits in encoded in the carbon and hydrogen atoms that make up chloroform molecules. They say this is the ﬁrst demonstration of a quantum tunnelling simulation using an NMR quantum computer.

That should open the floodgates for more digital quantum simulations in future. It's significant because this approach has the potential to simulate much more complex quantum phenomenon than is currently possible. Expect to see more of it.

Ref: arxiv.org/abs/1205.2421: Experimental Digital Simulation of Quantum Tunneling in a NMR Quantum Simulator

Antimatter Propulsion Engine Redesigned Using CERN's Particle Physics Simulation Toolkit

Mon, 14 May 2012 15:15:00 GMT

Latest simulation shows that the magnetic nozzles required for antimatter propulsion could be vastly more efficient than previously thought--and built with today's technologies

Smash a lump of matter into antimatter and it will release a thousand times more energy than the same mass of fuel in a nuclear fission reactor and some 2 billion times more than burning the equivalent in hydrocarbons.

So it's no wonder that antimatter is the dream fuel for science fiction fans.

The problem, of course, is that antimatter is in rather short supply making the prospect of ever building a rocket based on this technology somewhat remote.

But from time to time physicists put aside these concerns and have a little fun working out how good antimatter rocket engines can be. Today it's the turn of Ronan Keane at Western Reserve Academy and Wei-Ming Zhang at Kent State University, both in Ohio, who take a new approach to the problem with some interesting results.

First, some basic rocket science. The maximum speed of a rocket depends on its exhaust velocity, the fraction of mass devoted to fuel and the configuration of the rocket stages. "The latter two factors depend strongly on fine details of engineering and construction, and when considering space propulsion for the distant future, it seems appropriate to defer the study of such specifics," say Keane and Zhang.

So these guys focus on the exhaust velocity--the speed of the particles produced in matter-antimatter annihilations as they leave the rocket engine.

The thrust from these annihilations comes largely from using a magnetic field to deflect charged particles created in the annihilation. These guys focus on the annihilation of protons and antiprotons to produce charged pions.

So an important factor is how efficiently the magnetic field can channel these particles out of the nozzle.

In fact, the exhaust velocity of these pions depends on two factors--their average initial velocity when they are created and the efficiency of the magnetic nozzle design.

In the past, various physicists have calculated that the pions should travel at over 90 per cent the speed of light but that the nozzle would be only 36 per cent efficient. That translates into an average exhaust velocity of only a third of lightspeed, barely relativistic and somewhat of a disappointment for antimatter propulsion fans.

All that is set to change now, however. Keane and Zhang have come up with a different set of figures with the help of software developed by CERN that simulates the interaction between particles, matter and fields of various kinds.

CERN uses this software, called GEANT4 (short for Geometry and Tracking 4), to better understand how particles behave at the Large Hadron Collider, which itself collides beams of protons and antiprotons. So it's ideally suited to Keane and Zhang's task.

The new work produces some good news and some bad news. First the bad. The new simulations indicate that pions produced in this way will be significantly slower than previously thought, travelling at only 80 per cent of light speed.

The good news is that the GEANT4 simulations indicate that a magnetic nozzle can be much more efficient than previously envisioned, reaching 85 per cent efficiency. That translates into an average exhaust velocity of about 70 per cent light speed. That's much more promising. "True relativistic speeds once more become a possibility," say Keane and Zhang.

These guys have another surprise up their sleeve. Their nozzle has a magnetic field strength of around 12 Tesla. "Such a field could be produced with today’s technology, whereas prior nozzle designs anticipated and required major advances in this area," they say.

That will bring a smile to the face of many science fiction fans.

There is, of course, the small problem of gathering enough antimatter for a journey of any decent length. The number of antiatoms made at CERN is small enough to be countable. By one estimate, at this rate it will take a thousand years to make a single microgram of antimatter.

Keane and Zhang point out that all earlier estimates predate the PAMELA spacecraft's discovery last year that Earth is surrounded by a ring of antiprotons and suggest that this could mined for fuel. What they don't mention, however, is that PAMELA spotted only 28 antiprotons in two years--far less than the rate at which CERN makes them on a daily basis.

Keane and Zhang finish by noting that other fuel technologies have advanced at an exponential rate, liquid hydrogen production, for example. If antimatter manufacture turns out to follow a similar trajectory, who knows what could happen.

