Invisibility cloak

[Nihilist]

http://news.yahoo.com/s/ap/20061019/ap_on_sc/cloak_of_invisibility

Interesting stuff.

[nbk2000]

As it stands, it is 'invisible' to microwaves, which is what RADAR uses.

A lot of motion detectors use RADAR in conjunction with a PIR to eliminate falsing.

Foam takes care of the PIR, and now this can take care of the RADAR. :)

It'll be years before there's anything available to use, though. :(

[Skean Dhu]

The real hitch in the quest for cloaking, is being able to see after you've been cloaked. Because once you go bending electron waves around yourself or the vehicle you are occupying, you render yourself blind.

Since none of the waves are de/reflecting off of you, none of them are reaching your eye , FLIR, NVG ,or whatever optic system you are using. What ever set of waves you are invisible to you are also blind to.

[Hirudinea]

Interesting stuff.

I saw this on the news today as well (CBC Snoozeworld), the inventor said that while this worked in microwaves (around 3cm in wavelength) that it would be years before it would work in visible light (nanometers), but since it is more akin to a printing process and microchips are laid down in nanometers this seems like an engineering problem, not sci-fi. Another problem with these "cloaks" is that they are made of a rigid material (copper, as in a large copper tube) so don't order your Predator camo suit too soon. :D
The inventor also mentioned that the "cloak", since it directs the radiation around whatever it surrounds it could also be used to protect delicate equipment from electromagnetic interference, and (and I'm just supposing here) in the future, much lighter protection from radiation than traditional lead sheilding, since this relies on channeling the energy, not blocking it. All in all very interesting, it will be interesting to see what they come up with, or more the point not to see it. :rolleyes:

[Sausagemit]

I acctually read a really good article/interview about this in Electronic Engineering Times a while ago. After some searching I found it in electronic form though our school library. No link available though and to view it through EETimes.com you have to register.


Copyright 2006 CMP Media LLC
All Rights Reserved
ELECTRONIC ENGINEERING TIMES

August 14, 2006

SECTION: NEWS; The interview; Pg. 20

LENGTH: 1879 words

HEADLINE: Metamaterials hold key to cloak of invisibility

HIGHLIGHT:

A cloak of invisibility: It sounds like the stuff of comic book superheroes.
In fact, invisibility cloaks for any type of electromagnetic radiation-even
visible light-are something Duke University postdoctoral fellow David Schurig
believes are within grasp. Turig and the professors directing his research-David
Smith at Duke and Sir John Pendry at the Imperial College in London-maintain
that by the end of 2007, metamaterials will enable an invisibility cloak that
works in the microwave range. Further engineering effort will create such cloaks
for other types of light, the researchers say.

BODY:

Metamaterials, or engineered composites, substitute macroscopic objects for
atoms in a giant crystalline-like lattice, enabling the pitch of
passive-component arrays to set the wavelengths affected. The design of these
component arrays harks back to the first principles of electronics-simple R-C-L
(resistor-capacitor-inductor) circuits. The electromagnetic waves passing
through arrays of tiny resistors, capacitors, inductors and other dielectric
materials positioned in free space can be bent down any designer-specified path.

In 2000, Smith, who was then at the University of California-San Diego, and
his colleagues demonstrated a composite metamaterial that used embedded passive
resonators to bend microwaves backward. The circuit elements that the team used
were based on the theoretical analysis provided by Pendry and his colleagues in
London.

Schurig recently sat down with technology editor R. Colin Johnson to describe
how the cloaking device works, and what EEs can do to turn this science fiction
idea into fact.

EE Times: Where did the cloaking concept originate?

David Schurig: The cloaking concept came from Sir John Pendry's 1996 paper
with Andrew Ward. That work simplified finite-element codes by taking a
complicated geometry and transforming it into a simpler geometry before doing
simulations-this was before commercial codes made meshing so easy. What Pendry
realized, just last year, was that the material properties that he and Ward used
in these fictitious spaces were realizable now, thanks to this new field of
metamaterials.

EE Times: So it was a decade-from 1996 to 2006-between Pendry's seminal paper
and the cloaking device research that he collaborated on with you and professor
David Smith at Duke?

Schurig: Yes. John [Pendry] imagined this strange space that can be curved or
twisted or bent, or even have holes in it to hide things. What his new theory
told us was how to construct materials that make our normal space behave the
same way as this hypothetical space.

