Svi moji mladalački snovi na putu su da se ostvare. Ovo nađoh na blogu Warrena Ellisa:
http://news.yahoo.com/s/afp/20070406/sc_afp/ussciencephysicsinvisibility;_ylt=AqzJv7AbmCXR9NeichSy_KsPLBIF
Quote from: "Meho Krljic"Svi moji mladalački snovi na putu su da se ostvare.
prokletstvo je samo u tome što nam se mladalački snovi, if at all, ostvare pod stare dane... :( :cry:
prokleti mc fate! :evil:
Jeste, ali ipak ne treba pljuvati u usta ovom blagoslovu. Supervoajerske fantazije su na korak od ostvarivanja, nemojmo poklonu gledati u zube već budimo zahvalni Alahu na njegovoj bezgraničnoj milosti.
Jos ako, tako nevidljivi, uzjasemo i leteci cilim iz N.Pazara... Sky is the limit! Toliko zenskih svlacionica ce moci da se obidje za relativno kratko vreme.
Jes' samo je taj ugao nesto dalje od iduceg coska :mrgreen:
"...Calculations indicate the device would make an object invisible in a wavelength of 632.8 nanometers, which corresponds to the color red. The same design, however, could be used to create a cloak for any other single wavelength in the visible spectrum, Shalaev said...." - cela prica je ovde (http://news.uns.purdue.edu/x/2007a/070402ShalaevCloaking.html).
Elem, kako ne znam koliko tacno ima tih "any other single wavelength", mozda nije zgoreg podsetiti se kako izgleda spektralna osetljivost oka (x-osa talasna duznima u nanometrima, y-osa intenzitet, normiran na talasnu duzinu maksimalne osetljivosti)jos mo, te sagledati da je pomenut cose malo dalje:
(https://www.znaksagite.com/diskusije/proxy.php?request=http%3A%2F%2Fwww.rwc.uc.edu%2Fkoehler%2Fbiophys%2Fimages%2F6dimg3.gif&hash=865c4a1f2275c23e36d8c841a58b2f4fe4a2100a)
But, nada nikad ne umire, a snovi jos i redje, zar ne? :mrgreen:
***
Ima i nesto stariji hype o Harry Potter style ogrtacu, i tekst o tome, iz februara, bih stavila link, ali ne moze, treba passwd, pa ide kopirano dole u celini. Ko hoce:
Invisibility cloaks: Now you see them...
by Justin Mullins
Last September, news began leaking out of an extraordinary breakthrough. A team at Duke University in North Carolina had built a device that seemed more at home in a fantasy novel than a real lab. "First demonstration of a working invisibility cloak," screamed the press release when the news became official, invoking the garment J. K. Rowling gave her fictitious wizards. Sure enough, the resultant headlines were filled with references to Harry Potter, Star Wars and Klingon cloaking devices.
The truth was more prosaic. David Smith's team at Duke had indeed built a device that could hide an object from view, but only from the "eyes" of a microwave detector - and then only at a very specific microwave frequency. What's more, the device would work only for "flatlanders" living in two dimensions, and was hardly a cloak; it was more like a small invisibility barrel. Nevertheless, the team's work is not to be sniffed at. Hiding anything at any frequency is an impressive feat.
Smith hasn't stopped there. He is already rumoured to be building the kit he needs to test a 3D version of his device. You might be forgiven for thinking that it's only a matter of time before news of a Harry Potter-style cloak that works for visible light breaks into the headlines. In reality, the future is less clear. For some time, researchers have been saying that refining the "metamaterials" needed to make an invisibility cloak so that they work at optical frequencies is just too tricky.
A metamaterial is a manufactured structure whose internal configuration allows it to do things with light or other kinds of electromagnetic radiation that are beyond the powers of ordinary mirrors and lenses. While microwave metamaterials may be making good progress, in the optical region there are numerous problems, not least of which is that the necessary components absorb too much of the light they are meant to simply bend. It's possible that these problems are solvable, though nobody is sure how. It is also true that the laws of physics may prevent this kind of technology from ever working. The reality behind the headlines is that today an invisibility cloak that works in the optical part of the spectrum is nowhere in sight.
Setting aside such doubts, however, it's easy to imagine an invisibility cloak that works on the same principle as Smith's device. You just have to conceal yourself using a material that steers light around you so that someone looking in your direction sees only what lies behind you (see Diagram). What makes this fiendishly difficult in practice is the nature of light.
Light consists of oscillating electric and magnetic fields travelling in the form of electromagnetic waves. When such a wave passes through a material, the way that its magnetic component interacts with the material is called the material's "permeability", and the interaction of the electric component is its "permittivity". Permittivity and permeability are what you need to control if you want to make an invisibility cloak.