Interesting, entertaining and wildly ambitious--all good fun.

Ref: arxiv.org/abs/1205.2281: Beamed Core Antimatter Propulsion: Engine Design and Optimisation

Beams 'n' Bones

Sat, 12 May 2012 04:10:00 GMT

The best of the rest from the Physics arXiv this week

Three-Body Amplification Of Photon Heat Tunneling

Complexity and Information: Measuring Emergence, Self-organization, and Homeostasis at Multiple Scales

The Variability Of Tidewater-Glacier Calving: Origin Of Event-Size And Interval Distributions

Can Timeouts Change the Outcome of Basketball Games?

Parent-Offspring Conflict In Feral Dogs: A Bioassay

Is Eternal Inflation Past-Eternal? And What if It Is?

Chinese Physicists Smash Distance Record For Teleportation

Fri, 11 May 2012 13:06:00 GMT

The ability to teleport photons through 100 kilometres of free space opens the way for satellite-based quantum communications, say researchers

Teleportation is the extraordinary ability to transfer objects from one location to another without travelling through the intervening space.

The idea is not that the physical object is teleported but the information that describes it. This can then be applied to a similar object in a new location which effectively takes on the new identity.

And it is by no means science fiction. Physicists have been teleporting photons since 1997 and the technique is now standard in optics laboratories all over the world.

The phenomenon that makes this possible is known as quantum entanglement, the deep and mysterious link that occurs when two quantum objects share the same existence and yet are separated in space.

Teleportation turns out to be extremely useful. Because teleported information does not travel through the intervening space, it cannot be secretly accessed by an eavesdropper.

For that reason, teleportation is the enabling technology behind quantum cryptography, a way of sending information with close-to-perfect secrecy.

Unfortunately, entangled photons are fragile objects. They cannot travel further than a kilometre or so down optical fibres because the photons end up interacting with the glass breaking the entanglement. That severely limits quantum cryptography's usefulness.

However, physicists have had more success teleporting photons through the atmosphere. In 2010, a Chinese team announced that it had teleported single photons over a distance of 16 kilometres. Handy but not exactly Earth-shattering.

Now the same team says it has smashed this record. Juan Yin at the University of Science and Technology of China in Shanghai, and a bunch of mates say they have teleported entangled photons over a distance of 97 kilometres across a lake in China.

That's an impressive feat for several reasons. The trick these guys have perfected is to find a way to use a 1.3 Watt laser and some fancy optics to beam the light and receive it.

Inevitably photons get lost and entanglement is destroyed in such a process. Imperfections in the optics and air turbulence account for some of these losses but the biggest problem is beam widening (they did the experiment at an altitude of about 4000 metres). Since the beam spreads out as it travels, many of the photons simply miss the target altogether.

So the most important advance these guys have made is to develop a steering mechanism using a guide laser that keeps the beam precisely on target. As a result, they were able to teleport more than 1100 photons in 4 hours over a distance of 97 kilometres.

That's interesting because it's the same channel attenuation that you'd have to cope with when beaming photons to a satellite with, say, 20 centimetre optics orbiting at about 500 kilometres. "The successful quantum teleportation over such channel losses in combination with our high-frequency and high-accuracy [aiming] technique show the feasibility of satellite-based ultra-long-distance quantum teleportation," say Juan and co.

So these guys clearly have their eye on the possibility of satellite-based quantum cryptography which would provide ultra secure communications around the world. That's in stark contrast to the few kilometres that are possible with commercial quantum cryptography gear.

Of course, data rates are likely to be slow and the rapidly emerging technology of quantum repeaters will extend the reach of ground-based quantum cryptography so that it could reach around the world, in principle at least.

But a perfect, satellite-based security system might be a useful piece of kit to have on the roof of an embassy or distributed among the armed forces.

Something for western security experts to think about.

Ref: arxiv.org/abs/1205.2024: Teleporting Independent Qubits Through A 97 Km Free-Space Channel

Silicon Nanospheres Could Be Building Blocks of Optical Invisibility Cloaks

Thu, 10 May 2012 10:17:00 GMT

Invisibility cloaks that work for microwaves are easy to make using simple building blocks. Now engineers have created the equivalent building blocks for visible light.

Given the headlines associated with invisibility cloaks, you could be forgiven for thinking that a Star Trek-style cloaking device will be available in stores before the holiday season. Sadly, no.