EE Times: How should EEs imagine these transformations?

Schurig: They should imagine a piece of cloth woven with a hole in it made by
pushing a pointed object between the threads without tearing them. This hole is
where you hide something in the two-dimensional space. Electromagnetic fields
are confined to move along the threads, and can never access anything hidden in
the hole.

Of course, the trick is to come up with the material properties that make our
normal space, which doesn't have holes in it, behave as if it did. To do that,
you take a mathematical description of how the distorted thread pattern differs
from the normal weft and weave of the cloth. This is the coordinate
transformation. Then you ask, is there a set of material properties that will
give the same form for Maxwell's equations as you find for them under this
coordinate transformation?

Those material properties make electromagnetic fields in our boring, flat,
hole-free space behave as they do in the much more interesting, distorted space.

EE Times: Maxwell's four differential equations summarize all the properties
of electromagnetic fields. So I guess you guys spent the last year learning how
to transform them to handle odd-shaped spaces?

Schurig: Pendry did the first couple of transformations himself, then he
started to talk to David [Smith] and me about it, and we all agreed that it was
pretty interesting. So we started doing more transformations. Then I wrote some
ray-tracing code to give independent confirmation that these things really do
behave the way the theory says they are supposed to.

And that all proved out. Since then, another professor here, Steve Cummer,
has done full-wave simulations using Maxwell equation solvers. Those also worked
without a hitch, which to us was very surprising, because for the light to go
around an object and still be in phase on the other side, you would think it
would have to exceed the speed of light.

EE Times: I've read about that. The light has to travel a longer distance
than usual around the outside of the cloaked region, and since by definition it
is already traveling at the speed of light, the extra distance would mean it had
to go even faster to stay in phase with the light in the surroundings. But it
doesn't really transport energy faster than the speed of light, does it? Doesn't
it really just use stored energy built up in the steady state to make one
specific frequency's phase fronts exceed the speed of light?

Schurig: That's right. You've done your homework. You have a good
understanding of dispersion and relativity-that's exactly it. But it's not as
hard as it sounds. Remember, it's easier to hide things from the human eye than
[from] a spectrometer [laughs]. The eye only sees in three bands [red, green and
blue], and there are all kinds of ways to trick it. The easiest way is when the
environment is monochromatic, like in a jungle setting where most everything is
bathed in green light; there you could do very well, even though your cloak has
limited bandwidth.

EE Times: Don't these metamaterials work only for long wavelengths, such as
microwaves? Are metamaterials available for the visible region today?

Schurig: Metamaterials do not currently work on visible light; that's
probably 10 years off. But in principle, it's possible.

EE Times: I can imagine making a traditional lens-and-projection system to
route visible light around an object. Why, in principle, do you need
metamaterials at all?

Schurig: For one thing, the specifications that come from the transformation
theory require that the material be both inhomogeneous, meaning its properties
need to vary from point to point, and anisotropic, meaning that the properties
depend on the orientation of the fields, too. And finally, you also need
materials [with] permittivity and permeability that are less than that of free
space.

EE Times: Is that where the negative index of refraction comes from?
Permittivity and permeability are measures of a material's capacity to form
electrical and magnetic fields inside it, respectively. Aren't both always
positive for natural materials, meaning that the response of the material is
always in phase with applied fields?

Schurig: Usually, yes, but for an invisibility cloak we don't need negative
values, just values that are smaller than free space.

EE Times: But isn't free space defined as having an index of refraction equal
to 1?

Schurig: Yes, but in this case we don't need negative values. We just need
values between 0 and 1.

Metamaterials research has already shown that negative indices are difficult
if not impossible to do with natural materials. But it is also true that indices
of refraction that are positive, but less than one, are just as difficult to
achieve in natural materials as negative values, and for the same reason: Both
cases require a resonance, and in traditional materials the resonance is seldom
where you want it to be. But with engineered composites, or metamaterials, you
can put the resonance where you want it.

EE Times: A resonance at the working frequency, here at the wavelength you
are trying to cloak?

Schurig: Yes. It's relatively easy to engineer a metamaterial that has the
correct permittivity, permeability and anisotrophy you need at every point to
cloak something, but it's hard to imagine doing that with natural materials.