The basic principles are familiar. When a ray of light strikes the surface of a transparent material, part of the light is reflected, but the rest that enters the material is bent or refracted. That's why a straight stick dipped in water appears to be bent at the water's surface. The permeability and permittivity of a material determine how much it refracts light, and the degree of bending is called the refractive index.
The permeability and permittivity of water and glass are expressed as constants with positive values. That gives them a positive refractive index: in other words, if you imagine a line passing perpendicularly through the surface of a material at the point where a light ray hits the surface, the refracted ray will always be bent somewhere to the other side of the line from the side where the light strikes the material.
Until 1968, it was thought impossible for materials to behave in any other way. Then the Soviet physicist Victor Veselago showed that if a material had negative permittivity and permeability, it would have a negative refractive index, bending light "back" as if it had been reflected at that imaginary line
Metamaterial world
That was the first time that anyone had thought of a way that light could be steered in ways beyond the power of conventional lenses and mirrors. However, nobody took much notice of Veselago's ideas until the mid-1990s, when John Pendry, a theoretical physicist at Imperial College London, began to wonder about how light might interact with structures other than atoms and molecules.
Pendry calculated how microwaves would be affected when they passed through an array of thin, parallel conducting wires. Such an array would be a metamaterial because microwaves don't "see" the wires themselves, only the effects the wires have on electric and magnetic fields. Pendry found that the metamaterial would have a negative permittivity - the first time anyone had proved this was possible in an artificial material. By figuring out the properties of other arrays of electronic components, Pendry also calculated how to build a material with a negative permeability to microwaves.
In 2000, Smith constructed the metamaterial Pendry had dreamed up and finally demonstrated the strange properties that Veselago had imagined 40 years before, including bending microwaves in the opposite direction to every other material (New Scientist, 14 April 2001, page 35).
Early last year, Pendry took the research a stage further when he showed that it was possible to construct a material in which the permeability and permittivity vary continuously from point to point. This meant it might be possible to steer light around a central region within a material so that anything within this void would, to all intents and purposes, be invisible. Within hours of Pendry's paper being released, the notion of an invisibility cloak was hitting the news.
Building the dream
Although Pendry's work was theoretical, curiously Smith - an experimentalist - was named as a joint author. What nobody realised at the time was that all the speculation about invisibility was being brought to life in Smith's lab. By the time Pendry's paper appeared, Smith had built a working version of the microwave cloak that hit the headlines in September.
"Nobody realised that invisibility was about to be brought to life"
Unlike the heady few years for researchers studying the microwave part of the spectrum, until recently those toiling on the equivalent technologies in the optical region have struggled to get off the starting line.
So what's the problem - shouldn't optical metamaterials work in exactly the same way as their microwave counterparts? "If you shrink the components used in microwave metamaterials, you should get a material that works for visible light," says Ulf Leonhardt, a physicist at the University of St Andrews in the UK who studies metamaterials. Trouble is, that's easier said than done. The components of a metamaterial have to be significantly smaller than the wavelength of the radiation in question to avoid distorting the rays by diffraction, the process in which waves bend around an obstacle in their path.
To interact with microwaves that have a wavelength of just over 3 centimetres, Smith uses an array of electronic components each only a few millimetres in size; visible light has wavelengths ranging from about 400 nanometres for violet light to 700 nanometres for red. To repeat Smith's trick in the visible spectrum, you would need to create a metamaterial with components around 40 nanometres across. Components of that size have been made, and while assembling them into a 3D array that behaves like a metamaterial is still beyond researchers, it ought to be possible. "It shouldn't require a huge leap to get there," says Pendry.
There is a more formidable problem in the works. Even if you could build nanoscale versions of Smith's structures, they probably wouldn't work because the electromagnetic properties of metals change with scale. A structure a few millimetres in size has a different effect on microwaves than a nanoscale structure has on visible light. Worse, this change is not easy to predict. "We cannot be sure how these metals will behave in such a tiny structure," says Leonhardt.
It's down to the way that light's electromagnetic field excites the sea of free electrons within a metal. Microwaves do this, but to a negligible extent. At the shorter wavelengths of light the effect gets bigger. In most metals the frequency of such waves is close to a value known as the plasma frequency, which is particular to each metal. The result is that the photons are absorbed, setting up a resonance that sends huge waves of electrons called plasmons sloshing back and forth through the metal. This absorption process would prevents metamaterials from working in the optical part of the spectrum, or so researchers believed back in 2000 before Smith built the material Pendry dreamed up.
However, it turns out that another factor can have an important influence on the plasma frequency: the shape of the metal. The geometry of the metallic components affects the concentration of electrons on the surface of the metal, which in turn changes the plasma frequency. So physicists should be able to design metamaterials that transmit light at wavelengths which would otherwise be absorbed. Unfortunately, nobody knows how to do it. Designing geometries that will do the trick is more of an art than a science. "It's very hard to know in advance how a particular design will work," says Pendry.
