Archive for the ‘Mountain climbin'’ Category

Visible light metamaterials on the cheap

Tuesday, March 10th, 2009

visible-light-metamaterial

Only a couple of years, more than a few physicists doubted that it would ever be possible to build decent metamaterials with a negative refractive index for visible light.

Metamaterials have bulk properties that depend on the structure of their components rather than the bulk properties of the materials from which they are made. The thinking is that they can make light do all kinds of things that are no possible in naturally occurring stuff such as bending light backwards and imparting it with a reverse Doppler shift.

Metamaterials that bend microwaves backwards are straightforward to make: it’s just a question of arranging components, such as conducting wires and split rings, in a periodic 3D array on a centimetre scale.

It’s easy to think that similar structures would work for visible light were they shrunk to the nanometre scale. But, as many physicists have pointed out, the electrical properties of conducting metals do not scale with wavelength in quite the same way. Instead of transmitting light, many of these designs would be opaque to visible light.

Some people said it may never be possible to make efficient negative refraction index metamaterials for visible light. Others, who were a little more optimistic, were vindicated  last August, Xiang Zhang at the University of California, Berkeley, revealed that a periodic array of parallel silver nanowires embedded in aluminium oxide worked perfectly well as metamaterial with negative refractive index for visible light.

Now Akhlesh Lakhtakia  at Pennsylvania State University and pals have worked out how to make sheets of this stuff using a vapour deposition technique that is common in the optical industry.

So in a couple of years, we’ve gone from having little prospect of a negative refractive index material for visible light to a way of making sheets of it at extremely low cost.

That’ll make negative refractive index materials available to almost anybody who wants to play with them. Expect to see some ingenious applications in the coming months.

Ref: arxiv.org/abs/0903.1177: Vapor-deposited thin Films with Negative refractive Index in the Visible Regime

How superconducting sheets could reflect gravitational waves

Friday, March 6th, 2009

gravitational-wave-mirror

Gravitational waves are the elusive distortions in spacetime created by the universe’s most violent events–collisions between black holes, stars exploding and even the big bang itself.

Nobody has bagged a confirmed sighting of these waves but that may change thanks to an intriguing idea from Raymond Chiao and pals at the University of California, Merced. They propose the existence of a new kind of mirror that reflects gravitational waves and may even convert them into electromagnetic waves.

First some background. Theoretical physicists have long noticed that in certain circumstances, Einstein’s equations of general relativity, which predict the existence of gravitatonal waves, bear a remarkable similarity to Maxwell’s equations that describe the behaviour of electromagnetic radiation. That’s an important clue for understanding how gravitational waves  behave, says Chiao.

He points to the specific case in which a thin superconducting film reflects em waves. If that works for em waves, then the mathematics indicates that it must also work for gravitational waves.

Here’s the thinking. A gravitational wave stretches and squeezes space as it moves through the universe. Any object in its way will appear to be squashed  and stretched in the same way, the particles within this object will move with the distorted space in a specific trajectory (called geodesic motion).

The new idea comes from considering what happens to a superconducting sheet when a gravitational wave passes by. The Cooper pairs within the sheet are quantum objects governed by the uncertainty principle and so cannot have specific trajectory: they are entirely delocalised. On the other hand, the ions that make up the crystal structure of the superconductor are not delocalised and so can move along a geodesic trajectory when a gravitational wave passes.

This is the basis on which a gravitational wave can interact with a superconducting sheet. “Quantum delocalization causes the Cooper pairs of a superconductor to undergo non-geodesic motion relative to the geodesic motion of its ionic lattice,” says Chiao and buddies.

They speculate that this difference in motion causes the sheet to absorb energy from the  gravitational wave and then re-radiate it as gravitational wave travelling in the opposite direction–in other words specular reflection.

That’s an extraordinary claim which needs some further investigation, not least because there’s a fair amount of disagreement over the GR-Maxwell link in the first place.

Nevertheless, Chiao and co go even further by ending their paper with this:

This implies that two charged, levitated superconducing spheres in static mechanical equilibrium, such that their Coulombic repulsion balances their Newtonian attraction, should be an efficient transducer for converting EM waves into GR waves and vice versa. A Hertz-like experiment in which a transmitter and receiver of GR microwaves are constructed using two such transducers should therefore be practical to perform.

