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From the archive, originally posted by: [ spectre ]
Scientists Take Step Toward Invisibility
By JOHN SCHWARTZ
Published: October 20, 2006
Invisibility has long been the stuff of fantasy, from Plato’s story
of the ring of Gyges to Harry Potter’s mischief-enabling cloak. But
scientists led by a team at Duke University have demonstrated a
technology that could be a small step in the right misdirection.
The system, a set of concentric copper circles on fiberglass board,
deflects electromagnetic waves of a specific frequency that strike it,
without much of the scattering and absorption that make reflections and
shadows.
A result is that the microwaves slide around the structure like water
flowing around a smooth rock in a stream, said David R. Smith, a
professor of electrical and computer engineering at Duke and an author
of the paper published today in the journal Science.
The exact structure of the circles was described in an earlier paper by
Sir John Pendry of Imperial College in London, who worked with the Duke
group to see his theory etched into a working model by means of the
process used to print circuit boards. In the recent paper, researchers
said they had successfully cloaked a copper cylinder.
The findings were first reported in The Sun, a British newspaper.
“Boffin Invents Invisibility Cloak,” the headline stated, using the
British slang for a research scientist.
Enthusiasts have already suggested that the technology may someday be
useful for the military to create objects that are invisible to radar
or to shield equipment from cellphone signals.
But Dr. Smith warned against getting ahead of the day’s announcement
and envisioning the disappearing Romulan warbirds of “Star Trek” on
the horizon. The work “is really a scientific explanation,” he
said, adding, “Whether it’s useful is always a question.”
Creating a cloaking device in the visible spectrum would be vastly more
complex, he said, since the device would have to warp all of the
wavelengths of light. The chance of creating such a device is
“dim,” he said, but, “The theory doesn’t prevent it from an
electromagnetic point of view.”
Businesses are already looking at possible applications, said Nathan
Myhrvold, a former chief technology officer of the Microsoft
Corporation whose company, Intellectual Ventures, explores the
potential of new inventions.
“We hope it’s got some commercial potential,” Mr. Myhrvold said.
“It could easily take years to figure out what the stuff is really
good for from a practical, pragmatic standpoint. But, boy, it sure is
really cool from a short-term standpoint.”
–
http://www.sciencemag.org/cgi/content/abstract/1133628
Published Online October 19, 2006
Reports
Submitted on August 8, 2006
Accepted on October 3, 2006
Metamaterial Electromagnetic Cloak at Microwave Frequencies
D. Schurig 1, J. J. Mock 1, B. J. Justice 1, S. A. Cummer 1, J. B.
Pendry 2, A. F. Starr 3, D. R. Smith 1*
1 Department of Electrical and Computer Engineering, Duke University,
Box 90291, Durham, NC 27708, USA.
2 Department of Physics, The Blacket Laboratory, Imperial College,
London SW7 2AZ, UK.
3 SensorMetrix, San Diego, CA, USA.
* To whom correspondence should be addressed.
D. R. Smith , E-mail: drsmith [at] ee [dot] duke [dot] edu
Recently published theory has suggested that a cloak of invisibility is
in principle possible, at least over a narrow frequency band. We
present here the first practical realization of such a cloak: in our
demonstration, a copper cylinder is ‘hidden’ inside a cloak constructed
according to the previous theoretical prescription. The cloak is
constructed using artificially structured metamaterials, designed for
operation over a band of microwave frequencies. The cloak decreases
scattering from the hidden object whilst at the same time reducing its
shadow, so that the cloak and object combined begin to resemble free
space.
–
http://www.ee.duke.edu/~drsmith/cloaking.html
The Science Fact and Fiction of Invisibility
There is undeniably a link between science fact and the ideas that
emerge in science fiction and fantasy. Science fiction authors are
inspired by actual scientific and technological discoveries, but allow
themselves the freedom to project the possible future course of these
discoveries and their potential impact on society, perhaps remaining
only weakly tethered to the facts. And, when faced with obstacles
presented by the realities of actual technology, authors of fiction can
break free from the tethers entirely, inventing completely imaginary
technologies to move their storyline forward. However, the most skilled
practitioners of the genre create compelling stories by having the
fictional technology maintain some connection to either existing or at
least projected technologies.
Scientists, in turn, often derive inspiration from the imaginative
possibilities that exist in fictional worlds, but are constrained to
follow the laws of nature that apply in this world. The inventions in
fictional worlds seldom transition to the real world,–at least not in
the way they are first imagined. But it does happen. Jules Verne wrote
about space ships and submarines before either were demonstrated.
Planet colonization and terraforming, space elevators and “bionic”
replacement limbs are science fiction concepts that have not yet fully
materialized into reality, but that are taken seriously by researchers
and are very active research topics. The idea of semi-intelligent
servant robots, once restricted entirely to the realm of fiction, is
now actively being pursued by both academic and corporate researchers.