It's true that in recent years researchers have made great strides in their theoretical understanding of how these cloaks work and consequently built increasingly complex and impressive devices.

But these devices generally work in the microwave region of the electromagnetic spectrum, where wavelengths are measured in centimetres. A few teams have made devices that work in the optical realm but only in two dimensions and over extremely short distances.

The problem is simple. The building blocks of microwave cloaks are split ring resonators--c-shaped pieces of metal in which electric and magnetic fields resonate when they interact with a electromagnetic wave. This interaction is what steers microwaves in a way that cloaks objects

These split ring resonators need to be a little smaller than the wavelengths they are designed to interact with. For microwaves, that's the centimetre scale and so are relatively easy to make and assemble.

But that's much harder at optical wavelengths, where the scale is measured in nanometres..What's more, losses become much more significant on this scale because most metals tend to absorb visible light rather than transmit it.

Consequently, nobody has been able to make split ring resonators that work for visible light. The optical cloaks made so far all rely on other structures such as gold nanorods or holes drilled through silicon slabs.)

So anybody who finds a way to create the optical version of split ring resonators might reasonably be said to be sitting on an important breakthrough.

Enter Arseniy Kuznetsov at the Data Storage Institute in Singapore and a few pals. These guys say they've found an alternative to split ring resonators that work well at optical frequencies, with few losses.

This alternative is silicon nanospheres between 100 and 200 nm in diameter. It turns out that these spheres behave just like split ring resonators in the sense that they allow for the same kind of magnetic resonances when they interact with light.

The trouble of course is making these spheres. Chemists have been making much smaller spheres--between 2nm and 50nm in diameter--for some time now using various self assembly techniques to grow tiny crystals or 'quantum dots' as they are called. (These are useful because they fluoresce at well defined and controllable wavelengths.)

To make larger spheres, Kuznetsov blast a slab of silicon with a short pulse of high-powered laser light. This evaporates the silicon which then condenses into liquid balls. These solidify into spheres as they fall back on to the surface as a kind of silicon rain.

The end result is a surface covered with solid silicon nanospheres between 100 and 200nm in diameter.

Kuznetsov calculated how the magnetic fields inside these balls should resonate when they are zapped with light and then used a hi-tec tweezer and magnifying set to see whether the behaviour matched the theory.

Turns out it does. This magnetic resonance can be tuned to match any part of the visible spectrum simply selecting spheres of a specific size.

That's important because it opens up an entirely new way make invisibility cloaks that operate in the visible region. "These optical systems open up new perspectives for fabrication of low-loss optical metamaterials and nanophotonic devices," they say.

That could have a significant impact on the way optical cloaks are designed and made.

There are plenty of challenges ahead, however. Not least of these will be finding a reliable way to make nanospheres of a specific size.

Then there is the problem of assembling nanospheres into useful devices in a way that scales. That's not something that a tweezer and magnifying set can help with, no matter how hi-tec.

Having said that, there is huge interest and big money invested in cloaking research, which is one reason why progress has been so rapid in the last ten years. So it wouldn't be a total surprise if these problems were solved in double quick time.

Ref: arxiv.org/abs/1205.1610: Magnetic light

Physicists Store Short Movie In A Cloud of Gas

Wed, 09 May 2012 10:15:00 GMT

Researchers have been able to store single images in a cloud of rubidium atoms for several years. Now they've gone a step further

One of the enabling technologies for a quantum internet is the ability to store and retrieve quantum information in a reliable and repeatable way.

One of the more promising ways to do this involves photons and tiny clouds of rubidium gas. Rubidium atoms have an interesting property in that a magnetic field causes their electronic energy levels to split, creating a multitude of new levels. Switching the field off, returns the atoms to their normal state.

So one way to store photons, and the quantum information they carry, is to send them into a cloud of rubidium atoms and switch on the magnetic field. If the photons have a wavelength that is absorbed by the new electronic levels in the gas, they become trapped within it.

As long as the field remains on, that is. Switch the field off and the atoms are forced to emit the photons allowing the information they hold to be retrieved.

That immediately suggests a way of building a quantum memory.

Indeed various teams have spent the last few years testing this technique and others related to it. The results have been impressive. They can store not just single photons but entire images which they send into the gas by placing an image mask over the beam.