So a pair of levitating, superconducting spheres would act as an antenna for gravitational waves and convert them into electromagnetic waves.

Why wait for LIGO? What’s the betting that superconducting spheres can make the detection first?

Ref: arxiv.org/abs/0903.0661: Do Mirrors for Gravitational Waves Exist?

Were gravitational waves first detected in 1987?

Wednesday, March 4th, 2009

joe-weber

In 1987, Joe Weber, a physicist  at the University of  Maryland, claimed to have detected gravitational waves at exactly the same moment that other astronomers witnessed the famous supernova of that year, SN1987A.

His equipment consisted of several massive aluminium bars that were designed to vibrate in a unique way when a large enough gravitational wave passed by.

His claims were ignored largely because other physicists calculated that gravitational waves ought to be several orders of magnitude too weak to be picked up by this kind of gear. (And he’d made several similar claims throughout the 60s and 70s that others had failed to repeat.)

But Weber’s claims may have to be re-examined, says Asghar Qadir, a physicist at  the National University of Sciences and Technology in Rawalpindi, Pakistan. He points out that predicting the strength of a gravitational wave is by no means easy and until recently, only first order effects have been considered.

He and colleagues have now worked out that in certain circumstances, second order effects can enhance the  waves. But this only happens when there is a certain kind of assymetry in the event that created the waves.

But get this: the assymetry can enhance the waves by a factor of 10^4.

He also  points out that SN1987A is aspherical in exactly the way that might create this enhancement. So if SN1987A generated gravitational waves, Weber would have been perfectly able to detect them.

Qadir concludes: “The claim of Weber to have observed gravitational waves from [SN1987A] needs to be re-assessed”.

By all accounts, Weber was a careful experimenter who got something of a rough deal for his efforts (the most comprehensive telling of the tale is in a book called Gravity’s Shadow by Harry Collins) .

Weber died in 2000 but it wouldn’t do any harm to re-examine his work in the light of this development.

Ref: arxiv.org/abs/0903.0252: Gravitational Wave Sources May Be “Closer” Than We Think

Liquid film motors finally explained

Thursday, February 26th, 2009

liquid-film-motor-explained

Last year, a group of Iranian physicists made the extraordinary discovery that motors can be made of nothing more than a thin film of water sitting in a cell bathed in two perpendicular electric fields. The unexpected result of this set up is that the water begins to rotate. Divide the water into smaller cells and each rotates too.

The team at the Sharif University of Technology in Tehran have a number of  fascinating videos of it in action. They raise an interesting question: the electric fields are static, so what’s making the water move?

Now Vlad Vladimirov at York University in the UK and a couple of droogs from Russia have delved into the hydrodynamics to work out what’s putting the oomph in this motor. The key turns out to be the scale on which the effect takes place.

They say the flow is generated at the edge of the cell where the electric field crosses the  (dielectric) boundary between the water and the cell container. The change in field sets the water flowing along the boundary.  Crucially, this flow is opposite on the other side of the cell and this is what sets up the circular flow.

Vladimirov and co point out that this effect can only happen in a thin film where effects such as viscosity and friction play a large role in the dynamics. In larger bodies of water, these effects become insignificant and the rotation stops. Which is why these motors have only ever been seen in thin films.

That has important implications becaue it shows the scale dependency of important phenomena. In fact, liquid film motors may turn out to be a game-changers for anybody involved in microfluidics.

Ref: arxiv.org/abs/0902.3733: Rotating Electrohydrodynamic Flow in a Suspended Liquid Film

The frightening origins of glacial cycles

Thursday, February 12th, 2009

milankovitch-cycles

Climatologists have known for some time that the Earth’s motion around the Sun is not as regular as it might first appear. The orbit is subject to a number of periodic effects such as the precession of the Earth’s axis which varies over periods of 19, 22 and 24 thousand years, its axial tilt which varies over a period of 41,000 years and various other effects.

The combined effect of these variations are often cited to explain the 41,000 and 100,000 year glacial cycles the Earth appears to have gone through in the past.