Already, we can buy Roombas and Robovacs to tidy up around the house!
While the science in fiction seeks and achieves the same broad goals as
actual science seeks, it differs from actual science in that the
details are left out. Science reality is driven by a massive collection
of small details, mostly too difficult to describe easily except to
other experts in the field. To achieve the really big, exciting
results, scientists devote most of their time to what might seem like
small and even insignificant minutiae. Everyone can understand what a
cell phone is and what it does–conveying voice and other information
across space wirelessly–but only experts and the most devoted
enthusiasts really understand the underlying technologies such as
electromagnetic wave propagation or digital communication theory. As in
any technology, the details of wireless communication comprise volumes
of technical material representing decades of development.
So, the science in fiction necessarily short-cuts the scientific
process, providing largely only the “big” results. Of course, it has to
be this way, because a science fiction story is foremost a story, with
characters and a plot and all of the other things common to all
stories. The science, even if crucial to the plot, is secondary. If a
technology is introduced in a science fiction story, its purpose must
be easily understood by technical and non-technical readers alike. If
we know nothing else, we can understand that Death Star in the Star
Wars universe is a collosal weapon that has to be stopped; the warp
drive in the Star Trek universe enables faster-than-light travel, so
the Enterprise can zip around between galaxies in days rather than the
eternity it would take given our actual technology; an advanced
artificial intelligence computer chip enables the Terminator robot to
think and act in a nearly human way. In reality, scientists work on
topics that are nowhere near as easily conveyed to a non-technical
audience. It is especially difficult to communicate ideas originating
in the mathematical and physical sciences to non-experts, since most
concepts are conveyed using a language that amounts to a steady stream
of equations and formulas.
Our group studies the interesting electromagnetic properties of
artificially structured materials, or metamaterials. As
remarkable–sometimes even science-fiction like–as these metamaterials
are, we usually run into the problem that it is difficult to convey
their importance to non-experts. However, we have recently had the
opportunity to study a new type of electromagnetic metamaterial that
has a function easily understood by all. In a recent paper published in
the journal Science, we have outlined a theoretical method to design a
material that would render objects invisible. This topic has raised an
enormous amount of interest worldwide, and with that interest
inevitably comes rampant speculation about what might be possible with
this new paradigm. With all the speculation it can become easy to blur
the line between what is science fact and what remains science fiction!
So, we thought we’d describe here, in a non-technical way, our view on
the prospects of invisibility, and try to sort out the fact from the
fiction.
Invisibility in Science Fiction
Invisibility is a common theme in tales from science fiction, fantasy,
fables and mythology. If we exclude magic and supernatural mechanisms
for invisibility–sorry, but this includes Harry Potter’s cape and
Platform Nine and Three Quarters at King’s Crossing for the
moment–then we are left with a few more “scientific” routes to
invisibility that have been postulated in the science fiction genre. In
H. G. Well’s Invisible Man, a series of chemical experiments renders a
man invisible. Although the process is never fully described, one would
speculate that biochemistry has altered the invisible man’s molecules
so as to be inert to light passing by.
A chemical route to invisibility is pretty unlikely. The complex
molecules that make up human beings do absorb and scatter light, and
these interactions are often tied to other important biological
functions that would probably stop working if we tried to tinker too
much at the molecular level.
But, even the fictional invisible man suggests some of the issues that
would arise if invisibility were ever developed. For example, the
invisible man has the advantage only when his adversaries are not
expecting him to be there. If, on the other hand, we suspect an
invisible man is in our presence, we can simply toss paint or powder
everywhere randomly to reveal him!
In another fictional invisibility approach, mysterious “fields” are
created that can render people and objects undetectable, perhaps
routing the rays of light around the object to be concealed. Examples
of this sort of invisibility can easily be found science fiction: both
Susan Storm of the Fantastic Four as well as the Romulans in the Star
Trek universe are able to produce fields that can cloak people and
other objects. In both of these embodiments of cloaking, energy is
required to create the fields, which forms an important limitation of
the technology. The Romulans can shield themselves from detection, but
at the cost of not being able to utilize their power hungry weapons.
And Storm can vanish and make other objects invisible, but often not at
the same time as when she exercises some of her other “psionic” powers.
The speculative mechanisms of invisibility conjured up in Star Trek and
in the Fantastic Four contain a glimmer of reality within them.