The storage lasts for tens of microseconds and the images can be retrieved with accuracies approaching 90 per cent. (The storage duration is limited by the movement of the atoms in the gas which blurs the images over time.)

Today, Quentin Glorieux and pals at the National Institute of Standards and Technology in Maryland go a step further. These guys have used exactly this technique to store two images at the same time. That's clearly a very short movie but the important point is that it's a proof-of-principle demonstration of the technique.

The images are the letter T and the letter N and the sequence of pictures above shows the images being released from the gas, as recorded by a high speed camera in 100 nanosecond frames. "We have demonstrated that multiple images can be stored and retrieved at different times, allowing the storage of a short movie in an atomic memory," say Glorieux and co.

Interestingly, the images are released on a "last in, first out" basis, so this movie is running backwards.

That's an impressive feat. Until now, sequences of images have only ever been stored at the same time in solid state media, such as holographic memories.These seem to have impressive potential as quantum memory devices.

But it looks as if rubidium gas clouds will give holograms a run for their money in this race.

Ref: arxiv.org/abs/1205.1495: Temporally Multiplexed Storage of Images in a Gradient Echo Memory

The Quantum Biology Conundrum

Tue, 08 May 2012 10:33:00 GMT

If quantum mechanics plays an important role is biology, we'll want to copy it. If it doesn't, we'll want to know why not

One of the biggest questions in biology is whether the processes of life are able to exploit quantum effects to improve their lot.

Nobody questions whether living things are ultimately quantum at some level--we're all made of quantum objects called atoms and glued together by quantum forces. If you look closely enough at any biological process, you'll see quantum mechanics at work.

The question is whether nature exploits quantum mechanics to achieve things that are not possible in the ordinary, classical world.

There is a growing debate on this topic. On the one hand, evidence has begun to mount that quantum mechanics may play a role in processes such as photosynthesis, bird navigation and the sense of smell. On the other, critics say this evidence is far from conclusive and may simply show that reality always appears quantum in nature, if you look closely enough.

Today, Neill Lambert at the Japanese research institute RIKEN in Saitama and a few pals, provide a much needed review of the evidence in this area, focusing in particular on photosynthesis and bird navigation.

These guys point out that the efforts to find evidence of quantum effects in photosynthesis are largely focused on the fact that energy somehow crosses large protein molecules with an efficiency close to 100 per cent. That's hard to explain classically.

The evidence for quantum effects in bird navigation is a little more speculative but leaves less room for a classical explanation. It is based on the idea that that a weak magnetic field can influence the outcome of a certain type of chemical reaction in bird retinas involving radical ion pairs.

The details make for interesting reading.

This is an area that has gained huge attention in recent years. The promise, of course, is that if nature has found ways to exploit quantum mechanics, then it should be possible for us to copy those techniques. Think artificial photosynthesis, robotic noses and navigation systems, perhaps even artificial life.

But the alternative is just as interesting. If nature has not found a way to exploit quantum mechanics, an equally important question is: why not? Is it merely an oversight on the part of evolution or is there some other deeper reason why evolution cannot exploit quantum mechanics?

Important questions. And for answers, a good place to start is with a comprehensive overview of the research.

Ref: arxiv.org/abs/1205.0883: Functional Quantum Biology In Photosynthesis And Magnetoreception

The Single Theory That Could Explain Emergence, Organisation And The Origin of Life

Mon, 07 May 2012 12:27:00 GMT

Biochemists have long imagined that autocatalytic sets can explain the origin of life. Now a new mathematical approach to these sets has even broader implications

One of the most puzzling questions about the origin of life is how the rich chemical landscape that makes life possible came into existence.

This landscape would have consisted among other things of amino acids, proteins and complex RNA molecules. What's more, these molecules must have been part of a rich network of interrelated chemical reactions which generated them in a reliable way.

Clearly, all that must have happened before life itself emerged. But how?

One idea is that groups of molecules can form autocatalytic sets. These are self-sustaining chemical factories, in which the product of one reaction is the feedstock or catalyst for another. The result is a virtuous, self-contained cycle of chemical creation.

Today, Stuart Kauffman at the University of Vermont in Burlington and a couple of pals take a look at the broader mathematical properties of autocatalytic sets. In examining this bigger picture, they come to an astonishing conclusion that could have remarkable consequences for our understanding of complexity, evolution and the phenomenon of emergence.