But there is a problem with this idea: the change in the amount of sunlight that these variations cause is not enough to trigger glaciation. So some kind of non-linear effect must amplify the effects to cause widespread cooling.

That’s not so surprising given that we know that our climate appears to be influenced by all kinds of non-linear factors. Even still, nobody has been able to explain what kind of processes can account for the difference.

Now Peter Ditlevsen at the University of Copenhagen in Denmark thinks he knows what might have been going on. He says that the change in the amount of sunlight the Earth receives acts as a kind of forcing mechanism in a climatic resonant effect. The resulting system is not entirely stable but undergoes bifurcations in which the cycle switches from a period of 41,000 years to 100,000 years and back again, just as it seems to have done in Earth’s past.

“This makes the ice ages fundamentally unpredictable,” says Ditlevsen.

Quite, but the real worry is this: if bifurcations like this have happened in the past, then they will probably occur in the future. The trouble is that our current climate models are too primituve to allow for this kind of bifurcation and that means their predictions could be even more wildly innacurate than we know they already are .

Kinda frightening, don’t you think?

Ref: arxiv.org/abs/0902.1641:  The Bifurcation Structure and Noise Assisted Transitions in the Pleistocene Glacial Cycles

Watch this space for the chemistry of dust

Wednesday, February 11th, 2009

optical-doughnut

It’s not often that chemists get new tools with which to investigate the building blocks of the world around us, so  a paper on the arXiv today gives them good reason to pop a few corks.

Vladlen Shvedov at the Australian National University in Canberra and a few mates have today unveiled a way of confining and steering aerosol particles in a beam of light.

That hasn’t been possible until now because aerosols are slippery blighters. They absorb huge amounts of light that increases their temperature making them hard to hang on to.

That makes the conventional  way of handling small particles using optical tweezers more or less useless. Optical tweezers rely on radiation pressure to push their charges around. But this is dwarfed by thermal forces and so only works for particles that are relatively cool.

Thermal heating makes particles misbehave because  nonuniform heating makes one side of a particle hotter than the other, causing gas molecules on either side to bounce off with different velecities. This generates a force known as photophoresis.

What Shvedov and his cobbers have done is exploit the photophoretic force to trap partices using  two light beams in the shape of doughnuts.

The result is a device that can trap aerosol particles up to 10 micrometres across  and steer them along trajectories several millimetres long at a rate of around a centmetre per second.

All of a sudden that makes possible a whole host of experiments that were previosuly impossible: developing ecologically clean and safe nanotechnologies, modeleling the chemical proceses at work in the atmosphere and best of all (IMO) simulating interstellar dust .

If you see any chemists with smiles on their faces, you’ll know why. In the meantime look out for some fascinating insights into the chemistry of dust.

Ref:  arxiv.org/abs/0902.1205: Optical Guiding of Absorbing Nanoclusters in Air

Glider guns created in chemical Game of Life

Thursday, February 5th, 2009

glider-guns

If you’ve ever played Conway’s Game of Life, you’ll be familiar with cellular automata and, more importantly,  glider guns. So get this:  a team of British chemists and computer scientists  have  created a chemical cocktail that behaves like a cellular automata and which  reproduces this behavior: chemical  guns firing chemical gliders across a chemical grid.

For those who haven’t played with it, Conway’s  Game of Life is a two dimensional grid known as a cellular automata in which each square can be black or white. The game starts in an initial state–a pattern of black and white squares–and its evolution is determined by a set of rules that specify what color a square should become depending on the color of its neighbors.

The game was devised by the British mathematician John Conway in 1970 and has been studied in detail by countless generations of computer scientists, mathematicians and students ever since, not least because of the extraordinary patterns and structures that the game can produce.

One of these is the glider gun: a structure that periodically “fires” projectiles across the landscape.

Now Ben de Lacy Costello and pals from the University of West of England in Bristol have created a chemical version of all this. Their model is based on the famous Belousov-Zhabotinsky reactions in which a specific cocktail of chemicals  can produce complex patterns of oscillating colors.

The team set up a grid in which each square could change colour via a BZ reaction and in which the reaction diffused across the boundaries of each square in way that mimics Conway’s Game of Life.

