The best of the rest from the physics arXiv this week:
Archive for November, 2008
Steganophony is the term coined by Wojciech Mazurczyk and Józef Lubacz at the Warsaw University of Technology in Poland to describe the practice of hiding messages in internet telephony traffic (presumably the word is an amalgamation of the terms steganography and telephony).
The growing interest in this area is fueled by the fear that terrorist groups may be able to use services such as Skype to send messages secretly by embedding them in the data stream of internet telephony. At least that’s what Mazurczyk and Lubacz tell us.
The pair has developed a method for doing exactly that called Lost Audio PaCKets Steganography or LACKS and outline it on the arXiv today.
LACKS exploits a feature of internet telephony systems: they ignore data packets that are delayed by more than a certain time. LACKS plucks data packets out of the stream, changes the information they contain and then sends them on after a suitable delay. An ordinary receiver simply ignores these packets if they arrive after a certain time but the intended receiver collates them and extracts the information they contain.
That makes LACKS rather tricky to detect since dropped packets are a natural phenomenon of the internet traffic.
But is this really an area driven by the threat of terrorism? If anybody really wants to keep messages secret then there are plenty of easier ways to do it, such as Pretty Good Privacy.
There’s a far more powerful driver for this kind of work. It’s name? Paranoia
Ref: arxiv.org/abs/0811.4138: LACK – a VoIP Steganographic Method
“Two identical chaotic systems starting from almost identical initial states, end in completely uncorrelated trajectories. On the other hand, chaotic systems which are mutually coupled by some of their internal variables often synchronize to a collective dynamical behavior,” write Meital Zigzag at Bar-Ilan University in Israel and colleagues o the arXiv today.
And perhaps the most fascinating of these synchronized systems are those that show zero lag; that are perfectly synched. For example, in widely separated regions of the brain, zero lag synchronization of neural activity seems to be an important feature of the way we think.
This type of synchronization also turns out to be an important feature of chaotic communication. This is the process by which which information can be hidden in the evolution of a chaotic attractor and retrieved by substracting the same chaotic background to reveal the original message.
Obviously, this only works when the transmitter and receiver have are coupled so that they evolve in exactly the same way. For a long time physicists have wondered whether this effect can be used to send data securely and earlier this year, they proved that the security can only be guaranteed if the synchronisation has zero lag.
But how does zero lag occur and under what range of conditions?
Zero lag seems to occur when the delays in the mutual coupling and self feedback between two systems act to keep them in step. In effect, both systems lag but by exactly the same amount.
Until recently, this was thought to occur only for a very small subset of parameters in which the delays are identical or have a certain ratio. But these limits are so exact and constricting that it’s hard to imagine a wet system such as the brain ever achieving them.
Now Zigzag and friends have shown that it is possible to get around these strict limits by having more than one type of feedback between the systems. When that happens, it’s possible to have zero lag synchronisation over a much wider set of parameters.
That’s going to have important implications for our understanding of synchronisation in the brain and for the development of secure chaotic communication. Betcha!
Ref: arxiv.org/abs/0811.4066: Emergence of Zero-Lag Synchronization in Generic Mutually Coupled Chaotic Systems
How much force does it take to stab somebody to death? Strangely enough, forensic scientists do not know.
A number of groups have attempted to measure the forces necessary to penetrate skin but the results are difficult to apply to murder cases because of the sheer range of factors at work. The type and sharpness of the knife; the angle and speed at which it strikes; the strength of skin which varies with the age of the victim and the area of body involved; these are just a few of parameters that need to be taken into account.
So when giving evidence, forensic scientists have to resort to relative assessments of force.
“A mild level of force would typically be associated with penetration of skin and soft tissue whereas moderate force would be required to penetrate cartilage or rib bone. Severe force, on the other hand, would be typical of a knife impacting dense bone such as spine and sustaining visible damage to the blade,” says Michael Gilchrist at University College Dublin and pals who are hoping to change this state of affairs.
They’ve developed a machine that measures the force required to penetrate skin–either the animal kind or an artificial human skin made of polyurethane, foam and soap.
The surprise they’ve found is that the same knives from the same manufacturer can differ wildly in sharpness. And the force required for these knives to penetrate the skin can differ by more than 100 per cent.
That could have a significant bearing in some murder cases. And that’s important because in many European countries such as the UK, stabbing is the most common form of homicide.
Gilchrist and co say their work could even help tease apart what has happened in that most common of defences: “he ran onto the knife, your honour”.
The key thing here is the speed and angle of penetration. The angle can be measured easily enough but the speed is another matter altogether. Gilchrist and co say future work may throw some light on this.
Ref: arxiv.org/abs/0811.3955: Mechanics of Stabbing: Biaxial Measurement of Knife Stab Penetration of Skin Simulant
Almost 40 years ago, two Russian physicists predicted the existence of a new state of matter called a supersolid. They reasoned that at very low temperatures, the rules of quantum mechanics would allow a solid to move with zero resistance and that this would allow one solid to move through another like magician walking through a wall.
Like many quantum mechanical phenomenon, such behavior is entirely counterintuitive: how can the atoms that give a solid its rigidity also move with zero resistance?