EE Times: The metamaterials I'm familiar with work on microwaves and use
circuit boards with split-ring oscillators on them to provide resonance at the
working frequency. Is that the setup you are proposing too?

Schurig: Yes, that is mostly what we do here. In fact, our first
demonstration of a cloak will use those same circuit-board-based materials.

EE Times: Are you going to build a real working cloak?

Schurig: As you might guess, we are working on that right now.

EE Times: I am guessing that your first real cloaking device will provide a
smooth transition effect around a known volume at microwave frequencies. Is that
the case?

Schurig: Yes.

EE Times: Even that's got real applications. For example, why couldn't you
use one of those to cloak a spy satellite to keep radar from finding it?

Schurig: Absolutely. Hiding from radar is an application where metamaterials
are already working in the right frequency range. Visible light, for sure, is on
the order of 10 years off, but using [a cloaking device] to hide from radar
could be much sooner.

EE Times: For your initial experiments in the microwave range, how big a
volume are you going to try to cloak?

Schurig: Well, our measuring apparatus is only designed to measure things on
the order of tens of centimeters.

EE Times: So you will start with a volume that measures less than 100
centimeters on a side?

Schurig: Yes, our demonstration cloak will be on that kind of size scale.

EE Times: When do you expect to finish it?

Schurig: Originally we were telling people about a year, but things are going
pretty well, so now we expect to have a demonstration substantially sooner than
that.

EE Times: What advice do you have for EEs who might be interested in
experimenting with cloaking?

Schurig: A big part of the design process here keeps harking back to basic
circuit models. Those models that EEs learned in school are very powerful-we use
them over and over again to design our metamaterials. Just intuitively, when you
think about a metamaterial's split-ring resonators, you think about changing
their properties by changing the inductance and capacitance of the split rings,
which changes their resonant frequency.

EE Times: Does this mark a resurgence of basic electronic-engineering skills,
but applied at a different size scale?

Schurig: We think so. Most of us are physicists here, but we are using the EE
's basic circuit models all the time. In fact, we are all working in the EE
department here at Duke.

EE Times: After perfecting these circuitry principles at the scale of circuit
boards for microwaves, do you expect these structures to then migrate down to
microelectromechanical systems?

Schurig: Yes, I think that will happen over the next 10 years. Getting them
down to the nanoscale will enable cloaks to work at visible-light frequencies.

EE Times: The cloaking effect is always going to be limited in bandwidth-tied
to a frequency range. But could there be some way to use three parallel systems,
like RGB, to make a cloak work for the whole range of visible light?

Schurig: Yes, that's possible, or a single system could be dynamically
adjustable too. There are lots of little tricks that engineers will be able to
play.

If EEs are interested in these cloaking devices, then I suggest that they
start learning about metamaterials today. That's the technology that is going to
make the cloaking device a reality.

---

David Schurig

Born: March 19, 1966, Burbank, Calif.

Education: PhD, physics, 2002; BS, engineering physics, 1989; University of
California at San Diego

Here is another interesting article from the same publication...

Cloaking Device Postulated (http://www.eetimes.com/news/semi/showArticle.jhtml;jsessionid=3NFLK40EV1BZAQSNDLOSK HSCJUNN2JVN?articleID=188500975)

[Syke]

Imagine a world with inviso pigs (shudder)

[Jacks Complete]

http://www.newscientisttech.com/article/dn10334-working-invisibility-cloak-created-at-last.html gives a good account of it.

The limitations are noteworthy: Works at a single frequency, but can possibly be re-configured to another frequency in the same range electronically, rigid conductive surface to the item being cloaked, fairly heavy. It also doesn't tell us what the upper limit is to the total size of the object vs. the wavelength being distorted, either.

However, for something like a ship, this would be rather neat. Incoming radar pulses would be detected and then the skin would configure itself to deflect that frequency. The modern EW/ELINT gear has a threat library, and this tells the target the capabilities of the system they are up against, and so a system with a predictable pulse rate would be easy to hide from. If there were a second system, the capacitance and reactance of the skin could be reconfigured to avoid that, too, as long as the pulses were far enough apart that the system switched in time.
On the ship, you combine the microwave system with the water-spray optical cloaking system, and bingo, you are nearly invisible!

It might work for a tank too, but since I don't know of anyone who uses radar at ground level to detect tanks, it would be pointless unless you were facing radar guided anti-tank missiles or something.