The oscillating patterns produced by BZ reaction have long been thought of as similar to cellular automata but this is the first time that the Game of Life has been reproduced in a chemical system.

That’s impressive but it looks as if the best is yet to come. It is well known that cellular automata can operate as universal Turing machines . The next step, says  Costello and buddies, is to build a similar chemical grid capable of  arithmetic.

Beyond that, the question is this: if we can do this in the lab, might evolution also have harnessed these reactions in a similar way. Let the search begin.

Ref: arxiv.org/abs/0902.0587: Implementation of Glider Guns in the Light-Sensitive Belousov-Zhabotinsky Medium

Next generation search engines could rank sites by “talent”

Wednesday, January 7th, 2009

experience-v-talent

How will the next generation of search engines outperform Google’s all-conquering Pagerank algorithm?

One route might be to hire Vwani Roychowdhury at the University of California, Los Angeles and his buddies who have found a fascinating new way to tackle the problem of website rankings.

(more…)

How chaos could improve speech recognition

Wednesday, December 24th, 2008

a-sound

If you’ve ever used speech recognition software, you’ll know how often it fails to work well. Recognition rates are nowhere near what is needed for anything but the simplest applications.

So a new approach for analysing speech by Yuri Andreyev and Maxim Koroteev at the Institute of Radioengineering and Electronics of the Russian Academy of Sciences in Moscow is welcome. Their approach is to treat the production of speech as a chaotic phenomenon.

That’s a significant difference compared with previous approaches which predict the next point in a speech signal by extrapolating from previous points in a linear fashion.

That works because the organs that produce speech–the vocal cords–change over a much longer time period than the sound they produce. So they can be considered essentially stationary for this type of analysis.

Of course, one of the characteristics of chaos is that very small changes in starting conditions can produce large changes in output. And if that’s happening, what kind of chaos are we talking about?

Andreyev and Koroteev answer this question by measuring the frequency and amplitude of the sound a person makes when saying various vowels and consonants. They then use this data to reconstruct the multidimensional phase space in which the chaotic signal is produced.

The results are interesting because specific vowels appear to be linked to unique structures in the phase space. Andreyev and Koroteev call these structures phase portraits. The picture above is a phase portrait of the vowel sound ‘a’.

It’s a little harder to identify the shapes associated with consonants and the researchers haven’t yet tried with other sounds such as dipthongs.

It’s a long step from here to speech recognition but in principle, it could be done by looking for the phase portraits of specific phonemes and using them to spell out words.

The question, of course, is whether this would be easier or harder than current approaches.

Ref: arxiv.org/abs/0812.4172: On Chaotic Nature of Speech Signals

Entangled atoms could “sense” quantum gravity

Tuesday, December 23rd, 2008

entangled-atom-interferometry

The notion of quantum gravity has mystified many physicists, not least because there has never been a prospect of measuring the fabric of  the universe on this scale. That looks set to change.

A few years back, a number of physicists suggested that atom interferometry might do the trick. The thinking was that two atoms sent on different routes of equal length through space would then be made to interfere.

If spacetime is smooth and neat, the atoms should produce a certain set of fringes. But if spacetime on the plank scale were to be a maelstrom of quantum fluctuations, then these would force the atoms to travel slightly different paths and that would be picked up by the interferometer.

Sadly, it turns out that atom interferometers are nowhere near sensitive enough to detect these fluctuations and unlikely to become sensitive enough any time soon. The reason is that every three orders of magnitude increase in the sensitivity of the interferometer gives you only one order of magnitude increase in your ability to spot the fluctuations.

Which is why an idea floated by Mark Everitt and pals at the University of Leeds looks interesting. They say that the scaling problem effectively disappears if you use entangled atoms instead of ordinary ones.

And the improvement is such that the effect of quantum gravity should be detectable with current quantum optics technology.

They fall short of making any predictions so let’s fill in the blanks for them: somebody with a decent quantum optics lab will spot the first evidence of quantum gravity in 2009. Betcha!

Ref:  arxiv.org/abs/0812.3052:  Dephasing of entangled atoms as an improved test of quantum gravity