But In the absence of any experimental evidence to back up this claim, supersolids were more or less forgotten. That changed in 2004 when Moses Chan and pals at Pennsylvania State University said they had stumbled across the first evidence of supersolidity in helium cooled to within a whisker of absolute zero.
Since then, interest in supersolidity has skyrocketed. But our ideas about supersolids are as confused as ever because of a number of puzzling results. Supersolids, if that is indeed what Chan has seen, are more complicated and mysterious than we ever imagined.
Today Davide Galli and Luciano Reatto from the Universita degli Studi di Milano in Italy review the field, doing a sterling job of drawing together the disparate ideas and the puzzling experimental evidence.
Ref: arxiv.org/abs/0811.3598: Solid 4He and the Supersolid Phase: From Theoretical Speculation to the Discovery of a New State of Matter? A Review of the Past and Present Status of Research
Atomic matter waves have been generating a bit of interest of late. The thinking is that atom waves can be manipulated in much the same way as light waves and so could be used to directly print atoms onto microchips to create nanoscale features.
The question is: how small can these features be made and how accurately can they be put in place? An obvious answer is that these features can’t be smaller than the size of the atoms involved and placed no more accurately than the size of the wavelength of these atom ie diffraction limited. And until a few years ago, nobody would have argued with that.
Since then, numerous techniques have been been developed for manipulating light beyond the diffraction limit so it’s reasonable to assume that a similar thing can be done with matter waves.
And so it has come to pass. Jordi Mompart at the Universitat Autonoma de Barcelona in Spain and a few amigos have shown that it is possible to achieve single nanometre resolution with matter waves of rubidium. At least in theory.
All we need now is somebody to step up to the plate and do it for real. Judging by the speed of progress in this field right now, that shouldn’t take long.
And beyond that: the first microchips patterned using matter waves rather than light?
Ref: http://arxiv.org/abs/0811.3409: Coherent Patterning of Matter Waves with Subwavelength Localization
The best of the rest from the physics arXiv this week:
The Search for Extraterrestrial Intelligence is picking up steam. The folks over at the Berkeley SETI group now have 7 separate searches underway at infrared, visible and radio wavelengths.
Today, Andrew Siemiona and pals outline the two newest programs which have recently seen first light and are hunting for pulses just a few hundred nanosceonds long. By contrast, most searches up till now have looked only for pulses a few seconds long.
The first, a project called Fly’s Eye at the Allen Telescope Array in northern California, can watch huge areas of the sky up to 100 degrees square and spot pulses as short as 0.625 ms
The second is called Astropulse at the Arecibo Observatory in Puerto Rico and will be 30 times more sensitive than any search gone before.
The early results from these searches are being processed on the SETI@Home network which the authors claim is the second most powerful supercomputer on the planet.
Nevertheless, it looks as if they have long hard slog ahead of them: ET hasn’t revealed herself just yet.
Ref: arxiv.org/abs/0811.3046: New SETI Sky Surveys for Radio Pulses
The way snowflakes form is poorly understood. It seems clear that the process involves a subtle interplay of nonlinear effects in which small variations at the molecular level can produce large changes in the eventual shape. In particular, small levels of gaseous impurities are thought to have a major impact on the way these effects play out.
We all know the result: the amazing, beautiful and unique crystals that fall as snowflakes.
Watching and measuring the way snowflakes form is difficult for obvious reasons,a problem that has severely hampered our understanding of snow flake formation. But that looks set to change.
Kenneth Libbrecht and buddies at the California Institute of Technology in Pasadena have a built a machine that makes snowflakes in conditions that mimic those in the atmosphere. The crystals grow as they fall within this chamber and their size and thickness are measured when they land.
The work is the first systematic study of snowflake size and shape as a function of temperature and water vapor supersaturation. The results are workmanlike, merely confirming expectations. But they provide a baseline against which to measure other factors that influence snowflake formation, such as the levels of gaseous impurities. And when that happens we’ll be able to tease apart exactly what is going on when these crystals form for the first time.
Ref: arxiv.org/abs/0811.2994: Measurements of Snow Crystal Growth Dynamics in a Free-fall Convection Chamber
Graphene is the hottest property in materials science these days. Its extraordinary electronic, thermal and physical properties make it the most heavily studied substance on the plant right now.
But there is one thing that graphene can’t do and that is to fit easily into the silicon-based electronics industry. And while graphene based chips hold much promise, it’s hard to see chip makers re-tooling to use carbon instead of silicon in the near future.
That’s why a number of groups have become to look at the possibility of making silicon versions of grahene, a material called silicene. Silicon nanowires made their first appearance in 2005. And now Christelle Leandri at the Center for Interdisciplinary Nanoscience in Marseille, France, and a few buddies have made silicene for the first time, albeit in the form of stripes or nanoribbons.
What the team has done is create parallel stripes of silicene, just one atom thick on a silver substrate. The team says the physical and chemical properties of these nanoribbons is striking.
For a start silicon nanoribbons seem to be more chemically stable than their graphene cousins. In particular, graphene is highly reactive around its edges where carbon bonds dangle freely. This can make graphene hard to handle. The edges of silicene on the other hand seems to be naturally inert.
Leandri et amis have high hopes for silicene, saying that it could be incorporated into current manufacturing processes and thereby “help prolong the life of Moore’s law.”
Ref: arxiv.org/abs/0811.2611: Physics of Silicene Stripes