Normally, of course, light travels in a straight path. According to
Fermat’s Principle, the reason light travels in a straight line is
because that is the shortest distance between two points. However, it
is known from Einstein’s theory of general relativity that an
object–any object–warps the very fabric of space-time around it. If
space itself is curved, then the shortest distance between two points
in that space can become a curve instead of a line. So, the trajectory
of light that passes near very massive objects, like suns or black
holes, is actually bent, often resulting in lensing and mirage-like
optical effects. Unfortunately, Einstein’s theory of general relativity
requires enormously massive, stellar-sized objects to provide a
noticeable warping of space. We also have no way to control this
effect: there is no “on” or “off” switch! So, while the bending of
light rays due to distorting the fabric of space is science fact, the
control of this distortion remains firmly the domain of science
fiction.
Approaching Reality
Although far-fetched, Storm’s invisibility is worth considering in a
bit more detail. What if one could really warp space at will?
The animation below shows a lattice representing space. One could
imagine the lines as interwoven threads in a fabric. A beam of light
that starts out traveling along one of the lines is constrained to stay
on that line, in accordance with Fermat’s Principle. Now, imagine that
there exists another space. Every point in this new space can be
related to a point in the first space by a mathematical function or
transformation. The important point is that we don’t need to maintain
the same density of space–we can squeeze and expand different volumes
of this new space so as to open up voids. The animation below depicts a
possible transformation from our space to a new space in which a void
appears. Our lattice has been warped in this new space, with light
still constrained to follow the now-curved lines. Light that is
incident on the void is actually swept around the void, in the same way
that the threads of a fabric would be pushed around if you tried to
create a hole in a fabric without breaking any of the threads. Light
now circulates around the void–like water flowing past a rock in a
stream. We have mapped to a space where a particular region just does
not exist! Light can’t illuminate nor be scattered by this region,
because it is outside of the space where light can even exist.
All of this warped space is compelling and great for science fiction.
But, unless we can somehow carry black holes around, it’s not likely we
will be able to alter the flow of light in any signficant way. Susan
Storm’s route to invisibility is thus not likely; one shouldn’t pay
very much to obtain the secrets of Romulan cloaking technology, either.
But this is where things start to become interesting! When space is
warped by a massive object, all physical phenomena are likewise
modified in the new distorted space. But, if we are concerned with just
modifying electromagnetism and electromagnetic fields, then we can
restrict ourselves to Maxwell’s equations–the equations that govern
how electric and magnetic fields behave. And here we’re in luck,
because unlike many other equations of physics, Maxwell’s equations
have parameters that enter that can easily be modified: the electric
and magnetic material parameters.
What does this mean for invisibility? We’ve decided that making people
or other objects invisible by chemically altering their constituent
molecules won’t work. Nor is there any reasonable hope that we will be
able to shield objects from detection by curving space. This leaves our
third and most viable option for invisibility–and one familiar to
Harry Potter fans–creating a cloak using some sort of material whose
parameters have been suitably chosen. But we don’t need to attend
Hogwarts School of Witchcraft and Wizardry to conjure such a cloak; we
can combine the idea of transforming space with Maxwell’s equations,
which will reveal precisely the necessary material properties for our
invisibility cloak.
Here is how it works. We start by transforming space in a desired
manner. To achieve invisibility, for example, we would like to push
space outward creating a nice concealment volume, as in the animation
above. Now, we can’t actually transform space, but in Maxwell’s
equations the material properties enter in such a way that we can
achieve the same effect by transforming the material properties. We
thus replace the space outside the concealment volume by a material–a
cloak–in which light rays travel the exact same paths they would have
travelled in the warped space. When the dust has settled, we arrive at
a set of material properties. The resulting material parameters for our
invisibility cloak will be complicated, of course, but will be fully
consistent with the known laws of physics.
In our recent Science paper, we have presented a mathematical approach
that provides us with the expected material parameters needed to make a
cloak. Does it work? We can test out the idea by a variety of different
methods. One method, used by lens designers, is called ray-tracing.
Starting with a bunch of rays that represent light, the path of each
ray can be traced as it passes through an object made of any material.
Ray tracing provides a good test as to whether or not the mathematical
transformation has predicted the right set of material parameters. The
figure to the right shows the result of a ray-trace performed on a set
of rays that pass through a spherical cloak. The objects to be
concealed are assumed to lie within the inner sphere, while the cloak
occupies the region between the inner and outer spheres. If one knew
nothing about the method that was used to design the material, the
conclusion based on ray-tracing alone would be that a cloak had indeed
been found.
The Reality of Cloaking
We have now succeeded in taking the prospect of invisibility from the
realm of science fiction and fantasy to reality, providing what amounts
to a blueprint for a cloaking device. It is now fair to ask, as with
any technology, what are the limitations? The capabilities and
limitations of cloaking will continue to be sorted out in the coming
months and years, but there are some issues that are clear from the
outset.
The cloak is a complicated structure. Not just complicated, but one
that requires materials that are not known to exist! This appears to be
one difficulty we can surmount by the use of artificial micro- and
nano-structures that can substitute for the lack of conventional
materials having the right properties. And while the cloaking
structures are complex as materials go, they are nevertheless easily
fabricated using available technologies.