Against all odds, some progress is being made - but not without controversy. In 2005, Alexander Grigorenko and colleagues at the University of Manchester, UK, claimed to have used these techniques to make a metamaterial with a negative permeability, a first for the optical world. Grigorenko's metamaterial is a flat surface patterned with pairs of nanoscale gold rods that interact electromagnetically when light passes by, in a way that tends to reinforce the light's magnetic fields. This gives the material its negative permeability - and gold has a negative permittivity already.
This is by no means a technological slam dunk, however. Grigorenko's material absorbs too much light - it can penetrate but not pass through - and the work has come under fire from several quarters because his measurements give only indirect evidence that the material works. Proving that a material has a negative refractive index, the critics say, means actually seeing it bend light "backwards", or observing one of the other strange properties the theory predicts. Grigorenko agrees, but has yet to come up with the evidence.
In January this year, however, the matter was settled by Martin Wegener at the University of Karlsruhe in Germany and colleagues with a flat silver grid-like structure. Instead of measuring the way this material bent light, Wegener and colleagues looked at another theoretical prediction relating to differences in the phase of light as it enters the metamaterial and as it leaves. The results implied that the material has a negative refractive index of -0.6 for red light.
Before the back-slapping can begin in earnest, however, Wegener and his colleagues must deal with a number of shortcomings of the material. It is anisotropic, meaning that it works only when light passes through it in certain directions. Because of this, it looks different depending on which direction the light moves through it. Another problem is that less than half the light gets through, even though the structure is only a few tens of nanometres thick, making it useless for any practical invisibility cloak.
Whether these problems can be overcome is a matter of some debate. Pendry, at least, is optimistic. He says that while pure metals absorb so much light that they are almost unusable at optical frequencies, certain alloys are much more promising. He highlights in particular an alloy of gold and caesium that he believes should be far less likely to absorb light at optical frequencies than gold or caesium alone. "Nobody has studied the properties of this alloy. I'd like them to do that," he says.
Other materials are also being explored. In January, a group at the California Institute of Technology in Pasadena announced they had measured a negative refractive index by passing visible light through a sheet of silicon nitride sandwiched between two layers of silver. Again, opacity was a problem - only 1 per cent of the light entering the material actually leaves it.
Chameleon cloak
Costas Soukolis, a physicist at Iowa State University in Ames and one of Wegener's collaborators, thinks a better option is to stick with light-absorbing materials and continuously replace the absorbed light. This should be possible because the active metal components in optical metamaterials have to sit in a passive material known as a dielectric, which can be glass, air or any of a variety of other options. Soukoulis wants to use a lasing material as a dielectric so that light striking the metamaterial could stimulate the emission of more light. This would replace the light that would be absorbed, thereby ensuring that the light coming out of the material is as strong as the light that went in. He concedes that this will be tricky: "Nobody has done this experiment," he says.
Then there is the problem of designing and building structures that work for light travelling in any direction. "We have designed structures that we think are an improvement but we do not have the expertise to grow them," says Soukoulis, echoing a common concern. Nobody knows how to build these complex 3D structures on such a tiny scale.
Despite the excitement around the limited successes of metamaterials, it's possible that the solution to these problems may come from a different kind of technology. Another way to steer light inside a material is to carve channels within it to form a so-called photonic crystal. These can be used to diffract light in a controlled way and they need only be made to roughly the same scale as the wavelength of light, rather than 10 times smaller as is the case for metamaterials. A photonic crystal's ability to steer light on this scale could be a key advantage in building an invisibility cloak.
It would be fair to say that invisibility cloaks are the last thing on the minds of most photonic crystal researchers, however. Most are cautiously aiming for less ambitious goals, but even these require breakthroughs in the understanding of photonic crystals. Igor Smolyaninov at the University of Maryland, College Park, for instance, has designed a photonic crystal lens capable of capturing optical images of the kind you can normally only get with an electron microscope.
The trick here is to capture the fine details of the light emitted at a distance of less than a wavelength from the surface of the object you want to look at - the so-called near field. Smolyaninov's idea is to coat the object in concentric shells of polymethyl methacrylate (better known as plexiglas or perspex) like growing a pearl: each shell magnifies details of the near field. Smolyaninov has tested the idea and says it could one day be used to image DNA, viruses and even proteins. "These kinds of devices require only moderate engineering breakthroughs," says Pendry.
Many believe that building metamaterials and photonic crystals that can bend visible light in ways previously impossible is simply an engineering problem rather than a fundamental scientific challenge. Whether that will result in an optical invisibility shield, nobody is willing to say. What they will admit, though, is that the downright pessimism that reigned only five years ago is slowly being replaced by a cautious optimism.
From issue 2591 of New Scientist magazine, 16 February 2007, page 38-41