They begin by deriving some general mathematical properties of autocatalytic sets, showing that such a set can be made up of many autocatalytic subsets of different types, some of which can overlap.

In other words, autocatalytic sets can have a rich complex structure of their own.

They go on to show how evolution can work on a single autocatalytic set, producing new subsets within it that are mutually dependent on each other. This process sets up an environment in which newer subsets can evolve.

"In other words, self-sustaining, functionally closed structures can arise at a higher level (an autocatalytic set of autocatalytic sets), i.e., true emergence," they say.

That's an interesting view of emergence and certainly seems a sensible approach to the problem of the origin of life. It's not hard to imagine groups of molecules operating together like this. And indeed, biochemists have recently discovered simple autocatalytic sets that behave in exactly this way.

But what makes the approach so powerful is that the mathematics does not depend on the nature of chemistry--it is substrate independent. So the building blocks in an autocatalytic set need not be molecules at all but any units that can manipulate other units in the required way.

These units can be complex entities in themselves. "Perhaps it is not too far-fetched to think, for example, of the collection of bacterial species in your gut (several hundreds of them) as one big autocatalytic set," say Kauffman and co.

And they go even further. They point out that the economy is essentially the process of transforming raw materials into products such as hammers and spades that themselves facilitate further transformation of raw materials and so on. "Perhaps we can also view the economy as an (emergent) autocatalytic set, exhibiting some sort of functional closure," they speculate.

Could it be that the same idea--the general theory of autocatalytic sets--can help explain the origin of life, the nature of emergence and provide a mathematical foundation for organisation in economics?

As Kauffman and friends say with just a little understatement: "We believe that these ideas are worth pursuing and developing further."

We'll look forward to following the work as it progresses.

Ref: arxiv.org/abs/1205.0584: The Structure of Autocatalytic Sets: Evolvability, Enablement, and Emergence

Boom 'n' Bust

Sat, 05 May 2012 08:16:00 GMT

The best of the rest from the Physics arXiv this week

A New Perspective On Dark Energy Modeling Via Genetic Algorithms

Astrophysics Independent Bounds On The Annual Modulation Of Dark Matter Signals

The Effects Of Environmental Disturbances On Tumor Growth

Graphene Battery Made Of Low Cost Reduced Graphene Oxide

No Fundamental Limitation on Studying Living Organisms and Other Complex Systems with Statistical Methods

Predicting Fixation Tendencies of the H3N2 Influenza Virus by Free Energy Calculation

How A Private Data Market Could Ruin Facebook

Thu, 03 May 2012 04:10:00 GMT

The growing interest in a market for personal data that shares profits with the individuals who own the data could change the business landscape for companies like Facebook

Facebook's imminent IPO raises an interesting issue for many of its users. The company's value is based on its ability to exploit the online behaviours and interests of its users.

To justify its sky-high valuation, Facebook will have to increase its profit per user at rates that seem unlikely, even by the most generous predictions. Last year, we looked at just how unlikely this is.

The issue that concerns many Facebook users is this. The company is set profit from selling user data but the users whose data is being traded do not get paid at all. That seems unfair.

Today, Bernardo Huberman and Christina Aperjis at HP Labs in Palo Alto, say there is an alternative. Why not pay individuals for their data? TR looked at this idea earlier this week.

Setting up a market for private data won't be easy. Chief among the problems is that buyers will want unbiased samples--selections chosen at random from a certain subgroup of individuals. That's crucial for many kinds of statistical tests.

However, individuals will have different ideas about the value of their data. For example, one person might be willing to accept a few cents for their data while another might want several dollars.

If buyers choose only the cheapest data, the sample will be biased in favour of those who price their data cheaply. And if buyers pay everyone the highest price, they will be overpaying.

So how to get an unbiased sample without overpaying?

Huberman and Aperjis have an interesting straightforward solution. Their idea is that a middle man, such as Facebook or a healthcare provider, asks everyone in the database how much they want for their data. The middle man then chooses an unbiased sample and works out how much these individuals want in total, adding a service fee.

The buyer pays this price without knowing the breakdown of how much each individual will receive. The middle man then pays each individual what he or she asked, keeping the fee for the service provided.