There is an inherent limitation in bandwidth. This is actually clear
from the ray tracing figure above; note that rays that would normally
impinge on the cloaked sphere must instead be swept around the sphere,
essentially traversing a longer distance than they would have had they
passed directly through a volume of space. For all the rays to arrive
in step after swirling around the sphere, they must travel faster than
the speed of light in vacuum while in the cloak. This isn’t quite as
bad as it sounds. Without going into the details, it is possible for
electromagnetic waves to exceed the speed of light within a material,
but only at a particular wavelength (or, equivalently, frequency).
Thus, the material of which the cloak is made must disperse with
frequency–that is, our cloak can be designed to work optimally at a
targeted wavelength or bandwidth, but its performance will degrade
sharply away from the optimal bandwidth.
An ideal cloak would absorb no light whatsoever, since whatever amount
light is not transmitted by an object can be a signature that the
object is present. The artificial materials that we can currently
imagine using tend to absorb a significant amount of light that passes
through them, and this presents a very serious limitation that will
ultimately set the size of any object to be cloaked. At the moment, we
have a few strategies in mind that might help to soften the blow we
sustain from absorption in the material, but it is a problem we will
have to grapple with as we pursue cloaking.
Conclusions
We’ve provided a brief summary of the facts and fiction associated with
cloaking, but we’ve neglected an important point. While it would be
great if we could make things vanish entirely, a cloak needn’t
accomplish complete invisbility to be potentially useful. Remember how
easy it is to defeat the invisible man? Being invisible is only of
fleeting value at best if your adversary knows that you are there. The
advantage of invisibility comes about when your adversary has no idea
that you’re there. That is, invisibility is probably best thought of as
being a really good form of camouflage: it doesn’t have to be perfect
to work.
In perhaps one of the most realistic portrayels of invisibility, the
alien in the movie Predator possesses a cloaking device that renders it
nearly invisible. When cloaked, the predator is mostly transparent, but
there is a noticeable distortion of the transmitted light that just
vaguely outlines the shape of the predator. As depicted in the movie,
it is difficult to perceive the presence of the alien unless you know
it’s there and it’s in motion. Otherwise, the imperfect cloaking does a
pretty good job of keeping the alien well hidden. Although the
underlying fictional technology is not described in the movie, the
cloaking effect appears to be related to the alien’s armor, which would
make it more akin to the material cloak that we think might be
possible.
Along the lines of advanced camouflage, we should not overlook the
solutions that nature has provided. An object is invisible if it is
indistinguishable –or at least hard to distinguish–from its
surrounding environment. Many animals and insects have evolved in form
to blend into their enviornment, making it harder for predators
(animal, not alien) to find them. But some creatures, like chamelians
and cuttlefish, can change there appearance dynamically, virtually
being able to disappear in changing environments. This type of
camouflage/invisibility is similar to an optical device, invented by
Japanese scientist Susumi Tachi, that can provide a transparency effect
to people or objects.
A Route to Transparency: A “cloak” invented by Susumi Tachi suggests a
path to invisibility, or at least an interesting form of camouflage.
Invisibility of any sort will be a very difficult achievement, one that
will involve much more complication than we have even begun to delve
into here. As a result of the publication of our paper and several
others on the same topic, there have been reports in the media of Harry
Potter’s cape being “five to ten years off”; but those reports have to
be treated with some amount of realism. The physics of cloaking, as we
have outlined, is sound, and cloaks may be fabricated using the
artificial materials that have been introduced over the past several
years. But, as we have also described, there are serious and seemingly
unavoidable limitations on cloaking that will impair the performance of
any structure we can currently envision making. So, when we ourselves
project a demonstration will be possible, what we have in mind
initially is a very specific sort of structure that will most likely be
useable (but not necessarily useful!) at very long wavelengths in the
electromagnetic spectrum–radio frequencies for example, where
wavelengths are on the order of many centimeters to meters. It may not
be quite as exciting as making objects visibly disappear, but it will
be an important step. How much further might we go beyond this initial
demonstration is an open question, but given the tremendous interest in
the area, we can say for certain that the scientific community will do
its best!
One last point to consider is that the entire design paradigm that
leads to the cloak–starting by transforming space and then determining
the equivalent electromagnetic material–represents a new approach to
optics. Just five years ago this idea of transform optics might have
been abandoned because the resulting material requirements would have
been considered impractical. With the advent of metamaterials, that
conclusion has now changed, and we can envision entirely new classes of
optical devices, invisibility cloaks being just one example. So, while
we have been inspired by the invisibility of fictional worlds, perhaps
the discoveries that might follow from transformation optics will in
turn have an impact in fictional worlds–as well as in the actual world.