The clever bit is in how the middle man structures the payment to individuals. The trick here is to give each individual a choice. Something like this:

Option A: With probability 0.2, a buyer will get access to your data and you will receive a payment of $10. Otherwise, you’ll receive no payment.
Option B: With probability 0.2, a buyer will get access to your data. You’ll receive a payment of $1 irrespectively of whether or not a buyer gets access

So each time a selection of data is sold, individuals can choose to receive the higher amount if their data is selected or the lower amount whether or not it is selected.

The choice that individuals make will depend on their attitude to risk, say Huberman and Aperjis. Risk averse individuals are more likely to choose the second option, they say, so there will always be a mix of people expecting high and low prices.

The result is that the buyer gets an unbiased sample but doesn't have to pay the highest price to all individuals.

That's an interesting model which solves some of the problems that other data markets suffer from.

But not all of them. One problem is that individuals will quickly realise how the market works and work together to demand ever increasing returns.

Another problem is that the idea fails if a significant fraction of individuals choose to opt out altogether because the samples will then be biased towards those willing to sell their data. Huberman and Aperjis say this can be prevent by offering a high enough base price. Perhaps.

Such a market has an obvious downside for companies like Facebook which exploit individual's private data for profit. If they have to share their profit with the owners of the data, there is less for themselves.

And since Facebook will struggle to achieve the kind of profits per user it needs to justify its valuation, there is clearly trouble afoot.

Of course, Facebook may decide on an obvious way out of this conundrum--to not pay individuals for their data.

But that creates an interesting gap in the market for a social network that does pay a fair share to its users (perhaps using a different model to Huberman and Aperjis').

Is it possible that such a company could take a significant fraction of the market? You betcha!

Either way, Facebook loses out--it's only a question of when.

This kind of thinking must eventually filter through to the people who intend to buy and sell Facebook shares.

For the moment, however, the thinking is dominated by the greater fool theory of economics--buyers knowingly overpay on the basis that some other fool will pay even more. And there's only one outcome in that game.

Ref: arxiv.org/abs/1205.0030: A Market for Unbiased Private Data: Paying Individuals According to their Privacy Attitudes

Twitter Cannot Predict Elections Either

Wed, 02 May 2012 04:00:00 GMT

Claims that Twitter can predict the outcome of elections are riddled with flaws, according to a new analysis of research in this area

It wasn't so long ago that researchers were queuing up to explain Twitter's extraordinary ability to predict the future.

Tweets, we were told, reflect the sentiments of the people who send them. So it stands to reason that they should hold important clues about the things people intend to do, like buying or selling shares, voting in elections and even about paying to see a movie.

Indeed various researchers reported that social media can reliably predict the stock market, the results of elections and even box office revenues

But in recent months the mood has begun to change. Just a few weeks ago, we discussed new evidence indicating that this kind of social media is not so good at predicting box office revenues after all. Twitter's predictive crown is clearly slipping.

Today, Daniel Gayo-Avello, at the University of Oviedo in Spain, knocks the crown off altogether, at least as far as elections are concerned. His unequivocal conclusion: “No, you cannot predict elections with Twitter.”

Gayo-Avello backs up this statement by reviewing the work of researchers who claim to have seen Twitter's predictive power. These claims are riddled with flaws, he says.

For example, the work in this area assumes that all tweets are trustworthy and yet political statements are littered with rumours, propaganda and humour.

Neither does the research take demographics into account. Tweeters are overwhelmingly likely to be younger and this, of course, will bias any results. "Social media is not a representative and unbiased sample of the voting population," he says.

Then there is the problem of self selection. The people who make political remarks are those most interested in politics. The silent majority is a huge problem, says Gayo-Avello and more work needs to be done to understand this important group.

Most damning is the lack of a single actual prediction. Every analysis on elections so far has been done after the fact. "I have not found a single paper predicting a future result," says Gayo-Avello.

Clearly, Twitter is not all it has been cracked up to be when it comes to the art of prediction. Given the level of hype surrounding social media, it's not really surprising that the more sensational claims do not stand up to closer scrutiny. Perhaps we should have seen this coming (cough).

Gayo-Avello has a solution. He issues the following challenge to anybody working in this area: "There are elections virtually all the time, thus, if you are claiming you have a prediction method you should predict an election in the future!"

Ref: arxiv.org/abs/1204.6441: “I Wanted to Predict Elections with Twitter and all I got was this Lousy Paper”: A Balanced Survey on Election Prediction using Twitter Data